8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 1/739 The goal of this documentation is to comprehensively explain the Node.js API, both from a reference as well as a conceptual point of view. Each section describes a built-in module or high-level concept. Where appropriate, property types, method arguments, and the arguments provided to event handlers are detailed in a list underneath the topic heading. If errors are found in this documentation, please submit an issue or see the contributing guide for directions on how to submit a patch. Every file is generated based on the corresponding .md file in the doc/api/ folder in Node.js's source tree. The documentation is generated using the tools/doc/generate.js program. An HTML template is located at doc/template.html . Throughout the documentation are indications of a section's stability. The Node.js API is still somewhat changing, and as it matures, certain parts are more reliable than others. Some are so proven, and so relied upon, that they are unlikely to ever change at all. Others are brand new and experimental, or known to be hazardous and in the process of being redesigned. The stability indices are as follows: Stability: 0 - Deprecated. The feature may emit warnings. Backward compatibility is not guaranteed. Stability: 1 - Experimental. This feature is still under active development and subject to non-backward compatible changes or removal in any future version. Use of the feature is not recommended in production environments. Experimental features are not subject to the Node.js Semantic Versioning model. Stability: 2 - Stable. Compatibility with the npm ecosystem is a high priority. Caution must be used when making use of Experimental features, particularly within modules that may be used as dependencies (or dependencies of dependencies) within a Node.js application. End users may not be aware that experimental features are being used, and therefore may experience unexpected failures or behavior changes when API modifications occur. To help avoid such surprises, Experimental features may require a command-line flag to explicitly enable them, or may cause a process warning to be emitted. By default, such warnings are printed to stderr and may be handled by attaching a listener to the 'warning' event. Stability: 1 - Experimental Node.js v10.16.3 Documentation About this Documentation Contributing Stability Index JSON Output 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 2/739 Every .html document has a corresponding .json document presenting the same information in a structured manner. This feature is experimental, and added for the benefit of IDEs and other utilities that wish to do programmatic things with the documentation. System calls like open(2) and read(2) define the interface between user programs and the underlying operating system. Node.js functions which simply wrap a syscall, like fs.open() , will document that. The docs link to the corresponding man pages (short for manual pages) which describe how the syscalls work. Most Unix syscalls have Windows equivalents, but behavior may differ on Windows relative to Linux and macOS. For an example of the subtle ways in which it's sometimes impossible to replace Unix syscall semantics on Windows, see Node.js issue 4760 . node [options] [V8 options] [script.js | -e "script" | - ] [arguments] Please see the Command Line Options document for information about different options and ways to run scripts with Node.js. An example of a web server written with Node.js which responds with 'Hello, World!' : Commands displayed in this document are shown starting with $ or > to replicate how they would appear in a user's terminal. Do not include the $ and > characters. They are there to indicate the start of each command. There are many tutorials and examples that follow this convention: $ or > for commands run as a regular user, and # for commands that should be executed as an administrator. Lines that don’t start with $ or > character are typically showing the output of the previous command. Firstly, make sure to have downloaded and installed Node.js. See this guide for further install information. Now, create an empty project folder called projects , then navigate into it. The project folder can be named based on the user's current project title, but this example will use projects as the project folder. Linux and Mac: $ mkdir ~/projects $ cd ~/projects Windows CMD: > mkdir %USERPROFILE%\projects > cd %USERPROFILE%\projects Windows PowerShell: > mkdir $env:USERPROFILE\projects > cd $env:USERPROFILE\projects Next, create a new source file in the projects folder and call it hello-world.js . In Node.js it is considered good style to use hyphens ( - ) or underscores ( _ ) to separate multiple words in filenames. Syscalls and man pages Usage Example 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 3/739 Open hello-world.js in any preferred text editor and paste in the following content: const http = require('http'); const hostname = '127.0.0.1'; const port = 3000; const server = http.createServer((req, res) => { res.statusCode = 200; res.setHeader('Content-Type', 'text/plain'); res.end('Hello, World!\n'); }); server.listen(port, hostname, () => { console.log(`Server running at http://${hostname}:${port}/`); }); Save the file, go back to the terminal window enter the following command: $ node hello-world.js An output like this should appear in the terminal to indicate Node.js server is running: Server running at http://127.0.0.1:3000/ Now, open any preferred web browser and visit http://127.0.0.1:3000 . If the browser displays the string Hello, World! , that indicates the server is working. Many of the examples in the documentation can be run similarly. Stability: 2 - Stable The assert module provides a simple set of assertion tests that can be used to test invariants. A strict and a legacy mode exist, while it is recommended to only use strict mode . For more information about the used equality comparisons see MDN's guide on equality comparisons and sameness . A subclass of Error that indicates the failure of an assertion. All errors thrown by the assert module will be instances of the AssertionError class. options <Object> Assert Class: assert.AssertionError new assert.AssertionError(options) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 4/739 message <string> If provided, the error message is going to be set to this value. actual <any> The actual property on the error instance is going to contain this value. Internally used for the actual error input in case e.g., assert.strictEqual() is used. expected <any> The expected property on the error instance is going to contain this value. Internally used for the expected error input in case e.g., assert.strictEqual() is used. operator <string> The operator property on the error instance is going to contain this value. Internally used to indicate what operation was used for comparison (or what assertion function triggered the error). stackStartFn <Function> If provided, the generated stack trace is going to remove all frames up to the provided function. A subclass of Error that indicates the failure of an assertion. All instances contain the built-in Error properties ( message and name ) and: actual <any> Set to the actual value in case e.g., assert.strictEqual() is used. expected <any> Set to the expected value in case e.g., assert.strictEqual() is used. generatedMessage <boolean> Indicates if the message was auto-generated ( true ) or not. code <string> This is always set to the string ERR_ASSERTION to indicate that the error is actually an assertion error. operator <string> Set to the passed in operator value. const assert = require('assert'); // Generate an AssertionError to compare the error message later: const { message } = new assert.AssertionError({ actual: 1, expected: 2, operator: 'strictEqual' }); // Verify error output: try { assert.strictEqual(1, 2); } catch (err) { assert(err instanceof assert.AssertionError); assert.strictEqual(err.message, message); assert.strictEqual(err.name, 'AssertionError [ERR_ASSERTION]'); assert.strictEqual(err.actual, 1); assert.strictEqual(err.expected, 2); assert.strictEqual(err.code, 'ERR_ASSERTION'); assert.strictEqual(err.operator, 'strictEqual'); assert.strictEqual(err.generatedMessage, true); } When using the strict mode , any assert function will use the equality used in the strict function mode. So assert.deepEqual() will, for example, work the same as assert.deepStrictEqual() . On top of that, error messages which involve objects produce an error diff instead of displaying both objects. That is not the case for the legacy mode. It can be accessed using: Strict mode 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 5/739 const assert = require('assert').strict; Example error diff: const assert = require('assert').strict; assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]); // AssertionError: Input A expected to strictly deep-equal input B: // + expected - actual ... Lines skipped // // [ // [ // ... // 2, // - 3 // + '3' // ], // ... // 5 // ] To deactivate the colors, use the NODE_DISABLE_COLORS environment variable. Please note that this will also deactivate the colors in the REPL. Stability: 0 - Deprecated: Use strict mode instead. When accessing assert directly instead of using the strict property, the Abstract Equality Comparison will be used for any function without "strict" in its name, such as assert.deepEqual() . It can be accessed using: const assert = require('assert'); It is recommended to use the strict mode instead as the Abstract Equality Comparison can often have surprising results. This is especially true for assert.deepEqual() , where the comparison rules are lax: // WARNING: This does not throw an AssertionError! assert.deepEqual(/a/gi, new Date()); value <any> The input that is checked for being truthy. message <string> | <Error> An alias of assert.ok() . Legacy mode assert(value[, message]) assert.deepEqual(actual, expected[, message]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 6/739 actual <any> expected <any> message <string> | <Error> Strict mode An alias of assert.deepStrictEqual() . Legacy mode Stability: 0 - Deprecated: Use assert.deepStrictEqual() instead. Tests for deep equality between the actual and expected parameters. Primitive values are compared with the Abstract Equality Comparison ( == ). Only enumerable "own" properties are considered. The assert.deepEqual() implementation does not test the [[Prototype]] of objects or enumerable own Symbol properties. For such checks, consider using assert.deepStrictEqual() instead. assert.deepEqual() can have potentially surprising results. The following example does not throw an AssertionError because the properties on the RegExp object are not enumerable: // WARNING: This does not throw an AssertionError! assert.deepEqual(/a/gi, new Date()); An exception is made for Map and Set . Map s and Set s have their contained items compared too, as expected. "Deep" equality means that the enumerable "own" properties of child objects are evaluated also: const assert = require('assert'); const obj1 = { a: { b: 1 } }; const obj2 = { a: { b: 2 } }; const obj3 = { a: { b: 1 } }; const obj4 = Object.create(obj1); assert.deepEqual(obj1, obj1); // OK // Values of b are different: assert.deepEqual(obj1, obj2); // AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } } 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 7/739 assert.deepEqual(obj1, obj3); // OK // Prototypes are ignored: assert.deepEqual(obj1, obj4); // AssertionError: { a: { b: 1 } } deepEqual {} If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError . actual <any> expected <any> message <string> | <Error> Tests for deep equality between the actual and expected parameters. "Deep" equality means that the enumerable "own" properties of child objects are recursively evaluated also by the following rules. Primitive values are compared using the SameValue Comparison , used by Object.is() . Type tags of objects should be the same. [[Prototype]] of objects are compared using the Strict Equality Comparison . Only enumerable "own" properties are considered. Error names and messages are always compared, even if these are not enumerable properties. Enumerable own Symbol properties are compared as well. Object wrappers are compared both as objects and unwrapped values. Object properties are compared unordered. Map keys and Set items are compared unordered. Recursion stops when both sides differ or both sides encounter a circular reference. WeakMap and WeakSet comparison does not rely on their values. See below for further details. const assert = require('assert').strict; // This fails because 1 !== '1'. assert.deepStrictEqual({ a: 1 }, { a: '1' }); // AssertionError: Input A expected to strictly deep-equal input B: // + expected - actual // { // - a: 1 // + a: '1' // } // The following objects don't have own properties const date = new Date(); const object = {}; const fakeDate = {}; Object.setPrototypeOf(fakeDate, Date.prototype); assert.deepStrictEqual(actual, expected[, message]) Comparison details 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 8/739 // Different [[Prototype]]: assert.deepStrictEqual(object, fakeDate); // AssertionError: Input A expected to strictly deep-equal input B: // + expected - actual // - {} // + Date {} // Different type tags: assert.deepStrictEqual(date, fakeDate); // AssertionError: Input A expected to strictly deep-equal input B: // + expected - actual // - 2018-04-26T00:49:08.604Z // + Date {} assert.deepStrictEqual(NaN, NaN); // OK, because of the SameValue comparison // Different unwrapped numbers: assert.deepStrictEqual(new Number(1), new Number(2)); // AssertionError: Input A expected to strictly deep-equal input B: // + expected - actual // - [Number: 1] // + [Number: 2] assert.deepStrictEqual(new String('foo'), Object('foo')); // OK because the object and the string are identical when unwrapped. assert.deepStrictEqual(-0, -0); // OK // Different zeros using the SameValue Comparison: assert.deepStrictEqual(0, -0); // AssertionError: Input A expected to strictly deep-equal input B: // + expected - actual // - 0 // + -0 const symbol1 = Symbol(); const symbol2 = Symbol(); assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol1]: 1 }); // OK, because it is the same symbol on both objects. assert.deepStrictEqual({ [symbol1]: 1 }, { [symbol2]: 1 }); // AssertionError [ERR_ASSERTION]: Input objects not identical: // { // [Symbol()]: 1 // } const weakMap1 = new WeakMap(); const weakMap2 = new WeakMap([[{}, {}]]); const weakMap3 = new WeakMap(); weakMap3.unequal = true; assert.deepStrictEqual(weakMap1, weakMap2); 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 9/739 // OK, because it is impossible to compare the entries // Fails because weakMap3 has a property that weakMap1 does not contain: assert.deepStrictEqual(weakMap1, weakMap3); // AssertionError: Input A expected to strictly deep-equal input B: // + expected - actual // WeakMap { // - [items unknown] // + [items unknown], // + unequal: true // } If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError . asyncFn <Function> | <Promise> error <RegExp> | <Function> message <string> Awaits the asyncFn promise or, if asyncFn is a function, immediately calls the function and awaits the returned promise to complete. It will then check that the promise is not rejected. If asyncFn is a function and it throws an error synchronously, assert.doesNotReject() will return a rejected Promise with that error. If the function does not return a promise, assert.doesNotReject() will return a rejected Promise with an ERR_INVALID_RETURN_VALUE error. In both cases the error handler is skipped. Using assert.doesNotReject() is actually not useful because there is little benefit in catching a rejection and then rejecting it again. Instead, consider adding a comment next to the specific code path that should not reject and keep error messages as expressive as possible. If specified, error can be a Class , RegExp or a validation function. See assert.throws() for more details. Besides the async nature to await the completion behaves identically to assert.doesNotThrow() . (async () => { await assert.doesNotReject( async () => { throw new TypeError('Wrong value'); }, SyntaxError ); })(); assert.doesNotReject(Promise.reject(new TypeError('Wrong value'))) .then(() => { // ... }); assert.doesNotReject(asyncFn[, error][, message]) assert.doesNotThrow(fn[, error][, message]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 10/739 fn <Function> error <RegExp> | <Function> message <string> Asserts that the function fn does not throw an error. Using assert.doesNotThrow() is actually not useful because there is no benefit in catching an error and then rethrowing it. Instead, consider adding a comment next to the specific code path that should not throw and keep error messages as expressive as possible. When assert.doesNotThrow() is called, it will immediately call the fn function. If an error is thrown and it is the same type as that specified by the error parameter, then an AssertionError is thrown. If the error is of a different type, or if the error parameter is undefined, the error is propagated back to the caller. If specified, error can be a Class , RegExp or a validation function. See assert.throws() for more details. The following, for instance, will throw the TypeError because there is no matching error type in the assertion: assert.doesNotThrow( () => { throw new TypeError('Wrong value'); }, SyntaxError ); However, the following will result in an AssertionError with the message 'Got unwanted exception...': assert.doesNotThrow( () => { throw new TypeError('Wrong value'); }, TypeError ); If an AssertionError is thrown and a value is provided for the message parameter, the value of message will be appended to the AssertionError message: assert.doesNotThrow( () => { throw new TypeError('Wrong value'); }, /Wrong value/, 'Whoops' ); // Throws: AssertionError: Got unwanted exception: Whoops actual <any> expected <any> message <string> | <Error> assert.equal(actual, expected[, message]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 11/739 Strict mode An alias of assert.strictEqual() . Legacy mode Stability: 0 - Deprecated: Use assert.strictEqual() instead. Tests shallow, coercive equality between the actual and expected parameters using the Abstract Equality Comparison ( == ). const assert = require('assert'); assert.equal(1, 1); // OK, 1 == 1 assert.equal(1, '1'); // OK, 1 == '1' assert.equal(1, 2); // AssertionError: 1 == 2 assert.equal({ a: { b: 1 } }, { a: { b: 1 } }); // AssertionError: { a: { b: 1 } } == { a: { b: 1 } } If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError . message <string> | <Error> Default: 'Failed' Throws an AssertionError with the provided error message or a default error message. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError . const assert = require('assert').strict; assert.fail(); // AssertionError [ERR_ASSERTION]: Failed assert.fail('boom'); // AssertionError [ERR_ASSERTION]: boom assert.fail(new TypeError('need array')); // TypeError: need array Using assert.fail() with more than two arguments is possible but deprecated. See below for further details. actual <any> expected <any> message <string> | <Error> assert.fail([message]) assert.fail(actual, expected[, message[, operator[, stackStartFn]]]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 12/739 operator <string> Default: '!=' stackStartFn <Function> Default: assert.fail Stability: 0 - Deprecated: Use assert.fail([message]) or other assert functions instead. If message is falsy, the error message is set as the values of actual and expected separated by the provided operator . If just the two actual and expected arguments are provided, operator will default to '!=' . If message is provided as third argument it will be used as the error message and the other arguments will be stored as properties on the thrown object. If stackStartFn is provided, all stack frames above that function will be removed from stacktrace (see Error.captureStackTrace ). If no arguments are given, the default message Failed will be used. const assert = require('assert').strict; assert.fail('a', 'b'); // AssertionError [ERR_ASSERTION]: 'a' != 'b' assert.fail(1, 2, undefined, '>'); // AssertionError [ERR_ASSERTION]: 1 > 2 assert.fail(1, 2, 'fail'); // AssertionError [ERR_ASSERTION]: fail assert.fail(1, 2, 'whoops', '>'); // AssertionError [ERR_ASSERTION]: whoops assert.fail(1, 2, new TypeError('need array')); // TypeError: need array In the last three cases actual , expected , and operator have no influence on the error message. Example use of stackStartFn for truncating the exception's stacktrace: function suppressFrame() { assert.fail('a', 'b', undefined, '!==', suppressFrame); } suppressFrame(); // AssertionError [ERR_ASSERTION]: 'a' !== 'b' // at repl:1:1 // at ContextifyScript.Script.runInThisContext (vm.js:44:33) // ... value <any> Throws value if value is not undefined or null . This is useful when testing the error argument in callbacks. The stack trace contains all frames from the error passed to ifError() including the potential new frames for ifError() itself. const assert = require('assert').strict; assert.ifError(null); assert.ifError(value) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 13/739 asse t. o ( u ); // OK assert.ifError(0); // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 0 assert.ifError('error'); // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: 'error' assert.ifError(new Error()); // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: Error // Create some random error frames. let err; (function errorFrame() { err = new Error('test error'); })(); (function ifErrorFrame() { assert.ifError(err); })(); // AssertionError [ERR_ASSERTION]: ifError got unwanted exception: test error // at ifErrorFrame // at errorFrame actual <any> expected <any> message <string> | <Error> Strict mode An alias of assert.notDeepStrictEqual() . Legacy mode Stability: 0 - Deprecated: Use assert.notDeepStrictEqual() instead. Tests for any deep inequality. Opposite of assert.deepEqual() . const assert = require('assert'); const obj1 = { a: { b: 1 } }; const obj2 = { a: { b: 2 } }; const obj3 = { assert.notDeepEqual(actual, expected[, message]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 14/739 a: { b: 1 } }; const obj4 = Object.create(obj1); assert.notDeepEqual(obj1, obj1); // AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } } assert.notDeepEqual(obj1, obj2); // OK assert.notDeepEqual(obj1, obj3); // AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } } assert.notDeepEqual(obj1, obj4); // OK If the values are deeply equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError . actual <any> expected <any> message <string> | <Error> Tests for deep strict inequality. Opposite of assert.deepStrictEqual() . const assert = require('assert').strict; assert.notDeepStrictEqual({ a: 1 }, { a: '1' }); // OK If the values are deeply and strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError . actual <any> expected <any> message <string> | <Error> Strict mode An alias of assert.notStrictEqual() . Legacy mode assert.notDeepStrictEqual(actual, expected[, message]) assert.notEqual(actual, expected[, message]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 15/739 Stability: 0 - Deprecated: Use assert.notStrictEqual() instead. Tests shallow, coercive inequality with the Abstract Equality Comparison ( != ). const assert = require('assert'); assert.notEqual(1, 2); // OK assert.notEqual(1, 1); // AssertionError: 1 != 1 assert.notEqual(1, '1'); // AssertionError: 1 != '1' If the values are equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError . actual <any> expected <any> message <string> | <Error> Tests strict inequality between the actual and expected parameters as determined by the SameValue Comparison . const assert = require('assert').strict; assert.notStrictEqual(1, 2); // OK assert.notStrictEqual(1, 1); // AssertionError [ERR_ASSERTION]: Identical input passed to notStrictEqual: 1 assert.notStrictEqual(1, '1'); // OK If the values are strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError . value <any> message <string> | <Error> Tests if value is truthy. It is equivalent to assert.equal(!!value, true, message) . assert.notStrictEqual(actual, expected[, message]) assert.ok(value[, message]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 16/739 If value is not truthy, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined , a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError . If no arguments are passed in at all message will be set to the string: 'No value argument passed to `assert.ok()`' . Be aware that in the repl the error message will be different to the one thrown in a file! See below for further details. const assert = require('assert').strict; assert.ok(true); // OK assert.ok(1); // OK assert.ok(); // AssertionError: No value argument passed to `assert.ok()` assert.ok(false, 'it\'s false'); // AssertionError: it's false // In the repl: assert.ok(typeof 123 === 'string'); // AssertionError: false == true // In a file (e.g. test.js): assert.ok(typeof 123 === 'string'); // AssertionError: The expression evaluated to a falsy value: // // assert.ok(typeof 123 === 'string') assert.ok(false); // AssertionError: The expression evaluated to a falsy value: // // assert.ok(false) assert.ok(0); // AssertionError: The expression evaluated to a falsy value: // // assert.ok(0) // Using `assert()` works the same: assert(0); // AssertionError: The expression evaluated to a falsy value: // // assert(0) asyncFn <Function> | <Promise> error <RegExp> | <Function> | <Object> | <Error> message <string> assert.rejects(asyncFn[, error][, message]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 17/739 Awaits the asyncFn promise or, if asyncFn is a function, immediately calls the function and awaits the returned promise to complete. It will then check that the promise is rejected. If asyncFn is a function and it throws an error synchronously, assert.rejects() will return a rejected Promise with that error. If the function does not return a promise, assert.rejects() will return a rejected Promise with an ERR_INVALID_RETURN_VALUE error. In both cases the error handler is skipped. Besides the async nature to await the completion behaves identically to assert.throws() . If specified, error can be a Class , RegExp , a validation function, an object where each property will be tested for, or an instance of error where each property will be tested for including the non-enumerable message and name properties. If specified, message will be the message provided by the AssertionError if the asyncFn fails to reject. (async () => { await assert.rejects( async () => { throw new TypeError('Wrong value'); }, { name: 'TypeError', message: 'Wrong value' } ); })(); assert.rejects( Promise.reject(new Error('Wrong value')), Error ).then(() => { // ... }); Note that error cannot be a string. If a string is provided as the second argument, then error is assumed to be omitted and the string will be used for message instead. This can lead to easy-to-miss mistakes. Please read the example in assert.throws() carefully if using a string as the second argument gets considered. actual <any> expected <any> message <string> | <Error> Tests strict equality between the actual and expected parameters as determined by the SameValue Comparison . const assert = require('assert').strict; assert.strictEqual(1, 2); // AssertionError [ERR_ASSERTION]: Input A expected to strictly equal input B: // + expected - actual // - 1 // + 2 assert.strictEqual(actual, expected[, message]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 18/739 assert.strictEqual(1, 1); // OK assert.strictEqual(1, '1'); // AssertionError [ERR_ASSERTION]: Input A expected to strictly equal input B: // + expected - actual // - 1 // + '1' If the values are not strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned. If the message parameter is an instance of an Error then it will be thrown instead of the AssertionError . fn <Function> error <RegExp> | <Function> | <Object> | <Error> message <string> Expects the function fn to throw an error. If specified, error can be a Class , RegExp , a validation function, a validation object where each property will be tested for strict deep equality, or an instance of error where each property will be tested for strict deep equality including the non-enumerable message and name properties. When using an object, it is also possible to use a regular expression, when validating against a string property. See below for examples. If specified, message will be appended to the message provided by the AssertionError if the fn call fails to throw or in case the error validation fails. Custom validation object/error instance: const err = new TypeError('Wrong value'); err.code = 404; err.foo = 'bar'; err.info = { nested: true, baz: 'text' }; err.reg = /abc/i; assert.throws( () => { throw err; }, { name: 'TypeError', message: 'Wrong value', info: { nested: true, baz: 'text' } // Note that only properties on the validation object will be tested for. // Using nested objects requires all properties to be present. Otherwise // the validation is going to fail assert.throws(fn[, error][, message]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 19/739 // the validation is going to fail. } ); // Using regular expressions to validate error properties: assert.throws( () => { throw err; }, { // The `name` and `message` properties are strings and using regular // expressions on those will match against the string. If they fail, an // error is thrown. name: /^TypeError$/, message: /Wrong/, foo: 'bar', info: { nested: true, // It is not possible to use regular expressions for nested properties! baz: 'text' }, // The `reg` property contains a regular expression and only if the // validation object contains an identical regular expression, it is going // to pass. reg: /abc/i } ); // Fails due to the different `message` and `name` properties: assert.throws( () => { const otherErr = new Error('Not found'); otherErr.code = 404; throw otherErr; }, err // This tests for `message`, `name` and `code`. ); Validate instanceof using constructor: assert.throws( () => { throw new Error('Wrong value'); }, Error ); Validate error message using RegExp : Using a regular expression runs .toString on the error object, and will therefore also include the error name. 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 20/739 assert.throws( () => { throw new Error('Wrong value'); }, /^Error: Wrong value$/ ); Custom error validation: assert.throws( () => { throw new Error('Wrong value'); }, function(err) { if ((err instanceof Error) && /value/.test(err)) { return true; } }, 'unexpected error' ); Note that error cannot be a string. If a string is provided as the second argument, then error is assumed to be omitted and the string will be used for message instead. This can lead to easy-to-miss mistakes. Using the same message as the thrown error message is going to result in an ERR_AMBIGUOUS_ARGUMENT error. Please read the example below carefully if using a string as the second argument gets considered: function throwingFirst() { throw new Error('First'); } function throwingSecond() { throw new Error('Second'); } function notThrowing() {} // The second argument is a string and the input function threw an Error. // The first case will not throw as it does not match for the error message // thrown by the input function! assert.throws(throwingFirst, 'Second'); // In the next example the message has no benefit over the message from the // error and since it is not clear if the user intended to actually match // against the error message, Node.js thrown an `ERR_AMBIGUOUS_ARGUMENT` error. assert.throws(throwingSecond, 'Second'); // Throws an error: // TypeError [ERR_AMBIGUOUS_ARGUMENT] // The string is only used (as message) in case the function does not throw: assert.throws(notThrowing, 'Second'); // AssertionError [ERR_ASSERTION]: Missing expected exception: Second // If it was intended to match for the error message do this instead: assert.throws(throwingSecond, /Second$/); // Does not throw because the error messages match. assert.throws(throwingFirst, /Second$/); 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 21/739 asse t.t o s(t o g st, /Seco d$/); // Throws an error: // Error: First // at throwingFirst (repl:2:9) Due to the confusing notation, it is recommended not to use a string as the second argument. This might lead to difficult-to-spot errors. Stability: 1 - Experimental The async_hooks module provides an API to register callbacks tracking the lifetime of asynchronous resources created inside a Node.js application. It can be accessed using: const async_hooks = require('async_hooks'); An asynchronous resource represents an object with an associated callback. This callback may be called multiple times, for example, the 'connection' event in net.createServer() , or just a single time like in fs.open() . A resource can also be closed before the callback is called. AsyncHook does not explicitly distinguish between these different cases but will represent them as the abstract concept that is a resource. If Worker s are used, each thread has an independent async_hooks interface, and each thread will use a new set of async IDs. Following is a simple overview of the public API. const async_hooks = require('async_hooks'); // Return the ID of the current execution context. const eid = async_hooks.executionAsyncId(); // Return the ID of the handle responsible for triggering the callback of the // current execution scope to call. const tid = async_hooks.triggerAsyncId(); // Create a new AsyncHook instance. All of these callbacks are optional. const asyncHook = async_hooks.createHook({ init, before, after, destroy, promiseResolve }); // Allow callbacks of this AsyncHook instance to call. This is not an implicit // action after running the constructor, and must be explicitly run to begin // executing callbacks. asyncHook.enable(); // Disable listening for new asynchronous events. asyncHook.disable(); Async Hooks Terminology Public API Overview 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 22/739 // // The following are the callbacks that can be passed to createHook(). // // init is called during object construction. The resource may not have // completed construction when this callback runs, therefore all fields of the // resource referenced by "asyncId" may not have been populated. function init(asyncId, type, triggerAsyncId, resource) { } // before is called just before the resource's callback is called. It can be // called 0-N times for handles (e.g. TCPWrap), and will be called exactly 1 // time for requests (e.g. FSReqWrap). function before(asyncId) { } // after is called just after the resource's callback has finished. function after(asyncId) { } // destroy is called when an AsyncWrap instance is destroyed. function destroy(asyncId) { } // promiseResolve is called only for promise resources, when the // `resolve` function passed to the `Promise` constructor is invoked // (either directly or through other means of resolving a promise). function promiseResolve(asyncId) { } callbacks <Object> The Hook Callbacks to register init <Function> The init callback . before <Function> The before callback . after <Function> The after callback . destroy <Function> The destroy callback . Returns: <AsyncHook> Instance used for disabling and enabling hooks Registers functions to be called for different lifetime events of each async operation. The callbacks init() / before() / after() / destroy() are called for the respective asynchronous event during a resource's lifetime. All callbacks are optional. For example, if only resource cleanup needs to be tracked, then only the destroy callback needs to be passed. The specifics of all functions that can be passed to callbacks is in the Hook Callbacks section. const async_hooks = require('async_hooks'); const asyncHook = async_hooks.createHook({ init(asyncId, type, triggerAsyncId, resource) { }, destroy(asyncId) { } }); Note that the callbacks will be inherited via the prototype chain: async_hooks.createHook(callbacks) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 23/739 class MyAsyncCallbacks { init(asyncId, type, triggerAsyncId, resource) { } destroy(asyncId) {} } class MyAddedCallbacks extends MyAsyncCallbacks { before(asyncId) { } after(asyncId) { } } const asyncHook = async_hooks.createHook(new MyAddedCallbacks()); If any AsyncHook callbacks throw, the application will print the stack trace and exit. The exit path does follow that of an uncaught exception, but all 'uncaughtException' listeners are removed, thus forcing the process to exit. The 'exit' callbacks will still be called unless the application is run with --abort-on-uncaught-exception , in which case a stack trace will be printed and the application exits, leaving a core file. The reason for this error handling behavior is that these callbacks are running at potentially volatile points in an object's lifetime, for example during class construction and destruction. Because of this, it is deemed necessary to bring down the process quickly in order to prevent an unintentional abort in the future. This is subject to change in the future if a comprehensive analysis is performed to ensure an exception can follow the normal control flow without unintentional side effects. Because printing to the console is an asynchronous operation, console.log() will cause the AsyncHooks callbacks to be called. Using console.log() or similar asynchronous operations inside an AsyncHooks callback function will thus cause an infinite recursion. An easy solution to this when debugging is to use a synchronous logging operation such as fs.writeFileSync(file, msg, flag) . This will print to the file and will not invoke AsyncHooks recursively because it is synchronous. const fs = require('fs'); const util = require('util'); function debug(...args) { // use a function like this one when debugging inside an AsyncHooks callback fs.writeFileSync('log.out', `${util.format(...args)}\n`, { flag: 'a' }); } If an asynchronous operation is needed for logging, it is possible to keep track of what caused the asynchronous operation using the information provided by AsyncHooks itself. The logging should then be skipped when it was the logging itself that caused AsyncHooks callback to call. By doing this the otherwise infinite recursion is broken. Returns: <AsyncHook> A reference to asyncHook . Enable the callbacks for a given AsyncHook instance. If no callbacks are provided enabling is a noop. The AsyncHook instance is disabled by default. If the AsyncHook instance should be enabled immediately after creation, the following pattern can be used. Error Handling Printing in AsyncHooks callbacks asyncHook.enable() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 24/739 const async_hooks = require('async_hooks'); const hook = async_hooks.createHook(callbacks).enable(); Returns: <AsyncHook> A reference to asyncHook . Disable the callbacks for a given AsyncHook instance from the global pool of AsyncHook callbacks to be executed. Once a hook has been disabled it will not be called again until enabled. For API consistency disable() also returns the AsyncHook instance. Key events in the lifetime of asynchronous events have been categorized into four areas: instantiation, before/after the callback is called, and when the instance is destroyed. asyncId <number> A unique ID for the async resource. type <string> The type of the async resource. triggerAsyncId <number> The unique ID of the async resource in whose execution context this async resource was created. resource <Object> Reference to the resource representing the async operation, needs to be released during destroy. Called when a class is constructed that has the possibility to emit an asynchronous event. This does not mean the instance must call before / after before destroy is called, only that the possibility exists. This behavior can be observed by doing something like opening a resource then closing it before the resource can be used. The following snippet demonstrates this. require('net').createServer().listen(function() { this.close(); }); // OR clearTimeout(setTimeout(() => {}, 10)); Every new resource is assigned an ID that is unique within the scope of the current Node.js instance. The type is a string identifying the type of resource that caused init to be called. Generally, it will correspond to the name of the resource's constructor. FSEVENTWRAP, FSREQWRAP, GETADDRINFOREQWRAP, GETNAMEINFOREQWRAP, HTTPPARSER, JSSTREAM, PIPECONNECTWRAP, PIPEWRAP, PROCESSWRAP, QUERYWRAP, SHUTDOWNWRAP, SIGNALWRAP, STATWATCHER, TCPCONNECTWRAP, TCPSERVERWRAP, TCPWRAP, TIMERWRAP, TTYWRAP, UDPSENDWRAP, UDPWRAP, WRITEWRAP, ZLIB, SSLCONNECTION, PBKDF2REQUEST, RANDOMBYTESREQUEST, TLSWRAP, Timeout, Immediate, TickObject There is also the PROMISE resource type, which is used to track Promise instances and asynchronous work scheduled by them. Users are able to define their own type when using the public embedder API. asyncHook.disable() Hook Callbacks init(asyncId, type, triggerAsyncId, resource) type 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 25/739 It is possible to have type name collisions. Embedders are encouraged to use unique prefixes, such as the npm package name, to prevent collisions when listening to the hooks. triggerAsyncId is the asyncId of the resource that caused (or "triggered") the new resource to initialize and that caused init to call. This is different from async_hooks.executionAsyncId() that only shows when a resource was created, while triggerAsyncId shows why a resource was created. The following is a simple demonstration of triggerAsyncId : async_hooks.createHook({ init(asyncId, type, triggerAsyncId) { const eid = async_hooks.executionAsyncId(); fs.writeSync( 1, `${type}(${asyncId}): trigger: ${triggerAsyncId} execution: ${eid}\n`); } }).enable(); require('net').createServer((conn) => {}).listen(8080); Output when hitting the server with nc localhost 8080 : TCPSERVERWRAP(5): trigger: 1 execution: 1 TCPWRAP(7): trigger: 5 execution: 0 The TCPSERVERWRAP is the server which receives the connections. The TCPWRAP is the new connection from the client. When a new connection is made, the TCPWrap instance is immediately constructed. This happens outside of any JavaScript stack. (An executionAsyncId() of 0 means that it is being executed from C++ with no JavaScript stack above it.) With only that information, it would be impossible to link resources together in terms of what caused them to be created, so triggerAsyncId is given the task of propagating what resource is responsible for the new resource's existence. resource is an object that represents the actual async resource that has been initialized. This can contain useful information that can vary based on the value of type . For instance, for the GETADDRINFOREQWRAP resource type, resource provides the hostname used when looking up the IP address for the host in net.Server.listen() . The API for accessing this information is currently not considered public, but using the Embedder API, users can provide and document their own resource objects. For example, such a resource object could contain the SQL query being executed. In the case of Promises, the resource object will have promise property that refers to the Promise that is being initialized, and an isChainedPromise property, set to true if the promise has a parent promise, and false otherwise. For example, in the case of b = a.then(handler) , a is considered a parent Promise of b . Here, b is considered a chained promise. In some cases the resource object is reused for performance reasons, it is thus not safe to use it as a key in a WeakMap or add properties to it. The following is an example with additional information about the calls to init between the before and after calls, specifically what the callback to listen() will look like. The output formatting is slightly more elaborate to make calling context easier to see. let indent = 0; async hooks.createHook({ triggerAsyncId resource Asynchronous context example 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 26/739 asy c_ oo s.c eate oo ({ init(asyncId, type, triggerAsyncId) { const eid = async_hooks.executionAsyncId(); const indentStr = ' '.repeat(indent); fs.writeSync( 1, `${indentStr}${type}(${asyncId}):` + ` trigger: ${triggerAsyncId} execution: ${eid}\n`); }, before(asyncId) { const indentStr = ' '.repeat(indent); fs.writeFileSync('log.out', `${indentStr}before: ${asyncId}\n`, { flag: 'a' }); indent += 2; }, after(asyncId) { indent -= 2; const indentStr = ' '.repeat(indent); fs.writeFileSync('log.out', `${indentStr}after: ${asyncId}\n`, { flag: 'a' }); }, destroy(asyncId) { const indentStr = ' '.repeat(indent); fs.writeFileSync('log.out', `${indentStr}destroy: ${asyncId}\n`, { flag: 'a' }); }, }).enable(); require('net').createServer(() => {}).listen(8080, () => { // Let's wait 10ms before logging the server started. setTimeout(() => { console.log('>>>', async_hooks.executionAsyncId()); }, 10); }); Output from only starting the server: TCPSERVERWRAP(5): trigger: 1 execution: 1 TickObject(6): trigger: 5 execution: 1 before: 6 Timeout(7): trigger: 6 execution: 6 after: 6 destroy: 6 before: 7 >>> 7 TickObject(8): trigger: 7 execution: 7 after: 7 before: 8 after: 8 As illustrated in the example, executionAsyncId() and execution each specify the value of the current execution context; which is delineated by calls to before and after . Only using execution to graph resource allocation results in the following: 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 27/739 Timeout(7) -> TickObject(6) -> root(1) The TCPSERVERWRAP is not part of this graph, even though it was the reason for console.log() being called. This is because binding to a port without a hostname is a synchronous operation, but to maintain a completely asynchronous API the user's callback is placed in a process.nextTick() . The graph only shows when a resource was created, not why, so to track the why use triggerAsyncId . asyncId <number> When an asynchronous operation is initiated (such as a TCP server receiving a new connection) or completes (such as writing data to disk) a callback is called to notify the user. The before callback is called just before said callback is executed. asyncId is the unique identifier assigned to the resource about to execute the callback. The before callback will be called 0 to N times. The before callback will typically be called 0 times if the asynchronous operation was cancelled or, for example, if no connections are received by a TCP server. Persistent asynchronous resources like a TCP server will typically call the before callback multiple times, while other operations like fs.open() will call it only once. asyncId <number> Called immediately after the callback specified in before is completed. If an uncaught exception occurs during execution of the callback, then after will run after the 'uncaughtException' event is emitted or a domain 's handler runs. asyncId <number> Called after the resource corresponding to asyncId is destroyed. It is also called asynchronously from the embedder API emitDestroy() . Some resources depend on garbage collection for cleanup, so if a reference is made to the resource object passed to init it is possible that destroy will never be called, causing a memory leak in the application. If the resource does not depend on garbage collection, then this will not be an issue. asyncId <number> Called when the resolve function passed to the Promise constructor is invoked (either directly or through other means of resolving a promise). Note that resolve() does not do any observable synchronous work. The Promise is not necessarily fulfilled or rejected at this point if the Promise was resolved by assuming the state of another Promise . new Promise((resolve) => resolve(true)).then((a) => {}); calls the following callbacks: init for PROMISE with id 5, trigger id: 1 promise resolve 5 # corresponds to resolve(true) init for PROMISE with id 6, trigger id: 5 # the Promise returned by then() before(asyncId) after(asyncId) destroy(asyncId) promiseResolve(asyncId) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 28/739 before 6 # the then() callback is entered promise resolve 6 # the then() callback resolves the promise by returning after 6 Returns: <number> The asyncId of the current execution context. Useful to track when something calls. const async_hooks = require('async_hooks'); console.log(async_hooks.executionAsyncId()); // 1 - bootstrap fs.open(path, 'r', (err, fd) => { console.log(async_hooks.executionAsyncId()); // 6 - open() }); The ID returned from executionAsyncId() is related to execution timing, not causality (which is covered by triggerAsyncId() ): const server = net.createServer((conn) => { // Returns the ID of the server, not of the new connection, because the // callback runs in the execution scope of the server's MakeCallback(). async_hooks.executionAsyncId(); }).listen(port, () => { // Returns the ID of a TickObject (i.e. process.nextTick()) because all // callbacks passed to .listen() are wrapped in a nextTick(). async_hooks.executionAsyncId(); }); Note that promise contexts may not get precise executionAsyncIds by default. See the section on promise execution tracking . Returns: <number> The ID of the resource responsible for calling the callback that is currently being executed. const server = net.createServer((conn) => { // The resource that caused (or triggered) this callback to be called // was that of the new connection. Thus the return value of triggerAsyncId() // is the asyncId of "conn". async_hooks.triggerAsyncId(); }).listen(port, () => { // Even though all callbacks passed to .listen() are wrapped in a nextTick() // the callback itself exists because the call to the server's .listen() // was made. So the return value would be the ID of the server. async_hooks.triggerAsyncId(); }); Note that promise contexts may not get valid triggerAsyncId s by default. See the section on promise execution tracking . async_hooks.executionAsyncId() async_hooks.triggerAsyncId() Promise execution tracking 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 29/739 By default, promise executions are not assigned asyncId s due to the relatively expensive nature of the promise introspection API provided by V8. This means that programs using promises or async / await will not get correct execution and trigger ids for promise callback contexts by default. const ah = require('async_hooks'); Promise.resolve(1729).then(() => { console.log(`eid ${ah.executionAsyncId()} tid ${ah.triggerAsyncId()}`); }); // produces: // eid 1 tid 0 Observe that the then() callback claims to have executed in the context of the outer scope even though there was an asynchronous hop involved. Also note that the triggerAsyncId value is 0 , which means that we are missing context about the resource that caused (triggered) the then() callback to be executed. Installing async hooks via async_hooks.createHook enables promise execution tracking: const ah = require('async_hooks'); ah.createHook({ init() {} }).enable(); // forces PromiseHooks to be enabled. Promise.resolve(1729).then(() => { console.log(`eid ${ah.executionAsyncId()} tid ${ah.triggerAsyncId()}`); }); // produces: // eid 7 tid 6 In this example, adding any actual hook function enabled the tracking of promises. There are two promises in the example above; the promise created by Promise.resolve() and the promise returned by the call to then() . In the example above, the first promise got the asyncId 6 and the latter got asyncId 7 . During the execution of the then() callback, we are executing in the context of promise with asyncId 7 . This promise was triggered by async resource 6 . Another subtlety with promises is that before and after callbacks are run only on chained promises. That means promises not created by then() / catch() will not have the before and after callbacks fired on them. For more details see the details of the V8 PromiseHooks API. Library developers that handle their own asynchronous resources performing tasks like I/O, connection pooling, or managing callback queues may use the AsyncWrap JavaScript API so that all the appropriate callbacks are called. The class AsyncResource is designed to be extended by the embedder's async resources. Using this, users can easily trigger the lifetime events of their own resources. The init hook will trigger when an AsyncResource is instantiated. The following is an overview of the AsyncResource API. const { AsyncResource, executionAsyncId } = require('async_hooks'); // AsyncResource() is meant to be extended. Instantiating a // new AsyncResource() also triggers init. If triggerAsyncId is omitted then // async_hook.executionAsyncId() is used. const asyncResource = new AsyncResource( JavaScript Embedder API Class: AsyncResource 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 30/739 y y ( type, { triggerAsyncId: executionAsyncId(), requireManualDestroy: false } ); // Run a function in the execution context of the resource. This will // * establish the context of the resource // * trigger the AsyncHooks before callbacks // * call the provided function `fn` with the supplied arguments // * trigger the AsyncHooks after callbacks // * restore the original execution context asyncResource.runInAsyncScope(fn, thisArg, ...args); // Call AsyncHooks destroy callbacks. asyncResource.emitDestroy(); // Return the unique ID assigned to the AsyncResource instance. asyncResource.asyncId(); // Return the trigger ID for the AsyncResource instance. asyncResource.triggerAsyncId(); type <string> The type of async event. options <Object> triggerAsyncId <number> The ID of the execution context that created this async event. Default: executionAsyncId() . requireManualDestroy <boolean> Disables automatic emitDestroy when the object is garbage collected. This usually does not need to be set (even if emitDestroy is called manually), unless the resource's asyncId is retrieved and the sensitive API's emitDestroy is called with it. Default: false . Example usage: class DBQuery extends AsyncResource { constructor(db) { super('DBQuery'); this.db = db; } getInfo(query, callback) { this.db.get(query, (err, data) => { this.runInAsyncScope(callback, null, err, data); }); } close() { this.db = null; this.emitDestroy(); } } fn <Function> The function to call in the execution context of this async resource. new AsyncResource(type[, options]) asyncResource.runInAsyncScope(fn[, thisArg, ...args]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 31/739 thisArg <any> The receiver to be used for the function call. ...args <any> Optional arguments to pass to the function. Call the provided function with the provided arguments in the execution context of the async resource. This will establish the context, trigger the AsyncHooks before callbacks, call the function, trigger the AsyncHooks after callbacks, and then restore the original execution context. Stability: 0 - Deprecated: Use asyncResource.runInAsyncScope() instead. Call all before callbacks to notify that a new asynchronous execution context is being entered. If nested calls to emitBefore() are made, the stack of asyncId s will be tracked and properly unwound. before and after calls must be unwound in the same order that they are called. Otherwise, an unrecoverable exception will occur and the process will abort. For this reason, the emitBefore and emitAfter APIs are considered deprecated. Please use runInAsyncScope , as it provides a much safer alternative. Stability: 0 - Deprecated: Use asyncResource.runInAsyncScope() instead. Call all after callbacks. If nested calls to emitBefore() were made, then make sure the stack is unwound properly. Otherwise an error will be thrown. If the user's callback throws an exception, emitAfter() will automatically be called for all asyncId s on the stack if the error is handled by a domain or 'uncaughtException' handler. before and after calls must be unwound in the same order that they are called. Otherwise, an unrecoverable exception will occur and the process will abort. For this reason, the emitBefore and emitAfter APIs are considered deprecated. Please use runInAsyncScope , as it provides a much safer alternative. Returns: <AsyncResource> A reference to asyncResource . Call all destroy hooks. This should only ever be called once. An error will be thrown if it is called more than once. This must be manually called. If the resource is left to be collected by the GC then the destroy hooks will never be called. Returns: <number> The unique asyncId assigned to the resource. Returns: <number> The same triggerAsyncId that is passed to the AsyncResource constructor. Stability: 2 - Stable asyncResource.emitBefore() asyncResource.emitAfter() asyncResource.emitDestroy() asyncResource.asyncId() asyncResource.triggerAsyncId() Buffer 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 32/739 Prior to the introduction of TypedArray , the JavaScript language had no mechanism for reading or manipulating streams of binary data. The Buffer class was introduced as part of the Node.js API to enable interaction with octet streams in TCP streams, file system operations, and other contexts. With TypedArray now available, the Buffer class implements the Uint8Array API in a manner that is more optimized and suitable for Node.js. Instances of the Buffer class are similar to arrays of integers but correspond to fixed-sized, raw memory allocations outside the V8 heap. The size of the Buffer is established when it is created and cannot be changed. The Buffer class is within the global scope, making it unlikely that one would need to ever use require('buffer').Buffer . // Creates a zero-filled Buffer of length 10. const buf1 = Buffer.alloc(10); // Creates a Buffer of length 10, filled with 0x1. const buf2 = Buffer.alloc(10, 1); // Creates an uninitialized buffer of length 10. // This is faster than calling Buffer.alloc() but the returned // Buffer instance might contain old data that needs to be // overwritten using either fill() or write(). const buf3 = Buffer.allocUnsafe(10); // Creates a Buffer containing [0x1, 0x2, 0x3]. const buf4 = Buffer.from([1, 2, 3]); // Creates a Buffer containing UTF-8 bytes [0x74, 0xc3, 0xa9, 0x73, 0x74]. const buf5 = Buffer.from('tést'); // Creates a Buffer containing Latin-1 bytes [0x74, 0xe9, 0x73, 0x74]. const buf6 = Buffer.from('tést', 'latin1'); In versions of Node.js prior to 6.0.0, Buffer instances were created using the Buffer constructor function, which allocates the returned Buffer differently based on what arguments are provided: Passing a number as the first argument to Buffer() (e.g. new Buffer(10) ) allocates a new Buffer object of the specified size. Prior to Node.js 8.0.0, the memory allocated for such Buffer instances is not initialized and can contain sensitive data. Such Buffer instances must be subsequently initialized by using either buf.fill(0) or by writing to the entire Buffer . While this behavior is intentional to improve performance, development experience has demonstrated that a more explicit distinction is required between creating a fast-but- uninitialized Buffer versus creating a slower-but-safer Buffer . Starting in Node.js 8.0.0, Buffer(num) and new Buffer(num) will return a Buffer with initialized memory. Passing a string, array, or Buffer as the first argument copies the passed object's data into the Buffer . Passing an ArrayBuffer or a SharedArrayBuffer returns a Buffer that shares allocated memory with the given array buffer. Because the behavior of new Buffer() is different depending on the type of the first argument, security and reliability issues can be inadvertently introduced into applications when argument validation or Buffer initialization is not performed. To make the creation of Buffer instances more reliable and less error-prone, the various forms of the new Buffer() constructor have been deprecated and replaced by separate Buffer.from() , Buffer.alloc() , and Buffer.allocUnsafe() methods. Developers should migrate all existing uses of the new Buffer() constructors to one of these new APIs. Buffer.from() , Buffer.alloc() , and Buffer.allocUnsafe() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 33/739 Buffer.from(array) returns a new Buffer that contains a copy of the provided octets. Buffer.from(arrayBuffer[, byteOffset[, length]]) returns a new Buffer that shares the same allocated memory as the given ArrayBuffer . Buffer.from(buffer) returns a new Buffer that contains a copy of the contents of the given Buffer . Buffer.from(string[, encoding]) returns a new Buffer that contains a copy of the provided string. Buffer.alloc(size[, fill[, encoding]]) returns a new initialized Buffer of the specified size. This method is slower than Buffer.allocUnsafe(size) but guarantees that newly created Buffer instances never contain old data that is potentially sensitive. Buffer.allocUnsafe(size) and Buffer.allocUnsafeSlow(size) each return a new uninitialized Buffer of the specified size . Because the Buffer is uninitialized, the allocated segment of memory might contain old data that is potentially sensitive. Buffer instances returned by Buffer.allocUnsafe() may be allocated off a shared internal memory pool if size is less than or equal to half Buffer.poolSize . Instances returned by Buffer.allocUnsafeSlow() never use the shared internal memory pool. Node.js can be started using the --zero-fill-buffers command line option to cause all newly allocated Buffer instances to be zero-filled upon creation by default, including buffers returned by new Buffer(size) , Buffer.allocUnsafe() , Buffer.allocUnsafeSlow() , and new SlowBuffer(size) . Use of this flag can have a significant negative impact on performance. Use of the --zero-fill-buffers option is recommended only when necessary to enforce that newly allocated Buffer instances cannot contain old data that is potentially sensitive. $ node --zero-fill-buffers > Buffer.allocUnsafe(5); <Buffer 00 00 00 00 00> When calling Buffer.allocUnsafe() and Buffer.allocUnsafeSlow() , the segment of allocated memory is uninitialized (it is not zeroed-out). While this design makes the allocation of memory quite fast, the allocated segment of memory might contain old data that is potentially sensitive. Using a Buffer created by Buffer.allocUnsafe() without completely overwriting the memory can allow this old data to be leaked when the Buffer memory is read. While there are clear performance advantages to using Buffer.allocUnsafe() , extra care must be taken in order to avoid introducing security vulnerabilities into an application. When string data is stored in or extracted out of a Buffer instance, a character encoding may be specified. const buf = Buffer.from('hello world', 'ascii'); console.log(buf.toString('hex')); // Prints: 68656c6c6f20776f726c64 console.log(buf.toString('base64')); // Prints: aGVsbG8gd29ybGQ= console.log(Buffer.from('fhqwhgads', 'ascii')); // Prints: <Buffer 66 68 71 77 68 67 61 64 73> console.log(Buffer.from('fhqwhgads', 'utf16le')); // Prints: <Buffer 66 00 68 00 71 00 77 00 68 00 67 00 61 00 64 00 73 00> The --zero-fill-buffers command line option What makes Buffer.allocUnsafe() and Buffer.allocUnsafeSlow() "unsafe"? Buffers and Character Encodings 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 34/739 The character encodings currently supported by Node.js include: 'ascii' - For 7-bit ASCII data only. This encoding is fast and will strip the high bit if set. 'utf8' - Multibyte encoded Unicode characters. Many web pages and other document formats use UTF-8. 'utf16le' - 2 or 4 bytes, little-endian encoded Unicode characters. Surrogate pairs (U+10000 to U+10FFFF) are supported. 'ucs2' - Alias of 'utf16le' . 'base64' - Base64 encoding. When creating a Buffer from a string, this encoding will also correctly accept "URL and Filename Safe Alphabet" as specified in RFC4648, Section 5 . 'latin1' - A way of encoding the Buffer into a one-byte encoded string (as defined by the IANA in RFC1345 , page 63, to be the Latin-1 supplement block and C0/C1 control codes). 'binary' - Alias for 'latin1' . 'hex' - Encode each byte as two hexadecimal characters. Modern Web browsers follow the WHATWG Encoding Standard which aliases both 'latin1' and 'ISO-8859-1' to 'win-1252' . This means that while doing something like http.get() , if the returned charset is one of those listed in the WHATWG specification it is possible that the server actually returned 'win-1252' -encoded data, and using 'latin1' encoding may incorrectly decode the characters. Buffer instances are also Uint8Array instances. However, there are subtle incompatibilities with TypedArray . For example, while ArrayBuffer#slice() creates a copy of the slice, the implementation of Buffer#slice() creates a view over the existing Buffer without copying, making Buffer#slice() far more efficient. It is also possible to create new TypedArray instances from a Buffer with the following caveats: 1. The Buffer object's memory is copied to the TypedArray , not shared. 2. The Buffer object's memory is interpreted as an array of distinct elements, and not as a byte array of the target type. That is, new Uint32Array(Buffer.from([1, 2, 3, 4])) creates a 4-element Uint32Array with elements [1, 2, 3, 4] , not a Uint32Array with a single element [0x1020304] or [0x4030201] . It is possible to create a new Buffer that shares the same allocated memory as a TypedArray instance by using the TypedArray object's .buffer property. const arr = new Uint16Array(2); arr[0] = 5000; arr[1] = 4000; // Copies the contents of `arr` const buf1 = Buffer.from(arr); // Shares memory with `arr` const buf2 = Buffer.from(arr.buffer); console.log(buf1); // Prints: <Buffer 88 a0> console.log(buf2); // Prints: <Buffer 88 13 a0 0f> Buffers and TypedArray 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 35/739 arr[1] = 6000; console.log(buf1); // Prints: <Buffer 88 a0> console.log(buf2); // Prints: <Buffer 88 13 70 17> Note that when creating a Buffer using a TypedArray 's .buffer , it is possible to use only a portion of the underlying ArrayBuffer by passing in byteOffset and length parameters. const arr = new Uint16Array(20); const buf = Buffer.from(arr.buffer, 0, 16); console.log(buf.length); // Prints: 16 The Buffer.from() and TypedArray.from() have different signatures and implementations. Specifically, the TypedArray variants accept a second argument that is a mapping function that is invoked on every element of the typed array: TypedArray.from(source[, mapFn[, thisArg]]) The Buffer.from() method, however, does not support the use of a mapping function: Buffer.from(array) Buffer.from(buffer) Buffer.from(arrayBuffer[, byteOffset[, length]]) Buffer.from(string[, encoding]) Buffer instances can be iterated over using for..of syntax: const buf = Buffer.from([1, 2, 3]); // Prints: // 1 // 2 // 3 for (const b of buf) { console.log(b); } Additionally, the buf.values() , buf.keys() , and buf.entries() methods can be used to create iterators. The Buffer class is a global type for dealing with binary data directly. It can be constructed in a variety of ways. Buffers and iteration Class: Buffer new Buffer(array) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 36/739 Stability: 0 - Deprecated: Use Buffer.from(array) instead. array <integer[]> An array of bytes to copy from. Allocates a new Buffer using an array of octets. // Creates a new Buffer containing the UTF-8 bytes of the string 'buffer' const buf = new Buffer([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]); Stability: 0 - Deprecated: Use Buffer.from(arrayBuffer[, byteOffset[, length]]) instead. arrayBuffer <ArrayBuffer> | <SharedArrayBuffer> An ArrayBuffer , SharedArrayBuffer or the .buffer property of a TypedArray . byteOffset <integer> Index of first byte to expose. Default: 0 . length <integer> Number of bytes to expose. Default: arrayBuffer.length - byteOffset . This creates a view of the ArrayBuffer or SharedArrayBuffer without copying the underlying memory. For example, when passed a reference to the .buffer property of a TypedArray instance, the newly created Buffer will share the same allocated memory as the TypedArray . The optional byteOffset and length arguments specify a memory range within the arrayBuffer that will be shared by the Buffer . const arr = new Uint16Array(2); arr[0] = 5000; arr[1] = 4000; // Shares memory with `arr` const buf = new Buffer(arr.buffer); console.log(buf); // Prints: <Buffer 88 13 a0 0f> // Changing the original Uint16Array changes the Buffer also arr[1] = 6000; console.log(buf); // Prints: <Buffer 88 13 70 17> Stability: 0 - Deprecated: Use Buffer.from(buffer) instead. buffer <Buffer> | <Uint8Array> An existing Buffer or Uint8Array from which to copy data. Copies the passed buffer data onto a new Buffer instance. new Buffer(arrayBuffer[, byteOffset[, length]]) new Buffer(buffer) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 37/739 const buf1 = new Buffer('buffer'); const buf2 = new Buffer(buf1); buf1[0] = 0x61; console.log(buf1.toString()); // Prints: auffer console.log(buf2.toString()); // Prints: buffer Stability: 0 - Deprecated: Use Buffer.alloc() instead (also see Buffer.allocUnsafe() ). size <integer> The desired length of the new Buffer . Allocates a new Buffer of size bytes. If size is larger than buffer.constants.MAX_LENGTH or smaller than 0, ERR_INVALID_OPT_VALUE is thrown. A zero-length Buffer is created if size is 0. Prior to Node.js 8.0.0, the underlying memory for Buffer instances created in this way is not initialized. The contents of a newly created Buffer are unknown and may contain sensitive data. Use Buffer.alloc(size) instead to initialize a Buffer with zeroes. const buf = new Buffer(10); console.log(buf); // Prints: <Buffer 00 00 00 00 00 00 00 00 00 00> Stability: 0 - Deprecated: Use Buffer.from(string[, encoding]) instead. string <string> String to encode. encoding <string> The encoding of string . Default: 'utf8' . Creates a new Buffer containing string . The encoding parameter identifies the character encoding of string . const buf1 = new Buffer('this is a tést'); const buf2 = new Buffer('7468697320697320612074c3a97374', 'hex'); console.log(buf1.toString()); // Prints: this is a tést console.log(buf2.toString()); // Prints: this is a tést console.log(buf1.toString('ascii')); // Prints: this is a tC)st new Buffer(size) new Buffer(string[, encoding]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 38/739 size <integer> The desired length of the new Buffer . fill <string> | <Buffer> | <integer> A value to pre-fill the new Buffer with. Default: 0 . encoding <string> If fill is a string, this is its encoding. Default: 'utf8' . Allocates a new Buffer of size bytes. If fill is undefined , the Buffer will be zero-filled. const buf = Buffer.alloc(5); console.log(buf); // Prints: <Buffer 00 00 00 00 00> Allocates a new Buffer of size bytes. If size is larger than buffer.constants.MAX_LENGTH or smaller than 0, ERR_INVALID_OPT_VALUE is thrown. A zero-length Buffer is created if size is 0. If fill is specified, the allocated Buffer will be initialized by calling buf.fill(fill) . const buf = Buffer.alloc(5, 'a'); console.log(buf); // Prints: <Buffer 61 61 61 61 61> If both fill and encoding are specified, the allocated Buffer will be initialized by calling buf.fill(fill, encoding) . const buf = Buffer.alloc(11, 'aGVsbG8gd29ybGQ=', 'base64'); console.log(buf); // Prints: <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64> Calling Buffer.alloc() can be significantly slower than the alternative Buffer.allocUnsafe() but ensures that the newly created Buffer instance contents will never contain sensitive data. A TypeError will be thrown if size is not a number. size <integer> The desired length of the new Buffer . Allocates a new Buffer of size bytes. If size is larger than buffer.constants.MAX_LENGTH or smaller than 0, ERR_INVALID_OPT_VALUE is thrown. A zero-length Buffer is created if size is 0. The underlying memory for Buffer instances created in this way is not initialized. The contents of the newly created Buffer are unknown and may contain sensitive data. Use Buffer.alloc() instead to initialize Buffer instances with zeroes. const buf = Buffer.allocUnsafe(10); console.log(buf); // Prints: (contents may vary): <Buffer a0 8b 28 3f 01 00 00 00 50 32> buf.fill(0); console.log(buf); Class Method: Buffer.alloc(size[, fill[, encoding]]) Class Method: Buffer.allocUnsafe(size) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 39/739 co so e. og(bu ); // Prints: <Buffer 00 00 00 00 00 00 00 00 00 00> A TypeError will be thrown if size is not a number. Note that the Buffer module pre-allocates an internal Buffer instance of size Buffer.poolSize that is used as a pool for the fast allocation of new Buffer instances created using Buffer.allocUnsafe() and the deprecated new Buffer(size) constructor only when size is less than or equal to Buffer.poolSize >> 1 (floor of Buffer.poolSize divided by two). Use of this pre-allocated internal memory pool is a key difference between calling Buffer.alloc(size, fill) vs. Buffer.allocUnsafe(size).fill(fill) . Specifically, Buffer.alloc(size, fill) will never use the internal Buffer pool, while Buffer.allocUnsafe(size).fill(fill) will use the internal Buffer pool if size is less than or equal to half Buffer.poolSize . The difference is subtle but can be important when an application requires the additional performance that Buffer.allocUnsafe() provides. size <integer> The desired length of the new Buffer . Allocates a new Buffer of size bytes. If size is larger than buffer.constants.MAX_LENGTH or smaller than 0, ERR_INVALID_OPT_VALUE is thrown. A zero-length Buffer is created if size is 0. The underlying memory for Buffer instances created in this way is not initialized. The contents of the newly created Buffer are unknown and may contain sensitive data. Use buf.fill(0) to initialize such Buffer instances with zeroes. When using Buffer.allocUnsafe() to allocate new Buffer instances, allocations under 4KB are sliced from a single pre-allocated Buffer . This allows applications to avoid the garbage collection overhead of creating many individually allocated Buffer instances. This approach improves both performance and memory usage by eliminating the need to track and clean up as many persistent objects. However, in the case where a developer may need to retain a small chunk of memory from a pool for an indeterminate amount of time, it may be appropriate to create an un-pooled Buffer instance using Buffer.allocUnsafeSlow() and then copying out the relevant bits. // Need to keep around a few small chunks of memory const store = []; socket.on('readable', () => { let data; while (null !== (data = readable.read())) { // Allocate for retained data const sb = Buffer.allocUnsafeSlow(10); // Copy the data into the new allocation data.copy(sb, 0, 0, 10); store.push(sb); } }); Buffer.allocUnsafeSlow() should be used only as a last resort after a developer has observed undue memory retention in their applications. A TypeError will be thrown if size is not a number. string <string> | <Buffer> | <TypedArray> | <DataView> | <ArrayBuffer> | <SharedArrayBuffer> A value to calculate the length of. encoding <string> If string is a string, this is its encoding. Default: 'utf8' . Class Method: Buffer.allocUnsafeSlow(size) Class Method: Buffer.byteLength(string[, encoding]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 40/739 Returns: <integer> The number of bytes contained within string . Returns the actual byte length of a string. This is not the same as String.prototype.length since that returns the number of characters in a string. For 'base64' and 'hex' , this function assumes valid input. For strings that contain non-Base64/Hex-encoded data (e.g. whitespace), the return value might be greater than the length of a Buffer created from the string. const str = '\u00bd + \u00bc = \u00be'; console.log(`${str}: ${str.length} characters, ` + `${Buffer.byteLength(str, 'utf8')} bytes`); // Prints: ½ + ¼ = ¾: 9 characters, 12 bytes When string is a Buffer / DataView / TypedArray / ArrayBuffer / SharedArrayBuffer , the actual byte length is returned. buf1 <Buffer> | <Uint8Array> buf2 <Buffer> | <Uint8Array> Returns: <integer> Compares buf1 to buf2 typically for the purpose of sorting arrays of Buffer instances. This is equivalent to calling buf1.compare(buf2) . const buf1 = Buffer.from('1234'); const buf2 = Buffer.from('0123'); const arr = [buf1, buf2]; console.log(arr.sort(Buffer.compare)); // Prints: [ <Buffer 30 31 32 33>, <Buffer 31 32 33 34> ] // (This result is equal to: [buf2, buf1]) list <Buffer[]> | <Uint8Array[]> List of Buffer or Uint8Array instances to concat. totalLength <integer> Total length of the Buffer instances in list when concatenated. Returns: <Buffer> Returns a new Buffer which is the result of concatenating all the Buffer instances in the list together. If the list has no items, or if the totalLength is 0, then a new zero-length Buffer is returned. If totalLength is not provided, it is calculated from the Buffer instances in list . This however causes an additional loop to be executed in order to calculate the totalLength , so it is faster to provide the length explicitly if it is already known. If totalLength is provided, it is coerced to an unsigned integer. If the combined length of the Buffer s in list exceeds totalLength , the result is truncated to totalLength . // Create a single `Buffer` from a list of three `Buffer` instances. const buf1 = Buffer.alloc(10); const buf2 = Buffer.alloc(14); const buf3 = Buffer.alloc(18); Class Method: Buffer.compare(buf1, buf2) Class Method: Buffer.concat(list[, totalLength]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 41/739 const totalLength = buf1.length + buf2.length + buf3.length; console.log(totalLength); // Prints: 42 const bufA = Buffer.concat([buf1, buf2, buf3], totalLength); console.log(bufA); // Prints: <Buffer 00 00 00 00 ...> console.log(bufA.length); // Prints: 42 array <integer[]> Allocates a new Buffer using an array of octets. // Creates a new Buffer containing UTF-8 bytes of the string 'buffer' const buf = Buffer.from([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]); A TypeError will be thrown if array is not an Array . arrayBuffer <ArrayBuffer> | <SharedArrayBuffer> An ArrayBuffer , SharedArrayBuffer , or the .buffer property of a TypedArray . byteOffset <integer> Index of first byte to expose. Default: 0 . length <integer> Number of bytes to expose. Default: arrayBuffer.length - byteOffset . This creates a view of the ArrayBuffer without copying the underlying memory. For example, when passed a reference to the .buffer property of a TypedArray instance, the newly created Buffer will share the same allocated memory as the TypedArray . const arr = new Uint16Array(2); arr[0] = 5000; arr[1] = 4000; // Shares memory with `arr` const buf = Buffer.from(arr.buffer); console.log(buf); // Prints: <Buffer 88 13 a0 0f> // Changing the original Uint16Array changes the Buffer also arr[1] = 6000; console.log(buf); // Prints: <Buffer 88 13 70 17> The optional byteOffset and length arguments specify a memory range within the arrayBuffer that will be shared by the Buffer . Class Method: Buffer.from(array) Class Method: Buffer.from(arrayBuffer[, byteOffset[, length]]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 42/739 const ab = new ArrayBuffer(10); const buf = Buffer.from(ab, 0, 2); console.log(buf.length); // Prints: 2 A TypeError will be thrown if arrayBuffer is not an ArrayBuffer or a SharedArrayBuffer . buffer <Buffer> | <Uint8Array> An existing Buffer or Uint8Array from which to copy data. Copies the passed buffer data onto a new Buffer instance. const buf1 = Buffer.from('buffer'); const buf2 = Buffer.from(buf1); buf1[0] = 0x61; console.log(buf1.toString()); // Prints: auffer console.log(buf2.toString()); // Prints: buffer A TypeError will be thrown if buffer is not a Buffer . object <Object> An object supporting Symbol.toPrimitive or valueOf() offsetOrEncoding <number> | <string> A byte-offset or encoding, depending on the value returned either by object.valueOf() or object[Symbol.toPrimitive]() . length <number> A length, depending on the value returned either by object.valueOf() or object[Symbol.toPrimitive]() . For objects whose valueOf() function returns a value not strictly equal to object , returns Buffer.from(object.valueOf(), offsetOrEncoding, length) . const buf = Buffer.from(new String('this is a test')); // Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74> For objects that support Symbol.toPrimitive , returns Buffer.from(object[Symbol.toPrimitive](), offsetOrEncoding, length) . class Foo { [Symbol.toPrimitive]() { return 'this is a test'; } } const buf = Buffer.from(new Foo(), 'utf8'); // Prints: <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74> Class Method: Buffer.from(buffer) Class Method: Buffer.from(object[, offsetOrEncoding[, length]]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 43/739 string <string> A string to encode. encoding <string> The encoding of string . Default: 'utf8' . Creates a new Buffer containing string . The encoding parameter identifies the character encoding of string . const buf1 = Buffer.from('this is a tést'); const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex'); console.log(buf1.toString()); // Prints: this is a tést console.log(buf2.toString()); // Prints: this is a tést console.log(buf1.toString('ascii')); // Prints: this is a tC)st A TypeError will be thrown if string is not a string. obj <Object> Returns: <boolean> Returns true if obj is a Buffer , false otherwise. encoding <string> A character encoding name to check. Returns: <boolean> Returns true if encoding contains a supported character encoding, or false otherwise. <integer> Default: 8192 This is the size (in bytes) of pre-allocated internal Buffer instances used for pooling. This value may be modified. The index operator [index] can be used to get and set the octet at position index in buf . The values refer to individual bytes, so the legal value range is between 0x00 and 0xFF (hex) or 0 and 255 (decimal). This operator is inherited from Uint8Array , so its behavior on out-of-bounds access is the same as UInt8Array - that is, getting returns undefined and setting does nothing. // Copy an ASCII string into a `Buffer` one byte at a time. const str = 'Node.js'; const buf = Buffer.allocUnsafe(str.length); for (let i = 0; i < str.length; i++) { buf[i] = str.charCodeAt(i); Class Method: Buffer.from(string[, encoding]) Class Method: Buffer.isBuffer(obj) Class Method: Buffer.isEncoding(encoding) Class Property: Buffer.poolSize buf[index] 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 44/739 } console.log(buf.toString('ascii')); // Prints: Node.js <ArrayBuffer> The underlying ArrayBuffer object based on which this Buffer object is created. const arrayBuffer = new ArrayBuffer(16); const buffer = Buffer.from(arrayBuffer); console.log(buffer.buffer === arrayBuffer); // Prints: true <integer> The byteOffset on the underlying ArrayBuffer object based on which this Buffer object is created. When setting byteOffset in Buffer.from(ArrayBuffer, byteOffset, length) or sometimes when allocating a buffer smaller than Buffer.poolSize the buffer doesn't start from a zero offset on the underlying ArrayBuffer . This can cause problems when accessing the underlying ArrayBuffer directly using buf.buffer , as the first bytes in this ArrayBuffer may be unrelated to the buf object itself. A common issue is when casting a Buffer object to a TypedArray object, in this case one needs to specify the byteOffset correctly: // Create a buffer smaller than `Buffer.poolSize`. const nodeBuffer = new Buffer.from([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]); // When casting the Node.js Buffer to an Int8 TypedArray remember to use the // byteOffset. new Int8Array(nodeBuffer.buffer, nodeBuffer.byteOffset, nodeBuffer.length); target <Buffer> | <Uint8Array> A Buffer or Uint8Array with which to compare buf . targetStart <integer> The offset within target at which to begin comparison. Default: 0 . targetEnd <integer> The offset with target at which to end comparison (not inclusive). Default: target.length . sourceStart <integer> The offset within buf at which to begin comparison. Default: 0 . sourceEnd <integer> The offset within buf at which to end comparison (not inclusive). Default: buf.length . Returns: <integer> Compares buf with target and returns a number indicating whether buf comes before, after, or is the same as target in sort order. Comparison is based on the actual sequence of bytes in each Buffer . 0 is returned if target is the same as buf 1 is returned if target should come before buf when sorted. buf.buffer buf.byteOffset buf.compare(target[, targetStart[, targetEnd[, sourceStart[, sourceEnd]]]]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 45/739 -1 is returned if target should come after buf when sorted. const buf1 = Buffer.from('ABC'); const buf2 = Buffer.from('BCD'); const buf3 = Buffer.from('ABCD'); console.log(buf1.compare(buf1)); // Prints: 0 console.log(buf1.compare(buf2)); // Prints: -1 console.log(buf1.compare(buf3)); // Prints: -1 console.log(buf2.compare(buf1)); // Prints: 1 console.log(buf2.compare(buf3)); // Prints: 1 console.log([buf1, buf2, buf3].sort(Buffer.compare)); // Prints: [ <Buffer 41 42 43>, <Buffer 41 42 43 44>, <Buffer 42 43 44> ] // (This result is equal to: [buf1, buf3, buf2]) The optional targetStart , targetEnd , sourceStart , and sourceEnd arguments can be used to limit the comparison to specific ranges within target and buf respectively. const buf1 = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8, 9]); const buf2 = Buffer.from([5, 6, 7, 8, 9, 1, 2, 3, 4]); console.log(buf1.compare(buf2, 5, 9, 0, 4)); // Prints: 0 console.log(buf1.compare(buf2, 0, 6, 4)); // Prints: -1 console.log(buf1.compare(buf2, 5, 6, 5)); // Prints: 1 ERR_INDEX_OUT_OF_RANGE is thrown if targetStart < 0 , sourceStart < 0 , targetEnd > target.byteLength , or sourceEnd > source.byteLength . target <Buffer> | <Uint8Array> A Buffer or Uint8Array to copy into. targetStart <integer> The offset within target at which to begin writing. Default: 0 . sourceStart <integer> The offset within buf from which to begin copying. Default: 0 . sourceEnd <integer> The offset within buf at which to stop copying (not inclusive). Default: buf.length . Returns: <integer> The number of bytes copied. Copies data from a region of buf to a region in target even if the target memory region overlaps with buf . // Create two `Buffer` instances. const buf1 = Buffer.allocUnsafe(26); const buf2 = Buffer.allocUnsafe(26).fill('!'); for (let i = 0; i < 26; i++) { buf.copy(target[, targetStart[, sourceStart[, sourceEnd]]]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 46/739 // 97 is the decimal ASCII value for 'a' buf1[i] = i + 97; } // Copy `buf1` bytes 16 through 19 into `buf2` starting at byte 8 of `buf2` buf1.copy(buf2, 8, 16, 20); console.log(buf2.toString('ascii', 0, 25)); // Prints: !!!!!!!!qrst!!!!!!!!!!!!! // Create a `Buffer` and copy data from one region to an overlapping region // within the same `Buffer`. const buf = Buffer.allocUnsafe(26); for (let i = 0; i < 26; i++) { // 97 is the decimal ASCII value for 'a' buf[i] = i + 97; } buf.copy(buf, 0, 4, 10); console.log(buf.toString()); // Prints: efghijghijklmnopqrstuvwxyz Returns: <Iterator> Creates and returns an iterator of [index, byte] pairs from the contents of buf . // Log the entire contents of a `Buffer`. const buf = Buffer.from('buffer'); for (const pair of buf.entries()) { console.log(pair); } // Prints: // [0, 98] // [1, 117] // [2, 102] // [3, 102] // [4, 101] // [5, 114] otherBuffer <Buffer> A Buffer or Uint8Array with which to compare buf . Returns: <boolean> buf.entries() buf.equals(otherBuffer) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 47/739 Returns true if both buf and otherBuffer have exactly the same bytes, false otherwise. const buf1 = Buffer.from('ABC'); const buf2 = Buffer.from('414243', 'hex'); const buf3 = Buffer.from('ABCD'); console.log(buf1.equals(buf2)); // Prints: true console.log(buf1.equals(buf3)); // Prints: false value <string> | <Buffer> | <integer> The value with which to fill buf . offset <integer> Number of bytes to skip before starting to fill buf . Default: 0 . end <integer> Where to stop filling buf (not inclusive). Default: buf.length . encoding <string> The encoding for value if value is a string. Default: 'utf8' . Returns: <Buffer> A reference to buf . Fills buf with the specified value . If the offset and end are not given, the entire buf will be filled: // Fill a `Buffer` with the ASCII character 'h'. const b = Buffer.allocUnsafe(50).fill('h'); console.log(b.toString()); // Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh value is coerced to a uint32 value if it is not a string, Buffer , or integer. If the resulting integer is greater than 255 (decimal), buf will be filled with value & 255 . If the final write of a fill() operation falls on a multi-byte character, then only the bytes of that character that fit into buf are written: // Fill a `Buffer` with a two-byte character. console.log(Buffer.allocUnsafe(3).fill('\u0222')); // Prints: <Buffer c8 a2 c8> If value contains invalid characters, it is truncated; if no valid fill data remains, an exception is thrown: const buf = Buffer.allocUnsafe(5); console.log(buf.fill('a')); // Prints: <Buffer 61 61 61 61 61> console.log(buf.fill('aazz', 'hex')); // Prints: <Buffer aa aa aa aa aa> console.log(buf.fill('zz', 'hex')); // Throws an exception. buf.fill(value[, offset[, end]][, encoding]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 48/739 value <string> | <Buffer> | <integer> What to search for. byteOffset <integer> Where to begin searching in buf . Default: 0 . encoding <string> If value is a string, this is its encoding. Default: 'utf8' . Returns: <boolean> true if value was found in buf , false otherwise. Equivalent to buf.indexOf() !== -1 . const buf = Buffer.from('this is a buffer'); console.log(buf.includes('this')); // Prints: true console.log(buf.includes('is')); // Prints: true console.log(buf.includes(Buffer.from('a buffer'))); // Prints: true console.log(buf.includes(97)); // Prints: true (97 is the decimal ASCII value for 'a') console.log(buf.includes(Buffer.from('a buffer example'))); // Prints: false console.log(buf.includes(Buffer.from('a buffer example').slice(0, 8))); // Prints: true console.log(buf.includes('this', 4)); // Prints: false value <string> | <Buffer> | <Uint8Array> | <integer> What to search for. byteOffset <integer> Where to begin searching in buf . Default: 0 . encoding <string> If value is a string, this is the encoding used to determine the binary representation of the string that will be searched for in buf . Default: 'utf8' . Returns: <integer> The index of the first occurrence of value in buf , or -1 if buf does not contain value . If value is: a string, value is interpreted according to the character encoding in encoding . a Buffer or Uint8Array , value will be used in its entirety. To compare a partial Buffer , use buf.slice() . a number, value will be interpreted as an unsigned 8-bit integer value between 0 and 255 . const buf = Buffer.from('this is a buffer'); console.log(buf.indexOf('this')); // Prints: 0 console.log(buf.indexOf('is')); // Prints: 2 console.log(buf.indexOf(Buffer.from('a buffer'))); // Prints: 8 console.log(buf.indexOf(97)); // Prints: 8 (97 is the decimal ASCII value for 'a') console.log(buf.indexOf(Buffer.from('a buffer example'))); // Prints: 1 buf.includes(value[, byteOffset][, encoding]) buf.indexOf(value[, byteOffset][, encoding]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 49/739 // Prints: -1 console.log(buf.indexOf(Buffer.from('a buffer example').slice(0, 8))); // Prints: 8 const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le'); console.log(utf16Buffer.indexOf('\u03a3', 0, 'utf16le')); // Prints: 4 console.log(utf16Buffer.indexOf('\u03a3', -4, 'utf16le')); // Prints: 6 If value is not a string, number, or Buffer , this method will throw a TypeError . If value is a number, it will be coerced to a valid byte value, an integer between 0 and 255. If byteOffset is not a number, it will be coerced to a number. If the result of coercion is NaN or 0 , then the entire buffer will be searched. This behavior matches String#indexOf() . const b = Buffer.from('abcdef'); // Passing a value that's a number, but not a valid byte // Prints: 2, equivalent to searching for 99 or 'c' console.log(b.indexOf(99.9)); console.log(b.indexOf(256 + 99)); // Passing a byteOffset that coerces to NaN or 0 // Prints: 1, searching the whole buffer console.log(b.indexOf('b', undefined)); console.log(b.indexOf('b', {})); console.log(b.indexOf('b', null)); console.log(b.indexOf('b', [])); If value is an empty string or empty Buffer and byteOffset is less than buf.length , byteOffset will be returned. If value is empty and byteOffset is at least buf.length , buf.length will be returned. Returns: <Iterator> Creates and returns an iterator of buf keys (indices). const buf = Buffer.from('buffer'); for (const key of buf.keys()) { console.log(key); } // Prints: // 0 // 1 // 2 // 3 // 4 // 5 buf.keys() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 50/739 value <string> | <Buffer> | <Uint8Array> | <integer> What to search for. byteOffset <integer> Where to begin searching in buf . Default: buf.length - 1 . encoding <string> If value is a string, this is the encoding used to determine the binary representation of the string that will be searched for in buf . Default: 'utf8' . Returns: <integer> The index of the last occurrence of value in buf , or -1 if buf does not contain value . Identical to buf.indexOf() , except the last occurrence of value is found rather than the first occurrence. const buf = Buffer.from('this buffer is a buffer'); console.log(buf.lastIndexOf('this')); // Prints: 0 console.log(buf.lastIndexOf('buffer')); // Prints: 17 console.log(buf.lastIndexOf(Buffer.from('buffer'))); // Prints: 17 console.log(buf.lastIndexOf(97)); // Prints: 15 (97 is the decimal ASCII value for 'a') console.log(buf.lastIndexOf(Buffer.from('yolo'))); // Prints: -1 console.log(buf.lastIndexOf('buffer', 5)); // Prints: 5 console.log(buf.lastIndexOf('buffer', 4)); // Prints: -1 const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'utf16le'); console.log(utf16Buffer.lastIndexOf('\u03a3', undefined, 'utf16le')); // Prints: 6 console.log(utf16Buffer.lastIndexOf('\u03a3', -5, 'utf16le')); // Prints: 4 If value is not a string, number, or Buffer , this method will throw a TypeError . If value is a number, it will be coerced to a valid byte value, an integer between 0 and 255. If byteOffset is not a number, it will be coerced to a number. Any arguments that coerce to NaN , like {} or undefined , will search the whole buffer. This behavior matches String#lastIndexOf() . const b = Buffer.from('abcdef'); // Passing a value that's a number, but not a valid byte // Prints: 2, equivalent to searching for 99 or 'c' console.log(b.lastIndexOf(99.9)); console.log(b.lastIndexOf(256 + 99)); // Passing a byteOffset that coerces to NaN // Prints: 1, searching the whole buffer console.log(b.lastIndexOf('b', undefined)); console.log(b.lastIndexOf('b', {})); // Passing a byteOffset that coerces to 0 buf.lastIndexOf(value[, byteOffset][, encoding]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 51/739 // Prints: -1, equivalent to passing 0 console.log(b.lastIndexOf('b', null)); console.log(b.lastIndexOf('b', [])); If value is an empty string or empty Buffer , byteOffset will be returned. <integer> Returns the amount of memory allocated for buf in bytes. Note that this does not necessarily reflect the amount of "usable" data within buf . // Create a `Buffer` and write a shorter ASCII string to it. const buf = Buffer.alloc(1234); console.log(buf.length); // Prints: 1234 buf.write('some string', 0, 'ascii'); console.log(buf.length); // Prints: 1234 While the length property is not immutable, changing the value of length can result in undefined and inconsistent behavior. Applications that wish to modify the length of a Buffer should therefore treat length as read-only and use buf.slice() to create a new Buffer . let buf = Buffer.allocUnsafe(10); buf.write('abcdefghj', 0, 'ascii'); console.log(buf.length); // Prints: 10 buf = buf.slice(0, 5); console.log(buf.length); // Prints: 5 Stability: 0 - Deprecated: Use buf.buffer instead. The buf.parent property is a deprecated alias for buf.buffer . offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 8 . buf.length buf.parent buf.readDoubleBE(offset) buf.readDoubleLE(offset) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 52/739 Returns: <number> Reads a 64-bit double from buf at the specified offset with specified endian format ( readDoubleBE() returns big endian, readDoubleLE() returns little endian). const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]); console.log(buf.readDoubleBE(0)); // Prints: 8.20788039913184e-304 console.log(buf.readDoubleLE(0)); // Prints: 5.447603722011605e-270 console.log(buf.readDoubleLE(1)); // Throws ERR_OUT_OF_RANGE offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 4 . Returns: <number> Reads a 32-bit float from buf at the specified offset with specified endian format ( readFloatBE() returns big endian, readFloatLE() returns little endian). const buf = Buffer.from([1, 2, 3, 4]); console.log(buf.readFloatBE(0)); // Prints: 2.387939260590663e-38 console.log(buf.readFloatLE(0)); // Prints: 1.539989614439558e-36 console.log(buf.readFloatLE(1)); // Throws ERR_OUT_OF_RANGE offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 1 . Returns: <integer> Reads a signed 8-bit integer from buf at the specified offset . Integers read from a Buffer are interpreted as two's complement signed values. const buf = Buffer.from([-1, 5]); console.log(buf.readInt8(0)); // Prints: -1 console.log(buf.readInt8(1)); // Prints: 5 console.log(buf.readInt8(2)); // Throws ERR_OUT_OF_RANGE buf.readFloatBE(offset) buf.readFloatLE(offset) buf.readInt8(offset) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 53/739 offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 2 . Returns: <integer> Reads a signed 16-bit integer from buf at the specified offset with the specified endian format ( readInt16BE() returns big endian, readInt16LE() returns little endian). Integers read from a Buffer are interpreted as two's complement signed values. const buf = Buffer.from([0, 5]); console.log(buf.readInt16BE(0)); // Prints: 5 console.log(buf.readInt16LE(0)); // Prints: 1280 console.log(buf.readInt16LE(1)); // Throws ERR_OUT_OF_RANGE offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 4 . Returns: <integer> Reads a signed 32-bit integer from buf at the specified offset with the specified endian format ( readInt32BE() returns big endian, readInt32LE() returns little endian). Integers read from a Buffer are interpreted as two's complement signed values. const buf = Buffer.from([0, 0, 0, 5]); console.log(buf.readInt32BE(0)); // Prints: 5 console.log(buf.readInt32LE(0)); // Prints: 83886080 console.log(buf.readInt32LE(1)); // Throws ERR_OUT_OF_RANGE offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - byteLength . byteLength <integer> Number of bytes to read. Must satisfy 0 < byteLength <= 6 . Returns: <integer> Reads byteLength number of bytes from buf at the specified offset and interprets the result as a two's complement signed value. Supports up to 48 bits of accuracy. buf.readInt16BE(offset) buf.readInt16LE(offset) buf.readInt32BE(offset) buf.readInt32LE(offset) buf.readIntBE(offset, byteLength) buf.readIntLE(offset, byteLength) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 54/739 const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]); console.log(buf.readIntLE(0, 6).toString(16)); // Prints: -546f87a9cbee console.log(buf.readIntBE(0, 6).toString(16)); // Prints: 1234567890ab console.log(buf.readIntBE(1, 6).toString(16)); // Throws ERR_INDEX_OUT_OF_RANGE console.log(buf.readIntBE(1, 0).toString(16)); // Throws ERR_OUT_OF_RANGE offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 1 . Returns: <integer> Reads an unsigned 8-bit integer from buf at the specified offset . const buf = Buffer.from([1, -2]); console.log(buf.readUInt8(0)); // Prints: 1 console.log(buf.readUInt8(1)); // Prints: 254 console.log(buf.readUInt8(2)); // Throws ERR_OUT_OF_RANGE offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 2 . Returns: <integer> Reads an unsigned 16-bit integer from buf at the specified offset with specified endian format ( readUInt16BE() returns big endian, readUInt16LE() returns little endian). const buf = Buffer.from([0x12, 0x34, 0x56]); console.log(buf.readUInt16BE(0).toString(16)); // Prints: 1234 console.log(buf.readUInt16LE(0).toString(16)); // Prints: 3412 console.log(buf.readUInt16BE(1).toString(16)); // Prints: 3456 console.log(buf.readUInt16LE(1).toString(16)); // Prints: 5634 console.log(buf.readUInt16LE(2).toString(16)); // Throws ERR_OUT_OF_RANGE buf.readUInt8(offset) buf.readUInt16BE(offset) buf.readUInt16LE(offset) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 55/739 offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - 4 . Returns: <integer> Reads an unsigned 32-bit integer from buf at the specified offset with specified endian format ( readUInt32BE() returns big endian, readUInt32LE() returns little endian). const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]); console.log(buf.readUInt32BE(0).toString(16)); // Prints: 12345678 console.log(buf.readUInt32LE(0).toString(16)); // Prints: 78563412 console.log(buf.readUInt32LE(1).toString(16)); // Throws ERR_OUT_OF_RANGE offset <integer> Number of bytes to skip before starting to read. Must satisfy 0 <= offset <= buf.length - byteLength . byteLength <integer> Number of bytes to read. Must satisfy 0 < byteLength <= 6 . Returns: <integer> Reads byteLength number of bytes from buf at the specified offset and interprets the result as an unsigned integer. Supports up to 48 bits of accuracy. const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]); console.log(buf.readUIntBE(0, 6).toString(16)); // Prints: 1234567890ab console.log(buf.readUIntLE(0, 6).toString(16)); // Prints: ab9078563412 console.log(buf.readUIntBE(1, 6).toString(16)); // Throws ERR_OUT_OF_RANGE start <integer> Where the new Buffer will start. Default: 0 . end <integer> Where the new Buffer will end (not inclusive). Default: buf.length . Returns: <Buffer> Returns a new Buffer that references the same memory as the original, but offset and cropped by the start and end indices. Specifying end greater than buf.length will return the same result as that of end equal to buf.length . Modifying the new Buffer slice will modify the memory in the original Buffer because the allocated memory of the two objects overlap. // Create a `Buffer` with the ASCII alphabet, take a slice, and modify one byte // from the original`Buffer` buf.readUInt32BE(offset) buf.readUInt32LE(offset) buf.readUIntBE(offset, byteLength) buf.readUIntLE(offset, byteLength) buf.slice([start[, end]]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 56/739 // from the original Buffer . const buf1 = Buffer.allocUnsafe(26); for (let i = 0; i < 26; i++) { // 97 is the decimal ASCII value for 'a' buf1[i] = i + 97; } const buf2 = buf1.slice(0, 3); console.log(buf2.toString('ascii', 0, buf2.length)); // Prints: abc buf1[0] = 33; console.log(buf2.toString('ascii', 0, buf2.length)); // Prints: !bc Specifying negative indexes causes the slice to be generated relative to the end of buf rather than the beginning. const buf = Buffer.from('buffer'); console.log(buf.slice(-6, -1).toString()); // Prints: buffe // (Equivalent to buf.slice(0, 5)) console.log(buf.slice(-6, -2).toString()); // Prints: buff // (Equivalent to buf.slice(0, 4)) console.log(buf.slice(-5, -2).toString()); // Prints: uff // (Equivalent to buf.slice(1, 4)) Returns: <Buffer> A reference to buf . Interprets buf as an array of unsigned 16-bit integers and swaps the byte order in-place. Throws ERR_INVALID_BUFFER_SIZE if buf.length is not a multiple of 2. const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]); console.log(buf1); // Prints: <Buffer 01 02 03 04 05 06 07 08> buf1.swap16(); console.log(buf1); // Prints: <Buffer 02 01 04 03 06 05 08 07> const buf2 = Buffer.from([0x1, 0x2, 0x3]); buf.swap16() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 57/739 co st bu u e . o ([0 , 0 , 0 3]); buf2.swap16(); // Throws ERR_INVALID_BUFFER_SIZE One convenient use of buf.swap16() is to perform a fast in-place conversion between UTF-16 little-endian and UTF-16 big-endian: const buf = Buffer.from('This is little-endian UTF-16', 'utf16le'); buf.swap16(); // Convert to big-endian UTF-16 text. Returns: <Buffer> A reference to buf . Interprets buf as an array of unsigned 32-bit integers and swaps the byte order in-place. Throws ERR_INVALID_BUFFER_SIZE if buf.length is not a multiple of 4. const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]); console.log(buf1); // Prints: <Buffer 01 02 03 04 05 06 07 08> buf1.swap32(); console.log(buf1); // Prints: <Buffer 04 03 02 01 08 07 06 05> const buf2 = Buffer.from([0x1, 0x2, 0x3]); buf2.swap32(); // Throws ERR_INVALID_BUFFER_SIZE Returns: <Buffer> A reference to buf . Interprets buf as an array of 64-bit numbers and swaps byte order in-place. Throws ERR_INVALID_BUFFER_SIZE if buf.length is not a multiple of 8. const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]); console.log(buf1); // Prints: <Buffer 01 02 03 04 05 06 07 08> buf1.swap64(); console.log(buf1); // Prints: <Buffer 08 07 06 05 04 03 02 01> const buf2 = Buffer.from([0x1, 0x2, 0x3]); buf2.swap64(); // Throws ERR INVALID BUFFER SIZE buf.swap32() buf.swap64() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 58/739 // Throws ERR_INVALID_BUFFER_SIZE Note that JavaScript cannot encode 64-bit integers. This method is intended for working with 64-bit floats. Returns: <Object> Returns a JSON representation of buf . JSON.stringify() implicitly calls this function when stringifying a Buffer instance. const buf = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5]); const json = JSON.stringify(buf); console.log(json); // Prints: {"type":"Buffer","data":[1,2,3,4,5]} const copy = JSON.parse(json, (key, value) => { return value && value.type === 'Buffer' ? Buffer.from(value.data) : value; }); console.log(copy); // Prints: <Buffer 01 02 03 04 05> encoding <string> The character encoding to use. Default: 'utf8' . start <integer> The byte offset to start decoding at. Default: 0 . end <integer> The byte offset to stop decoding at (not inclusive). Default: buf.length . Returns: <string> Decodes buf to a string according to the specified character encoding in encoding . start and end may be passed to decode only a subset of buf . The maximum length of a string instance (in UTF-16 code units) is available as buffer.constants.MAX_STRING_LENGTH . const buf1 = Buffer.allocUnsafe(26); for (let i = 0; i < 26; i++) { // 97 is the decimal ASCII value for 'a' buf1[i] = i + 97; } console.log(buf1.toString('ascii')); // Prints: abcdefghijklmnopqrstuvwxyz console.log(buf1.toString('ascii', 0, 5)); // Prints: abcde const buf2 = Buffer.from('tést'); console.log(buf2.toString('hex')); // Prints: 74c3a97374 buf.toJSON() buf.toString([encoding[, start[, end]]]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 59/739 console.log(buf2.toString('utf8', 0, 3)); // Prints: té console.log(buf2.toString(undefined, 0, 3)); // Prints: té Returns: <Iterator> Creates and returns an iterator for buf values (bytes). This function is called automatically when a Buffer is used in a for..of statement. const buf = Buffer.from('buffer'); for (const value of buf.values()) { console.log(value); } // Prints: // 98 // 117 // 102 // 102 // 101 // 114 for (const value of buf) { console.log(value); } // Prints: // 98 // 117 // 102 // 102 // 101 // 114 string <string> String to write to buf . offset <integer> Number of bytes to skip before starting to write string . Default: 0 . length <integer> Number of bytes to write. Default: buf.length - offset . encoding <string> The character encoding of string . Default: 'utf8' . Returns: <integer> Number of bytes written. Writes string to buf at offset according to the character encoding in encoding . The length parameter is the number of bytes to write. If buf did not contain enough space to fit the entire string, only part of string will be written. However, partially encoded characters will not be written. const buf = Buffer.alloc(256); const len = buf.write('\u00bd + \u00bc = \u00be', 0); buf.values() buf.write(string[, offset[, length]][, encoding]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 60/739 console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`); // Prints: 12 bytes: ½ + ¼ = ¾ value <number> Number to be written to buf . offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 8 . Returns: <integer> offset plus the number of bytes written. Writes value to buf at the specified offset with specified endian format ( writeDoubleBE() writes big endian, writeDoubleLE() writes little endian). value should be a valid 64-bit double. Behavior is undefined when value is anything other than a 64-bit double. const buf = Buffer.allocUnsafe(8); buf.writeDoubleBE(123.456, 0); console.log(buf); // Prints: <Buffer 40 5e dd 2f 1a 9f be 77> buf.writeDoubleLE(123.456, 0); console.log(buf); // Prints: <Buffer 77 be 9f 1a 2f dd 5e 40> value <number> Number to be written to buf . offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 4 . Returns: <integer> offset plus the number of bytes written. Writes value to buf at the specified offset with specified endian format ( writeFloatBE() writes big endian, writeFloatLE() writes little endian). value should be a valid 32-bit float. Behavior is undefined when value is anything other than a 32-bit float. const buf = Buffer.allocUnsafe(4); buf.writeFloatBE(0xcafebabe, 0); console.log(buf); // Prints: <Buffer 4f 4a fe bb> buf.writeFloatLE(0xcafebabe, 0); console.log(buf); // Prints: <Buffer bb fe 4a 4f> buf.writeDoubleBE(value, offset) buf.writeDoubleLE(value, offset) buf.writeFloatBE(value, offset) buf.writeFloatLE(value, offset) buf.writeInt8(value, offset) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 61/739 value <integer> Number to be written to buf . offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 1 . Returns: <integer> offset plus the number of bytes written. Writes value to buf at the specified offset . value should be a valid signed 8-bit integer. Behavior is undefined when value is anything other than a signed 8-bit integer. value is interpreted and written as a two's complement signed integer. const buf = Buffer.allocUnsafe(2); buf.writeInt8(2, 0); buf.writeInt8(-2, 1); console.log(buf); // Prints: <Buffer 02 fe> value <integer> Number to be written to buf . offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 2 . Returns: <integer> offset plus the number of bytes written. Writes value to buf at the specified offset with specified endian format ( writeInt16BE() writes big endian, writeInt16LE() writes little endian). value should be a valid signed 16-bit integer. Behavior is undefined when value is anything other than a signed 16-bit integer. value is interpreted and written as a two's complement signed integer. const buf = Buffer.allocUnsafe(4); buf.writeInt16BE(0x0102, 0); buf.writeInt16LE(0x0304, 2); console.log(buf); // Prints: <Buffer 01 02 04 03> value <integer> Number to be written to buf . offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 4 . Returns: <integer> offset plus the number of bytes written. Writes value to buf at the specified offset with specified endian format ( writeInt32BE() writes big endian, writeInt32LE() writes little endian). value should be a valid signed 32-bit integer. Behavior is undefined when value is anything other than a signed 32-bit integer. value is interpreted and written as a two's complement signed integer. buf.writeInt16BE(value, offset) buf.writeInt16LE(value, offset) buf.writeInt32BE(value, offset) buf.writeInt32LE(value, offset) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 62/739 const buf = Buffer.allocUnsafe(8); buf.writeInt32BE(0x01020304, 0); buf.writeInt32LE(0x05060708, 4); console.log(buf); // Prints: <Buffer 01 02 03 04 08 07 06 05> value <integer> Number to be written to buf . offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - byteLength . byteLength <integer> Number of bytes to write. Must satisfy 0 < byteLength <= 6 . Returns: <integer> offset plus the number of bytes written. Writes byteLength bytes of value to buf at the specified offset . Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than a signed integer. const buf = Buffer.allocUnsafe(6); buf.writeIntBE(0x1234567890ab, 0, 6); console.log(buf); // Prints: <Buffer 12 34 56 78 90 ab> buf.writeIntLE(0x1234567890ab, 0, 6); console.log(buf); // Prints: <Buffer ab 90 78 56 34 12> value <integer> Number to be written to buf . offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 1 . Returns: <integer> offset plus the number of bytes written. Writes value to buf at the specified offset . value should be a valid unsigned 8-bit integer. Behavior is undefined when value is anything other than an unsigned 8-bit integer. const buf = Buffer.allocUnsafe(4); buf.writeUInt8(0x3, 0); buf.writeUInt8(0x4, 1); buf.writeUInt8(0x23, 2); buf.writeUInt8(0x42, 3); console.log(buf); // Prints: <Buffer 03 04 23 42> buf.writeIntBE(value, offset, byteLength) buf.writeIntLE(value, offset, byteLength) buf.writeUInt8(value, offset) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 63/739 value <integer> Number to be written to buf . offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 2 . Returns: <integer> offset plus the number of bytes written. Writes value to buf at the specified offset with specified endian format ( writeUInt16BE() writes big endian, writeUInt16LE() writes little endian). value should be a valid unsigned 16-bit integer. Behavior is undefined when value is anything other than an unsigned 16-bit integer. const buf = Buffer.allocUnsafe(4); buf.writeUInt16BE(0xdead, 0); buf.writeUInt16BE(0xbeef, 2); console.log(buf); // Prints: <Buffer de ad be ef> buf.writeUInt16LE(0xdead, 0); buf.writeUInt16LE(0xbeef, 2); console.log(buf); // Prints: <Buffer ad de ef be> value <integer> Number to be written to buf . offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - 4 . Returns: <integer> offset plus the number of bytes written. Writes value to buf at the specified offset with specified endian format ( writeUInt32BE() writes big endian, writeUInt32LE() writes little endian). value should be a valid unsigned 32-bit integer. Behavior is undefined when value is anything other than an unsigned 32-bit integer. const buf = Buffer.allocUnsafe(4); buf.writeUInt32BE(0xfeedface, 0); console.log(buf); // Prints: <Buffer fe ed fa ce> buf.writeUInt32LE(0xfeedface, 0); console.log(buf); // Prints: <Buffer ce fa ed fe> buf.writeUInt16BE(value, offset) buf.writeUInt16LE(value, offset) buf.writeUInt32BE(value, offset) buf.writeUInt32LE(value, offset) buf.writeUIntBE(value, offset, byteLength) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 64/739 value <integer> Number to be written to buf . offset <integer> Number of bytes to skip before starting to write. Must satisfy 0 <= offset <= buf.length - byteLength . byteLength <integer> Number of bytes to write. Must satisfy 0 < byteLength <= 6 . Returns: <integer> offset plus the number of bytes written. Writes byteLength bytes of value to buf at the specified offset . Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than an unsigned integer. const buf = Buffer.allocUnsafe(6); buf.writeUIntBE(0x1234567890ab, 0, 6); console.log(buf); // Prints: <Buffer 12 34 56 78 90 ab> buf.writeUIntLE(0x1234567890ab, 0, 6); console.log(buf); // Prints: <Buffer ab 90 78 56 34 12> <integer> Default: 50 Returns the maximum number of bytes that will be returned when buf.inspect() is called. This can be overridden by user modules. See util.inspect() for more details on buf.inspect() behavior. Note that this is a property on the buffer module returned by require('buffer') , not on the Buffer global or a Buffer instance. <integer> The largest size allowed for a single Buffer instance. An alias for buffer.constants.MAX_LENGTH . Note that this is a property on the buffer module returned by require('buffer') , not on the Buffer global or a Buffer instance. source <Buffer> | <Uint8Array> A Buffer or Uint8Array instance. fromEnc <string> The current encoding. toEnc <string> To target encoding. Re-encodes the given Buffer or Uint8Array instance from one character encoding to another. Returns a new Buffer instance. Throws if the fromEnc or toEnc specify invalid character encodings or if conversion from fromEnc to toEnc is not permitted. Encodings supported by buffer.transcode() are: 'ascii' , 'utf8' , 'utf16le' , 'ucs2' , 'latin1' , and 'binary' . The transcoding process will use substitution characters if a given byte sequence cannot be adequately represented in the target encoding. For instance: const buffer = require('buffer'); buf.writeUIntLE(value, offset, byteLength) buffer.INSPECT_MAX_BYTES buffer.kMaxLength buffer.transcode(source, fromEnc, toEnc) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 65/739 q ( ); const newBuf = buffer.transcode(Buffer.from('€'), 'utf8', 'ascii'); console.log(newBuf.toString('ascii')); // Prints: '?' Because the Euro ( € ) sign is not representable in US-ASCII, it is replaced with ? in the transcoded Buffer . Note that this is a property on the buffer module returned by require('buffer') , not on the Buffer global or a Buffer instance. Stability: 0 - Deprecated: Use Buffer.allocUnsafeSlow() instead. Returns an un-pooled Buffer . In order to avoid the garbage collection overhead of creating many individually allocated Buffer instances, by default allocations under 4KB are sliced from a single larger allocated object. In the case where a developer may need to retain a small chunk of memory from a pool for an indeterminate amount of time, it may be appropriate to create an un-pooled Buffer instance using SlowBuffer then copy out the relevant bits. // Need to keep around a few small chunks of memory const store = []; socket.on('readable', () => { let data; while (null !== (data = readable.read())) { // Allocate for retained data const sb = SlowBuffer(10); // Copy the data into the new allocation data.copy(sb, 0, 0, 10); store.push(sb); } }); Use of SlowBuffer should be used only as a last resort after a developer has observed undue memory retention in their applications. Stability: 0 - Deprecated: Use Buffer.allocUnsafeSlow() instead. size <integer> The desired length of the new SlowBuffer . Allocates a new Buffer of size bytes. If size is larger than buffer.constants.MAX_LENGTH or smaller than 0, ERR_INVALID_OPT_VALUE is thrown. A zero-length Buffer is created if size is 0. The underlying memory for SlowBuffer instances is not initialized. The contents of a newly created SlowBuffer are unknown and may contain sensitive data. Use buf.fill(0) to initialize a SlowBuffer with zeroes. Class: SlowBuffer new SlowBuffer(size) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 66/739 const { SlowBuffer } = require('buffer'); const buf = new SlowBuffer(5); console.log(buf); // Prints: (contents may vary): <Buffer 78 e0 82 02 01> buf.fill(0); console.log(buf); // Prints: <Buffer 00 00 00 00 00> Note that buffer.constants is a property on the buffer module returned by require('buffer') , not on the Buffer global or a Buffer instance. <integer> The largest size allowed for a single Buffer instance. On 32-bit architectures, this value is (2^30)-1 (~1GB). On 64-bit architectures, this value is (2^31)-1 (~2GB). This value is also available as buffer.kMaxLength . <integer> The largest length allowed for a single string instance. Represents the largest length that a string primitive can have, counted in UTF-16 code units. This value may depend on the JS engine that is being used. Node.js Addons are dynamically-linked shared objects, written in C++, that can be loaded into Node.js using the require() function, and used just as if they were an ordinary Node.js module. They are used primarily to provide an interface between JavaScript running in Node.js and C/C++ libraries. At the moment, the method for implementing Addons is rather complicated, involving knowledge of several components and APIs: V8: the C++ library Node.js currently uses to provide the JavaScript implementation. V8 provides the mechanisms for creating objects, calling functions, etc. V8's API is documented mostly in the v8.h header file ( deps/v8/include/v8.h in the Node.js source tree), which is also available online . libuv : The C library that implements the Node.js event loop, its worker threads and all of the asynchronous behaviors of the platform. It also serves as a cross-platform abstraction library, giving easy, POSIX-like access across all major operating systems to many common system tasks, such as interacting with the filesystem, sockets, timers, and system events. libuv also provides a pthreads-like threading abstraction that may be used to power more sophisticated asynchronous Addons that need to move beyond the standard event loop. Addon authors are encouraged to think about how to avoid blocking the event loop with I/O or other time-intensive tasks by off-loading work via libuv to non-blocking system operations, worker threads or a custom use of libuv's threads. Internal Node.js libraries. Node.js itself exports a number of C++ APIs that Addons can use — the most important of which is the node::ObjectWrap class. Buffer Constants buffer.constants.MAX_LENGTH buffer.constants.MAX_STRING_LENGTH C++ Addons 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 67/739 Node.js includes a number of other statically linked libraries including OpenSSL. These other libraries are located in the deps/ directory in the Node.js source tree. Only the libuv, OpenSSL, V8 and zlib symbols are purposefully re-exported by Node.js and may be used to various extents by Addons. See Linking to Node.js' own dependencies for additional information. All of the following examples are available for download and may be used as the starting-point for an Addon. This "Hello world" example is a simple Addon, written in C++, that is the equivalent of the following JavaScript code: module.exports.hello = () => 'world'; First, create the file hello.cc : // hello.cc #include <node.h> namespace demo { using v8::FunctionCallbackInfo; using v8::Isolate; using v8::Local; using v8::NewStringType; using v8::Object; using v8::String; using v8::Value; void Method(const FunctionCallbackInfo<Value>& args) { Isolate* isolate = args.GetIsolate(); args.GetReturnValue().Set(String::NewFromUtf8( isolate, "world", NewStringType::kNormal).ToLocalChecked()); } void Initialize(Local<Object> exports) { NODE_SET_METHOD(exports, "hello", Method); } NODE_MODULE(NODE_GYP_MODULE_NAME, Initialize) } // namespace demo Note that all Node.js Addons must export an initialization function following the pattern: void Initialize(Local<Object> exports); NODE_MODULE(NODE_GYP_MODULE_NAME, Initialize) There is no semi-colon after NODE_MODULE as it's not a function (see node.h ). The module_name must match the filename of the final binary (excluding the .node suffix). In the hello.cc example, then, the initialization function is Initialize and the addon module name is addon . Hello world 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 68/739 When building addons with node-gyp , using the macro NODE_GYP_MODULE_NAME as the first parameter of NODE_MODULE() will ensure that the name of the final binary will be passed to NODE_MODULE() . There are environments in which Node.js addons may need to be loaded multiple times in multiple contexts. For example, the Electron runtime runs multiple instances of Node.js in a single process. Each instance will have its own require() cache, and thus each instance will need a native addon to behave correctly when loaded via require() . From the addon's perspective, this means that it must support multiple initializations. A context-aware addon can be constructed by using the macro NODE_MODULE_INITIALIZER , which expands to the name of a function which Node.js will expect to find when it loads an addon. An addon can thus be initialized as in the following example: using namespace v8; extern "C" NODE_MODULE_EXPORT void NODE_MODULE_INITIALIZER(Local<Object> exports, Local<Value> module, Local<Context> context) { /* Perform addon initialization steps here. */ } Another option is to use the macro NODE_MODULE_INIT() , which will also construct a context-aware addon. Unlike NODE_MODULE() , which is used to construct an addon around a given addon initializer function, NODE_MODULE_INIT() serves as the declaration of such an initializer to be followed by a function body. The following three variables may be used inside the function body following an invocation of NODE_MODULE_INIT() : Local<Object> exports , Local<Value> module , and Local<Context> context The choice to build a context-aware addon carries with it the responsibility of carefully managing global static data. Since the addon may be loaded multiple times, potentially even from different threads, any global static data stored in the addon must be properly protected, and must not contain any persistent references to JavaScript objects. The reason for this is that JavaScript objects are only valid in one context, and will likely cause a crash when accessed from the wrong context or from a different thread than the one on which they were created. The context-aware addon can be structured to avoid global static data by performing the following steps: defining a class which will hold per-addon-instance data. Such a class should include a v8::Persistent<v8::Object> which will hold a weak reference to the addon's exports object. The callback associated with the weak reference will then destroy the instance of the class. constructing an instance of this class in the addon initializer such that the v8::Persistent<v8::Object> is set to the exports object. storing the instance of the class in a v8::External , and passing the v8::External to all methods exposed to JavaScript by passing it to the v8::FunctionTemplate constructor which creates the native-backed JavaScript functions. The v8::FunctionTemplate constructor's third parameter accepts the v8::External . This will ensure that the per-addon-instance data reaches each binding that can be called from JavaScript. The per-addon-instance data must also be passed into any asynchronous callbacks the addon may create. The following example illustrates the implementation of a context-aware addon: #include <node.h> Context-aware addons 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 69/739 using namespace v8; class AddonData { public: AddonData(Isolate* isolate, Local<Object> exports): call_count(0) { // Link the existence of this object instance to the existence of exports. exports_.Reset(isolate, exports); exports_.SetWeak(this, DeleteMe, WeakCallbackType::kParameter); } ~AddonData() { if (!exports_.IsEmpty()) { // Reset the reference to avoid leaking data. exports_.ClearWeak(); exports_.Reset(); } } // Per-addon data. int call_count; private: // Method to call when "exports" is about to be garbage-collected. static void DeleteMe(const WeakCallbackInfo<AddonData>& info) { delete info.GetParameter(); } // Weak handle to the "exports" object. An instance of this class will be // destroyed along with the exports object to which it is weakly bound. v8::Persistent<v8::Object> exports_; }; static void Method(const v8::FunctionCallbackInfo<v8::Value>& info) { // Retrieve the per-addon-instance data. AddonData* data = reinterpret_cast<AddonData*>(info.Data().As<External>()->Value()); data->call_count++; info.GetReturnValue().Set((double)data->call_count); } // Initialize this addon to be context-aware. NODE_MODULE_INIT(/* exports, module, context */) { Isolate* isolate = context->GetIsolate(); // Create a new instance of AddonData for this instance of the addon. AddonData* data = new AddonData(isolate, exports); // Wrap the data in a v8::External so we can pass it to the method we expose. Local<External> external = External::New(isolate, data); // Expose the method "Method" to JavaScript, and make sure it receives the // per-addon-instance data we created above by passing `external` as the // third parameter to the FunctionTemplate constructor. exports->Set(context, 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 70/739 exports >Set(context, String::NewFromUtf8(isolate, "method", NewStringType::kNormal) .ToLocalChecked(), FunctionTemplate::New(isolate, Method, external) ->GetFunction(context).ToLocalChecked()).FromJust(); } Once the source code has been written, it must be compiled into the binary addon.node file. To do so, create a file called binding.gyp in the top-level of the project describing the build configuration of the module using a JSON-like format. This file is used by node-gyp — a tool written specifically to compile Node.js Addons. { "targets": [ { "target_name": "addon", "sources": [ "hello.cc" ] } ] } A version of the node-gyp utility is bundled and distributed with Node.js as part of npm . This version is not made directly available for developers to use and is intended only to support the ability to use the npm install command to compile and install Addons. Developers who wish to use node-gyp directly can install it using the command npm install -g node-gyp . See the node-gyp installation instructions for more information, including platform-specific requirements. Once the binding.gyp file has been created, use node-gyp configure to generate the appropriate project build files for the current platform. This will generate either a Makefile (on Unix platforms) or a vcxproj file (on Windows) in the build/ directory. Next, invoke the node-gyp build command to generate the compiled addon.node file. This will be put into the build/Release/ directory. When using npm install to install a Node.js Addon, npm uses its own bundled version of node-gyp to perform this same set of actions, generating a compiled version of the Addon for the user's platform on demand. Once built, the binary Addon can be used from within Node.js by pointing require() to the built addon.node module: // hello.js const addon = require('./build/Release/addon'); console.log(addon.hello()); // Prints: 'world' Please see the examples below for further information or https://github.com/arturadib/node-qt for an example in production. Because the exact path to the compiled Addon binary can vary depending on how it is compiled (i.e. sometimes it may be in ./build/Debug/ ), Addons can use the bindings package to load the compiled module. Note that while the bindings package implementation is more sophisticated in how it locates Addon modules, it is essentially using a try- catch pattern similar to: try { return require('./build/Release/addon.node'); Building 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 71/739 } catch (err) { return require('./build/Debug/addon.node'); } Node.js uses a number of statically linked libraries such as V8, libuv and OpenSSL. All Addons are required to link to V8 and may link to any of the other dependencies as well. Typically, this is as simple as including the appropriate #include <...> statements (e.g. #include <v8.h> ) and node-gyp will locate the appropriate headers automatically. However, there are a few caveats to be aware of: When node-gyp runs, it will detect the specific release version of Node.js and download either the full source tarball or just the headers. If the full source is downloaded, Addons will have complete access to the full set of Node.js dependencies. However, if only the Node.js headers are downloaded, then only the symbols exported by Node.js will be available. node-gyp can be run using the --nodedir flag pointing at a local Node.js source image. Using this option, the Addon will have access to the full set of dependencies. The filename extension of the compiled Addon binary is .node (as opposed to .dll or .so ). The require() function is written to look for files with the .node file extension and initialize those as dynamically-linked libraries. When calling require() , the .node extension can usually be omitted and Node.js will still find and initialize the Addon. One caveat, however, is that Node.js will first attempt to locate and load modules or JavaScript files that happen to share the same base name. For instance, if there is a file addon.js in the same directory as the binary addon.node , then require('addon') will give precedence to the addon.js file and load it instead. Each of the examples illustrated in this document make direct use of the Node.js and V8 APIs for implementing Addons. It is important to understand that the V8 API can, and has, changed dramatically from one V8 release to the next (and one major Node.js release to the next). With each change, Addons may need to be updated and recompiled in order to continue functioning. The Node.js release schedule is designed to minimize the frequency and impact of such changes but there is little that Node.js can do currently to ensure stability of the V8 APIs. The Native Abstractions for Node.js (or nan ) provide a set of tools that Addon developers are recommended to use to keep compatibility between past and future releases of V8 and Node.js. See the nan examples for an illustration of how it can be used. Stability: 2 - Stable N-API is an API for building native Addons. It is independent from the underlying JavaScript runtime (e.g. V8) and is maintained as part of Node.js itself. This API will be Application Binary Interface (ABI) stable across versions of Node.js. It is intended to insulate Addons from changes in the underlying JavaScript engine and allow modules compiled for one version to run on later versions of Node.js without recompilation. Addons are built/packaged with the same approach/tools outlined in this document (node-gyp, etc.). The only difference is the set of APIs that are used by the native code. Instead of using the V8 or Native Abstractions for Node.js APIs, the functions available in the N- API are used. Creating and maintaining an addon that benefits from the ABI stability provided by N-API carries with it certain implementation considerations . To use N-API in the above "Hello world" example, replace the content of hello.cc with the following. All other instructions remain the same. Linking to Node.js' own dependencies Loading Addons using require() Native Abstractions for Node.js N-API 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 72/739 // hello.cc using N-API #include <node_api.h> namespace demo { napi_value Method(napi_env env, napi_callback_info args) { napi_value greeting; napi_status status; status = napi_create_string_utf8(env, "world", NAPI_AUTO_LENGTH, &greeting); if (status != napi_ok) return nullptr; return greeting; } napi_value init(napi_env env, napi_value exports) { napi_status status; napi_value fn; status = napi_create_function(env, nullptr, 0, Method, nullptr, &fn); if (status != napi_ok) return nullptr; status = napi_set_named_property(env, exports, "hello", fn); if (status != napi_ok) return nullptr; return exports; } NAPI_MODULE(NODE_GYP_MODULE_NAME, init) } // namespace demo The functions available and how to use them are documented in the section titled C/C++ Addons - N-API . Following are some example Addons intended to help developers get started. The examples make use of the V8 APIs. Refer to the online V8 reference for help with the various V8 calls, and V8's Embedder's Guide for an explanation of several concepts used such as handles, scopes, function templates, etc. Each of these examples using the following binding.gyp file: { "targets": [ { "target_name": "addon", "sources": [ "addon.cc" ] } ] } In cases where there is more than one .cc file, simply add the additional filename to the sources array: "sources": ["addon.cc", "myexample.cc"] Addon examples 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 73/739 Once the binding.gyp file is ready, the example Addons can be configured and built using node-gyp : $ node-gyp configure build Addons will typically expose objects and functions that can be accessed from JavaScript running within Node.js. When functions are invoked from JavaScript, the input arguments and return value must be mapped to and from the C/C++ code. The following example illustrates how to read function arguments passed from JavaScript and how to return a result: // addon.cc #include <node.h> namespace demo { using v8::Exception; using v8::FunctionCallbackInfo; using v8::Isolate; using v8::Local; using v8::NewStringType; using v8::Number; using v8::Object; using v8::String; using v8::Value; // This is the implementation of the "add" method // Input arguments are passed using the // const FunctionCallbackInfo<Value>& args struct void Add(const FunctionCallbackInfo<Value>& args) { Isolate* isolate = args.GetIsolate(); // Check the number of arguments passed. if (args.Length() < 2) { // Throw an Error that is passed back to JavaScript isolate->ThrowException(Exception::TypeError( String::NewFromUtf8(isolate, "Wrong number of arguments", NewStringType::kNormal).ToLocalChecked())); return; } // Check the argument types if (!args[0]->IsNumber() || !args[1]->IsNumber()) { isolate->ThrowException(Exception::TypeError( String::NewFromUtf8(isolate, "Wrong arguments", NewStringType::kNormal).ToLocalChecked())); return; } // P f th ti Function arguments 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 74/739 // Perform the operation double value = args[0].As<Number>()->Value() + args[1].As<Number>()->Value(); Local<Number> num = Number::New(isolate, value); // Set the return value (using the passed in // FunctionCallbackInfo<Value>&) args.GetReturnValue().Set(num); } void Init(Local<Object> exports) { NODE_SET_METHOD(exports, "add", Add); } NODE_MODULE(NODE_GYP_MODULE_NAME, Init) } // namespace demo Once compiled, the example Addon can be required and used from within Node.js: // test.js const addon = require('./build/Release/addon'); console.log('This should be eight:', addon.add(3, 5)); It is common practice within Addons to pass JavaScript functions to a C++ function and execute them from there. The following example illustrates how to invoke such callbacks: // addon.cc #include <node.h> namespace demo { using v8::Context; using v8::Function; using v8::FunctionCallbackInfo; using v8::Isolate; using v8::Local; using v8::NewStringType; using v8::Null; using v8::Object; using v8::String; using v8::Value; void RunCallback(const FunctionCallbackInfo<Value>& args) { Isolate* isolate = args.GetIsolate(); Local<Context> context = isolate->GetCurrentContext(); Local<Function> cb = Local<Function>::Cast(args[0]); const unsigned argc = 1; Local<Value> argv[argc] = { String::NewFromUtf8(isolate Callbacks 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 75/739 String::NewFromUtf8(isolate, "hello world", NewStringType::kNormal).ToLocalChecked() }; cb->Call(context, Null(isolate), argc, argv).ToLocalChecked(); } void Init(Local<Object> exports, Local<Object> module) { NODE_SET_METHOD(module, "exports", RunCallback); } NODE_MODULE(NODE_GYP_MODULE_NAME, Init) } // namespace demo Note that this example uses a two-argument form of Init() that receives the full module object as the second argument. This allows the Addon to completely overwrite exports with a single function instead of adding the function as a property of exports . To test it, run the following JavaScript: // test.js const addon = require('./build/Release/addon'); addon((msg) => { console.log(msg); // Prints: 'hello world' }); Note that, in this example, the callback function is invoked synchronously. Addons can create and return new objects from within a C++ function as illustrated in the following example. An object is created and returned with a property msg that echoes the string passed to createObject() : // addon.cc #include <node.h> namespace demo { using v8::Context; using v8::FunctionCallbackInfo; using v8::Isolate; using v8::Local; using v8::NewStringType; using v8::Object; using v8::String; using v8::Value; void CreateObject(const FunctionCallbackInfo<Value>& args) { Isolate* isolate = args.GetIsolate(); Local<Context> context = isolate->GetCurrentContext(); Local<Object> obj = Object::New(isolate); obj->Set(context, Object factory 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 76/739 obj >Set(co te t, String::NewFromUtf8(isolate, "msg", NewStringType::kNormal).ToLocalChecked(), args[0]->ToString(context).ToLocalChecked()) .FromJust(); args.GetReturnValue().Set(obj); } void Init(Local<Object> exports, Local<Object> module) { NODE_SET_METHOD(module, "exports", CreateObject); } NODE_MODULE(NODE_GYP_MODULE_NAME, Init) } // namespace demo To test it in JavaScript: // test.js const addon = require('./build/Release/addon'); const obj1 = addon('hello'); const obj2 = addon('world'); console.log(obj1.msg, obj2.msg); // Prints: 'hello world' Another common scenario is creating JavaScript functions that wrap C++ functions and returning those back to JavaScript: // addon.cc #include <node.h> namespace demo { using v8::Context; using v8::Function; using v8::FunctionCallbackInfo; using v8::FunctionTemplate; using v8::Isolate; using v8::Local; using v8::NewStringType; using v8::Object; using v8::String; using v8::Value; void MyFunction(const FunctionCallbackInfo<Value>& args) { Isolate* isolate = args.GetIsolate(); args.GetReturnValue().Set(String::NewFromUtf8( isolate, "hello world", NewStringType::kNormal).ToLocalChecked()); } Function factory 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 77/739 void CreateFunction(const FunctionCallbackInfo<Value>& args) { Isolate* isolate = args.GetIsolate(); Local<Context> context = isolate->GetCurrentContext(); Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, MyFunction); Local<Function> fn = tpl->GetFunction(context).ToLocalChecked(); // omit this to make it anonymous fn->SetName(String::NewFromUtf8( isolate, "theFunction", NewStringType::kNormal).ToLocalChecked()); args.GetReturnValue().Set(fn); } void Init(Local<Object> exports, Local<Object> module) { NODE_SET_METHOD(module, "exports", CreateFunction); } NODE_MODULE(NODE_GYP_MODULE_NAME, Init) } // namespace demo To test: // test.js const addon = require('./build/Release/addon'); const fn = addon(); console.log(fn()); // Prints: 'hello world' It is also possible to wrap C++ objects/classes in a way that allows new instances to be created using the JavaScript new operator: // addon.cc #include <node.h> #include "myobject.h" namespace demo { using v8::Local; using v8::Object; void InitAll(Local<Object> exports) { MyObject::Init(exports); } NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll) } // namespace demo Wrapping C++ objects 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 78/739 Then, in myobject.h , the wrapper class inherits from node::ObjectWrap : // myobject.h #ifndef MYOBJECT_H #define MYOBJECT_H #include <node.h> #include <node_object_wrap.h> namespace demo { class MyObject : public node::ObjectWrap { public: static void Init(v8::Local<v8::Object> exports); private: explicit MyObject(double value = 0); ~MyObject(); static void New(const v8::FunctionCallbackInfo<v8::Value>& args); static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args); static v8::Persistent<v8::Function> constructor; double value_; }; } // namespace demo #endif In myobject.cc , implement the various methods that are to be exposed. Below, the method plusOne() is exposed by adding it to the constructor's prototype: // myobject.cc #include "myobject.h" namespace demo { using v8::Context; using v8::Function; using v8::FunctionCallbackInfo; using v8::FunctionTemplate; using v8::Isolate; using v8::Local; using v8::NewStringType; using v8::Number; using v8::Object; using v8::Persistent; using v8::String; using v8::Value; Persistent<Function> MyObject::constructor; 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 79/739 MyObject::MyObject(double value) : value_(value) { } MyObject::~MyObject() { } void MyObject::Init(Local<Object> exports) { Isolate* isolate = exports->GetIsolate(); // Prepare constructor template Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New); tpl->SetClassName(String::NewFromUtf8( isolate, "MyObject", NewStringType::kNormal).ToLocalChecked()); tpl->InstanceTemplate()->SetInternalFieldCount(1); // Prototype NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne); Local<Context> context = isolate->GetCurrentContext(); constructor.Reset(isolate, tpl->GetFunction(context).ToLocalChecked()); exports->Set(context, String::NewFromUtf8( isolate, "MyObject", NewStringType::kNormal).ToLocalChecked(), tpl->GetFunction(context).ToLocalChecked()).FromJust(); } void MyObject::New(const FunctionCallbackInfo<Value>& args) { Isolate* isolate = args.GetIsolate(); Local<Context> context = isolate->GetCurrentContext(); if (args.IsConstructCall()) { // Invoked as constructor: `new MyObject(...)` double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue(context).FromMaybe(0); MyObject* obj = new MyObject(value); obj->Wrap(args.This()); args.GetReturnValue().Set(args.This()); } else { // Invoked as plain function `MyObject(...)`, turn into construct call. const int argc = 1; Local<Value> argv[argc] = { args[0] }; Local<Function> cons = Local<Function>::New(isolate, constructor); Local<Object> result = cons->NewInstance(context, argc, argv).ToLocalChecked(); args.GetReturnValue().Set(result); } } void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) { Isolate* isolate = args.GetIsolate(); MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder()); obj->value_ += 1; args.GetReturnValue().Set(Number::New(isolate, obj->value_)); 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 80/739 } } // namespace demo To build this example, the myobject.cc file must be added to the binding.gyp : { "targets": [ { "target_name": "addon", "sources": [ "addon.cc", "myobject.cc" ] } ] } Test it with: // test.js const addon = require('./build/Release/addon'); const obj = new addon.MyObject(10); console.log(obj.plusOne()); // Prints: 11 console.log(obj.plusOne()); // Prints: 12 console.log(obj.plusOne()); // Prints: 13 The destructor for a wrapper object will run when the object is garbage-collected. For destructor testing, there are command-line flags that can be used to make it possible to force garbage collection. These flags are provided by the underlying V8 JavaScript engine. They are subject to change or removal at any time. They are not documented by Node.js or V8, and they should never be used outside of testing. Alternatively, it is possible to use a factory pattern to avoid explicitly creating object instances using the JavaScript new operator: const obj = addon.createObject(); // instead of: // const obj = new addon.Object(); First, the createObject() method is implemented in addon.cc : // addon.cc #include <node.h> #include "myobject.h" namespace demo { Factory of wrapped objects 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 81/739 using v8::FunctionCallbackInfo; using v8::Isolate; using v8::Local; using v8::Object; using v8::String; using v8::Value; void CreateObject(const FunctionCallbackInfo<Value>& args) { MyObject::NewInstance(args); } void InitAll(Local<Object> exports, Local<Object> module) { MyObject::Init(exports->GetIsolate()); NODE_SET_METHOD(module, "exports", CreateObject); } NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll) } // namespace demo In myobject.h , the static method NewInstance() is added to handle instantiating the object. This method takes the place of using new in JavaScript: // myobject.h #ifndef MYOBJECT_H #define MYOBJECT_H #include <node.h> #include <node_object_wrap.h> namespace demo { class MyObject : public node::ObjectWrap { public: static void Init(v8::Isolate* isolate); static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args); private: explicit MyObject(double value = 0); ~MyObject(); static void New(const v8::FunctionCallbackInfo<v8::Value>& args); static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args); static v8::Persistent<v8::Function> constructor; double value_; }; } // namespace demo #endif 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 82/739 The implementation in myobject.cc is similar to the previous example: // myobject.cc #include <node.h> #include "myobject.h" namespace demo { using v8::Context; using v8::Function; using v8::FunctionCallbackInfo; using v8::FunctionTemplate; using v8::Isolate; using v8::Local; using v8::NewStringType; using v8::Number; using v8::Object; using v8::Persistent; using v8::String; using v8::Value; Persistent<Function> MyObject::constructor; MyObject::MyObject(double value) : value_(value) { } MyObject::~MyObject() { } void MyObject::Init(Isolate* isolate) { // Prepare constructor template Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New); tpl->SetClassName(String::NewFromUtf8( isolate, "MyObject", NewStringType::kNormal).ToLocalChecked()); tpl->InstanceTemplate()->SetInternalFieldCount(1); // Prototype NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne); Local<Context> context = isolate->GetCurrentContext(); constructor.Reset(isolate, tpl->GetFunction(context).ToLocalChecked()); } void MyObject::New(const FunctionCallbackInfo<Value>& args) { Isolate* isolate = args.GetIsolate(); Local<Context> context = isolate->GetCurrentContext(); if (args.IsConstructCall()) { // Invoked as constructor: `new MyObject(...)` double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue(context).FromMaybe(0); MyObject* obj = new MyObject(value); obj->Wrap(args.This()); 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 83/739 args.GetReturnValue().Set(args.This()); } else { // Invoked as plain function `MyObject(...)`, turn into construct call. const int argc = 1; Local<Value> argv[argc] = { args[0] }; Local<Function> cons = Local<Function>::New(isolate, constructor); Local<Object> instance = cons->NewInstance(context, argc, argv).ToLocalChecked(); args.GetReturnValue().Set(instance); } } void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) { Isolate* isolate = args.GetIsolate(); const unsigned argc = 1; Local<Value> argv[argc] = { args[0] }; Local<Function> cons = Local<Function>::New(isolate, constructor); Local<Context> context = isolate->GetCurrentContext(); Local<Object> instance = cons->NewInstance(context, argc, argv).ToLocalChecked(); args.GetReturnValue().Set(instance); } void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) { Isolate* isolate = args.GetIsolate(); MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder()); obj->value_ += 1; args.GetReturnValue().Set(Number::New(isolate, obj->value_)); } } // namespace demo Once again, to build this example, the myobject.cc file must be added to the binding.gyp : { "targets": [ { "target_name": "addon", "sources": [ "addon.cc", "myobject.cc" ] } ] } Test it with: 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 84/739 // test.js const createObject = require('./build/Release/addon'); const obj = createObject(10); console.log(obj.plusOne()); // Prints: 11 console.log(obj.plusOne()); // Prints: 12 console.log(obj.plusOne()); // Prints: 13 const obj2 = createObject(20); console.log(obj2.plusOne()); // Prints: 21 console.log(obj2.plusOne()); // Prints: 22 console.log(obj2.plusOne()); // Prints: 23 In addition to wrapping and returning C++ objects, it is possible to pass wrapped objects around by unwrapping them with the Node.js helper function node::ObjectWrap::Unwrap . The following examples shows a function add() that can take two MyObject objects as input arguments: // addon.cc #include <node.h> #include <node_object_wrap.h> #include "myobject.h" namespace demo { using v8::Context; using v8::FunctionCallbackInfo; using v8::Isolate; using v8::Local; using v8::Number; using v8::Object; using v8::String; using v8::Value; void CreateObject(const FunctionCallbackInfo<Value>& args) { MyObject::NewInstance(args); } void Add(const FunctionCallbackInfo<Value>& args) { Isolate* isolate = args.GetIsolate(); Local<Context> context = isolate->GetCurrentContext(); MyObject* obj1 = node::ObjectWrap::Unwrap<MyObject>( args[0]->ToObject(context).ToLocalChecked()); MyObject* obj2 = node::ObjectWrap::Unwrap<MyObject>( Passing wrapped objects around 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 85/739 args[1]->ToObject(context).ToLocalChecked()); double sum = obj1->value() + obj2->value(); args.GetReturnValue().Set(Number::New(isolate, sum)); } void InitAll(Local<Object> exports) { MyObject::Init(exports->GetIsolate()); NODE_SET_METHOD(exports, "createObject", CreateObject); NODE_SET_METHOD(exports, "add", Add); } NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll) } // namespace demo In myobject.h , a new public method is added to allow access to private values after unwrapping the object. // myobject.h #ifndef MYOBJECT_H #define MYOBJECT_H #include <node.h> #include <node_object_wrap.h> namespace demo { class MyObject : public node::ObjectWrap { public: static void Init(v8::Isolate* isolate); static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args); inline double value() const { return value_; } private: explicit MyObject(double value = 0); ~MyObject(); static void New(const v8::FunctionCallbackInfo<v8::Value>& args); static v8::Persistent<v8::Function> constructor; double value_; }; } // namespace demo #endif The implementation of myobject.cc is similar to before: // myobject.cc #include <node.h> #include "myobject.h" 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 86/739 namespace demo { using v8::Context; using v8::Function; using v8::FunctionCallbackInfo; using v8::FunctionTemplate; using v8::Isolate; using v8::Local; using v8::NewStringType; using v8::Object; using v8::Persistent; using v8::String; using v8::Value; Persistent<Function> MyObject::constructor; MyObject::MyObject(double value) : value_(value) { } MyObject::~MyObject() { } void MyObject::Init(Isolate* isolate) { // Prepare constructor template Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New); tpl->SetClassName(String::NewFromUtf8( isolate, "MyObject", NewStringType::kNormal).ToLocalChecked()); tpl->InstanceTemplate()->SetInternalFieldCount(1); Local<Context> context = isolate->GetCurrentContext(); constructor.Reset(isolate, tpl->GetFunction(context).ToLocalChecked()); } void MyObject::New(const FunctionCallbackInfo<Value>& args) { Isolate* isolate = args.GetIsolate(); Local<Context> context = isolate->GetCurrentContext(); if (args.IsConstructCall()) { // Invoked as constructor: `new MyObject(...)` double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue(context).FromMaybe(0); MyObject* obj = new MyObject(value); obj->Wrap(args.This()); args.GetReturnValue().Set(args.This()); } else { // Invoked as plain function `MyObject(...)`, turn into construct call. const int argc = 1; Local<Value> argv[argc] = { args[0] }; Local<Function> cons = Local<Function>::New(isolate, constructor); Local<Object> instance = cons->NewInstance(context, argc, argv).ToLocalChecked(); args.GetReturnValue().Set(instance); } } 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 87/739 } void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) { Isolate* isolate = args.GetIsolate(); const unsigned argc = 1; Local<Value> argv[argc] = { args[0] }; Local<Function> cons = Local<Function>::New(isolate, constructor); Local<Context> context = isolate->GetCurrentContext(); Local<Object> instance = cons->NewInstance(context, argc, argv).ToLocalChecked(); args.GetReturnValue().Set(instance); } } // namespace demo Test it with: // test.js const addon = require('./build/Release/addon'); const obj1 = addon.createObject(10); const obj2 = addon.createObject(20); const result = addon.add(obj1, obj2); console.log(result); // Prints: 30 An AtExit hook is a function that is invoked after the Node.js event loop has ended but before the JavaScript VM is terminated and Node.js shuts down. AtExit hooks are registered using the node::AtExit API. callback <void (*)(void*)> A pointer to the function to call at exit. args <void*> A pointer to pass to the callback at exit. Registers exit hooks that run after the event loop has ended but before the VM is killed. AtExit takes two parameters: a pointer to a callback function to run at exit, and a pointer to untyped context data to be passed to that callback. Callbacks are run in last-in first-out order. The following addon.cc implements AtExit : // addon.cc #include <assert.h> #include <stdlib.h> #include <node.h> AtExit hooks void AtExit(callback, args) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 88/739 namespace demo { using node::AtExit; using v8::HandleScope; using v8::Isolate; using v8::Local; using v8::Object; static char cookie[] = "yum yum"; static int at_exit_cb1_called = 0; static int at_exit_cb2_called = 0; static void at_exit_cb1(void* arg) { Isolate* isolate = static_cast<Isolate*>(arg); HandleScope scope(isolate); Local<Object> obj = Object::New(isolate); assert(!obj.IsEmpty()); // assert VM is still alive assert(obj->IsObject()); at_exit_cb1_called++; } static void at_exit_cb2(void* arg) { assert(arg == static_cast<void*>(cookie)); at_exit_cb2_called++; } static void sanity_check(void*) { assert(at_exit_cb1_called == 1); assert(at_exit_cb2_called == 2); } void init(Local<Object> exports) { AtExit(at_exit_cb2, cookie); AtExit(at_exit_cb2, cookie); AtExit(at_exit_cb1, exports->GetIsolate()); AtExit(sanity_check); } NODE_MODULE(NODE_GYP_MODULE_NAME, init) } // namespace demo Test in JavaScript by running: // test.js require('./build/Release/addon'); Stability: 2 - Stable N-API 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 89/739 N-API (pronounced N as in the letter, followed by API) is an API for building native Addons. It is independent from the underlying JavaScript runtime (ex V8) and is maintained as part of Node.js itself. This API will be Application Binary Interface (ABI) stable across versions of Node.js. It is intended to insulate Addons from changes in the underlying JavaScript engine and allow modules compiled for one major version to run on later major versions of Node.js without recompilation. The ABI Stability guide provides a more in-depth explanation. Addons are built/packaged with the same approach/tools outlined in the section titled C++ Addons . The only difference is the set of APIs that are used by the native code. Instead of using the V8 or Native Abstractions for Node.js APIs, the functions available in the N-API are used. APIs exposed by N-API are generally used to create and manipulate JavaScript values. Concepts and operations generally map to ideas specified in the ECMA262 Language Specification. The APIs have the following properties: All N-API calls return a status code of type napi_status . This status indicates whether the API call succeeded or failed. The API's return value is passed via an out parameter. All JavaScript values are abstracted behind an opaque type named napi_value . In case of an error status code, additional information can be obtained using napi_get_last_error_info . More information can be found in the error handling section Error Handling . The N-API is a C API that ensures ABI stability across Node.js versions and different compiler levels. A C++ API can be easier to use. To support using C++, the project maintains a C++ wrapper module called node-addon-api . This wrapper provides an inlineable C++ API. Binaries built with node-addon-api will depend on the symbols for the N-API C-based functions exported by Node.js. node-addon-api is a more efficient way to write code that calls N-API. Take, for example, the following node-addon-api code. The first section shows the node-addon-api code and the second section shows what actually gets used in the addon. Object obj = Object::New(env); obj["foo"] = String::New(env, "bar"); napi_status status; napi_value object, string; status = napi_create_object(env, &object); if (status != napi_ok) { napi_throw_error(env, ...); return; } status = napi_create_string_utf8(env, "bar", NAPI_AUTO_LENGTH, &string); if (status != napi_ok) { napi_throw_error(env, ...); return; } status = napi_set_named_property(env, object, "foo", string); if (status != napi_ok) { napi_throw_error(env, ...); return; } The end result is that the addon only uses the exported C APIs. As a result, it still gets the benefits of the ABI stability provided by the C API. When using node-addon-api instead of the C APIs, start with the API docs for node-addon-api . Implications of ABI Stability 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 90/739 Although N-API provides an ABI stability guarantee, other parts of Node.js do not, and any external libraries used from the addon may not. In particular, none of the following APIs provide an ABI stability guarantee across major versions: the Node.js C++ APIs available via any of #include <node.h> #include <node_buffer.h> #include <node_version.h> #include <node_object_wrap.h> the libuv APIs which are also included with Node.js and available via #include <uv.h> the V8 API available via #include <v8.h> Thus, for an addon to remain ABI-compatible across Node.js major versions, it must make use exclusively of N-API by restricting itself to using #include <node_api.h> and by checking, for all external libraries that it uses, that the external library makes ABI stability guarantees similar to N-API. In order to use the N-API functions, include the file node_api.h which is located in the src directory in the node development tree: #include <node_api.h> This will opt into the default NAPI_VERSION for the given release of Node.js. In order to ensure compatibility with specific versions of N-API, the version can be specified explicitly when including the header: #define NAPI_VERSION 3 #include <node_api.h> This restricts the N-API surface to just the functionality that was available in the specified (and earlier) versions. Some of the N-API surface is considered experimental and requires explicit opt-in to access those APIs: #define NAPI_EXPERIMENTAL #include <node_api.h> In this case the entire API surface, including any experimental APIs, will be available to the module code. 1 2 3 4 Usage N-API Version Matrix 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 91/739 1 2 3 4 v6.x v6.14.2* v8.x v8.0.0* v8.10.0* v8.11.2 v9.x v9.0.0* v9.3.0* v9.11.0* v10.x v10.0.0 v10.16.0 v11.x v11.0.0 v11.8.0 * Indicates that the N-API version was released as experimental N-API exposes the following fundamental datatypes as abstractions that are consumed by the various APIs. These APIs should be treated as opaque, introspectable only with other N-API calls. Integral status code indicating the success or failure of a N-API call. Currently, the following status codes are supported. typedef enum { napi_ok, napi_invalid_arg, napi_object_expected, napi_string_expected, napi_name_expected, napi_function_expected, napi_number_expected, napi_boolean_expected, napi_array_expected, napi_generic_failure, napi_pending_exception, napi_cancelled, napi_escape_called_twice, napi_handle_scope_mismatch, napi_callback_scope_mismatch, napi_queue_full, napi_closing, napi_bigint_expected, } napi_status; If additional information is required upon an API returning a failed status, it can be obtained by calling napi_get_last_error_info . typedef struct { const char* error_message; void* engine_reserved; uint32_t engine_error_code; Basic N-API Data Types napi_status napi_extended_error_info 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 92/739 napi_status error_code; } napi_extended_error_info; error_message : UTF8-encoded string containing a VM-neutral description of the error. engine_reserved : Reserved for VM-specific error details. This is currently not implemented for any VM. engine_error_code : VM-specific error code. This is currently not implemented for any VM. error_code : The N-API status code that originated with the last error. See the Error Handling section for additional information. napi_env is used to represent a context that the underlying N-API implementation can use to persist VM-specific state. This structure is passed to native functions when they're invoked, and it must be passed back when making N-API calls. Specifically, the same napi_env that was passed in when the initial native function was called must be passed to any subsequent nested N-API calls. Caching the napi_env for the purpose of general reuse is not allowed. This is an opaque pointer that is used to represent a JavaScript value. Stability: 2 - Stable This is an opaque pointer that represents a JavaScript function which can be called asynchronously from multiple threads via napi_call_threadsafe_function() . Stability: 2 - Stable A value to be given to napi_release_threadsafe_function() to indicate whether the thread-safe function is to be closed immediately ( napi_tsfn_abort ) or merely released ( napi_tsfn_release ) and thus available for subsequent use via napi_acquire_threadsafe_function() and napi_call_threadsafe_function() . typedef enum { napi_tsfn_release, napi_tsfn_abort } napi_threadsafe_function_release_mode; Stability: 2 - Stable napi_env napi_value napi_threadsafe_function napi_threadsafe_function_release_mode napi_threadsafe_function_call_mode 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 93/739 A value to be given to napi_call_threadsafe_function() to indicate whether the call should block whenever the queue associated with the thread-safe function is full. typedef enum { napi_tsfn_nonblocking, napi_tsfn_blocking } napi_threadsafe_function_call_mode; This is an abstraction used to control and modify the lifetime of objects created within a particular scope. In general, N-API values are created within the context of a handle scope. When a native method is called from JavaScript, a default handle scope will exist. If the user does not explicitly create a new handle scope, N-API values will be created in the default handle scope. For any invocations of code outside the execution of a native method (for instance, during a libuv callback invocation), the module is required to create a scope before invoking any functions that can result in the creation of JavaScript values. Handle scopes are created using napi_open_handle_scope and are destroyed using napi_close_handle_scope . Closing the scope can indicate to the GC that all napi_value s created during the lifetime of the handle scope are no longer referenced from the current stack frame. For more details, review the Object Lifetime Management . Escapable handle scopes are a special type of handle scope to return values created within a particular handle scope to a parent scope. This is the abstraction to use to reference a napi_value . This allows for users to manage the lifetimes of JavaScript values, including defining their minimum lifetimes explicitly. For more details, review the Object Lifetime Management . Opaque datatype that is passed to a callback function. It can be used for getting additional information about the context in which the callback was invoked. Function pointer type for user-provided native functions which are to be exposed to JavaScript via N-API. Callback functions should satisfy the following signature: typedef napi_value (*napi_callback)(napi_env, napi_callback_info); Function pointer type for add-on provided functions that allow the user to be notified when externally-owned data is ready to be cleaned up because the object with which it was associated with, has been garbage-collected. The user must provide a function satisfying the following N-API Memory Management types napi_handle_scope napi_escapable_handle_scope napi_ref N-API Callback types napi_callback_info napi_callback napi_finalize 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 94/739 signature which would get called upon the object's collection. Currently, napi_finalize can be used for finding out when objects that have external data are collected. typedef void (*napi_finalize)(napi_env env, void* finalize_data, void* finalize_hint); Function pointer used with functions that support asynchronous operations. Callback functions must statisfy the following signature: typedef void (*napi_async_execute_callback)(napi_env env, void* data); Implementations of this type of function should avoid making any N-API calls that could result in the execution of JavaScript or interaction with JavaScript objects. Most often, any code that needs to make N-API calls should be made in napi_async_complete_callback instead. Function pointer used with functions that support asynchronous operations. Callback functions must statisfy the following signature: typedef void (*napi_async_complete_callback)(napi_env env, napi_status status, void* data); Stability: 2 - Stable Function pointer used with asynchronous thread-safe function calls. The callback will be called on the main thread. Its purpose is to use a data item arriving via the queue from one of the secondary threads to construct the parameters necessary for a call into JavaScript, usually via napi_call_function , and then make the call into JavaScript. The data arriving from the secondary thread via the queue is given in the data parameter and the JavaScript function to call is given in the js_callback parameter. N-API sets up the environment prior to calling this callback, so it is sufficient to call the JavaScript function via napi_call_function rather than via napi_make_callback . Callback functions must satisfy the following signature: typedef void (*napi_threadsafe_function_call_js)(napi_env env, napi_value js_callback, void* context, void* data); [in] env : The environment to use for API calls, or NULL if the thread-safe function is being torn down and data may need to be freed. [in] js_callback : The JavaScript function to call, or NULL if the thread-safe function is being torn down and data may need to be freed. [in] context : The optional data with which the thread-safe function was created. napi_async_execute_callback napi_async_complete_callback napi_threadsafe_function_call_js 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 95/739 [in] data : Data created by the secondary thread. It is the responsibility of the callback to convert this native data to JavaScript values (with N-API functions) that can be passed as parameters when js_callback is invoked. This pointer is managed entirely by the threads and this callback. Thus this callback should free the data. N-API uses both return values and JavaScript exceptions for error handling. The following sections explain the approach for each case. All of the N-API functions share the same error handling pattern. The return type of all API functions is napi_status . The return value will be napi_ok if the request was successful and no uncaught JavaScript exception was thrown. If an error occurred AND an exception was thrown, the napi_status value for the error will be returned. If an exception was thrown, and no error occurred, napi_pending_exception will be returned. In cases where a return value other than napi_ok or napi_pending_exception is returned, napi_is_exception_pending must be called to check if an exception is pending. See the section on exceptions for more details. The full set of possible napi_status values is defined in napi_api_types.h . The napi_status return value provides a VM-independent representation of the error which occurred. In some cases it is useful to be able to get more detailed information, including a string representing the error as well as VM (engine)-specific information. In order to retrieve this information napi_get_last_error_info is provided which returns a napi_extended_error_info structure. The format of the napi_extended_error_info structure is as follows: typedef struct napi_extended_error_info { const char* error_message; void* engine_reserved; uint32_t engine_error_code; napi_status error_code; }; error_message : Textual representation of the error that occurred. engine_reserved : Opaque handle reserved for engine use only. engine_error_code : VM specific error code. error_code : n-api status code for the last error. napi_get_last_error_info returns the information for the last N-API call that was made. Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes. napi_status napi_get_last_error_info(napi_env env, const napi_extended_error_info** result); [in] env : The environment that the API is invoked under. [out] result : The napi_extended_error_info structure with more information about the error. Returns napi_ok if the API succeeded. Error Handling Return values napi_get_last_error_info 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 96/739 This API retrieves a napi_extended_error_info structure with information about the last error that occurred. The content of the napi_extended_error_info returned is only valid up until an n-api function is called on the same env . Do not rely on the content or format of any of the extended information as it is not subject to SemVer and may change at any time. It is intended only for logging purposes. This API can be called even if there is a pending JavaScript exception. Any N-API function call may result in a pending JavaScript exception. This is obviously the case for any function that may cause the execution of JavaScript, but N-API specifies that an exception may be pending on return from any of the API functions. If the napi_status returned by a function is napi_ok then no exception is pending and no additional action is required. If the napi_status returned is anything other than napi_ok or napi_pending_exception , in order to try to recover and continue instead of simply returning immediately, napi_is_exception_pending must be called in order to determine if an exception is pending or not. In many cases when an N-API function is called and an exception is already pending, the function will return immediately with a napi_status of napi_pending_exception . However, this is not the case for all functions. N-API allows a subset of the functions to be called to allow for some minimal cleanup before returning to JavaScript. In that case, napi_status will reflect the status for the function. It will not reflect previous pending exceptions. To avoid confusion, check the error status after every function call. When an exception is pending one of two approaches can be employed. The first approach is to do any appropriate cleanup and then return so that execution will return to JavaScript. As part of the transition back to JavaScript the exception will be thrown at the point in the JavaScript code where the native method was invoked. The behavior of most N-API calls is unspecified while an exception is pending, and many will simply return napi_pending_exception , so it is important to do as little as possible and then return to JavaScript where the exception can be handled. The second approach is to try to handle the exception. There will be cases where the native code can catch the exception, take the appropriate action, and then continue. This is only recommended in specific cases where it is known that the exception can be safely handled. In these cases napi_get_and_clear_last_exception can be used to get and clear the exception. On success, result will contain the handle to the last JavaScript Object thrown. If it is determined, after retrieving the exception, the exception cannot be handled after all it can be re-thrown it with napi_throw where error is the JavaScript Error object to be thrown. The following utility functions are also available in case native code needs to throw an exception or determine if a napi_value is an instance of a JavaScript Error object: napi_throw_error , napi_throw_type_error , napi_throw_range_error and napi_is_error . The following utility functions are also available in case native code needs to create an Error object: napi_create_error , napi_create_type_error , and napi_create_range_error , where result is the napi_value that refers to the newly created JavaScript Error object. The Node.js project is adding error codes to all of the errors generated internally. The goal is for applications to use these error codes for all error checking. The associated error messages will remain, but will only be meant to be used for logging and display with the expectation that the message can change without SemVer applying. In order to support this model with N-API, both in internal functionality and for module specific functionality (as its good practice), the throw_ and create_ functions take an optional code parameter which is the string for the code to be added to the error object. If the optional parameter is NULL then no code will be associated with the error. If a code is provided, the name associated with the error is also updated to be: originalName [code] where originalName is the original name associated with the error and code is the code that was provided. For example, if the code is 'ERR_ERROR_1' and a TypeError is being created the name will be: Exceptions 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 97/739 TypeError [ERR_ERROR_1] NAPI_EXTERN napi_status napi_throw(napi_env env, napi_value error); [in] env : The environment that the API is invoked under. [in] error : The JavaScript value to be thrown. Returns napi_ok if the API succeeded. This API throws the JavaScript value provided. NAPI_EXTERN napi_status napi_throw_error(napi_env env, const char* code, const char* msg); [in] env : The environment that the API is invoked under. [in] code : Optional error code to be set on the error. [in] msg : C string representing the text to be associated with the error. Returns napi_ok if the API succeeded. This API throws a JavaScript Error with the text provided. NAPI_EXTERN napi_status napi_throw_type_error(napi_env env, const char* code, const char* msg); [in] env : The environment that the API is invoked under. [in] code : Optional error code to be set on the error. [in] msg : C string representing the text to be associated with the error. Returns napi_ok if the API succeeded. This API throws a JavaScript TypeError with the text provided. NAPI_EXTERN napi_status napi_throw_range_error(napi_env env, const char* code, const char* msg); [in] env : The environment that the API is invoked under. [in] code : Optional error code to be set on the error. [in] msg : C string representing the text to be associated with the error. napi_throw napi_throw_error napi_throw_type_error napi_throw_range_error 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 98/739 Returns napi_ok if the API succeeded. This API throws a JavaScript RangeError with the text provided. NAPI_EXTERN napi_status napi_is_error(napi_env env, napi_value value, bool* result); [in] env : The environment that the API is invoked under. [in] value : The napi_value to be checked. [out] result : Boolean value that is set to true if napi_value represents an error, false otherwise. Returns napi_ok if the API succeeded. This API queries a napi_value to check if it represents an error object. NAPI_EXTERN napi_status napi_create_error(napi_env env, napi_value code, napi_value msg, napi_value* result); [in] env : The environment that the API is invoked under. [in] code : Optional napi_value with the string for the error code to be associated with the error. [in] msg : napi_value that references a JavaScript String to be used as the message for the Error . [out] result : napi_value representing the error created. Returns napi_ok if the API succeeded. This API returns a JavaScript Error with the text provided. NAPI_EXTERN napi_status napi_create_type_error(napi_env env, napi_value code, napi_value msg, napi_value* result); [in] env : The environment that the API is invoked under. [in] code : Optional napi_value with the string for the error code to be associated with the error. [in] msg : napi_value that references a JavaScript String to be used as the message for the Error . [out] result : napi_value representing the error created. Returns napi_ok if the API succeeded. This API returns a JavaScript TypeError with the text provided. napi_is_error napi_create_error napi_create_type_error napi_create_range_error 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 99/739 NAPI_EXTERN napi_status napi_create_range_error(napi_env env, napi_value code, napi_value msg, napi_value* result); [in] env : The environment that the API is invoked under. [in] code : Optional napi_value with the string for the error code to be associated with the error. [in] msg : napi_value that references a JavaScript String to be used as the message for the Error . [out] result : napi_value representing the error created. Returns napi_ok if the API succeeded. This API returns a JavaScript RangeError with the text provided. napi_status napi_get_and_clear_last_exception(napi_env env, napi_value* result); [in] env : The environment that the API is invoked under. [out] result : The exception if one is pending, NULL otherwise. Returns napi_ok if the API succeeded. This API returns true if an exception is pending. This API can be called even if there is a pending JavaScript exception. napi_status napi_is_exception_pending(napi_env env, bool* result); [in] env : The environment that the API is invoked under. [out] result : Boolean value that is set to true if an exception is pending. Returns napi_ok if the API succeeded. This API returns true if an exception is pending. This API can be called even if there is a pending JavaScript exception. napi_status napi_fatal_exception(napi_env env, napi_value err); [in] env : The environment that the API is invoked under. [in] err : The error that is passed to 'uncaughtException' . Trigger an 'uncaughtException' in JavaScript. Useful if an async callback throws an exception with no way to recover. In the event of an unrecoverable error in a native module, a fatal error can be thrown to immediately terminate the process. napi_get_and_clear_last_exception napi_is_exception_pending napi_fatal_exception Fatal Errors 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 100/739 NAPI_NO_RETURN void napi_fatal_error(const char* location, size_t location_len, const char* message, size_t message_len); [in] location : Optional location at which the error occurred. [in] location_len : The length of the location in bytes, or NAPI_AUTO_LENGTH if it is null-terminated. [in] message : The message associated with the error. [in] message_len : The length of the message in bytes, or NAPI_AUTO_LENGTH if it is null-terminated. The function call does not return, the process will be terminated. This API can be called even if there is a pending JavaScript exception. As N-API calls are made, handles to objects in the heap for the underlying VM may be returned as napi_values . These handles must hold the objects 'live' until they are no longer required by the native code, otherwise the objects could be collected before the native code was finished using them. As object handles are returned they are associated with a 'scope'. The lifespan for the default scope is tied to the lifespan of the native method call. The result is that, by default, handles remain valid and the objects associated with these handles will be held live for the lifespan of the native method call. In many cases, however, it is necessary that the handles remain valid for either a shorter or longer lifespan than that of the native method. The sections which follow describe the N-API functions that can be used to change the handle lifespan from the default. It is often necessary to make the lifespan of handles shorter than the lifespan of a native method. For example, consider a native method that has a loop which iterates through the elements in a large array: for (int i = 0; i < 1000000; i++) { napi_value result; napi_status status = napi_get_element(env, object, i, &result); if (status != napi_ok) { break; } // do something with element } This would result in a large number of handles being created, consuming substantial resources. In addition, even though the native code could only use the most recent handle, all of the associated objects would also be kept alive since they all share the same scope. To handle this case, N-API provides the ability to establish a new 'scope' to which newly created handles will be associated. Once those handles are no longer required, the scope can be 'closed' and any handles associated with the scope are invalidated. The methods available to open/close scopes are napi_open_handle_scope and napi_close_handle_scope . N-API only supports a single nested hierarchy of scopes. There is only one active scope at any time, and all new handles will be associated with that scope while it is active. Scopes must be closed in the reverse order from which they are opened. In addition, all scopes created within a native method must be closed before returning from that method. napi_fatal_error Object Lifetime management Making handle lifespan shorter than that of the native method 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 101/739 Taking the earlier example, adding calls to napi_open_handle_scope and napi_close_handle_scope would ensure that at most a single handle is valid throughout the execution of the loop: for (int i = 0; i < 1000000; i++) { napi_handle_scope scope; napi_status status = napi_open_handle_scope(env, &scope); if (status != napi_ok) { break; } napi_value result; status = napi_get_element(env, object, i, &result); if (status != napi_ok) { break; } // do something with element status = napi_close_handle_scope(env, scope); if (status != napi_ok) { break; } } When nesting scopes, there are cases where a handle from an inner scope needs to live beyond the lifespan of that scope. N-API supports an 'escapable scope' in order to support this case. An escapable scope allows one handle to be 'promoted' so that it 'escapes' the current scope and the lifespan of the handle changes from the current scope to that of the outer scope. The methods available to open/close escapable scopes are napi_open_escapable_handle_scope and napi_close_escapable_handle_scope . The request to promote a handle is made through napi_escape_handle which can only be called once. NAPI_EXTERN napi_status napi_open_handle_scope(napi_env env, napi_handle_scope* result); [in] env : The environment that the API is invoked under. [out] result : napi_value representing the new scope. Returns napi_ok if the API succeeded. This API open a new scope. NAPI_EXTERN napi_status napi_close_handle_scope(napi_env env, napi_handle_scope scope); [in] env : The environment that the API is invoked under. [in] scope : napi_value representing the scope to be closed. Returns napi_ok if the API succeeded. This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created. napi_open_handle_scope napi_close_handle_scope 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 102/739 This API can be called even if there is a pending JavaScript exception. NAPI_EXTERN napi_status napi_open_escapable_handle_scope(napi_env env, napi_handle_scope* result); [in] env : The environment that the API is invoked under. [out] result : napi_value representing the new scope. Returns napi_ok if the API succeeded. This API open a new scope from which one object can be promoted to the outer scope. NAPI_EXTERN napi_status napi_close_escapable_handle_scope(napi_env env, napi_handle_scope scope); [in] env : The environment that the API is invoked under. [in] scope : napi_value representing the scope to be closed. Returns napi_ok if the API succeeded. This API closes the scope passed in. Scopes must be closed in the reverse order from which they were created. This API can be called even if there is a pending JavaScript exception. napi_status napi_escape_handle(napi_env env, napi_escapable_handle_scope scope, napi_value escapee, napi_value* result); [in] env : The environment that the API is invoked under. [in] scope : napi_value representing the current scope. [in] escapee : napi_value representing the JavaScript Object to be escaped. [out] result : napi_value representing the handle to the escaped Object in the outer scope. Returns napi_ok if the API succeeded. This API promotes the handle to the JavaScript object so that it is valid for the lifetime of the outer scope. It can only be called once per scope. If it is called more than once an error will be returned. This API can be called even if there is a pending JavaScript exception. In some cases an addon will need to be able to create and reference objects with a lifespan longer than that of a single native method invocation. For example, to create a constructor and later use that constructor in a request to creates instances, it must be possible to napi_open_escapable_handle_scope napi_close_escapable_handle_scope napi_escape_handle References to objects with a lifespan longer than that of the native method 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 103/739 reference the constructor object across many different instance creation requests. This would not be possible with a normal handle returned as a napi_value as described in the earlier section. The lifespan of a normal handle is managed by scopes and all scopes must be closed before the end of a native method. N-API provides methods to create persistent references to an object. Each persistent reference has an associated count with a value of 0 or higher. The count determines if the reference will keep the corresponding object live. References with a count of 0 do not prevent the object from being collected and are often called 'weak' references. Any count greater than 0 will prevent the object from being collected. References can be created with an initial reference count. The count can then be modified through napi_reference_ref and napi_reference_unref . If an object is collected while the count for a reference is 0, all subsequent calls to get the object associated with the reference napi_get_reference_value will return NULL for the returned napi_value . An attempt to call napi_reference_ref for a reference whose object has been collected will result in an error. References must be deleted once they are no longer required by the addon. When a reference is deleted it will no longer prevent the corresponding object from being collected. Failure to delete a persistent reference will result in a 'memory leak' with both the native memory for the persistent reference and the corresponding object on the heap being retained forever. There can be multiple persistent references created which refer to the same object, each of which will either keep the object live or not based on its individual count. NAPI_EXTERN napi_status napi_create_reference(napi_env env, napi_value value, int initial_refcount, napi_ref* result); [in] env : The environment that the API is invoked under. [in] value : napi_value representing the Object to which we want a reference. [in] initial_refcount : Initial reference count for the new reference. [out] result : napi_ref pointing to the new reference. Returns napi_ok if the API succeeded. This API create a new reference with the specified reference count to the Object passed in. NAPI_EXTERN napi_status napi_delete_reference(napi_env env, napi_ref ref); [in] env : The environment that the API is invoked under. [in] ref : napi_ref to be deleted. Returns napi_ok if the API succeeded. This API deletes the reference passed in. This API can be called even if there is a pending JavaScript exception. NAPI_EXTERN napi_status napi_reference_ref(napi_env env, napi_ref ref, int* result); napi_create_reference napi_delete_reference napi_reference_ref 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 104/739 [in] env : The environment that the API is invoked under. [in] ref : napi_ref for which the reference count will be incremented. [out] result : The new reference count. Returns napi_ok if the API succeeded. This API increments the reference count for the reference passed in and returns the resulting reference count. NAPI_EXTERN napi_status napi_reference_unref(napi_env env, napi_ref ref, int* result); [in] env : The environment that the API is invoked under. [in] ref : napi_ref for which the reference count will be decremented. [out] result : The new reference count. Returns napi_ok if the API succeeded. This API decrements the reference count for the reference passed in and returns the resulting reference count. NAPI_EXTERN napi_status napi_get_reference_value(napi_env env, napi_ref ref, napi_value* result); the napi_value passed in or out of these methods is a handle to the object to which the reference is related. [in] env : The environment that the API is invoked under. [in] ref : napi_ref for which we requesting the corresponding Object . [out] result : The napi_value for the Object referenced by the napi_ref . Returns napi_ok if the API succeeded. If still valid, this API returns the napi_value representing the JavaScript Object associated with the napi_ref . Otherwise, result will be NULL. While a Node.js process typically releases all its resources when exiting, embedders of Node.js, or future Worker support, may require addons to register clean-up hooks that will be run once the current Node.js instance exits. N-API provides functions for registering and un-registering such callbacks. When those callbacks are run, all resources that are being held by the addon should be freed up. NODE_EXTERN napi_status napi_add_env_cleanup_hook(napi_env env, void (*fun)(void* arg), void* arg); napi_reference_unref napi_get_reference_value Cleanup on exit of the current Node.js instance napi_add_env_cleanup_hook 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 105/739 Registers fun as a function to be run with the arg parameter once the current Node.js environment exits. A function can safely be specified multiple times with different arg values. In that case, it will be called multiple times as well. Providing the same fun and arg values multiple times is not allowed and will lead the process to abort. The hooks will be called in reverse order, i.e. the most recently added one will be called first. Removing this hook can be done by using napi_remove_env_cleanup_hook . Typically, that happens when the resource for which this hook was added is being torn down anyway. NAPI_EXTERN napi_status napi_remove_env_cleanup_hook(napi_env env, void (*fun)(void* arg), void* arg); Unregisters fun as a function to be run with the arg parameter once the current Node.js environment exits. Both the argument and the function value need to be exact matches. The function must have originally been registered with napi_add_env_cleanup_hook , otherwise the process will abort. N-API modules are registered in a manner similar to other modules except that instead of using the NODE_MODULE macro the following is used: NAPI_MODULE(NODE_GYP_MODULE_NAME, Init) The next difference is the signature for the Init method. For a N-API module it is as follows: napi_value Init(napi_env env, napi_value exports); The return value from Init is treated as the exports object for the module. The Init method is passed an empty object via the exports parameter as a convenience. If Init returns NULL, the parameter passed as exports is exported by the module. N-API modules cannot modify the module object but can specify anything as the exports property of the module. To add the method hello as a function so that it can be called as a method provided by the addon: napi_value Init(napi_env env, napi_value exports) { napi_status status; napi_property_descriptor desc = {"hello", NULL, Method, NULL, NULL, NULL, napi_default, NULL}; status = napi_define_properties(env, exports, 1, &desc); if (status != napi_ok) return NULL; return exports; } To set a function to be returned by the require() for the addon: napi_value Init(napi_env env, napi_value exports) { napi_value method; napi status status; napi_remove_env_cleanup_hook Module registration 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 106/739 napi_status status; status = napi_create_function(env, "exports", NAPI_AUTO_LENGTH, Method, NULL, &method); if (status != napi_ok) return NULL; return method; } To define a class so that new instances can be created (often used with Object Wrap ): // NOTE: partial example, not all referenced code is included napi_value Init(napi_env env, napi_value exports) { napi_status status; napi_property_descriptor properties[] = { { "value", NULL, NULL, GetValue, SetValue, NULL, napi_default, NULL }, DECLARE_NAPI_METHOD("plusOne", PlusOne), DECLARE_NAPI_METHOD("multiply", Multiply), }; napi_value cons; status = napi_define_class(env, "MyObject", New, NULL, 3, properties, &cons); if (status != napi_ok) return NULL; status = napi_create_reference(env, cons, 1, &constructor); if (status != napi_ok) return NULL; status = napi_set_named_property(env, exports, "MyObject", cons); if (status != napi_ok) return NULL; return exports; } If the module will be loaded multiple times during the lifetime of the Node.js process, use the NAPI_MODULE_INIT macro to initialize the module: NAPI_MODULE_INIT() { napi_value answer; napi_status result; status = napi_create_int64(env, 42, &answer); if (status != napi_ok) return NULL; status = napi_set_named_property(env, exports, "answer", answer); if (status != napi_ok) return NULL; return exports; } This macro includes NAPI_MODULE , and declares an Init function with a special name and with visibility beyond the addon. This will allow Node.js to initialize the module even if it is loaded multiple times. There are a few design considerations when declaring a module that may be loaded multiple times. The documentation of context-aware addons provides more details. 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 107/739 The variables env and exports will be available inside the function body following the macro invocation. For more details on setting properties on objects, see the section on Working with JavaScript Properties . For more details on building addon modules in general, refer to the existing API. N-API exposes a set of APIs to create all types of JavaScript values. Some of these types are documented under Section 6 of the ECMAScript Language Specification . Fundamentally, these APIs are used to do one of the following: 1. Create a new JavaScript object 2. Convert from a primitive C type to an N-API value 3. Convert from N-API value to a primitive C type 4. Get global instances including undefined and null N-API values are represented by the type napi_value . Any N-API call that requires a JavaScript value takes in a napi_value . In some cases, the API does check the type of the napi_value up-front. However, for better performance, it's better for the caller to make sure that the napi_value in question is of the JavaScript type expected by the API. typedef enum { // ES6 types (corresponds to typeof) napi_undefined, napi_null, napi_boolean, napi_number, napi_string, napi_symbol, napi_object, napi_function, napi_external, napi_bigint, } napi_valuetype; Describes the type of a napi_value . This generally corresponds to the types described in Section 6.1 of the ECMAScript Language Specification. In addition to types in that section, napi_valuetype can also represent Function s and Object s with external data. A JavaScript value of type napi_external appears in JavaScript as a plain object such that no properties can be set on it, and no prototype. typedef enum { napi_int8_array, napi_uint8_array, napi_uint8_clamped_array, napi_int16_array, napi_uint16_array, napi_int32_array, napi_uint32_array, napi_float32_array, napi_float64_array, Working with JavaScript Values Enum types napi_valuetype napi_typedarray_type 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 108/739 napi_bigint64_array, napi_biguint64_array, } napi_typedarray_type; This represents the underlying binary scalar datatype of the TypedArray . Elements of this enum correspond to Section 22.2 of the ECMAScript Language Specification . napi_status napi_create_array(napi_env env, napi_value* result) [in] env : The environment that the N-API call is invoked under. [out] result : A napi_value representing a JavaScript Array . Returns napi_ok if the API succeeded. This API returns an N-API value corresponding to a JavaScript Array type. JavaScript arrays are described in Section 22.1 of the ECMAScript Language Specification. napi_status napi_create_array_with_length(napi_env env, size_t length, napi_value* result) [in] env : The environment that the API is invoked under. [in] length : The initial length of the Array . [out] result : A napi_value representing a JavaScript Array . Returns napi_ok if the API succeeded. This API returns an N-API value corresponding to a JavaScript Array type. The Array 's length property is set to the passed-in length parameter. However, the underlying buffer is not guaranteed to be pre-allocated by the VM when the array is created - that behavior is left to the underlying VM implementation. If the buffer must be a contiguous block of memory that can be directly read and/or written via C, consider using napi_create_external_arraybuffer . JavaScript arrays are described in Section 22.1 of the ECMAScript Language Specification. napi_status napi_create_arraybuffer(napi_env env, size_t byte_length, void** data, napi_value* result) [in] env : The environment that the API is invoked under. [in] length : The length in bytes of the array buffer to create. [out] data : Pointer to the underlying byte buffer of the ArrayBuffer . [out] result : A napi_value representing a JavaScript ArrayBuffer . Object Creation Functions napi_create_array napi_create_array_with_length napi_create_arraybuffer 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 109/739 Returns napi_ok if the API succeeded. This API returns an N-API value corresponding to a JavaScript ArrayBuffer . ArrayBuffer s are used to represent fixed-length binary data buffers. They are normally used as a backing-buffer for TypedArray objects. The ArrayBuffer allocated will have an underlying byte buffer whose size is determined by the length parameter that's passed in. The underlying buffer is optionally returned back to the caller in case the caller wants to directly manipulate the buffer. This buffer can only be written to directly from native code. To write to this buffer from JavaScript, a typed array or DataView object would need to be created. JavaScript ArrayBuffer objects are described in Section 24.1 of the ECMAScript Language Specification. napi_status napi_create_buffer(napi_env env, size_t size, void** data, napi_value* result) [in] env : The environment that the API is invoked under. [in] size : Size in bytes of the underlying buffer. [out] data : Raw pointer to the underlying buffer. [out] result : A napi_value representing a node::Buffer . Returns napi_ok if the API succeeded. This API allocates a node::Buffer object. While this is still a fully-supported data structure, in most cases using a TypedArray will suffice. napi_status napi_create_buffer_copy(napi_env env, size_t length, const void* data, void** result_data, napi_value* result) [in] env : The environment that the API is invoked under. [in] size : Size in bytes of the input buffer (should be the same as the size of the new buffer). [in] data : Raw pointer to the underlying buffer to copy from. [out] result_data : Pointer to the new Buffer 's underlying data buffer. [out] result : A napi_value representing a node::Buffer . Returns napi_ok if the API succeeded. This API allocates a node::Buffer object and initializes it with data copied from the passed-in buffer. While this is still a fully-supported data structure, in most cases using a TypedArray will suffice. napi_status napi_create_external(napi_env env, void* data, napi_finalize finalize_cb, void* finalize_hint, napi_value* result) napi_create_buffer napi_create_buffer_copy napi_create_external 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 110/739 [in] env : The environment that the API is invoked under. [in] data : Raw pointer to the external data. [in] finalize_cb : Optional callback to call when the external value is being collected. [in] finalize_hint : Optional hint to pass to the finalize callback during collection. [out] result : A napi_value representing an external value. Returns napi_ok if the API succeeded. This API allocates a JavaScript value with external data attached to it. This is used to pass external data through JavaScript code, so it can be retrieved later by native code. The API allows the caller to pass in a finalize callback, in case the underlying native resource needs to be cleaned up when the external JavaScript value gets collected. The created value is not an object, and therefore does not support additional properties. It is considered a distinct value type: calling napi_typeof() with an external value yields napi_external . napi_status napi_create_external_arraybuffer(napi_env env, void* external_data, size_t byte_length, napi_finalize finalize_cb, void* finalize_hint, napi_value* result) [in] env : The environment that the API is invoked under. [in] external_data : Pointer to the underlying byte buffer of the ArrayBuffer . [in] byte_length : The length in bytes of the underlying buffer. [in] finalize_cb : Optional callback to call when the ArrayBuffer is being collected. [in] finalize_hint : Optional hint to pass to the finalize callback during collection. [out] result : A napi_value representing a JavaScript ArrayBuffer . Returns napi_ok if the API succeeded. This API returns an N-API value corresponding to a JavaScript ArrayBuffer . The underlying byte buffer of the ArrayBuffer is externally allocated and managed. The caller must ensure that the byte buffer remains valid until the finalize callback is called. JavaScript ArrayBuffer s are described in Section 24.1 of the ECMAScript Language Specification. napi_status napi_create_external_buffer(napi_env env, size_t length, void* data, napi_finalize finalize_cb, void* finalize_hint, napi_value* result) [in] env : The environment that the API is invoked under. [in] length : Size in bytes of the input buffer (should be the same as the size of the new buffer). napi_create_external_arraybuffer napi_create_external_buffer 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 111/739 [in] data : Raw pointer to the underlying buffer to copy from. [in] finalize_cb : Optional callback to call when the ArrayBuffer is being collected. [in] finalize_hint : Optional hint to pass to the finalize callback during collection. [out] result : A napi_value representing a node::Buffer . Returns napi_ok if the API succeeded. This API allocates a node::Buffer object and initializes it with data backed by the passed in buffer. While this is still a fully-supported data structure, in most cases using a TypedArray will suffice. For Node.js >=4 Buffers are Uint8Array s. napi_status napi_create_object(napi_env env, napi_value* result) [in] env : The environment that the API is invoked under. [out] result : A napi_value representing a JavaScript Object . Returns napi_ok if the API succeeded. This API allocates a default JavaScript Object . It is the equivalent of doing new Object() in JavaScript. The JavaScript Object type is described in Section 6.1.7 of the ECMAScript Language Specification. napi_status napi_create_symbol(napi_env env, napi_value description, napi_value* result) [in] env : The environment that the API is invoked under. [in] description : Optional napi_value which refers to a JavaScript String to be set as the description for the symbol. [out] result : A napi_value representing a JavaScript Symbol . Returns napi_ok if the API succeeded. This API creates a JavaScript Symbol object from a UTF8-encoded C string. The JavaScript Symbol type is described in Section 19.4 of the ECMAScript Language Specification. napi_status napi_create_typedarray(napi_env env, napi_typedarray_type type, size_t length, napi_value arraybuffer, size_t byte_offset, napi_value* result) [in] env : The environment that the API is invoked under. [in] type : Scalar datatype of the elements within the TypedArray . napi_create_object napi_create_symbol napi_create_typedarray 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 112/739 [in] length : Number of elements in the TypedArray . [in] arraybuffer : ArrayBuffer underlying the typed array. [in] byte_offset : The byte offset within the ArrayBuffer from which to start projecting the TypedArray . [out] result : A napi_value representing a JavaScript TypedArray . Returns napi_ok if the API succeeded. This API creates a JavaScript TypedArray object over an existing ArrayBuffer . TypedArray objects provide an array-like view over an underlying data buffer where each element has the same underlying binary scalar datatype. It's required that (length * size_of_element) + byte_offset should be <= the size in bytes of the array passed in. If not, a RangeError exception is raised. JavaScript TypedArray objects are described in Section 22.2 of the ECMAScript Language Specification. napi_status napi_create_dataview(napi_env env, size_t byte_length, napi_value arraybuffer, size_t byte_offset, napi_value* result) [in] env : The environment that the API is invoked under. [in] length : Number of elements in the DataView . [in] arraybuffer : ArrayBuffer underlying the DataView . [in] byte_offset : The byte offset within the ArrayBuffer from which to start projecting the DataView . [out] result : A napi_value representing a JavaScript DataView . Returns napi_ok if the API succeeded. This API creates a JavaScript DataView object over an existing ArrayBuffer . DataView objects provide an array-like view over an underlying data buffer, but one which allows items of different size and type in the ArrayBuffer . It is required that byte_length + byte_offset is less than or equal to the size in bytes of the array passed in. If not, a RangeError exception is raised. JavaScript DataView objects are described in Section 24.3 of the ECMAScript Language Specification. napi_status napi_create_int32(napi_env env, int32_t value, napi_value* result) [in] env : The environment that the API is invoked under. [in] value : Integer value to be represented in JavaScript. [out] result : A napi_value representing a JavaScript Number . Returns napi_ok if the API succeeded. This API is used to convert from the C int32_t type to the JavaScript Number type. napi_create_dataview Functions to convert from C types to N-API napi_create_int32 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 113/739 The JavaScript Number type is described in Section 6.1.6 of the ECMAScript Language Specification. napi_status napi_create_uint32(napi_env env, uint32_t value, napi_value* result) [in] env : The environment that the API is invoked under. [in] value : Unsigned integer value to be represented in JavaScript. [out] result : A napi_value representing a JavaScript Number . Returns napi_ok if the API succeeded. This API is used to convert from the C uint32_t type to the JavaScript Number type. The JavaScript Number type is described in Section 6.1.6 of the ECMAScript Language Specification. napi_status napi_create_int64(napi_env env, int64_t value, napi_value* result) [in] env : The environment that the API is invoked under. [in] value : Integer value to be represented in JavaScript. [out] result : A napi_value representing a JavaScript Number . Returns napi_ok if the API succeeded. This API is used to convert from the C int64_t type to the JavaScript Number type. The JavaScript Number type is described in Section 6.1.6 of the ECMAScript Language Specification. Note the complete range of int64_t cannot be represented with full precision in JavaScript. Integer values outside the range of Number.MIN_SAFE_INTEGER -(2^53 - 1) - Number.MAX_SAFE_INTEGER (2^53 - 1) will lose precision. napi_status napi_create_double(napi_env env, double value, napi_value* result) [in] env : The environment that the API is invoked under. [in] value : Double-precision value to be represented in JavaScript. [out] result : A napi_value representing a JavaScript Number . Returns napi_ok if the API succeeded. This API is used to convert from the C double type to the JavaScript Number type. The JavaScript Number type is described in Section 6.1.6 of the ECMAScript Language Specification. Stability: 1 - Experimental napi_create_uint32 napi_create_int64 napi_create_double napi_create_bigint_int64 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 114/739 napi_status napi_create_bigint_int64(napi_env env, int64_t value, napi_value* result); [in] env : The environment that the API is invoked under. [in] value : Integer value to be represented in JavaScript. [out] result : A napi_value representing a JavaScript BigInt . Returns napi_ok if the API succeeded. This API converts the C int64_t type to the JavaScript BigInt type. Stability: 1 - Experimental napi_status napi_create_bigint_uint64(napi_env env, uint64_t value, napi_value* result); [in] env : The environment that the API is invoked under. [in] value : Unsigned integer value to be represented in JavaScript. [out] result : A napi_value representing a JavaScript BigInt . Returns napi_ok if the API succeeded. This API converts the C uint64_t type to the JavaScript BigInt type. Stability: 1 - Experimental napi_status napi_create_bigint_words(napi_env env, int sign_bit, size_t word_count, const uint64_t* words, napi_value* result); [in] env : The environment that the API is invoked under. [in] sign_bit : Determines if the resulting BigInt will be positive or negative. [in] word_count : The length of the words array. [in] words : An array of uint64_t little-endian 64-bit words. [out] result : A napi_value representing a JavaScript BigInt . Returns napi_ok if the API succeeded. This API converts an array of unsigned 64-bit words into a single BigInt value. napi_create_bigint_uint64 napi_create_bigint_words 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 115/739 The resulting BigInt is calculated as: (–1) sign_bit ( words[0] × (264)0 + words[1] × (264)1 + …) napi_status napi_create_string_latin1(napi_env env, const char* str, size_t length, napi_value* result); [in] env : The environment that the API is invoked under. [in] str : Character buffer representing an ISO-8859-1-encoded string. [in] length : The length of the string in bytes, or NAPI_AUTO_LENGTH if it is null-terminated. [out] result : A napi_value representing a JavaScript String . Returns napi_ok if the API succeeded. This API creates a JavaScript String object from an ISO-8859-1-encoded C string. The native string is copied. The JavaScript String type is described in Section 6.1.4 of the ECMAScript Language Specification. napi_status napi_create_string_utf16(napi_env env, const char16_t* str, size_t length, napi_value* result) [in] env : The environment that the API is invoked under. [in] str : Character buffer representing a UTF16-LE-encoded string. [in] length : The length of the string in two-byte code units, or NAPI_AUTO_LENGTH if it is null-terminated. [out] result : A napi_value representing a JavaScript String . Returns napi_ok if the API succeeded. This API creates a JavaScript String object from a UTF16-LE-encoded C string. The native string is copied. The JavaScript String type is described in Section 6.1.4 of the ECMAScript Language Specification. napi_status napi_create_string_utf8(napi_env env, const char* str, size_t length, napi_value* result) [in] env : The environment that the API is invoked under. [in] str : Character buffer representing a UTF8-encoded string. [in] length : The length of the string in bytes, or NAPI_AUTO_LENGTH if it is null-terminated. [out] result : A napi_value representing a JavaScript String . Returns napi_ok if the API succeeded. napi_create_string_latin1 napi_create_string_utf16 napi_create_string_utf8 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 116/739 This API creates a JavaScript String object from a UTF8-encoded C string. The native string is copied. The JavaScript String type is described in Section 6.1.4 of the ECMAScript Language Specification. napi_status napi_get_array_length(napi_env env, napi_value value, uint32_t* result) [in] env : The environment that the API is invoked under. [in] value : napi_value representing the JavaScript Array whose length is being queried. [out] result : uint32 representing length of the array. Returns napi_ok if the API succeeded. This API returns the length of an array. Array length is described in Section 22.1.4.1 of the ECMAScript Language Specification. napi_status napi_get_arraybuffer_info(napi_env env, napi_value arraybuffer, void** data, size_t* byte_length) [in] env : The environment that the API is invoked under. [in] arraybuffer : napi_value representing the ArrayBuffer being queried. [out] data : The underlying data buffer of the ArrayBuffer . [out] byte_length : Length in bytes of the underlying data buffer. Returns napi_ok if the API succeeded. This API is used to retrieve the underlying data buffer of an ArrayBuffer and its length. WARNING: Use caution while using this API. The lifetime of the underlying data buffer is managed by the ArrayBuffer even after it's returned. A possible safe way to use this API is in conjunction with napi_create_reference , which can be used to guarantee control over the lifetime of the ArrayBuffer . It's also safe to use the returned data buffer within the same callback as long as there are no calls to other APIs that might trigger a GC. napi_status napi_get_buffer_info(napi_env env, napi_value value, void** data, size_t* length) [in] env : The environment that the API is invoked under. [in] value : napi_value representing the node::Buffer being queried. Functions to convert from N-API to C types napi_get_array_length napi_get_arraybuffer_info napi_get_buffer_info 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 117/739 [out] data : The underlying data buffer of the node::Buffer . [out] length : Length in bytes of the underlying data buffer. Returns napi_ok if the API succeeded. This API is used to retrieve the underlying data buffer of a node::Buffer and it's length. Warning: Use caution while using this API since the underlying data buffer's lifetime is not guaranteed if it's managed by the VM. napi_status napi_get_prototype(napi_env env, napi_value object, napi_value* result) [in] env : The environment that the API is invoked under. [in] object : napi_value representing JavaScript Object whose prototype to return. This returns the equivalent of Object.getPrototypeOf (which is not the same as the function's prototype property). [out] result : napi_value representing prototype of the given object. Returns napi_ok if the API succeeded. napi_status napi_get_typedarray_info(napi_env env, napi_value typedarray, napi_typedarray_type* type, size_t* length, void** data, napi_value* arraybuffer, size_t* byte_offset) [in] env : The environment that the API is invoked under. [in] typedarray : napi_value representing the TypedArray whose properties to query. [out] type : Scalar datatype of the elements within the TypedArray . [out] length : The number of elements in the TypedArray . [out] data : The data buffer underlying the TypedArray adjusted by the byte_offset value so that it points to the first element in the TypedArray . [out] arraybuffer : The ArrayBuffer underlying the TypedArray . [out] byte_offset : The byte offset within the underlying native array at which the first element of the arrays is located. The value for the data parameter has already been adjusted so that data points to the first element in the array. Therefore, the first byte of the native array would be at data - byte_offset . Returns napi_ok if the API succeeded. This API returns various properties of a typed array. Warning: Use caution while using this API since the underlying data buffer is managed by the VM. napi_get_prototype napi_get_typedarray_info napi_get_dataview_info 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 118/739 napi_status napi_get_dataview_info(napi_env env, napi_value dataview, size_t* byte_length, void** data, napi_value* arraybuffer, size_t* byte_offset) [in] env : The environment that the API is invoked under. [in] dataview : napi_value representing the DataView whose properties to query. [out] byte_length : Number of bytes in the DataView . [out] data : The data buffer underlying the DataView . [out] arraybuffer : ArrayBuffer underlying the DataView . [out] byte_offset : The byte offset within the data buffer from which to start projecting the DataView . Returns napi_ok if the API succeeded. This API returns various properties of a DataView . napi_status napi_get_value_bool(napi_env env, napi_value value, bool* result) [in] env : The environment that the API is invoked under. [in] value : napi_value representing JavaScript Boolean . [out] result : C boolean primitive equivalent of the given JavaScript Boolean . Returns napi_ok if the API succeeded. If a non-boolean napi_value is passed in it returns napi_boolean_expected . This API returns the C boolean primitive equivalent of the given JavaScript Boolean . napi_status napi_get_value_double(napi_env env, napi_value value, double* result) [in] env : The environment that the API is invoked under. [in] value : napi_value representing JavaScript Number . [out] result : C double primitive equivalent of the given JavaScript Number . Returns napi_ok if the API succeeded. If a non-number napi_value is passed in it returns napi_number_expected . This API returns the C double primitive equivalent of the given JavaScript Number . Stability: 1 - Experimental napi_get_value_bool napi_get_value_double napi_get_value_bigint_int64 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 119/739 napi_status napi_get_value_bigint_int64(napi_env env, napi_value value, int64_t* result, bool* lossless); [in] env : The environment that the API is invoked under [in] value : napi_value representing JavaScript BigInt . [out] result : C int64_t primitive equivalent of the given JavaScript BigInt . [out] lossless : Indicates whether the BigInt value was converted losslessly. Returns napi_ok if the API succeeded. If a non- BigInt is passed in it returns napi_bigint_expected . This API returns the C int64_t primitive equivalent of the given JavaScript BigInt . If needed it will truncate the value, setting lossless to false . Stability: 1 - Experimental napi_status napi_get_value_bigint_uint64(napi_env env, napi_value value, uint64_t* result, bool* lossless); [in] env : The environment that the API is invoked under. [in] value : napi_value representing JavaScript BigInt . [out] result : C uint64_t primitive equivalent of the given JavaScript BigInt . [out] lossless : Indicates whether the BigInt value was converted losslessly. Returns napi_ok if the API succeeded. If a non- BigInt is passed in it returns napi_bigint_expected . This API returns the C uint64_t primitive equivalent of the given JavaScript BigInt . If needed it will truncate the value, setting lossless to false . Stability: 1 - Experimental napi_status napi_get_value_bigint_words(napi_env env, napi_value value, size_t* word_count, int* sign_bit, uint64_t* words); [in] env : The environment that the API is invoked under. [in] value : napi_value representing JavaScript BigInt . [out] sign_bit : Integer representing if the JavaScript BigInt is positive or negative. napi_get_value_bigint_uint64 napi_get_value_bigint_words 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 120/739 [in/out] word_count : Must be initialized to the length of the words array. Upon return, it will be set to the actual number of words that would be needed to store this BigInt . [out] words : Pointer to a pre-allocated 64-bit word array. Returns napi_ok if the API succeeded. This API converts a single BigInt value into a sign bit, 64-bit little-endian array, and the number of elements in the array. sign_bit and words may be both set to NULL , in order to get only word_count . napi_status napi_get_value_external(napi_env env, napi_value value, void** result) [in] env : The environment that the API is invoked under. [in] value : napi_value representing JavaScript external value. [out] result : Pointer to the data wrapped by the JavaScript external value. Returns napi_ok if the API succeeded. If a non-external napi_value is passed in it returns napi_invalid_arg . This API retrieves the external data pointer that was previously passed to napi_create_external() . napi_status napi_get_value_int32(napi_env env, napi_value value, int32_t* result) [in] env : The environment that the API is invoked under. [in] value : napi_value representing JavaScript Number . [out] result : C int32 primitive equivalent of the given JavaScript Number . Returns napi_ok if the API succeeded. If a non-number napi_value is passed in napi_number_expected . This API returns the C int32 primitive equivalent of the given JavaScript Number . If the number exceeds the range of the 32 bit integer, then the result is truncated to the equivalent of the bottom 32 bits. This can result in a large positive number becoming a negative number if the value is > 2^31 -1. Non-finite number values ( NaN , +Infinity , or -Infinity ) set the result to zero. napi_status napi_get_value_int64(napi_env env, napi_value value, int64_t* result) [in] env : The environment that the API is invoked under. [in] value : napi_value representing JavaScript Number . [out] result : C int64 primitive equivalent of the given JavaScript Number . Returns napi_ok if the API succeeded. If a non-number napi_value is passed in it returns napi_number_expected . napi_get_value_external napi_get_value_int32 napi_get_value_int64 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 121/739 This API returns the C int64 primitive equivalent of the given JavaScript Number . Number values outside the range of Number.MIN_SAFE_INTEGER -(2^53 - 1) - Number.MAX_SAFE_INTEGER (2^53 - 1) will lose precision. Non-finite number values ( NaN , +Infinity , or -Infinity ) set the result to zero. napi_status napi_get_value_string_latin1(napi_env env, napi_value value, char* buf, size_t bufsize, size_t* result) [in] env : The environment that the API is invoked under. [in] value : napi_value representing JavaScript string. [in] buf : Buffer to write the ISO-8859-1-encoded string into. If NULL is passed in, the length of the string (in bytes) is returned. [in] bufsize : Size of the destination buffer. When this value is insufficient, the returned string will be truncated. [out] result : Number of bytes copied into the buffer, excluding the null terminator. Returns napi_ok if the API succeeded. If a non- String napi_value is passed in it returns napi_string_expected . This API returns the ISO-8859-1-encoded string corresponding the value passed in. napi_status napi_get_value_string_utf8(napi_env env, napi_value value, char* buf, size_t bufsize, size_t* result) [in] env : The environment that the API is invoked under. [in] value : napi_value representing JavaScript string. [in] buf : Buffer to write the UTF8-encoded string into. If NULL is passed in, the length of the string (in bytes) is returned. [in] bufsize : Size of the destination buffer. When this value is insufficient, the returned string will be truncated. [out] result : Number of bytes copied into the buffer, excluding the null terminator. Returns napi_ok if the API succeeded. If a non- String napi_value is passed in it returns napi_string_expected . This API returns the UTF8-encoded string corresponding the value passed in. napi_status napi_get_value_string_utf16(napi_env env, napi_value value, char16_t* buf, size_t bufsize, size_t* result) [in] env : The environment that the API is invoked under. napi_get_value_string_latin1 napi_get_value_string_utf8 napi_get_value_string_utf16 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 122/739 [in] value : napi_value representing JavaScript string. [in] buf : Buffer to write the UTF16-LE-encoded string into. If NULL is passed in, the length of the string (in 2-byte code units) is returned. [in] bufsize : Size of the destination buffer. When this value is insufficient, the returned string will be truncated. [out] result : Number of 2-byte code units copied into the buffer, excluding the null terminator. Returns napi_ok if the API succeeded. If a non- String napi_value is passed in it returns napi_string_expected . This API returns the UTF16-encoded string corresponding the value passed in. napi_status napi_get_value_uint32(napi_env env, napi_value value, uint32_t* result) [in] env : The environment that the API is invoked under. [in] value : napi_value representing JavaScript Number . [out] result : C primitive equivalent of the given napi_value as a uint32_t . Returns napi_ok if the API succeeded. If a non-number napi_value is passed in it returns napi_number_expected . This API returns the C primitive equivalent of the given napi_value as a uint32_t . napi_status napi_get_boolean(napi_env env, bool value, napi_value* result) [in] env : The environment that the API is invoked under. [in] value : The value of the boolean to retrieve. [out] result : napi_value representing JavaScript Boolean singleton to retrieve. Returns napi_ok if the API succeeded. This API is used to return the JavaScript singleton object that is used to represent the given boolean value. napi_status napi_get_global(napi_env env, napi_value* result) [in] env : The environment that the API is invoked under. [out] result : napi_value representing JavaScript global object. Returns napi_ok if the API succeeded. This API returns the global object. napi_get_value_uint32 Functions to get global instances napi_get_boolean napi_get_global napi_get_null 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 123/739 napi_status napi_get_null(napi_env env, napi_value* result) [in] env : The environment that the API is invoked under. [out] result : napi_value representing JavaScript null object. Returns napi_ok if the API succeeded. This API returns the null object. napi_status napi_get_undefined(napi_env env, napi_value* result) [in] env : The environment that the API is invoked under. [out] result : napi_value representing JavaScript Undefined value. Returns napi_ok if the API succeeded. This API returns the Undefined object. N-API exposes a set of APIs to perform some abstract operations on JavaScript values. Some of these operations are documented under Section 7 of the ECMAScript Language Specification . These APIs support doing one of the following: 1. Coerce JavaScript values to specific JavaScript types (such as Number or String ). 2. Check the type of a JavaScript value. 3. Check for equality between two JavaScript values. napi_status napi_coerce_to_bool(napi_env env, napi_value value, napi_value* result) [in] env : The environment that the API is invoked under. [in] value : The JavaScript value to coerce. [out] result : napi_value representing the coerced JavaScript Boolean . Returns napi_ok if the API succeeded. This API implements the abstract operation ToBoolean() as defined in Section 7.1.2 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object . napi_status napi_coerce_to_number(napi_env env, napi_value value, napi_value* result) [in] env : The environment that the API is invoked under. [in] value : The JavaScript value to coerce. napi_get_undefined Working with JavaScript Values - Abstract Operations napi_coerce_to_bool napi_coerce_to_number 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 124/739 [out] result : napi_value representing the coerced JavaScript Number . Returns napi_ok if the API succeeded. This API implements the abstract operation ToNumber() as defined in Section 7.1.3 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object . napi_status napi_coerce_to_object(napi_env env, napi_value value, napi_value* result) [in] env : The environment that the API is invoked under. [in] value : The JavaScript value to coerce. [out] result : napi_value representing the coerced JavaScript Object . Returns napi_ok if the API succeeded. This API implements the abstract operation ToObject() as defined in Section 7.1.13 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object . napi_status napi_coerce_to_string(napi_env env, napi_value value, napi_value* result) [in] env : The environment that the API is invoked under. [in] value : The JavaScript value to coerce. [out] result : napi_value representing the coerced JavaScript String . Returns napi_ok if the API succeeded. This API implements the abstract operation ToString() as defined in Section 7.1.13 of the ECMAScript Language Specification. This API can be re-entrant if getters are defined on the passed-in Object . napi_status napi_typeof(napi_env env, napi_value value, napi_valuetype* result) [in] env : The environment that the API is invoked under. [in] value : The JavaScript value whose type to query. [out] result : The type of the JavaScript value. Returns napi_ok if the API succeeded. napi_invalid_arg if the type of value is not a known ECMAScript type and value is not an External value. This API represents behavior similar to invoking the typeof Operator on the object as defined in Section 12.5.5 of the ECMAScript Language Specification. However, it has support for detecting an External value. If value has a type that is invalid, an error is returned. napi_coerce_to_object napi_coerce_to_string napi_typeof 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 125/739 napi_status napi_instanceof(napi_env env, napi_value object, napi_value constructor, bool* result) [in] env : The environment that the API is invoked under. [in] object : The JavaScript value to check. [in] constructor : The JavaScript function object of the constructor function to check against. [out] result : Boolean that is set to true if object instanceof constructor is true. Returns napi_ok if the API succeeded. This API represents invoking the instanceof Operator on the object as defined in Section 12.10.4 of the ECMAScript Language Specification. napi_status napi_is_array(napi_env env, napi_value value, bool* result) [in] env : The environment that the API is invoked under. [in] value : The JavaScript value to check. [out] result : Whether the given object is an array. Returns napi_ok if the API succeeded. This API represents invoking the IsArray operation on the object as defined in Section 7.2.2 of the ECMAScript Language Specification. napi_status napi_is_arraybuffer(napi_env env, napi_value value, bool* result) [in] env : The environment that the API is invoked under. [in] value : The JavaScript value to check. [out] result : Whether the given object is an ArrayBuffer . Returns napi_ok if the API succeeded. This API checks if the Object passed in is an array buffer. napi_status napi_is_buffer(napi_env env, napi_value value, bool* result) [in] env : The environment that the API is invoked under. [in] value : The JavaScript value to check. [out] result : Whether the given napi_value represents a node::Buffer object. Returns napi_ok if the API succeeded. napi_instanceof napi_is_array napi_is_arraybuffer napi_is_buffer 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 126/739 This API checks if the Object passed in is a buffer. napi_status napi_is_error(napi_env env, napi_value value, bool* result) [in] env : The environment that the API is invoked under. [in] value : The JavaScript value to check. [out] result : Whether the given napi_value represents an Error object. Returns napi_ok if the API succeeded. This API checks if the Object passed in is an Error . napi_status napi_is_typedarray(napi_env env, napi_value value, bool* result) [in] env : The environment that the API is invoked under. [in] value : The JavaScript value to check. [out] result : Whether the given napi_value represents a TypedArray . Returns napi_ok if the API succeeded. This API checks if the Object passed in is a typed array. napi_status napi_is_dataview(napi_env env, napi_value value, bool* result) [in] env : The environment that the API is invoked under. [in] value : The JavaScript value to check. [out] result : Whether the given napi_value represents a DataView . Returns napi_ok if the API succeeded. This API checks if the Object passed in is a DataView . napi_status napi_strict_equals(napi_env env, napi_value lhs, napi_value rhs, bool* result) [in] env : The environment that the API is invoked under. [in] lhs : The JavaScript value to check. [in] rhs : The JavaScript value to check against. [out] result : Whether the two napi_value objects are equal. napi_is_error napi_is_typedarray napi_is_dataview napi_strict_equals 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 127/739 Returns napi_ok if the API succeeded. This API represents the invocation of the Strict Equality algorithm as defined in Section 7.2.14 of the ECMAScript Language Specification. N-API exposes a set of APIs to get and set properties on JavaScript objects. Some of these types are documented under Section 7 of the ECMAScript Language Specification . Properties in JavaScript are represented as a tuple of a key and a value. Fundamentally, all property keys in N-API can be represented in one of the following forms: Named: a simple UTF8-encoded string Integer-Indexed: an index value represented by uint32_t JavaScript value: these are represented in N-API by napi_value . This can be a napi_value representing a String , Number , or Symbol . N-API values are represented by the type napi_value . Any N-API call that requires a JavaScript value takes in a napi_value . However, it's the caller's responsibility to make sure that the napi_value in question is of the JavaScript type expected by the API. The APIs documented in this section provide a simple interface to get and set properties on arbitrary JavaScript objects represented by napi_value . For instance, consider the following JavaScript code snippet: const obj = {}; obj.myProp = 123; The equivalent can be done using N-API values with the following snippet: napi_status status = napi_generic_failure; // const obj = {} napi_value obj, value; status = napi_create_object(env, &obj); if (status != napi_ok) return status; // Create a napi_value for 123 status = napi_create_int32(env, 123, &value); if (status != napi_ok) return status; // obj.myProp = 123 status = napi_set_named_property(env, obj, "myProp", value); if (status != napi_ok) return status; Indexed properties can be set in a similar manner. Consider the following JavaScript snippet: const arr = []; arr[123] = 'hello'; The equivalent can be done using N-API values with the following snippet: napi_status status = napi_generic_failure; Working with JavaScript Properties 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 128/739 // const arr = []; napi_value arr, value; status = napi_create_array(env, &arr); if (status != napi_ok) return status; // Create a napi_value for 'hello' status = napi_create_string_utf8(env, "hello", NAPI_AUTO_LENGTH, &value); if (status != napi_ok) return status; // arr[123] = 'hello'; status = napi_set_element(env, arr, 123, value); if (status != napi_ok) return status; Properties can be retrieved using the APIs described in this section. Consider the following JavaScript snippet: const arr = []; const value = arr[123]; The following is the approximate equivalent of the N-API counterpart: napi_status status = napi_generic_failure; // const arr = [] napi_value arr, value; status = napi_create_array(env, &arr); if (status != napi_ok) return status; // const value = arr[123] status = napi_get_element(env, arr, 123, &value); if (status != napi_ok) return status; Finally, multiple properties can also be defined on an object for performance reasons. Consider the following JavaScript: const obj = {}; Object.defineProperties(obj, { 'foo': { value: 123, writable: true, configurable: true, enumerable: true }, 'bar': { value: 456, writable: true, configurable: true, enumerable: true } }); The following is the approximate equivalent of the N-API counterpart: napi_status status = napi_status_generic_failure; // const obj = {}; napi_value obj; status = napi_create_object(env, &obj); if (status != napi_ok) return status; // Create napi_values for 123 and 456 napi_value fooValue, barValue; status = napi_create_int32(env, 123, &fooValue); 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 129/739 if (status != napi_ok) return status; status = napi_create_int32(env, 456, &barValue); if (status != napi_ok) return status; // Set the properties napi_property_descriptor descriptors[] = { { "foo", NULL, NULL, NULL, NULL, fooValue, napi_default, NULL }, { "bar", NULL, NULL, NULL, NULL, barValue, napi_default, NULL } } status = napi_define_properties(env, obj, sizeof(descriptors) / sizeof(descriptors[0]), descriptors); if (status != napi_ok) return status; typedef enum { napi_default = 0, napi_writable = 1 << 0, napi_enumerable = 1 << 1, napi_configurable = 1 << 2, // Used with napi_define_class to distinguish static properties // from instance properties. Ignored by napi_define_properties. napi_static = 1 << 10, } napi_property_attributes; napi_property_attributes are flags used to control the behavior of properties set on a JavaScript object. Other than napi_static they correspond to the attributes listed in Section 6.1.7.1 of the ECMAScript Language Specification . They can be one or more of the following bitflags: napi_default - Used to indicate that no explicit attributes are set on the given property. By default, a property is read only, not enumerable and not configurable. napi_writable - Used to indicate that a given property is writable. napi_enumerable - Used to indicate that a given property is enumerable. napi_configurable - Used to indicate that a given property is configurable, as defined in Section 6.1.7.1 of the ECMAScript Language Specification . napi_static - Used to indicate that the property will be defined as a static property on a class as opposed to an instance property, which is the default. This is used only by napi_define_class . It is ignored by napi_define_properties . typedef struct { // One of utf8name or name should be NULL. const char* utf8name; napi_value name; napi_callback method; Structures napi_property_attributes napi_property_descriptor 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 130/739 napi_callback getter; napi_callback setter; napi_value value; napi_property_attributes attributes; void* data; } napi_property_descriptor; utf8name : Optional String describing the key for the property, encoded as UTF8. One of utf8name or name must be provided for the property. name : Optional napi_value that points to a JavaScript string or symbol to be used as the key for the property. One of utf8name or name must be provided for the property. value : The value that's retrieved by a get access of the property if the property is a data property. If this is passed in, set getter , setter , method and data to NULL (since these members won't be used). getter : A function to call when a get access of the property is performed. If this is passed in, set value and method to NULL (since these members won't be used). The given function is called implicitly by the runtime when the property is accessed from JavaScript code (or if a get on the property is performed using a N-API call). setter : A function to call when a set access of the property is performed. If this is passed in, set value and method to NULL (since these members won't be used). The given function is called implicitly by the runtime when the property is set from JavaScript code (or if a set on the property is performed using a N-API call). method : Set this to make the property descriptor object's value property to be a JavaScript function represented by method . If this is passed in, set value , getter and setter to NULL (since these members won't be used). attributes : The attributes associated with the particular property. See napi_property_attributes . data : The callback data passed into method , getter and setter if this function is invoked. napi_status napi_get_property_names(napi_env env, napi_value object, napi_value* result); [in] env : The environment that the N-API call is invoked under. [in] object : The object from which to retrieve the properties. [out] result : A napi_value representing an array of JavaScript values that represent the property names of the object. The API can be used to iterate over result using napi_get_array_length and napi_get_element . Returns napi_ok if the API succeeded. This API returns the names of the enumerable properties of object as an array of strings. The properties of object whose key is a symbol will not be included. napi_status napi_set_property(napi_env env, napi_value object, napi_value key, napi_value value); [in] env : The environment that the N-API call is invoked under. Functions napi_get_property_names napi_set_property 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 131/739 [in] object : The object on which to set the property. [in] key : The name of the property to set. [in] value : The property value. Returns napi_ok if the API succeeded. This API set a property on the Object passed in. napi_status napi_get_property(napi_env env, napi_value object, napi_value key, napi_value* result); [in] env : The environment that the N-API call is invoked under. [in] object : The object from which to retrieve the property. [in] key : The name of the property to retrieve. [out] result : The value of the property. Returns napi_ok if the API succeeded. This API gets the requested property from the Object passed in. napi_status napi_has_property(napi_env env, napi_value object, napi_value key, bool* result); [in] env : The environment that the N-API call is invoked under. [in] object : The object to query. [in] key : The name of the property whose existence to check. [out] result : Whether the property exists on the object or not. Returns napi_ok if the API succeeded. This API checks if the Object passed in has the named property. napi_status napi_delete_property(napi_env env, napi_value object, napi_value key, bool* result); [in] env : The environment that the N-API call is invoked under. [in] object : The object to query. [in] key : The name of the property to delete. napi_get_property napi_has_property napi_delete_property 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 132/739 [out] result : Whether the property deletion succeeded or not. result can optionally be ignored by passing NULL . Returns napi_ok if the API succeeded. This API attempts to delete the key own property from object . napi_status napi_has_own_property(napi_env env, napi_value object, napi_value key, bool* result); [in] env : The environment that the N-API call is invoked under. [in] object : The object to query. [in] key : The name of the own property whose existence to check. [out] result : Whether the own property exists on the object or not. Returns napi_ok if the API succeeded. This API checks if the Object passed in has the named own property. key must be a string or a Symbol , or an error will be thrown. N-API will not perform any conversion between data types. napi_status napi_set_named_property(napi_env env, napi_value object, const char* utf8Name, napi_value value); [in] env : The environment that the N-API call is invoked under. [in] object : The object on which to set the property. [in] utf8Name : The name of the property to set. [in] value : The property value. Returns napi_ok if the API succeeded. This method is equivalent to calling napi_set_property with a napi_value created from the string passed in as utf8Name . napi_status napi_get_named_property(napi_env env, napi_value object, const char* utf8Name, napi_value* result); [in] env : The environment that the N-API call is invoked under. [in] object : The object from which to retrieve the property. [in] utf8Name : The name of the property to get. [out] result : The value of the property. Returns napi_ok if the API succeeded. napi_has_own_property napi_set_named_property napi_get_named_property 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 133/739 This method is equivalent to calling napi_get_property with a napi_value created from the string passed in as utf8Name . napi_status napi_has_named_property(napi_env env, napi_value object, const char* utf8Name, bool* result); [in] env : The environment that the N-API call is invoked under. [in] object : The object to query. [in] utf8Name : The name of the property whose existence to check. [out] result : Whether the property exists on the object or not. Returns napi_ok if the API succeeded. This method is equivalent to calling napi_has_property with a napi_value created from the string passed in as utf8Name . napi_status napi_set_element(napi_env env, napi_value object, uint32_t index, napi_value value); [in] env : The environment that the N-API call is invoked under. [in] object : The object from which to set the properties. [in] index : The index of the property to set. [in] value : The property value. Returns napi_ok if the API succeeded. This API sets and element on the Object passed in. napi_status napi_get_element(napi_env env, napi_value object, uint32_t index, napi_value* result); [in] env : The environment that the N-API call is invoked under. [in] object : The object from which to retrieve the property. [in] index : The index of the property to get. [out] result : The value of the property. Returns napi_ok if the API succeeded. This API gets the element at the requested index. napi_has_named_property napi_set_element napi_get_element napi_has_element 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 134/739 napi_status napi_has_element(napi_env env, napi_value object, uint32_t index, bool* result); [in] env : The environment that the N-API call is invoked under. [in] object : The object to query. [in] index : The index of the property whose existence to check. [out] result : Whether the property exists on the object or not. Returns napi_ok if the API succeeded. This API returns if the Object passed in has an element at the requested index. napi_status napi_delete_element(napi_env env, napi_value object, uint32_t index, bool* result); [in] env : The environment that the N-API call is invoked under. [in] object : The object to query. [in] index : The index of the property to delete. [out] result : Whether the element deletion succeeded or not. result can optionally be ignored by passing NULL . Returns napi_ok if the API succeeded. This API attempts to delete the specified index from object . napi_status napi_define_properties(napi_env env, napi_value object, size_t property_count, const napi_property_descriptor* properties); [in] env : The environment that the N-API call is invoked under. [in] object : The object from which to retrieve the properties. [in] property_count : The number of elements in the properties array. [in] properties : The array of property descriptors. Returns napi_ok if the API succeeded. This method allows the efficient definition of multiple properties on a given object. The properties are defined using property descriptors (see napi_property_descriptor ). Given an array of such property descriptors, this API will set the properties on the object one at a time, as defined by DefineOwnProperty() (described in Section 9.1.6 of the ECMA262 specification). napi_delete_element napi_define_properties Working with JavaScript Functions 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 135/739 N-API provides a set of APIs that allow JavaScript code to call back into native code. N-API APIs that support calling back into native code take in a callback functions represented by the napi_callback type. When the JavaScript VM calls back to native code, the napi_callback function provided is invoked. The APIs documented in this section allow the callback function to do the following: Get information about the context in which the callback was invoked. Get the arguments passed into the callback. Return a napi_value back from the callback. Additionally, N-API provides a set of functions which allow calling JavaScript functions from native code. One can either call a function like a regular JavaScript function call, or as a constructor function. Any non- NULL data which is passed to this API via the data field of the napi_property_descriptor items can be associated with object and freed whenever object is garbage-collected by passing both object and the data to napi_add_finalizer . napi_status napi_call_function(napi_env env, napi_value recv, napi_value func, int argc, const napi_value* argv, napi_value* result) [in] env : The environment that the API is invoked under. [in] recv : The this object passed to the called function. [in] func : napi_value representing the JavaScript function to be invoked. [in] argc : The count of elements in the argv array. [in] argv : Array of napi_values representing JavaScript values passed in as arguments to the function. [out] result : napi_value representing the JavaScript object returned. Returns napi_ok if the API succeeded. This method allows a JavaScript function object to be called from a native add-on. This is the primary mechanism of calling back from the add- on's native code into JavaScript. For the special case of calling into JavaScript after an async operation, see napi_make_callback . A sample use case might look as follows. Consider the following JavaScript snippet: function AddTwo(num) { return num + 2; } Then, the above function can be invoked from a native add-on using the following code: // Get the function named "AddTwo" on the global object napi_value global, add_two, arg; napi_status status = napi_get_global(env, &global); if (status != napi_ok) return; status = napi_get_named_property(env, global, "AddTwo", &add_two); if (status != napi_ok) return; // const arg = 1337 t t i t i t32( 1337 & ) napi_call_function 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 136/739 status = napi_create_int32(env, 1337, &arg); if (status != napi_ok) return; napi_value* argv = &arg; size_t argc = 1; // AddTwo(arg); napi_value return_val; status = napi_call_function(env, global, add_two, argc, argv, &return_val); if (status != napi_ok) return; // Convert the result back to a native type int32_t result; status = napi_get_value_int32(env, return_val, &result); if (status != napi_ok) return; napi_status napi_create_function(napi_env env, const char* utf8name, size_t length, napi_callback cb, void* data, napi_value* result); [in] env : The environment that the API is invoked under. [in] utf8Name : The name of the function encoded as UTF8. This is visible within JavaScript as the new function object's name property. [in] length : The length of the utf8name in bytes, or NAPI_AUTO_LENGTH if it is null-terminated. [in] cb : The native function which should be called when this function object is invoked. [in] data : User-provided data context. This will be passed back into the function when invoked later. [out] result : napi_value representing the JavaScript function object for the newly created function. Returns napi_ok if the API succeeded. This API allows an add-on author to create a function object in native code. This is the primary mechanism to allow calling into the add-on's native code from JavaScript. The newly created function is not automatically visible from script after this call. Instead, a property must be explicitly set on any object that is visible to JavaScript, in order for the function to be accessible from script. In order to expose a function as part of the add-on's module exports, set the newly created function on the exports object. A sample module might look as follows: napi_value SayHello(napi_env env, napi_callback_info info) { printf("Hello\n"); return NULL; } napi_value Init(napi_env env, napi_value exports) { napi_status status; napi_create_function 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 137/739 napi_value fn; status = napi_create_function(env, NULL, 0, SayHello, NULL, &fn); if (status != napi_ok) return NULL; status = napi_set_named_property(env, exports, "sayHello", fn); if (status != napi_ok) return NULL; return exports; } NAPI_MODULE(NODE_GYP_MODULE_NAME, Init) Given the above code, the add-on can be used from JavaScript as follows: const myaddon = require('./addon'); myaddon.sayHello(); The string passed to require() is the name of the target in binding.gyp responsible for creating the .node file. Any non- NULL data which is passed to this API via the data parameter can be associated with the resulting JavaScript function (which is returned in the result parameter) and freed whenever the function is garbage-collected by passing both the JavaScript function and the data to napi_add_finalizer . JavaScript Function s are described in Section 19.2 of the ECMAScript Language Specification. napi_status napi_get_cb_info(napi_env env, napi_callback_info cbinfo, size_t* argc, napi_value* argv, napi_value* thisArg, void** data) [in] env : The environment that the API is invoked under. [in] cbinfo : The callback info passed into the callback function. [in-out] argc : Specifies the size of the provided argv array and receives the actual count of arguments. [out] argv : Buffer to which the napi_value representing the arguments are copied. If there are more arguments than the provided count, only the requested number of arguments are copied. If there are fewer arguments provided than claimed, the rest of argv is filled with napi_value values that represent undefined . [out] this : Receives the JavaScript this argument for the call. [out] data : Receives the data pointer for the callback. Returns napi_ok if the API succeeded. This method is used within a callback function to retrieve details about the call like the arguments and the this pointer from a given callback info. napi_get_cb_info napi_get_new_target 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 138/739 napi_status napi_get_new_target(napi_env env, napi_callback_info cbinfo, napi_value* result) [in] env : The environment that the API is invoked under. [in] cbinfo : The callback info passed into the callback function. [out] result : The new.target of the constructor call. Returns napi_ok if the API succeeded. This API returns the new.target of the constructor call. If the current callback is not a constructor call, the result is NULL . napi_status napi_new_instance(napi_env env, napi_value cons, size_t argc, napi_value* argv, napi_value* result) [in] env : The environment that the API is invoked under. [in] cons : napi_value representing the JavaScript function to be invoked as a constructor. [in] argc : The count of elements in the argv array. [in] argv : Array of JavaScript values as napi_value representing the arguments to the constructor. [out] result : napi_value representing the JavaScript object returned, which in this case is the constructed object. This method is used to instantiate a new JavaScript value using a given napi_value that represents the constructor for the object. For example, consider the following snippet: function MyObject(param) { this.param = param; } const arg = 'hello'; const value = new MyObject(arg); The following can be approximated in N-API using the following snippet: // Get the constructor function MyObject napi_value global, constructor, arg, value; napi_status status = napi_get_global(env, &global); if (status != napi_ok) return; status = napi_get_named_property(env, global, "MyObject", &constructor); if (status != napi_ok) return; // const arg = "hello" status = napi_create_string_utf8(env, "hello", NAPI_AUTO_LENGTH, &arg); if (status != napi_ok) return; napi value* argv = &arg; napi_new_instance 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 139/739 ap _ a ue a g &a g; size_t argc = 1; // const value = new MyObject(arg) status = napi_new_instance(env, constructor, argc, argv, &value); Returns napi_ok if the API succeeded. N-API offers a way to "wrap" C++ classes and instances so that the class constructor and methods can be called from JavaScript. 1. The napi_define_class API defines a JavaScript class with constructor, static properties and methods, and instance properties and methods that correspond to the C++ class. 2. When JavaScript code invokes the constructor, the constructor callback uses napi_wrap to wrap a new C++ instance in a JavaScript object, then returns the wrapper object. 3. When JavaScript code invokes a method or property accessor on the class, the corresponding napi_callback C++ function is invoked. For an instance callback, napi_unwrap obtains the C++ instance that is the target of the call. For wrapped objects it may be difficult to distinguish between a function called on a class prototype and a function called on an instance of a class. A common pattern used to address this problem is to save a persistent reference to the class constructor for later instanceof checks. napi_value MyClass_constructor = NULL; status = napi_get_reference_value(env, MyClass::es_constructor, &MyClass_constructor); assert(napi_ok == status); bool is_instance = false; status = napi_instanceof(env, es_this, MyClass_constructor, &is_instance); assert(napi_ok == status); if (is_instance) { // napi_unwrap() ... } else { // otherwise... } The reference must be freed once it is no longer needed. napi_status napi_define_class(napi_env env, const char* utf8name, size_t length, napi_callback constructor, void* data, size_t property_count, const napi_property_descriptor* properties, napi_value* result); [in] env : The environment that the API is invoked under. [in] utf8name : Name of the JavaScript constructor function; this is not required to be the same as the C++ class name, though it is recommended for clarity. [in] length : The length of the utf8name in bytes, or NAPI_AUTO_LENGTH if it is null-terminated. Object Wrap napi_define_class 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 140/739 [in] constructor : Callback function that handles constructing instances of the class. (This should be a static method on the class, not an actual C++ constructor function.) [in] data : Optional data to be passed to the constructor callback as the data property of the callback info. [in] property_count : Number of items in the properties array argument. [in] properties : Array of property descriptors describing static and instance data properties, accessors, and methods on the class See napi_property_descriptor . [out] result : A napi_value representing the constructor function for the class. Returns napi_ok if the API succeeded. Defines a JavaScript class that corresponds to a C++ class, including: A JavaScript constructor function that has the class name and invokes the provided C++ constructor callback. Properties on the constructor function corresponding to static data properties, accessors, and methods of the C++ class (defined by property descriptors with the napi_static attribute). Properties on the constructor function's prototype object corresponding to non-static data properties, accessors, and methods of the C++ class (defined by property descriptors without the napi_static attribute). The C++ constructor callback should be a static method on the class that calls the actual class constructor, then wraps the new C++ instance in a JavaScript object, and returns the wrapper object. See napi_wrap() for details. The JavaScript constructor function returned from napi_define_class is often saved and used later, to construct new instances of the class from native code, and/or check whether provided values are instances of the class. In that case, to prevent the function value from being garbage-collected, create a persistent reference to it using napi_create_reference and ensure the reference count is kept >= 1. Any non- NULL data which is passed to this API via the data parameter or via the data field of the napi_property_descriptor array items can be associated with the resulting JavaScript constructor (which is returned in the result parameter) and freed whenever the class is garbage-collected by passing both the JavaScript function and the data to napi_add_finalizer . napi_status napi_wrap(napi_env env, napi_value js_object, void* native_object, napi_finalize finalize_cb, void* finalize_hint, napi_ref* result); [in] env : The environment that the API is invoked under. [in] js_object : The JavaScript object that will be the wrapper for the native object. [in] native_object : The native instance that will be wrapped in the JavaScript object. [in] finalize_cb : Optional native callback that can be used to free the native instance when the JavaScript object is ready for garbage- collection. [in] finalize_hint : Optional contextual hint that is passed to the finalize callback. [out] result : Optional reference to the wrapped object. Returns napi_ok if the API succeeded. Wraps a native instance in a JavaScript object. The native instance can be retrieved later using napi_unwrap() . When JavaScript code invokes a constructor for a class that was defined using napi_define_class() , the napi_callback for the constructor is invoked. After constructing an instance of the native class, the callback must then call napi_wrap() to wrap the newly constructed instance napi_wrap 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 141/739 in the already-created JavaScript object that is the this argument to the constructor callback. (That this object was created from the constructor function's prototype , so it already has definitions of all the instance properties and methods.) Typically when wrapping a class instance, a finalize callback should be provided that simply deletes the native instance that is received as the data argument to the finalize callback. The optional returned reference is initially a weak reference, meaning it has a reference count of 0. Typically this reference count would be incremented temporarily during async operations that require the instance to remain valid. Caution: The optional returned reference (if obtained) should be deleted via napi_delete_reference ONLY in response to the finalize callback invocation. If it is deleted before then, then the finalize callback may never be invoked. Therefore, when obtaining a reference a finalize callback is also required in order to enable correct disposal of the reference. Calling napi_wrap() a second time on an object will return an error. To associate another native instance with the object, use napi_remove_wrap() first. napi_status napi_unwrap(napi_env env, napi_value js_object, void** result); [in] env : The environment that the API is invoked under. [in] js_object : The object associated with the native instance. [out] result : Pointer to the wrapped native instance. Returns napi_ok if the API succeeded. Retrieves a native instance that was previously wrapped in a JavaScript object using napi_wrap() . When JavaScript code invokes a method or property accessor on the class, the corresponding napi_callback is invoked. If the callback is for an instance method or accessor, then the this argument to the callback is the wrapper object; the wrapped C++ instance that is the target of the call can be obtained then by calling napi_unwrap() on the wrapper object. napi_status napi_remove_wrap(napi_env env, napi_value js_object, void** result); [in] env : The environment that the API is invoked under. [in] js_object : The object associated with the native instance. [out] result : Pointer to the wrapped native instance. Returns napi_ok if the API succeeded. Retrieves a native instance that was previously wrapped in the JavaScript object js_object using napi_wrap() and removes the wrapping. If a finalize callback was associated with the wrapping, it will no longer be called when the JavaScript object becomes garbage-collected. napi_unwrap napi_remove_wrap napi_add_finalizer 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 142/739 Stability: 1 - Experimental napi_status napi_add_finalizer(napi_env env, napi_value js_object, void* native_object, napi_finalize finalize_cb, void* finalize_hint, napi_ref* result); [in] env : The environment that the API is invoked under. [in] js_object : The JavaScript object to which the native data will be attached. [in] native_object : The native data that will be attached to the JavaScript object. [in] finalize_cb : Native callback that will be used to free the native data when the JavaScript object is ready for garbage-collection. [in] finalize_hint : Optional contextual hint that is passed to the finalize callback. [out] result : Optional reference to the JavaScript object. Returns napi_ok if the API succeeded. Adds a napi_finalize callback which will be called when the JavaScript object in js_object is ready for garbage collection. This API is similar to napi_wrap() except that the native data cannot be retrieved later using napi_unwrap() , nor can it be removed later using napi_remove_wrap() , and the API can be called multiple times with different data items in order to attach each of them to the JavaScript object. Caution: The optional returned reference (if obtained) should be deleted via napi_delete_reference ONLY in response to the finalize callback invocation. If it is deleted before then, then the finalize callback may never be invoked. Therefore, when obtaining a reference a finalize callback is also required in order to enable correct disposal of the reference. Addon modules often need to leverage async helpers from libuv as part of their implementation. This allows them to schedule work to be executed asynchronously so that their methods can return in advance of the work being completed. This is important in order to allow them to avoid blocking overall execution of the Node.js application. N-API provides an ABI-stable interface for these supporting functions which covers the most common asynchronous use cases. N-API defines the napi_work structure which is used to manage asynchronous workers. Instances are created/deleted with napi_create_async_work and napi_delete_async_work . The execute and complete callbacks are functions that will be invoked when the executor is ready to execute and when it completes its task respectively. The execute function should avoid making any N-API calls that could result in the execution of JavaScript or interaction with JavaScript objects. Most often, any code that needs to make N-API calls should be made in complete callback instead. These functions implement the following interfaces: typedef void (*napi_async_execute_callback)(napi_env env, void* data); typedef void (*napi_async_complete_callback)(napi_env env, Simple Asynchronous Operations 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 143/739 napi_status status, void* data); When these methods are invoked, the data parameter passed will be the addon-provided void* data that was passed into the napi_create_async_work call. Once created the async worker can be queued for execution using the napi_queue_async_work function: napi_status napi_queue_async_work(napi_env env, napi_async_work work); napi_cancel_async_work can be used if the work needs to be cancelled before the work has started execution. After calling napi_cancel_async_work , the complete callback will be invoked with a status value of napi_cancelled . The work should not be deleted before the complete callback invocation, even when it was cancelled. napi_status napi_create_async_work(napi_env env, napi_value async_resource, napi_value async_resource_name, napi_async_execute_callback execute, napi_async_complete_callback complete, void* data, napi_async_work* result); [in] env : The environment that the API is invoked under. [in] async_resource : An optional object associated with the async work that will be passed to possible async_hooks init hooks . [in] async_resource_name : Identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API. [in] execute : The native function which should be called to execute the logic asynchronously. The given function is called from a worker pool thread and can execute in parallel with the main event loop thread. [in] complete : The native function which will be called when the asynchronous logic is completed or is cancelled. The given function is called from the main event loop thread. [in] data : User-provided data context. This will be passed back into the execute and complete functions. [out] result : napi_async_work* which is the handle to the newly created async work. Returns napi_ok if the API succeeded. This API allocates a work object that is used to execute logic asynchronously. It should be freed using napi_delete_async_work once the work is no longer required. async_resource_name should be a null-terminated, UTF-8-encoded string. The async_resource_name identifier is provided by the user and should be representative of the type of async work being performed. It is also recommended to apply namespacing to the identifier, e.g. by including the module name. See the async_hooks documentation for more information. napi_create_async_work napi_delete_async_work 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 144/739 napi_status napi_delete_async_work(napi_env env, napi_async_work work); [in] env : The environment that the API is invoked under. [in] work : The handle returned by the call to napi_create_async_work . Returns napi_ok if the API succeeded. This API frees a previously allocated work object. This API can be called even if there is a pending JavaScript exception. napi_status napi_queue_async_work(napi_env env, napi_async_work work); [in] env : The environment that the API is invoked under. [in] work : The handle returned by the call to napi_create_async_work . Returns napi_ok if the API succeeded. This API requests that the previously allocated work be scheduled for execution. napi_status napi_cancel_async_work(napi_env env, napi_async_work work); [in] env : The environment that the API is invoked under. [in] work : The handle returned by the call to napi_create_async_work . Returns napi_ok if the API succeeded. This API cancels queued work if it has not yet been started. If it has already started executing, it cannot be cancelled and napi_generic_failure will be returned. If successful, the complete callback will be invoked with a status value of napi_cancelled . The work should not be deleted before the complete callback invocation, even if it has been successfully cancelled. This API can be called even if there is a pending JavaScript exception. The simple asynchronous work APIs above may not be appropriate for every scenario. When using any other asynchronous mechanism, the following APIs are necessary to ensure an asynchronous operation is properly tracked by the runtime. napi_status napi_async_init(napi_env env, napi_value async_resource, napi_value async_resource_name, napi_async_context* result) napi_queue_async_work napi_cancel_async_work Custom Asynchronous Operations napi_async_init 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 145/739 [in] env : The environment that the API is invoked under. [in] async_resource : An optional object associated with the async work that will be passed to possible async_hooks init hooks . [in] async_resource_name : Identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API. [out] result : The initialized async context. Returns napi_ok if the API succeeded. napi_status napi_async_destroy(napi_env env, napi_async_context async_context); [in] env : The environment that the API is invoked under. [in] async_context : The async context to be destroyed. Returns napi_ok if the API succeeded. This API can be called even if there is a pending JavaScript exception. napi_status napi_make_callback(napi_env env, napi_async_context async_context, napi_value recv, napi_value func, int argc, const napi_value* argv, napi_value* result) [in] env : The environment that the API is invoked under. [in] async_context : Context for the async operation that is invoking the callback. This should normally be a value previously obtained from napi_async_init . However NULL is also allowed, which indicates the current async context (if any) is to be used for the callback. [in] recv : The this object passed to the called function. [in] func : napi_value representing the JavaScript function to be invoked. [in] argc : The count of elements in the argv array. [in] argv : Array of JavaScript values as napi_value representing the arguments to the function. [out] result : napi_value representing the JavaScript object returned. Returns napi_ok if the API succeeded. This method allows a JavaScript function object to be called from a native add-on. This API is similar to napi_call_function . However, it is used to call from native code back into JavaScript after returning from an async operation (when there is no other script on the stack). It is a fairly simple wrapper around node::MakeCallback . Note it is not necessary to use napi_make_callback from within a napi_async_complete_callback ; in that situation the callback's async context has already been set up, so a direct call to napi_call_function is sufficient and appropriate. Use of the napi_make_callback function may be required when implementing custom async behavior that does not use napi_create_async_work . napi_async_destroy napi_make_callback napi_open_callback_scope 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 146/739 NAPI_EXTERN napi_status napi_open_callback_scope(napi_env env, napi_value resource_object, napi_async_context context, napi_callback_scope* result) [in] env : The environment that the API is invoked under. [in] resource_object : An object associated with the async work that will be passed to possible async_hooks init hooks . [in] context : Context for the async operation that is invoking the callback. This should be a value previously obtained from napi_async_init . [out] result : The newly created scope. There are cases (for example, resolving promises) where it is necessary to have the equivalent of the scope associated with a callback in place when making certain N-API calls. If there is no other script on the stack the napi_open_callback_scope and napi_close_callback_scope functions can be used to open/close the required scope. NAPI_EXTERN napi_status napi_close_callback_scope(napi_env env, napi_callback_scope scope) [in] env : The environment that the API is invoked under. [in] scope : The scope to be closed. This API can be called even if there is a pending JavaScript exception. typedef struct { uint32_t major; uint32_t minor; uint32_t patch; const char* release; } napi_node_version; napi_status napi_get_node_version(napi_env env, const napi_node_version** version); [in] env : The environment that the API is invoked under. [out] version : A pointer to version information for Node.js itself. Returns napi_ok if the API succeeded. This function fills the version struct with the major, minor, and patch version of Node.js that is currently running, and the release field with the value of process.release.name . The returned buffer is statically allocated and does not need to be freed. napi_close_callback_scope Version Management napi_get_node_version napi_get_version 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 147/739 napi_status napi_get_version(napi_env env, uint32_t* result); [in] env : The environment that the API is invoked under. [out] result : The highest version of N-API supported. Returns napi_ok if the API succeeded. This API returns the highest N-API version supported by the Node.js runtime. N-API is planned to be additive such that newer releases of Node.js may support additional API functions. In order to allow an addon to use a newer function when running with versions of Node.js that support it, while providing fallback behavior when running with Node.js versions that don't support it: Call napi_get_version() to determine if the API is available. If available, dynamically load a pointer to the function using uv_dlsym() . Use the dynamically loaded pointer to invoke the function. If the function is not available, provide an alternate implementation that does not use the function. NAPI_EXTERN napi_status napi_adjust_external_memory(napi_env env, int64_t change_in_bytes, int64_t* result); [in] env : The environment that the API is invoked under. [in] change_in_bytes : The change in externally allocated memory that is kept alive by JavaScript objects. [out] result : The adjusted value Returns napi_ok if the API succeeded. This function gives V8 an indication of the amount of externally allocated memory that is kept alive by JavaScript objects (i.e. a JavaScript object that points to its own memory allocated by a native module). Registering externally allocated memory will trigger global garbage collections more often than it would otherwise. N-API provides facilities for creating Promise objects as described in Section 25.4 of the ECMA specification. It implements promises as a pair of objects. When a promise is created by napi_create_promise() , a "deferred" object is created and returned alongside the Promise . The deferred object is bound to the created Promise and is the only means to resolve or reject the Promise using napi_resolve_deferred() or napi_reject_deferred() . The deferred object that is created by napi_create_promise() is freed by napi_resolve_deferred() or napi_reject_deferred() . The Promise object may be returned to JavaScript where it can be used in the usual fashion. For example, to create a promise and pass it to an asynchronous worker: napi_deferred deferred; napi_value promise; napi_status status; // Create the promise. status = napi_create_promise(env, &deferred, &promise); if (status != napi_ok) return NULL; Memory Management napi_adjust_external_memory Promises 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 148/739 // Pass the deferred to a function that performs an asynchronous action. do_something_asynchronous(deferred); // Return the promise to JS return promise; The above function do_something_asynchronous() would perform its asynchronous action and then it would resolve or reject the deferred, thereby concluding the promise and freeing the deferred: napi_deferred deferred; napi_value undefined; napi_status status; // Create a value with which to conclude the deferred. status = napi_get_undefined(env, &undefined); if (status != napi_ok) return NULL; // Resolve or reject the promise associated with the deferred depending on // whether the asynchronous action succeeded. if (asynchronous_action_succeeded) { status = napi_resolve_deferred(env, deferred, undefined); } else { status = napi_reject_deferred(env, deferred, undefined); } if (status != napi_ok) return NULL; // At this point the deferred has been freed, so we should assign NULL to it. deferred = NULL; napi_status napi_create_promise(napi_env env, napi_deferred* deferred, napi_value* promise); [in] env : The environment that the API is invoked under. [out] deferred : A newly created deferred object which can later be passed to napi_resolve_deferred() or napi_reject_deferred() to resolve resp. reject the associated promise. [out] promise : The JavaScript promise associated with the deferred object. Returns napi_ok if the API succeeded. This API creates a deferred object and a JavaScript promise. napi_status napi_resolve_deferred(napi_env env, napi_deferred deferred, napi_value resolution); napi_create_promise napi_resolve_deferred 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 149/739 [in] env : The environment that the API is invoked under. [in] deferred : The deferred object whose associated promise to resolve. [in] resolution : The value with which to resolve the promise. This API resolves a JavaScript promise by way of the deferred object with which it is associated. Thus, it can only be used to resolve JavaScript promises for which the corresponding deferred object is available. This effectively means that the promise must have been created using napi_create_promise() and the deferred object returned from that call must have been retained in order to be passed to this API. The deferred object is freed upon successful completion. napi_status napi_reject_deferred(napi_env env, napi_deferred deferred, napi_value rejection); [in] env : The environment that the API is invoked under. [in] deferred : The deferred object whose associated promise to resolve. [in] rejection : The value with which to reject the promise. This API rejects a JavaScript promise by way of the deferred object with which it is associated. Thus, it can only be used to reject JavaScript promises for which the corresponding deferred object is available. This effectively means that the promise must have been created using napi_create_promise() and the deferred object returned from that call must have been retained in order to be passed to this API. The deferred object is freed upon successful completion. napi_status napi_is_promise(napi_env env, napi_value promise, bool* is_promise); [in] env : The environment that the API is invoked under. [in] promise : The promise to examine [out] is_promise : Flag indicating whether promise is a native promise object - that is, a promise object created by the underlying engine. N-API provides an API for executing a string containing JavaScript using the underlying JavaScript engine. NAPI_EXTERN napi_status napi_run_script(napi_env env, napi_value script, napi_value* result); [in] env : The environment that the API is invoked under. [in] script : A JavaScript string containing the script to execute. [out] result : The value resulting from having executed the script. napi_reject_deferred napi_is_promise Script execution napi_run_script 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 150/739 N-API provides a function for getting the current event loop associated with a specific napi_env . NAPI_EXTERN napi_status napi_get_uv_event_loop(napi_env env, uv_loop_t** loop); [in] env : The environment that the API is invoked under. [out] loop : The current libuv loop instance. Stability: 1 - Experimental JavaScript functions can normally only be called from a native addon's main thread. If an addon creates additional threads, then N-API functions that require a napi_env , napi_value , or napi_ref must not be called from those threads. When an addon has additional threads and JavaScript functions need to be invoked based on the processing completed by those threads, those threads must communicate with the addon's main thread so that the main thread can invoke the JavaScript function on their behalf. The thread-safe function APIs provide an easy way to do this. These APIs provide the type napi_threadsafe_function as well as APIs to create, destroy, and call objects of this type. napi_create_threadsafe_function() creates a persistent reference to a napi_value that holds a JavaScript function which can be called from multiple threads. The calls happen asynchronously. This means that values with which the JavaScript callback is to be called will be placed in a queue, and, for each value in the queue, a call will eventually be made to the JavaScript function. Upon creation of a napi_threadsafe_function a napi_finalize callback can be provided. This callback will be invoked on the main thread when the thread-safe function is about to be destroyed. It receives the context and the finalize data given during construction, and provides an opportunity for cleaning up after the threads e.g. by calling uv_thread_join() . It is important that, aside from the main loop thread, there be no threads left using the thread-safe function after the finalize callback completes. The context given during the call to napi_create_threadsafe_function() can be retrieved from any thread with a call to napi_get_threadsafe_function_context() . napi_call_threadsafe_function() can then be used for initiating a call into JavaScript. napi_call_threadsafe_function() accepts a parameter which controls whether the API behaves blockingly. If set to napi_tsfn_nonblocking , the API behaves non-blockingly, returning napi_queue_full if the queue was full, preventing data from being successfully added to the queue. If set to napi_tsfn_blocking , the API blocks until space becomes available in the queue. napi_call_threadsafe_function() never blocks if the thread-safe function was created with a maximum queue size of 0. The actual call into JavaScript is controlled by the callback given via the call_js_cb parameter. call_js_cb is invoked on the main thread once for each value that was placed into the queue by a successful call to napi_call_threadsafe_function() . If such a callback is not given, a default callback will be used, and the resulting JavaScript call will have no arguments. The call_js_cb callback receives the JavaScript function to call as a napi_value in its parameters, as well as the void* context pointer used when creating the napi_threadsafe_function , and the next data pointer that was created by one of the secondary threads. The callback can then use an API such as napi_call_function() to call into JavaScript. The callback may also be invoked with env and call_js_cb both set to NULL to indicate that calls into JavaScript are no longer possible, while items remain in the queue that may need to be freed. This normally occurs when the Node.js process exits while there is a thread-safe function still active. libuv event loop napi_get_uv_event_loop Asynchronous Thread-safe Function Calls 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 151/739 It is not necessary to call into JavaScript via napi_make_callback() because N-API runs call_js_cb in a context appropriate for callbacks. Threads can be added to and removed from a napi_threadsafe_function object during its existence. Thus, in addition to specifying an initial number of threads upon creation, napi_acquire_threadsafe_function can be called to indicate that a new thread will start making use of the thread-safe function. Similarly, napi_release_threadsafe_function can be called to indicate that an existing thread will stop making use of the thread-safe function. napi_threadsafe_function objects are destroyed when every thread which uses the object has called napi_release_threadsafe_function() or has received a return status of napi_closing in response to a call to napi_call_threadsafe_function . The queue is emptied before the napi_threadsafe_function is destroyed. It is important that napi_release_threadsafe_function() be the last API call made in conjunction with a given napi_threadsafe_function , because after the call completes, there is no guarantee that the napi_threadsafe_function is still allocated. For the same reason it is also important that no more use be made of a thread-safe function after receiving a return value of napi_closing in response to a call to napi_call_threadsafe_function . Data associated with the napi_threadsafe_function can be freed in its napi_finalize callback which was passed to napi_create_threadsafe_function() . Once the number of threads making use of a napi_threadsafe_function reaches zero, no further threads can start making use of it by calling napi_acquire_threadsafe_function() . In fact, all subsequent API calls associated with it, except napi_release_threadsafe_function() , will return an error value of napi_closing . The thread-safe function can be "aborted" by giving a value of napi_tsfn_abort to napi_release_threadsafe_function() . This will cause all subsequent APIs associated with the thread-safe function except napi_release_threadsafe_function() to return napi_closing even before its reference count reaches zero. In particular, napi_call_threadsafe_function() will return napi_closing , thus informing the threads that it is no longer possible to make asynchronous calls to the thread-safe function. This can be used as a criterion for terminating the thread. Upon receiving a return value of napi_closing from napi_call_threadsafe_function() a thread must make no further use of the thread-safe function because it is no longer guaranteed to be allocated. Similarly to libuv handles, thread-safe functions can be "referenced" and "unreferenced". A "referenced" thread-safe function will cause the event loop on the thread on which it is created to remain alive until the thread-safe function is destroyed. In contrast, an "unreferenced" thread-safe function will not prevent the event loop from exiting. The APIs napi_ref_threadsafe_function and napi_unref_threadsafe_function exist for this purpose. Stability: 2 - Stable NAPI_EXTERN napi_status napi_create_threadsafe_function(napi_env env, napi_value func, napi_value async_resource, napi_value async_resource_name, size_t max_queue_size, size_t initial_thread_count, void* thread_finalize_data, napi_finalize thread_finalize_cb, void* context, napi_threadsafe_function_call_js call_js_cb, napi_threadsafe_function* result); [in] env : The environment that the API is invoked under. [in] func : The JavaScript function to call from another thread. napi_create_threadsafe_function 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 152/739 [in] async_resource : An optional object associated with the async work that will be passed to possible async_hooks init hooks . [in] async_resource_name : A JavaScript string to provide an identifier for the kind of resource that is being provided for diagnostic information exposed by the async_hooks API. [in] max_queue_size : Maximum size of the queue. 0 for no limit. [in] initial_thread_count : The initial number of threads, including the main thread, which will be making use of this function. [in] thread_finalize_data : Optional data to be passed to thread_finalize_cb . [in] thread_finalize_cb : Optional function to call when the napi_threadsafe_function is being destroyed. [in] context : Optional data to attach to the resulting napi_threadsafe_function . [in] call_js_cb : Optional callback which calls the JavaScript function in response to a call on a different thread. This callback will be called on the main thread. If not given, the JavaScript function will be called with no parameters and with undefined as its this value. [out] result : The asynchronous thread-safe JavaScript function. Stability: 2 - Stable NAPI_EXTERN napi_status napi_get_threadsafe_function_context(napi_threadsafe_function func, void** result); [in] func : The thread-safe function for which to retrieve the context. [out] result : The location where to store the context. This API may be called from any thread which makes use of func . Stability: 2 - Stable NAPI_EXTERN napi_status napi_call_threadsafe_function(napi_threadsafe_function func, void* data, napi_threadsafe_function_call_mode is_blocking); [in] func : The asynchronous thread-safe JavaScript function to invoke. [in] data : Data to send into JavaScript via the callback call_js_cb provided during the creation of the thread-safe JavaScript function. [in] is_blocking : Flag whose value can be either napi_tsfn_blocking to indicate that the call should block if the queue is full or napi_tsfn_nonblocking to indicate that the call should return immediately with a status of napi_queue_full whenever the queue is full. This API will return napi_closing if napi_release_threadsafe_function() was called with abort set to napi_tsfn_abort from any thread. The value is only added to the queue if the API returns napi_ok . This API may be called from any thread which makes use of func . napi_get_threadsafe_function_context napi_call_threadsafe_function napi_acquire_threadsafe_function 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 153/739 Stability: 2 - Stable NAPI_EXTERN napi_status napi_acquire_threadsafe_function(napi_threadsafe_function func); [in] func : The asynchronous thread-safe JavaScript function to start making use of. A thread should call this API before passing func to any other thread-safe function APIs to indicate that it will be making use of func . This prevents func from being destroyed when all other threads have stopped making use of it. This API may be called from any thread which will start making use of func . Stability: 2 - Stable NAPI_EXTERN napi_status napi_release_threadsafe_function(napi_threadsafe_function func, napi_threadsafe_function_release_mode mode); [in] func : The asynchronous thread-safe JavaScript function whose reference count to decrement. [in] mode : Flag whose value can be either napi_tsfn_release to indicate that the current thread will make no further calls to the thread-safe function, or napi_tsfn_abort to indicate that in addition to the current thread, no other thread should make any further calls to the thread-safe function. If set to napi_tsfn_abort , further calls to napi_call_threadsafe_function() will return napi_closing , and no further values will be placed in the queue. A thread should call this API when it stops making use of func . Passing func to any thread-safe APIs after having called this API has undefined results, as func may have been destroyed. This API may be called from any thread which will stop making use of func . Stability: 2 - Stable NAPI_EXTERN napi_status napi_ref_threadsafe_function(napi_env env, napi_threadsafe_function func); [in] env : The environment that the API is invoked under. [in] func : The thread-safe function to reference. This API is used to indicate that the event loop running on the main thread should not exit until func has been destroyed. Similar to uv_ref it is also idempotent. This API may only be called from the main thread. napi_release_threadsafe_function napi_ref_threadsafe_function napi_unref_threadsafe_function 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 154/739 Stability: 2 - Stable NAPI_EXTERN napi_status napi_unref_threadsafe_function(napi_env env, napi_threadsafe_function func); [in] env : The environment that the API is invoked under. [in] func : The thread-safe function to unreference. This API is used to indicate that the event loop running on the main thread may exit before func is destroyed. Similar to uv_unref it is also idempotent. This API may only be called from the main thread. Stability: 2 - Stable The child_process module provides the ability to spawn child processes in a manner that is similar, but not identical, to popen(3) . This capability is primarily provided by the child_process.spawn() function: const { spawn } = require('child_process'); const ls = spawn('ls', ['-lh', '/usr']); ls.stdout.on('data', (data) => { console.log(`stdout: ${data}`); }); ls.stderr.on('data', (data) => { console.log(`stderr: ${data}`); }); ls.on('close', (code) => { console.log(`child process exited with code ${code}`); }); By default, pipes for stdin , stdout , and stderr are established between the parent Node.js process and the spawned child. These pipes have limited (and platform-specific) capacity. If the child process writes to stdout in excess of that limit without the output being captured, the child process will block waiting for the pipe buffer to accept more data. This is identical to the behavior of pipes in the shell. Use the { stdio: 'ignore' } option if the output will not be consumed. The child_process.spawn() method spawns the child process asynchronously, without blocking the Node.js event loop. The child_process.spawnSync() function provides equivalent functionality in a synchronous manner that blocks the event loop until the spawned process either exits or is terminated. For convenience, the child_process module provides a handful of synchronous and asynchronous alternatives to child_process.spawn() and child_process.spawnSync() . Note that each of these alternatives are implemented on top of child_process.spawn() or child_process.spawnSync() . Child Process 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 155/739 child_process.exec() : spawns a shell and runs a command within that shell, passing the stdout and stderr to a callback function when complete. child_process.execFile() : similar to child_process.exec() except that it spawns the command directly without first spawning a shell by default. child_process.fork() : spawns a new Node.js process and invokes a specified module with an IPC communication channel established that allows sending messages between parent and child. child_process.execSync() : a synchronous version of child_process.exec() that will block the Node.js event loop. child_process.execFileSync() : a synchronous version of child_process.execFile() that will block the Node.js event loop. For certain use cases, such as automating shell scripts, the synchronous counterparts may be more convenient. In many cases, however, the synchronous methods can have significant impact on performance due to stalling the event loop while spawned processes complete. The child_process.spawn() , child_process.fork() , child_process.exec() , and child_process.execFile() methods all follow the idiomatic asynchronous programming pattern typical of other Node.js APIs. Each of the methods returns a ChildProcess instance. These objects implement the Node.js EventEmitter API, allowing the parent process to register listener functions that are called when certain events occur during the life cycle of the child process. The child_process.exec() and child_process.execFile() methods additionally allow for an optional callback function to be specified that is invoked when the child process terminates. The importance of the distinction between child_process.exec() and child_process.execFile() can vary based on platform. On Unix- type operating systems (Unix, Linux, macOS) child_process.execFile() can be more efficient because it does not spawn a shell by default. On Windows, however, .bat and .cmd files are not executable on their own without a terminal, and therefore cannot be launched using child_process.execFile() . When running on Windows, .bat and .cmd files can be invoked using child_process.spawn() with the shell option set, with child_process.exec() , or by spawning cmd.exe and passing the .bat or .cmd file as an argument (which is what the shell option and child_process.exec() do). In any case, if the script filename contains spaces it needs to be quoted. // On Windows Only ... const { spawn } = require('child_process'); const bat = spawn('cmd.exe', ['/c', 'my.bat']); bat.stdout.on('data', (data) => { console.log(data.toString()); }); bat.stderr.on('data', (data) => { console.log(data.toString()); }); bat.on('exit', (code) => { console.log(`Child exited with code ${code}`); }); // OR... const { exec } = require('child_process'); exec('my.bat', (err, stdout, stderr) => { if (err) { console.error(err); Asynchronous Process Creation Spawning .bat and .cmd files on Windows 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 156/739 return; } console.log(stdout); }); // Script with spaces in the filename: const bat = spawn('"my script.cmd"', ['a', 'b'], { shell: true }); // or: exec('"my script.cmd" a b', (err, stdout, stderr) => { // ... }); command <string> The command to run, with space-separated arguments. options <Object> cwd <string> Current working directory of the child process. Default: null . env <Object> Environment key-value pairs. Default: null . encoding <string> Default: 'utf8' shell <string> Shell to execute the command with. See Shell Requirements and Default Windows Shell . Default: '/bin/sh' on UNIX, process.env.ComSpec on Windows. timeout <number> Default: 0 maxBuffer <number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at maxBuffer and Unicode . Default: 200 * 1024 . killSignal <string> | <integer> Default: 'SIGTERM' uid <number> Sets the user identity of the process (see setuid(2) ). gid <number> Sets the group identity of the process (see setgid(2) ). windowsHide <boolean> Hide the subprocess console window that would normally be created on Windows systems. Default: false . callback <Function> called with the output when process terminates. error <Error> stdout <string> | <Buffer> stderr <string> | <Buffer> Returns: <ChildProcess> Spawns a shell then executes the command within that shell, buffering any generated output. The command string passed to the exec function is processed directly by the shell and special characters (vary based on shell ) need to be dealt with accordingly: exec('"/path/to/test file/test.sh" arg1 arg2'); // Double quotes are used so that the space in the path is not interpreted as // multiple arguments exec('echo "The \\$HOME variable is $HOME"'); // The $HOME variable is escaped in the first instance, but not in the second child_process.exec(command[, options][, callback]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 157/739 Never pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution. If a callback function is provided, it is called with the arguments (error, stdout, stderr) . On success, error will be null . On error, error will be an instance of Error . The error.code property will be the exit code of the child process while error.signal will be set to the signal that terminated the process. Any exit code other than 0 is considered to be an error. The stdout and stderr arguments passed to the callback will contain the stdout and stderr output of the child process. By default, Node.js will decode the output as UTF-8 and pass strings to the callback. The encoding option can be used to specify the character encoding used to decode the stdout and stderr output. If encoding is 'buffer' , or an unrecognized character encoding, Buffer objects will be passed to the callback instead. const { exec } = require('child_process'); exec('cat *.js missing_file | wc -l', (error, stdout, stderr) => { if (error) { console.error(`exec error: ${error}`); return; } console.log(`stdout: ${stdout}`); console.log(`stderr: ${stderr}`); }); If timeout is greater than 0 , the parent will send the signal identified by the killSignal property (the default is 'SIGTERM' ) if the child runs longer than timeout milliseconds. Unlike the exec(3) POSIX system call, child_process.exec() does not replace the existing process and uses a shell to execute the command. If this method is invoked as its util.promisify() ed version, it returns a Promise for an Object with stdout and stderr properties. In case of an error (including any error resulting in an exit code other than 0), a rejected promise is returned, with the same error object given in the callback, but with an additional two properties stdout and stderr . const util = require('util'); const exec = util.promisify(require('child_process').exec); async function lsExample() { const { stdout, stderr } = await exec('ls'); console.log('stdout:', stdout); console.log('stderr:', stderr); } lsExample(); file <string> The name or path of the executable file to run. args <string[]> List of string arguments. options <Object> cwd <string> Current working directory of the child process. env <Object> Environment key-value pairs. encoding <string> Default: 'utf8' timeout <number> Default: 0 child_process.execFile(file[, args][, options][, callback]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 158/739 maxBuffer <number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at maxBuffer and Unicode . Default: 200 * 1024 . killSignal <string> | <integer> Default: 'SIGTERM' uid <number> Sets the user identity of the process (see setuid(2) ). gid <number> Sets the group identity of the process (see setgid(2) ). windowsHide <boolean> Hide the subprocess console window that would normally be created on Windows systems. Default: false . windowsVerbatimArguments <boolean> No quoting or escaping of arguments is done on Windows. Ignored on Unix. Default: false . shell <boolean> | <string> If true , runs command inside of a shell. Uses '/bin/sh' on UNIX, and process.env.ComSpec on Windows. A different shell can be specified as a string. See Shell Requirements and Default Windows Shell . Default: false (no shell). callback <Function> Called with the output when process terminates. error <Error> stdout <string> | <Buffer> stderr <string> | <Buffer> Returns: <ChildProcess> The child_process.execFile() function is similar to child_process.exec() except that it does not spawn a shell by default. Rather, the specified executable file is spawned directly as a new process making it slightly more efficient than child_process.exec() . The same options as child_process.exec() are supported. Since a shell is not spawned, behaviors such as I/O redirection and file globbing are not supported. const { execFile } = require('child_process'); const child = execFile('node', ['--version'], (error, stdout, stderr) => { if (error) { throw error; } console.log(stdout); }); The stdout and stderr arguments passed to the callback will contain the stdout and stderr output of the child process. By default, Node.js will decode the output as UTF-8 and pass strings to the callback. The encoding option can be used to specify the character encoding used to decode the stdout and stderr output. If encoding is 'buffer' , or an unrecognized character encoding, Buffer objects will be passed to the callback instead. If this method is invoked as its util.promisify() ed version, it returns a Promise for an Object with stdout and stderr properties. In case of an error (including any error resulting in an exit code other than 0), a rejected promise is returned, with the same error object given in the callback, but with an additional two properties stdout and stderr . const util = require('util'); const execFile = util.promisify(require('child_process').execFile); async function getVersion() { const { stdout } = await execFile('node', ['--version']); console.log(stdout); } getVersion(); 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 159/739 If the shell option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution. modulePath <string> The module to run in the child. args <string[]> List of string arguments. options <Object> cwd <string> Current working directory of the child process. detached <boolean> Prepare child to run independently of its parent process. Specific behavior depends on the platform, see options.detached ). env <Object> Environment key-value pairs. execPath <string> Executable used to create the child process. execArgv <string[]> List of string arguments passed to the executable. Default: process.execArgv . silent <boolean> If true , stdin, stdout, and stderr of the child will be piped to the parent, otherwise they will be inherited from the parent, see the 'pipe' and 'inherit' options for child_process.spawn() 's stdio for more details. Default: false . stdio <Array> | <string> See child_process.spawn() 's stdio . When this option is provided, it overrides silent . If the array variant is used, it must contain exactly one item with value 'ipc' or an error will be thrown. For instance [0, 1, 2, 'ipc'] . windowsVerbatimArguments <boolean> No quoting or escaping of arguments is done on Windows. Ignored on Unix. Default: false . uid <number> Sets the user identity of the process (see setuid(2) ). gid <number> Sets the group identity of the process (see setgid(2) ). Returns: <ChildProcess> The child_process.fork() method is a special case of child_process.spawn() used specifically to spawn new Node.js processes. Like child_process.spawn() , a ChildProcess object is returned. The returned ChildProcess will have an additional communication channel built-in that allows messages to be passed back and forth between the parent and child. See subprocess.send() for details. It is important to keep in mind that spawned Node.js child processes are independent of the parent with exception of the IPC communication channel that is established between the two. Each process has its own memory, with their own V8 instances. Because of the additional resource allocations required, spawning a large number of child Node.js processes is not recommended. By default, child_process.fork() will spawn new Node.js instances using the process.execPath of the parent process. The execPath property in the options object allows for an alternative execution path to be used. Node.js processes launched with a custom execPath will communicate with the parent process using the file descriptor (fd) identified using the environment variable NODE_CHANNEL_FD on the child process. Unlike the fork(2) POSIX system call, child_process.fork() does not clone the current process. The shell option available in child_process.spawn() is not supported by child_process.fork() and will be ignored if set. command <string> The command to run. args <string[]> List of string arguments. options <Object> cwd <string> Current working directory of the child process. env <Object> Environment key-value pairs. argv0 <string> Explicitly set the value of argv[0] sent to the child process. This will be set to command if not specified. child_process.fork(modulePath[, args][, options]) child_process.spawn(command[, args][, options]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 160/739 stdio <Array> | <string> Child's stdio configuration (see options.stdio ). detached <boolean> Prepare child to run independently of its parent process. Specific behavior depends on the platform, see options.detached ). uid <number> Sets the user identity of the process (see setuid(2) ). gid <number> Sets the group identity of the process (see setgid(2) ). shell <boolean> | <string> If true , runs command inside of a shell. Uses '/bin/sh' on UNIX, and process.env.ComSpec on Windows. A different shell can be specified as a string. See Shell Requirements and Default Windows Shell . Default: false (no shell). windowsVerbatimArguments <boolean> No quoting or escaping of arguments is done on Windows. Ignored on Unix. This is set to true automatically when shell is specified. Default: false . windowsHide <boolean> Hide the subprocess console window that would normally be created on Windows systems. Default: false . Returns: <ChildProcess> The child_process.spawn() method spawns a new process using the given command , with command line arguments in args . If omitted, args defaults to an empty array. If the shell option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution. A third argument may be used to specify additional options, with these defaults: const defaults = { cwd: undefined, env: process.env }; Use cwd to specify the working directory from which the process is spawned. If not given, the default is to inherit the current working directory. Use env to specify environment variables that will be visible to the new process, the default is process.env . undefined values in env will be ignored. Example of running ls -lh /usr , capturing stdout , stderr , and the exit code: const { spawn } = require('child_process'); const ls = spawn('ls', ['-lh', '/usr']); ls.stdout.on('data', (data) => { console.log(`stdout: ${data}`); }); ls.stderr.on('data', (data) => { console.log(`stderr: ${data}`); }); ls.on('close', (code) => { console.log(`child process exited with code ${code}`); }); Example: A very elaborate way to run ps ax | grep ssh 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 161/739 const { spawn } = require('child_process'); const ps = spawn('ps', ['ax']); const grep = spawn('grep', ['ssh']); ps.stdout.on('data', (data) => { grep.stdin.write(data); }); ps.stderr.on('data', (data) => { console.log(`ps stderr: ${data}`); }); ps.on('close', (code) => { if (code !== 0) { console.log(`ps process exited with code ${code}`); } grep.stdin.end(); }); grep.stdout.on('data', (data) => { console.log(data.toString()); }); grep.stderr.on('data', (data) => { console.log(`grep stderr: ${data}`); }); grep.on('close', (code) => { if (code !== 0) { console.log(`grep process exited with code ${code}`); } }); Example of checking for failed spawn : const { spawn } = require('child_process'); const subprocess = spawn('bad_command'); subprocess.on('error', (err) => { console.log('Failed to start subprocess.'); }); Certain platforms (macOS, Linux) will use the value of argv[0] for the process title while others (Windows, SunOS) will use command . Node.js currently overwrites argv[0] with process.execPath on startup, so process.argv[0] in a Node.js child process will not match the argv0 parameter passed to spawn from the parent, retrieve it with the process.argv0 property instead. On Windows, setting options.detached to true makes it possible for the child process to continue running after the parent exits. The child will have its own console window. Once enabled for a child process, it cannot be disabled. options.detached 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 162/739 On non-Windows platforms, if options.detached is set to true , the child process will be made the leader of a new process group and session. Note that child processes may continue running after the parent exits regardless of whether they are detached or not. See setsid(2) for more information. By default, the parent will wait for the detached child to exit. To prevent the parent from waiting for a given subprocess to exit, use the subprocess.unref() method. Doing so will cause the parent's event loop to not include the child in its reference count, allowing the parent to exit independently of the child, unless there is an established IPC channel between the child and the parent. When using the detached option to start a long-running process, the process will not stay running in the background after the parent exits unless it is provided with a stdio configuration that is not connected to the parent. If the parent's stdio is inherited, the child will remain attached to the controlling terminal. Example of a long-running process, by detaching and also ignoring its parent stdio file descriptors, in order to ignore the parent's termination: const { spawn } = require('child_process'); const subprocess = spawn(process.argv[0], ['child_program.js'], { detached: true, stdio: 'ignore' }); subprocess.unref(); Alternatively one can redirect the child process' output into files: const fs = require('fs'); const { spawn } = require('child_process'); const out = fs.openSync('./out.log', 'a'); const err = fs.openSync('./out.log', 'a'); const subprocess = spawn('prg', [], { detached: true, stdio: [ 'ignore', out, err ] }); subprocess.unref(); The options.stdio option is used to configure the pipes that are established between the parent and child process. By default, the child's stdin, stdout, and stderr are redirected to corresponding subprocess.stdin , subprocess.stdout , and subprocess.stderr streams on the ChildProcess object. This is equivalent to setting the options.stdio equal to ['pipe', 'pipe', 'pipe'] . For convenience, options.stdio may be one of the following strings: 'pipe' - equivalent to ['pipe', 'pipe', 'pipe'] (the default) 'ignore' - equivalent to ['ignore', 'ignore', 'ignore'] 'inherit' - equivalent to ['inherit', 'inherit', 'inherit'] or [0, 1, 2] Otherwise, the value of options.stdio is an array where each index corresponds to an fd in the child. The fds 0, 1, and 2 correspond to stdin, stdout, and stderr, respectively. Additional fds can be specified to create additional pipes between the parent and child. The value is one of the following: options.stdio 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 163/739 1. 'pipe' - Create a pipe between the child process and the parent process. The parent end of the pipe is exposed to the parent as a property on the child_process object as subprocess.stdio[fd] . Pipes created for fds 0 - 2 are also available as subprocess.stdin , subprocess.stdout and subprocess.stderr , respectively. 2. 'ipc' - Create an IPC channel for passing messages/file descriptors between parent and child. A ChildProcess may have at most one IPC stdio file descriptor. Setting this option enables the subprocess.send() method. If the child is a Node.js process, the presence of an IPC channel will enable process.send() and process.disconnect() methods, as well as 'disconnect' and 'message' events within the child. Accessing the IPC channel fd in any way other than process.send() or using the IPC channel with a child process that is not a Node.js instance is not supported. 3. 'ignore' - Instructs Node.js to ignore the fd in the child. While Node.js will always open fds 0 - 2 for the processes it spawns, setting the fd to 'ignore' will cause Node.js to open /dev/null and attach it to the child's fd. 4. 'inherit' - Pass through the corresponding stdio stream to/from the parent process. In the first three positions, this is equivalent to process.stdin , process.stdout , and process.stderr , respectively. In any other position, equivalent to 'ignore' . 5. <Stream> object - Share a readable or writable stream that refers to a tty, file, socket, or a pipe with the child process. The stream's underlying file descriptor is duplicated in the child process to the fd that corresponds to the index in the stdio array. Note that the stream must have an underlying descriptor (file streams do not until the 'open' event has occurred). 6. Positive integer - The integer value is interpreted as a file descriptor that is currently open in the parent process. It is shared with the child process, similar to how <Stream> objects can be shared. Passing sockets is not supported on Windows. 7. null , undefined - Use default value. For stdio fds 0, 1, and 2 (in other words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the default is 'ignore' . const { spawn } = require('child_process'); // Child will use parent's stdios spawn('prg', [], { stdio: 'inherit' }); // Spawn child sharing only stderr spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] }); // Open an extra fd=4, to interact with programs presenting a // startd-style interface. spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] }); It is worth noting that when an IPC channel is established between the parent and child processes, and the child is a Node.js process, the child is launched with the IPC channel unreferenced (using unref() ) until the child registers an event handler for the 'disconnect' event or the 'message' event. This allows the child to exit normally without the process being held open by the open IPC channel. On UNIX-like operating systems, the child_process.spawn() method performs memory operations synchronously before decoupling the event loop from the child. Applications with a large memory footprint may find frequent child_process.spawn() calls to be a bottleneck. For more information, see V8 issue 7381 . See also: child_process.exec() and child_process.fork() . The child_process.spawnSync() , child_process.execSync() , and child_process.execFileSync() methods are synchronous and WILL block the Node.js event loop, pausing execution of any additional code until the spawned process exits. Synchronous Process Creation 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 164/739 Blocking calls like these are mostly useful for simplifying general-purpose scripting tasks and for simplifying the loading/processing of application configuration at startup. file <string> The name or path of the executable file to run. args <string[]> List of string arguments. options <Object> cwd <string> Current working directory of the child process. input <string> | <Buffer> | <TypedArray> | <DataView> The value which will be passed as stdin to the spawned process. Supplying this value will override stdio[0] . stdio <string> | <Array> Child's stdio configuration. stderr by default will be output to the parent process' stderr unless stdio is specified. Default: 'pipe' . env <Object> Environment key-value pairs. uid <number> Sets the user identity of the process (see setuid(2) ). gid <number> Sets the group identity of the process (see setgid(2) ). timeout <number> In milliseconds the maximum amount of time the process is allowed to run. Default: undefined . killSignal <string> | <integer> The signal value to be used when the spawned process will be killed. Default: 'SIGTERM' . maxBuffer <number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated. See caveat at maxBuffer and Unicode . Default: 200 * 1024 . encoding <string> The encoding used for all stdio inputs and outputs. Default: 'buffer' . windowsHide <boolean> Hide the subprocess console window that would normally be created on Windows systems. Default: false . shell <boolean> | <string> If true , runs command inside of a shell. Uses '/bin/sh' on UNIX, and process.env.ComSpec on Windows. A different shell can be specified as a string. See Shell Requirements and Default Windows Shell . Default: false (no shell). Returns: <Buffer> | <string> The stdout from the command. The child_process.execFileSync() method is generally identical to child_process.execFile() with the exception that the method will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. If the child process intercepts and handles the SIGTERM signal and does not exit, the parent process will still wait until the child process has exited. If the process times out or has a non-zero exit code, this method will throw an Error that will include the full result of the underlying child_process.spawnSync() . If the shell option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution. command <string> The command to run. options <Object> cwd <string> Current working directory of the child process. input <string> | <Buffer> | <TypedArray> | <DataView> The value which will be passed as stdin to the spawned process. Supplying this value will override stdio[0] . child_process.execFileSync(file[, args][, options]) child_process.execSync(command[, options]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 165/739 stdio <string> | <Array> Child's stdio configuration. stderr by default will be output to the parent process' stderr unless stdio is specified. Default: 'pipe' . env <Object> Environment key-value pairs. shell <string> Shell to execute the command with. See Shell Requirements and Default Windows Shell . Default: '/bin/sh' on UNIX, process.env.ComSpec on Windows. uid <number> Sets the user identity of the process. (See setuid(2) ). gid <number> Sets the group identity of the process. (See setgid(2) ). timeout <number> In milliseconds the maximum amount of time the process is allowed to run. Default: undefined . killSignal <string> | <integer> The signal value to be used when the spawned process will be killed. Default: 'SIGTERM' . maxBuffer <number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at maxBuffer and Unicode . Default: 200 * 1024 . encoding <string> The encoding used for all stdio inputs and outputs. Default: 'buffer' . windowsHide <boolean> Hide the subprocess console window that would normally be created on Windows systems. Default: false . Returns: <Buffer> | <string> The stdout from the command. The child_process.execSync() method is generally identical to child_process.exec() with the exception that the method will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. Note that if the child process intercepts and handles the SIGTERM signal and doesn't exit, the parent process will wait until the child process has exited. If the process times out or has a non-zero exit code, this method will throw. The Error object will contain the entire result from child_process.spawnSync() . Never pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution. command <string> The command to run. args <string[]> List of string arguments. options <Object> cwd <string> Current working directory of the child process. input <string> | <Buffer> | <TypedArray> | <DataView> The value which will be passed as stdin to the spawned process. Supplying this value will override stdio[0] . argv0 <string> Explicitly set the value of argv[0] sent to the child process. This will be set to command if not specified. stdio <string> | <Array> Child's stdio configuration. env <Object> Environment key-value pairs. uid <number> Sets the user identity of the process (see setuid(2) ). gid <number> Sets the group identity of the process (see setgid(2) ). timeout <number> In milliseconds the maximum amount of time the process is allowed to run. Default: undefined . killSignal <string> | <integer> The signal value to be used when the spawned process will be killed. Default: 'SIGTERM' . maxBuffer <number> Largest amount of data in bytes allowed on stdout or stderr. If exceeded, the child process is terminated and any output is truncated. See caveat at maxBuffer and Unicode . Default: 200 * 1024 . encoding <string> The encoding used for all stdio inputs and outputs. Default: 'buffer' . child_process.spawnSync(command[, args][, options]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 166/739 shell <boolean> | <string> If true , runs command inside of a shell. Uses '/bin/sh' on UNIX, and process.env.ComSpec on Windows. A different shell can be specified as a string. See Shell Requirements and Default Windows Shell . Default: false (no shell). windowsVerbatimArguments <boolean> No quoting or escaping of arguments is done on Windows. Ignored on Unix. This is set to true automatically when shell is specified. Default: false . windowsHide <boolean> Hide the subprocess console window that would normally be created on Windows systems. Default: false . Returns: <Object> pid <number> Pid of the child process. output <Array> Array of results from stdio output. stdout <Buffer> | <string> The contents of output[1] . stderr <Buffer> | <string> The contents of output[2] . status <number> | <null> The exit code of the subprocess, or null if the subprocess terminated due to a signal. signal <string> | <null> The signal used to kill the subprocess, or null if the subprocess did not terminate due to a signal. error <Error> The error object if the child process failed or timed out. The child_process.spawnSync() method is generally identical to child_process.spawn() with the exception that the function will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. Note that if the process intercepts and handles the SIGTERM signal and doesn't exit, the parent process will wait until the child process has exited. If the shell option is enabled, do not pass unsanitized user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution. Instances of the ChildProcess class are EventEmitters that represent spawned child processes. Instances of ChildProcess are not intended to be created directly. Rather, use the child_process.spawn() , child_process.exec() , child_process.execFile() , or child_process.fork() methods to create instances of ChildProcess . code <number> The exit code if the child exited on its own. signal <string> The signal by which the child process was terminated. The 'close' event is emitted when the stdio streams of a child process have been closed. This is distinct from the 'exit' event, since multiple processes might share the same stdio streams. The 'disconnect' event is emitted after calling the subprocess.disconnect() method in parent process or process.disconnect() in child process. After disconnecting it is no longer possible to send or receive messages, and the subprocess.connected property is false . err <Error> The error. The 'error' event is emitted whenever: 1. The process could not be spawned, or 2. The process could not be killed, or Class: ChildProcess Event: 'close' Event: 'disconnect' Event: 'error' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 167/739 3. Sending a message to the child process failed. The 'exit' event may or may not fire after an error has occurred. When listening to both the 'exit' and 'error' events, it is important to guard against accidentally invoking handler functions multiple times. See also subprocess.kill() and subprocess.send() . code <number> The exit code if the child exited on its own. signal <string> The signal by which the child process was terminated. The 'exit' event is emitted after the child process ends. If the process exited, code is the final exit code of the process, otherwise null . If the process terminated due to receipt of a signal, signal is the string name of the signal, otherwise null . One of the two will always be non- null. Note that when the 'exit' event is triggered, child process stdio streams might still be open. Also, note that Node.js establishes signal handlers for SIGINT and SIGTERM and Node.js processes will not terminate immediately due to receipt of those signals. Rather, Node.js will perform a sequence of cleanup actions and then will re-raise the handled signal. See waitpid(2) . message <Object> A parsed JSON object or primitive value. sendHandle <Handle> A net.Socket or net.Server object, or undefined. The 'message' event is triggered when a child process uses process.send() to send messages. The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent. <Object> A pipe representing the IPC channel to the child process. The subprocess.channel property is a reference to the child's IPC channel. If no IPC channel currently exists, this property is undefined . <boolean> Set to false after subprocess.disconnect() is called. The subprocess.connected property indicates whether it is still possible to send and receive messages from a child process. When subprocess.connected is false , it is no longer possible to send or receive messages. Closes the IPC channel between parent and child, allowing the child to exit gracefully once there are no other connections keeping it alive. After calling this method the subprocess.connected and process.connected properties in both the parent and child (respectively) will be set to false , and it will be no longer possible to pass messages between the processes. The 'disconnect' event will be emitted when there are no messages in the process of being received. This will most often be triggered immediately after calling subprocess.disconnect() . Note that when the child process is a Node.js instance (e.g. spawned using child_process.fork() ), the process.disconnect() method can be invoked within the child process to close the IPC channel as well. Event: 'exit' Event: 'message' subprocess.channel subprocess.connected subprocess.disconnect() subprocess.kill([signal]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 168/739 signal <string> The subprocess.kill() method sends a signal to the child process. If no argument is given, the process will be sent the 'SIGTERM' signal. See signal(7) for a list of available signals. const { spawn } = require('child_process'); const grep = spawn('grep', ['ssh']); grep.on('close', (code, signal) => { console.log( `child process terminated due to receipt of signal ${signal}`); }); // Send SIGHUP to process grep.kill('SIGHUP'); The ChildProcess object may emit an 'error' event if the signal cannot be delivered. Sending a signal to a child process that has already exited is not an error but may have unforeseen consequences. Specifically, if the process identifier (PID) has been reassigned to another process, the signal will be delivered to that process instead which can have unexpected results. Note that while the function is called kill , the signal delivered to the child process may not actually terminate the process. See kill(2) for reference. On Linux, child processes of child processes will not be terminated when attempting to kill their parent. This is likely to happen when running a new process in a shell or with the use of the shell option of ChildProcess : 'use strict'; const { spawn } = require('child_process'); const subprocess = spawn( 'sh', [ '-c', `node -e "setInterval(() => { console.log(process.pid, 'is alive') }, 500);"` ], { stdio: ['inherit', 'inherit', 'inherit'] } ); setTimeout(() => { subprocess.kill(); // does not terminate the node process in the shell }, 2000); <boolean> Set to true after subprocess.kill() is used to successfully send a signal to the child process. The subprocess.killed property indicates whether the child process successfully received a signal from subprocess.kill() . The killed property does not indicate that the child process has been terminated. subprocess.killed 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 169/739 <integer> Returns the process identifier (PID) of the child process. const { spawn } = require('child_process'); const grep = spawn('grep', ['ssh']); console.log(`Spawned child pid: ${grep.pid}`); grep.stdin.end(); Calling subprocess.ref() after making a call to subprocess.unref() will restore the removed reference count for the child process, forcing the parent to wait for the child to exit before exiting itself. const { spawn } = require('child_process'); const subprocess = spawn(process.argv[0], ['child_program.js'], { detached: true, stdio: 'ignore' }); subprocess.unref(); subprocess.ref(); message <Object> sendHandle <Handle> options <Object> The options argument, if present, is an object used to parameterize the sending of certain types of handles. options supports the following properties: keepOpen <boolean> A value that can be used when passing instances of net.Socket . When true , the socket is kept open in the sending process. Default: false . callback <Function> Returns: <boolean> When an IPC channel has been established between the parent and child ( i.e. when using child_process.fork() ), the subprocess.send() method can be used to send messages to the child process. When the child process is a Node.js instance, these messages can be received via the 'message' event. The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent. For example, in the parent script: const cp = require('child_process'); const n = cp.fork(`${__dirname}/sub.js`); n.on('message', (m) => { console.log('PARENT got message:', m); subprocess.pid subprocess.ref() subprocess.send(message[, sendHandle[, options]][, callback]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 170/739 console.log( PARENT got message: , m); }); // Causes the child to print: CHILD got message: { hello: 'world' } n.send({ hello: 'world' }); And then the child script, 'sub.js' might look like this: process.on('message', (m) => { console.log('CHILD got message:', m); }); // Causes the parent to print: PARENT got message: { foo: 'bar', baz: null } process.send({ foo: 'bar', baz: NaN }); Child Node.js processes will have a process.send() method of their own that allows the child to send messages back to the parent. There is a special case when sending a {cmd: 'NODE_foo'} message. Messages containing a NODE_ prefix in the cmd property are reserved for use within Node.js core and will not be emitted in the child's 'message' event. Rather, such messages are emitted using the 'internalMessage' event and are consumed internally by Node.js. Applications should avoid using such messages or listening for 'internalMessage' events as it is subject to change without notice. The optional sendHandle argument that may be passed to subprocess.send() is for passing a TCP server or socket object to the child process. The child will receive the object as the second argument passed to the callback function registered on the 'message' event. Any data that is received and buffered in the socket will not be sent to the child. The optional callback is a function that is invoked after the message is sent but before the child may have received it. The function is called with a single argument: null on success, or an Error object on failure. If no callback function is provided and the message cannot be sent, an 'error' event will be emitted by the ChildProcess object. This can happen, for instance, when the child process has already exited. subprocess.send() will return false if the channel has closed or when the backlog of unsent messages exceeds a threshold that makes it unwise to send more. Otherwise, the method returns true . The callback function can be used to implement flow control. The sendHandle argument can be used, for instance, to pass the handle of a TCP server object to the child process as illustrated in the example below: const subprocess = require('child_process').fork('subprocess.js'); // Open up the server object and send the handle. const server = require('net').createServer(); server.on('connection', (socket) => { socket.end('handled by parent'); }); server.listen(1337, () => { subprocess.send('server', server); }); The child would then receive the server object as: Example: sending a server object 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 171/739 process.on('message', (m, server) => { if (m === 'server') { server.on('connection', (socket) => { socket.end('handled by child'); }); } }); Once the server is now shared between the parent and child, some connections can be handled by the parent and some by the child. While the example above uses a server created using the net module, dgram module servers use exactly the same workflow with the exceptions of listening on a 'message' event instead of 'connection' and using server.bind() instead of server.listen() . This is, however, currently only supported on UNIX platforms. Similarly, the sendHandler argument can be used to pass the handle of a socket to the child process. The example below spawns two children that each handle connections with "normal" or "special" priority: const { fork } = require('child_process'); const normal = fork('subprocess.js', ['normal']); const special = fork('subprocess.js', ['special']); // Open up the server and send sockets to child. Use pauseOnConnect to prevent // the sockets from being read before they are sent to the child process. const server = require('net').createServer({ pauseOnConnect: true }); server.on('connection', (socket) => { // If this is special priority if (socket.remoteAddress === '74.125.127.100') { special.send('socket', socket); return; } // This is normal priority normal.send('socket', socket); }); server.listen(1337); The subprocess.js would receive the socket handle as the second argument passed to the event callback function: process.on('message', (m, socket) => { if (m === 'socket') { if (socket) { // Check that the client socket exists. // It is possible for the socket to be closed between the time it is // sent and the time it is received in the child process. socket.end(`Request handled with ${process.argv[2]} priority`); } } }); Example: sending a socket object 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 172/739 Once a socket has been passed to a child, the parent is no longer capable of tracking when the socket is destroyed. To indicate this, the .connections property becomes null . It is recommended not to use .maxConnections when this occurs. It is also recommended that any 'message' handlers in the child process verify that socket exists, as the connection may have been closed during the time it takes to send the connection to the child. <stream.Readable> A Readable Stream that represents the child process's stderr . If the child was spawned with stdio[2] set to anything other than 'pipe' , then this will be null . subprocess.stderr is an alias for subprocess.stdio[2] . Both properties will refer to the same value. <stream.Writable> A Writable Stream that represents the child process's stdin . Note that if a child process waits to read all of its input, the child will not continue until this stream has been closed via end() . If the child was spawned with stdio[0] set to anything other than 'pipe' , then this will be null . subprocess.stdin is an alias for subprocess.stdio[0] . Both properties will refer to the same value. <Array> A sparse array of pipes to the child process, corresponding with positions in the stdio option passed to child_process.spawn() that have been set to the value 'pipe' . Note that subprocess.stdio[0] , subprocess.stdio[1] , and subprocess.stdio[2] are also available as subprocess.stdin , subprocess.stdout , and subprocess.stderr , respectively. In the following example, only the child's fd 1 (stdout) is configured as a pipe, so only the parent's subprocess.stdio[1] is a stream, all other values in the array are null . const assert = require('assert'); const fs = require('fs'); const child_process = require('child_process'); const subprocess = child_process.spawn('ls', { stdio: [ 0, // Use parent's stdin for child 'pipe', // Pipe child's stdout to parent fs.openSync('err.out', 'w') // Direct child's stderr to a file ] }); assert.strictEqual(subprocess.stdio[0], null); assert.strictEqual(subprocess.stdio[0], subprocess.stdin); assert(subprocess.stdout); assert.strictEqual(subprocess.stdio[1], subprocess.stdout); subprocess.stderr subprocess.stdin subprocess.stdio 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 173/739 assert.strictEqual(subprocess.stdio[2], null); assert.strictEqual(subprocess.stdio[2], subprocess.stderr); <stream.Readable> A Readable Stream that represents the child process's stdout . If the child was spawned with stdio[1] set to anything other than 'pipe' , then this will be null . subprocess.stdout is an alias for subprocess.stdio[1] . Both properties will refer to the same value. By default, the parent will wait for the detached child to exit. To prevent the parent from waiting for a given subprocess to exit, use the subprocess.unref() method. Doing so will cause the parent's event loop to not include the child in its reference count, allowing the parent to exit independently of the child, unless there is an established IPC channel between the child and the parent. const { spawn } = require('child_process'); const subprocess = spawn(process.argv[0], ['child_program.js'], { detached: true, stdio: 'ignore' }); subprocess.unref(); The maxBuffer option specifies the largest number of bytes allowed on stdout or stderr . If this value is exceeded, then the child process is terminated. This impacts output that includes multibyte character encodings such as UTF-8 or UTF-16. For instance, console.log('中⽂测 试') will send 13 UTF-8 encoded bytes to stdout although there are only 4 characters. The shell should understand the -c switch on UNIX or /d /s /c on Windows. On Windows, command line parsing should be compatible with 'cmd.exe' . Although Microsoft specifies %COMSPEC% must contain the path to 'cmd.exe' in the root environment, child processes are not always subject to the same requirement. Thus, in child_process functions where a shell can be spawned, 'cmd.exe' is used as a fallback if process.env.ComSpec is unavailable. Stability: 2 - Stable subprocess.stdout subprocess.unref() maxBuffer and Unicode Shell Requirements Default Windows Shell Cluster 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 174/739 A single instance of Node.js runs in a single thread. To take advantage of multi-core systems, the user will sometimes want to launch a cluster of Node.js processes to handle the load. The cluster module allows easy creation of child processes that all share server ports. const cluster = require('cluster'); const http = require('http'); const numCPUs = require('os').cpus().length; if (cluster.isMaster) { console.log(`Master ${process.pid} is running`); // Fork workers. for (let i = 0; i < numCPUs; i++) { cluster.fork(); } cluster.on('exit', (worker, code, signal) => { console.log(`worker ${worker.process.pid} died`); }); } else { // Workers can share any TCP connection // In this case it is an HTTP server http.createServer((req, res) => { res.writeHead(200); res.end('hello world\n'); }).listen(8000); console.log(`Worker ${process.pid} started`); } Running Node.js will now share port 8000 between the workers: $ node server.js Master 3596 is running Worker 4324 started Worker 4520 started Worker 6056 started Worker 5644 started Please note that on Windows, it is not yet possible to set up a named pipe server in a worker. The worker processes are spawned using the child_process.fork() method, so that they can communicate with the parent via IPC and pass server handles back and forth. The cluster module supports two methods of distributing incoming connections. The first one (and the default one on all platforms except Windows), is the round-robin approach, where the master process listens on a port, accepts new connections and distributes them across the workers in a round-robin fashion, with some built-in smarts to avoid overloading a worker process. How It Works 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 175/739 The second approach is where the master process creates the listen socket and sends it to interested workers. The workers then accept incoming connections directly. The second approach should, in theory, give the best performance. In practice however, distribution tends to be very unbalanced due to operating system scheduler vagaries. Loads have been observed where over 70% of all connections ended up in just two processes, out of a total of eight. Because server.listen() hands off most of the work to the master process, there are three cases where the behavior between a normal Node.js process and a cluster worker differs: 1. server.listen({fd: 7}) Because the message is passed to the master, file descriptor 7 in the parent will be listened on, and the handle passed to the worker, rather than listening to the worker's idea of what the number 7 file descriptor references. 2. server.listen(handle) Listening on handles explicitly will cause the worker to use the supplied handle, rather than talk to the master process. 3. server.listen(0) Normally, this will cause servers to listen on a random port. However, in a cluster, each worker will receive the same "random" port each time they do listen(0) . In essence, the port is random the first time, but predictable thereafter. To listen on a unique port, generate a port number based on the cluster worker ID. Node.js does not provide routing logic. It is, therefore important to design an application such that it does not rely too heavily on in-memory data objects for things like sessions and login. Because workers are all separate processes, they can be killed or re-spawned depending on a program's needs, without affecting other workers. As long as there are some workers still alive, the server will continue to accept connections. If no workers are alive, existing connections will be dropped and new connections will be refused. Node.js does not automatically manage the number of workers, however. It is the application's responsibility to manage the worker pool based on its own needs. Although a primary use case for the cluster module is networking, it can also be used for other use cases requiring worker processes. A Worker object contains all public information and method about a worker. In the master it can be obtained using cluster.workers . In a worker it can be obtained using cluster.worker . Similar to the cluster.on('disconnect') event, but specific to this worker. cluster.fork().on('disconnect', () => { // Worker has disconnected }); This event is the same as the one provided by child_process.fork() . Within a worker, process.on('error') may also be used. code <number> The exit code, if it exited normally. signal <string> The name of the signal (e.g. 'SIGHUP' ) that caused the process to be killed. Similar to the cluster.on('exit') event, but specific to this worker. Class: Worker Event: 'disconnect' Event: 'error' Event: 'exit' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 176/739 const worker = cluster.fork(); worker.on('exit', (code, signal) => { if (signal) { console.log(`worker was killed by signal: ${signal}`); } else if (code !== 0) { console.log(`worker exited with error code: ${code}`); } else { console.log('worker success!'); } }); address <Object> Similar to the cluster.on('listening') event, but specific to this worker. cluster.fork().on('listening', (address) => { // Worker is listening }); It is not emitted in the worker. message <Object> handle <undefined> | <Object> Similar to the 'message' event of cluster , but specific to this worker. Within a worker, process.on('message') may also be used. See process event: 'message' . Here is an example using the message system. It keeps a count in the master process of the number of HTTP requests received by the workers: const cluster = require('cluster'); const http = require('http'); if (cluster.isMaster) { // Keep track of http requests let numReqs = 0; setInterval(() => { console.log(`numReqs = ${numReqs}`); }, 1000); // Count requests function messageHandler(msg) { if (msg.cmd && msg.cmd === 'notifyRequest') { numReqs += 1; } } Event: 'listening' Event: 'message' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 177/739 // Start workers and listen for messages containing notifyRequest const numCPUs = require('os').cpus().length; for (let i = 0; i < numCPUs; i++) { cluster.fork(); } for (const id in cluster.workers) { cluster.workers[id].on('message', messageHandler); } } else { // Worker processes have a http server. http.Server((req, res) => { res.writeHead(200); res.end('hello world\n'); // notify master about the request process.send({ cmd: 'notifyRequest' }); }).listen(8000); } Similar to the cluster.on('online') event, but specific to this worker. cluster.fork().on('online', () => { // Worker is online }); It is not emitted in the worker. Returns: <cluster.Worker> A reference to worker . In a worker, this function will close all servers, wait for the 'close' event on those servers, and then disconnect the IPC channel. In the master, an internal message is sent to the worker causing it to call .disconnect() on itself. Causes .exitedAfterDisconnect to be set. Note that after a server is closed, it will no longer accept new connections, but connections may be accepted by any other listening worker. Existing connections will be allowed to close as usual. When no more connections exist, see server.close() , the IPC channel to the worker will close allowing it to die gracefully. The above applies only to server connections, client connections are not automatically closed by workers, and disconnect does not wait for them to close before exiting. Note that in a worker, process.disconnect exists, but it is not this function, it is disconnect . Event: 'online' worker.disconnect() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 178/739 Because long living server connections may block workers from disconnecting, it may be useful to send a message, so application specific actions may be taken to close them. It also may be useful to implement a timeout, killing a worker if the 'disconnect' event has not been emitted after some time. if (cluster.isMaster) { const worker = cluster.fork(); let timeout; worker.on('listening', (address) => { worker.send('shutdown'); worker.disconnect(); timeout = setTimeout(() => { worker.kill(); }, 2000); }); worker.on('disconnect', () => { clearTimeout(timeout); }); } else if (cluster.isWorker) { const net = require('net'); const server = net.createServer((socket) => { // connections never end }); server.listen(8000); process.on('message', (msg) => { if (msg === 'shutdown') { // initiate graceful close of any connections to server } }); } <boolean> Set by calling .kill() or .disconnect() . Until then, it is undefined . The boolean worker.exitedAfterDisconnect allows distinguishing between voluntary and accidental exit, the master may choose not to respawn a worker based on this value. cluster.on('exit', (worker, code, signal) => { if (worker.exitedAfterDisconnect === true) { console.log('Oh, it was just voluntary – no need to worry'); } }); // kill worker worker.kill(); worker.exitedAfterDisconnect 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 179/739 <number> Each new worker is given its own unique id, this id is stored in the id . While a worker is alive, this is the key that indexes it in cluster.workers . This function returns true if the worker is connected to its master via its IPC channel, false otherwise. A worker is connected to its master after it has been created. It is disconnected after the 'disconnect' event is emitted. This function returns true if the worker's process has terminated (either because of exiting or being signaled). Otherwise, it returns false . signal <string> Name of the kill signal to send to the worker process. This function will kill the worker. In the master, it does this by disconnecting the worker.process , and once disconnected, killing with signal . In the worker, it does it by disconnecting the channel, and then exiting with code 0 . Because kill() attempts to gracefully disconnect the worker process, it is susceptible to waiting indefinitely for the disconnect to complete. For example, if the worker enters an infinite loop, a graceful disconnect will never occur. If the graceful disconnect behavior is not needed, use worker.process.kill() . Causes .exitedAfterDisconnect to be set. This method is aliased as worker.destroy() for backwards compatibility. Note that in a worker, process.kill() exists, but it is not this function, it is kill . <ChildProcess> All workers are created using child_process.fork() , the returned object from this function is stored as .process . In a worker, the global process is stored. See: Child Process module . Note that workers will call process.exit(0) if the 'disconnect' event occurs on process and .exitedAfterDisconnect is not true . This protects against accidental disconnection. message <Object> sendHandle <Handle> callback <Function> Returns: <boolean> Send a message to a worker or master, optionally with a handle. In the master this sends a message to a specific worker. It is identical to ChildProcess.send() . worker.id worker.isConnected() worker.isDead() worker.kill([signal='SIGTERM']) worker.process worker.send(message[, sendHandle][, callback]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 180/739 In a worker this sends a message to the master. It is identical to process.send() . This example will echo back all messages from the master: if (cluster.isMaster) { const worker = cluster.fork(); worker.send('hi there'); } else if (cluster.isWorker) { process.on('message', (msg) => { process.send(msg); }); } worker <cluster.Worker> Emitted after the worker IPC channel has disconnected. This can occur when a worker exits gracefully, is killed, or is disconnected manually (such as with worker.disconnect() ). There may be a delay between the 'disconnect' and 'exit' events. These events can be used to detect if the process is stuck in a cleanup or if there are long-living connections. cluster.on('disconnect', (worker) => { console.log(`The worker #${worker.id} has disconnected`); }); worker <cluster.Worker> code <number> The exit code, if it exited normally. signal <string> The name of the signal (e.g. 'SIGHUP' ) that caused the process to be killed. When any of the workers die the cluster module will emit the 'exit' event. This can be used to restart the worker by calling .fork() again. cluster.on('exit', (worker, code, signal) => { console.log('worker %d died (%s). restarting...', worker.process.pid, signal || code); cluster.fork(); }); See child_process event: 'exit' . worker <cluster.Worker> When a new worker is forked the cluster module will emit a 'fork' event. This can be used to log worker activity, and create a custom timeout. Event: 'disconnect' Event: 'exit' Event: 'fork' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 181/739 const timeouts = []; function errorMsg() { console.error('Something must be wrong with the connection ...'); } cluster.on('fork', (worker) => { timeouts[worker.id] = setTimeout(errorMsg, 2000); }); cluster.on('listening', (worker, address) => { clearTimeout(timeouts[worker.id]); }); cluster.on('exit', (worker, code, signal) => { clearTimeout(timeouts[worker.id]); errorMsg(); }); worker <cluster.Worker> address <Object> After calling listen() from a worker, when the 'listening' event is emitted on the server a 'listening' event will also be emitted on cluster in the master. The event handler is executed with two arguments, the worker contains the worker object and the address object contains the following connection properties: address , port and addressType . This is very useful if the worker is listening on more than one address. cluster.on('listening', (worker, address) => { console.log( `A worker is now connected to ${address.address}:${address.port}`); }); The addressType is one of: 4 (TCPv4) 6 (TCPv6) -1 (unix domain socket) 'udp4' or 'udp6' (UDP v4 or v6) worker <cluster.Worker> message <Object> handle <undefined> | <Object> Emitted when the cluster master receives a message from any worker. See child_process event: 'message' . Event: 'listening' Event: 'message' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 182/739 Before Node.js v6.0, this event emitted only the message and the handle, but not the worker object, contrary to what the documentation stated. If support for older versions is required but a worker object is not required, it is possible to work around the discrepancy by checking the number of arguments: cluster.on('message', (worker, message, handle) => { if (arguments.length === 2) { handle = message; message = worker; worker = undefined; } // ... }); worker <cluster.Worker> After forking a new worker, the worker should respond with an online message. When the master receives an online message it will emit this event. The difference between 'fork' and 'online' is that fork is emitted when the master forks a worker, and 'online' is emitted when the worker is running. cluster.on('online', (worker) => { console.log('Yay, the worker responded after it was forked'); }); settings <Object> Emitted every time .setupMaster() is called. The settings object is the cluster.settings object at the time .setupMaster() was called and is advisory only, since multiple calls to .setupMaster() can be made in a single tick. If accuracy is important, use cluster.settings . callback <Function> Called when all workers are disconnected and handles are closed. Calls .disconnect() on each worker in cluster.workers . When they are disconnected all internal handles will be closed, allowing the master process to die gracefully if no other event is waiting. The method takes an optional callback argument which will be called when finished. This can only be called from the master process. env <Object> Key/value pairs to add to worker process environment. Returns: <cluster.Worker> Event: 'online' Event: 'setup' cluster.disconnect([callback]) cluster.fork([env]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 183/739 Spawn a new worker process. This can only be called from the master process. <boolean> True if the process is a master. This is determined by the process.env.NODE_UNIQUE_ID . If process.env.NODE_UNIQUE_ID is undefined, then isMaster is true . <boolean> True if the process is not a master (it is the negation of cluster.isMaster ). The scheduling policy, either cluster.SCHED_RR for round-robin or cluster.SCHED_NONE to leave it to the operating system. This is a global setting and effectively frozen once either the first worker is spawned, or cluster.setupMaster() is called, whichever comes first. SCHED_RR is the default on all operating systems except Windows. Windows will change to SCHED_RR once libuv is able to effectively distribute IOCP handles without incurring a large performance hit. cluster.schedulingPolicy can also be set through the NODE_CLUSTER_SCHED_POLICY environment variable. Valid values are 'rr' and 'none' . <Object> execArgv <string[]> List of string arguments passed to the Node.js executable. Default: process.execArgv . exec <string> File path to worker file. Default: process.argv[1] . args <string[]> String arguments passed to worker. Default: process.argv.slice(2) . cwd <string> Current working directory of the worker process. Default: undefined (inherits from parent process). silent <boolean> Whether or not to send output to parent's stdio. Default: false . stdio <Array> Configures the stdio of forked processes. Because the cluster module relies on IPC to function, this configuration must contain an 'ipc' entry. When this option is provided, it overrides silent . uid <number> Sets the user identity of the process. (See setuid(2) .) gid <number> Sets the group identity of the process. (See setgid(2) .) inspectPort <number> | <Function> Sets inspector port of worker. This can be a number, or a function that takes no arguments and returns a number. By default each worker gets its own port, incremented from the master's process.debugPort . windowsHide <boolean> Hide the forked processes console window that would normally be created on Windows systems. Default: false . After calling .setupMaster() (or .fork() ) this settings object will contain the settings, including the default values. This object is not intended to be changed or set manually. settings <Object> See cluster.settings . cluster.isMaster cluster.isWorker cluster.schedulingPolicy cluster.settings cluster.setupMaster([settings]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 184/739 setupMaster is used to change the default 'fork' behavior. Once called, the settings will be present in cluster.settings . Note that: Any settings changes only affect future calls to .fork() and have no effect on workers that are already running. The only attribute of a worker that cannot be set via .setupMaster() is the env passed to .fork() . The defaults above apply to the first call only, the defaults for later calls is the current value at the time of cluster.setupMaster() is called. const cluster = require('cluster'); cluster.setupMaster({ exec: 'worker.js', args: ['--use', 'https'], silent: true }); cluster.fork(); // https worker cluster.setupMaster({ exec: 'worker.js', args: ['--use', 'http'] }); cluster.fork(); // http worker This can only be called from the master process. <Object> A reference to the current worker object. Not available in the master process. const cluster = require('cluster'); if (cluster.isMaster) { console.log('I am master'); cluster.fork(); cluster.fork(); } else if (cluster.isWorker) { console.log(`I am worker #${cluster.worker.id}`); } <Object> A hash that stores the active worker objects, keyed by id field. Makes it easy to loop through all the workers. It is only available in the master process. A worker is removed from cluster.workers after the worker has disconnected and exited. The order between these two events cannot be determined in advance. However, it is guaranteed that the removal from the cluster.workers list happens before last 'disconnect' or 'exit' event is emitted. // Go through all workers function eachWorker(callback) { cluster.worker cluster.workers 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 185/739 for (const id in cluster.workers) { callback(cluster.workers[id]); } } eachWorker((worker) => { worker.send('big announcement to all workers'); }); Using the worker's unique id is the easiest way to locate the worker. socket.on('data', (id) => { const worker = cluster.workers[id]; }); Node.js comes with a variety of CLI options. These options expose built-in debugging, multiple ways to execute scripts, and other helpful runtime options. To view this documentation as a manual page in a terminal, run man node . node [options] [V8 options] [script.js | -e "script" | -] [--] [arguments] node inspect [script.js | -e "script" | <host>:<port>] … node --v8-options Execute without arguments to start the REPL . For more info about node inspect , please see the debugger documentation. All options, including V8 options, allow words to be separated by both dashes ( - ) or underscores ( _ ). For example, --pending-deprecation is equivalent to --pending_deprecation . Alias for stdin, analogous to the use of - in other command line utilities, meaning that the script will be read from stdin, and the rest of the options are passed to that script. Indicate the end of node options. Pass the rest of the arguments to the script. If no script filename or eval/print script is supplied prior to this, then the next argument will be used as a script filename. Aborting instead of exiting causes a core file to be generated for post-mortem analysis using a debugger (such as lldb , gdb , and mdb ). Command Line Options Synopsis Options - -- --abort-on-uncaught-exception 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 186/739 If this flag is passed, the behavior can still be set to not abort through process.setUncaughtExceptionCaptureCallback() (and through usage of the domain module that uses it). Print source-able bash completion script for Node.js. $ node --completion-bash > node_bash_completion $ source node_bash_completion Enable FIPS-compliant crypto at startup. (Requires Node.js to be built with ./configure --openssl-fips .) Enable experimental ES module support and caching modules. Enable experimental top-level await keyword support in REPL. Enable experimental ES Module support in the vm module. Enable experimental worker threads using the worker_threads module. Force FIPS-compliant crypto on startup. (Cannot be disabled from script code.) (Same requirements as --enable-fips .) Specify ICU data load path. (Overrides NODE_ICU_DATA .) Activate inspector on host:port and break at start of user script. Default host:port is 127.0.0.1:9229 . Set the host:port to be used when the inspector is activated. Useful when activating the inspector by sending the SIGUSR1 signal. Default host is 127.0.0.1 . See the security warning below regarding the host parameter usage. Activate inspector on host:port . Default is 127.0.0.1:9229 . --completion-bash --enable-fips --experimental-modules --experimental-repl-await --experimental-vm-modules --experimental-worker --force-fips --icu-data-dir=file --inspect-brk[=[host:]port] --inspect-port=[host:]port --inspect[=[host:]port] 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 187/739 V8 inspector integration allows tools such as Chrome DevTools and IDEs to debug and profile Node.js instances. The tools attach to Node.js instances via a tcp port and communicate using the Chrome DevTools Protocol . Binding the inspector to a public IP (including 0.0.0.0 ) with an open port is insecure, as it allows external hosts to connect to the inspector and perform a remote code execution attack. If you specify a host, make sure that at least one of the following is true: either the host is not public, or the port is properly firewalled to disallow unwanted connections. More specifically, --inspect=0.0.0.0 is insecure if the port ( 9229 by default) is not firewall-protected. See the debugging security implications section for more information. Specify the file of the custom experimental ECMAScript Module loader. Specify the maximum size, in bytes, of HTTP headers. Defaults to 8KB. This option is a no-op. It is kept for compatibility. Silence deprecation warnings. Disables runtime checks for async_hooks . These will still be enabled dynamically when async_hooks is enabled. Silence all process warnings (including deprecations). Load an OpenSSL configuration file on startup. Among other uses, this can be used to enable FIPS-compliant crypto if Node.js is built with ./configure --openssl-fips . Emit pending deprecation warnings. Pending deprecations are generally identical to a runtime deprecation with the notable exception that they are turned off by default and will not be emitted unless either the --pending-deprecation command line flag, or the NODE_PENDING_DEPRECATION=1 environment variable, is set. Pending deprecations are used to provide a kind of selective "early warning" mechanism that developers may leverage to detect deprecated API usage. Warning: binding inspector to a public IP:port combination is insecure --loader=file --max-http-header-size=size --napi-modules --no-deprecation --no-force-async-hooks-checks --no-warnings --openssl-config=file --pending-deprecation --preserve-symlinks 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 188/739 Instructs the module loader to preserve symbolic links when resolving and caching modules. By default, when Node.js loads a module from a path that is symbolically linked to a different on-disk location, Node.js will dereference the link and use the actual on-disk "real path" of the module as both an identifier and as a root path to locate other dependency modules. In most cases, this default behavior is acceptable. However, when using symbolically linked peer dependencies, as illustrated in the example below, the default behavior causes an exception to be thrown if moduleA attempts to require moduleB as a peer dependency: {appDir} ├── app │ ├── index.js │ └── node_modules │ ├── moduleA -> {appDir}/moduleA │ └── moduleB │ ├── index.js │ └── package.json └── moduleA ├── index.js └── package.json The --preserve-symlinks command line flag instructs Node.js to use the symlink path for modules as opposed to the real path, allowing symbolically linked peer dependencies to be found. Note, however, that using --preserve-symlinks can have other side effects. Specifically, symbolically linked native modules can fail to load if those are linked from more than one location in the dependency tree (Node.js would see those as two separate modules and would attempt to load the module multiple times, causing an exception to be thrown). The --preserve-symlinks flag does not apply to the main module, which allows node --preserve-symlinks node_module/.bin/<foo> to work. To apply the same behavior for the main module, also use --preserve-symlinks-main . Instructs the module loader to preserve symbolic links when resolving and caching the main module ( require.main ). This flag exists so that the main module can be opted-in to the same behavior that --preserve-symlinks gives to all other imports; they are separate flags, however, for backward compatibility with older Node.js versions. Note that --preserve-symlinks-main does not imply --preserve-symlinks ; it is expected that --preserve-symlinks-main will be used in addition to --preserve-symlinks when it is not desirable to follow symlinks before resolving relative paths. See --preserve-symlinks for more information. Generate V8 profiler output. Process V8 profiler output generated using the V8 option --prof . Write process warnings to the given file instead of printing to stderr. The file will be created if it does not exist, and will be appended to if it does. If an error occurs while attempting to write the warning to the file, the warning will be written to stderr instead. --preserve-symlinks-main --prof --prof-process --redirect-warnings=file 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 189/739 Throw errors for deprecations. Set process.title on startup. Specify an alternative default TLS cipher list. Requires Node.js to be built with crypto support (default). Print stack traces for deprecations. A comma separated list of categories that should be traced when trace event tracing is enabled using --trace-events-enabled . Template string specifying the filepath for the trace event data, it supports ${rotation} and ${pid} . Enables the collection of trace event tracing information. Prints a stack trace whenever synchronous I/O is detected after the first turn of the event loop. Print stack traces for process warnings (including deprecations). Track heap object allocations for heap snapshots. Use bundled Mozilla CA store as supplied by current Node.js version or use OpenSSL's default CA store. The default store is selectable at build- time. The bundled CA store, as supplied by Node.js, is a snapshot of Mozilla CA store that is fixed at release time. It is identical on all supported platforms. Using OpenSSL store allows for external modifications of the store. For most Linux and BSD distributions, this store is maintained by the distribution maintainers and system administrators. OpenSSL CA store location is dependent on configuration of the OpenSSL library but this can be altered at runtime using environment variables. See SSL_CERT_DIR and SSL_CERT_FILE . --throw-deprecation --title=title --tls-cipher-list=list --trace-deprecation --trace-event-categories --trace-event-file-pattern --trace-events-enabled --trace-sync-io --trace-warnings --track-heap-objects --use-bundled-ca , --use-openssl-ca --v8-options 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 190/739 Print V8 command line options. Set V8's thread pool size which will be used to allocate background jobs. If set to 0 then V8 will choose an appropriate size of the thread pool based on the number of online processors. If the value provided is larger than V8's maximum, then the largest value will be chosen. Automatically zero-fills all newly allocated Buffer and SlowBuffer instances. Syntax check the script without executing. Evaluate the following argument as JavaScript. The modules which are predefined in the REPL can also be used in script . On Windows, using cmd.exe a single quote will not work correctly because it only recognizes double " for quoting. In Powershell or Git bash, both ' and " are usable. Print node command line options. The output of this option is less detailed than this document. Opens the REPL even if stdin does not appear to be a terminal. Identical to -e but prints the result. Preload the specified module at startup. Follows require() 's module resolution rules. module may be either a path to a file, or a node module name. Print node's version. ',' -separated list of core modules that should print debug information. --v8-pool-size=num --zero-fill-buffers -c , --check -e , --eval "script" -h , --help -i , --interactive -p , --print "script" -r , --require module -v , --version Environment Variables NODE_DEBUG=module[,…] NODE_DEBUG_NATIVE=module[,…] 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 191/739 ',' -separated list of core C++ modules that should print debug information. When set to 1 colors will not be used in the REPL. When set, the well known "root" CAs (like VeriSign) will be extended with the extra certificates in file . The file should consist of one or more trusted certificates in PEM format. A message will be emitted (once) with process.emitWarning() if the file is missing or malformed, but any errors are otherwise ignored. Note that neither the well known nor extra certificates are used when the ca options property is explicitly specified for a TLS or HTTPS client or server. This environment variable is ignored when node runs as setuid root or has Linux file capabilities set. Data path for ICU ( Intl object) data. Will extend linked-in data when compiled with small-icu support. When set to 1 , process warnings are silenced. A space-separated list of command line options. options... are interpreted as if they had been specified on the command line before the actual command line (so they can be overridden). Node.js will exit with an error if an option that is not allowed in the environment is used, such as -p or a script file. Node.js options that are allowed are: --enable-fips --experimental-modules --experimental-repl-await --experimental-vm-modules --experimental-worker --force-fips --icu-data-dir --inspect --inspect-brk --inspect-port --loader --max-http-header-size --napi-modules --no-deprecation --no-force-async-hooks-checks --no-warnings --openssl-config NODE_DISABLE_COLORS=1 NODE_EXTRA_CA_CERTS=file NODE_ICU_DATA=file NODE_NO_WARNINGS=1 NODE_OPTIONS=options... 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 192/739 --pending-deprecation --redirect-warnings --require , -r --throw-deprecation --title --tls-cipher-list --trace-deprecation --trace-event-categories --trace-event-file-pattern --trace-events-enabled --trace-sync-io --trace-warnings --track-heap-objects --use-bundled-ca --use-openssl-ca --v8-pool-size --zero-fill-buffers V8 options that are allowed are: --abort-on-uncaught-exception --max-old-space-size --perf-basic-prof --perf-prof --stack-trace-limit ':' -separated list of directories prefixed to the module search path. On Windows, this is a ';' -separated list instead. When set to 1 , emit pending deprecation warnings. Pending deprecations are generally identical to a runtime deprecation with the notable exception that they are turned off by default and will not be emitted unless either the --pending-deprecation command line flag, or the NODE_PENDING_DEPRECATION=1 environment variable, is set. Pending deprecations are used to provide a kind of selective "early warning" mechanism that developers may leverage to detect deprecated API usage. When set to 1 , instructs the module loader to preserve symbolic links when resolving and caching modules. When set, process warnings will be emitted to the given file instead of printing to stderr. The file will be created if it does not exist, and will be appended to if it does. If an error occurs while attempting to write the warning to the file, the warning will be written to stderr instead. This is NODE_PATH=path[:…] NODE_PENDING_DEPRECATION=1 NODE_PRESERVE_SYMLINKS=1 NODE_REDIRECT_WARNINGS=file 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 193/739 equivalent to using the --redirect-warnings=file command-line flag. Path to the file used to store the persistent REPL history. The default path is ~/.node_repl_history , which is overridden by this variable. Setting the value to an empty string ( '' or ' ' ) disables persistent REPL history. If value equals '0' , certificate validation is disabled for TLS connections. This makes TLS, and HTTPS by extension, insecure. The use of this environment variable is strongly discouraged. When set, Node.js will begin outputting V8 JavaScript code coverage to the directory provided as an argument. Coverage is output as an array of ScriptCoverage objects: { "result": [ { "scriptId": "67", "url": "internal/tty.js", "functions": [] } ] } NODE_V8_COVERAGE will automatically propagate to subprocesses, making it easier to instrument applications that call the child_process.spawn() family of functions. NODE_V8_COVERAGE can be set to an empty string, to prevent propagation. At this time coverage is only collected in the main thread and will not be output for code executed by worker threads. Load an OpenSSL configuration file on startup. Among other uses, this can be used to enable FIPS-compliant crypto if Node.js is built with ./configure --openssl-fips . If the --openssl-config command line option is used, the environment variable is ignored. If --use-openssl-ca is enabled, this overrides and sets OpenSSL's directory containing trusted certificates. Be aware that unless the child environment is explicitly set, this environment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node. If --use-openssl-ca is enabled, this overrides and sets OpenSSL's file containing trusted certificates. Be aware that unless the child environment is explicitly set, this environment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node. NODE_REPL_HISTORY=file NODE_TLS_REJECT_UNAUTHORIZED=value NODE_V8_COVERAGE=dir OPENSSL_CONF=file SSL_CERT_DIR=dir SSL_CERT_FILE=file UV_THREADPOOL_SIZE=size 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 194/739 Set the number of threads used in libuv's threadpool to size threads. Asynchronous system APIs are used by Node.js whenever possible, but where they do not exist, libuv's threadpool is used to create asynchronous node APIs based on synchronous system APIs. Node.js APIs that use the threadpool are: all fs APIs, other than the file watcher APIs and those that are explicitly synchronous crypto.pbkdf2() crypto.randomBytes() , unless it is used without a callback crypto.randomFill() dns.lookup() all zlib APIs, other than those that are explicitly synchronous Because libuv's threadpool has a fixed size, it means that if for whatever reason any of these APIs takes a long time, other (seemingly unrelated) APIs that run in libuv's threadpool will experience degraded performance. In order to mitigate this issue, one potential solution is to increase the size of libuv's threadpool by setting the 'UV_THREADPOOL_SIZE' environment variable to a value greater than 4 (its current default value). For more information, see the libuv threadpool documentation . Stability: 2 - Stable The console module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers. The module exports two specific components: A Console class with methods such as console.log() , console.error() and console.warn() that can be used to write to any Node.js stream. A global console instance configured to write to process.stdout and process.stderr . The global console can be used without calling require('console') . Warning: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. See the note on process I/O for more information. Example using the global console : console.log('hello world'); // Prints: hello world, to stdout console.log('hello %s', 'world'); // Prints: hello world, to stdout console.error(new Error('Whoops, something bad happened')); // Prints: [Error: Whoops, something bad happened], to stderr const name = 'Will Robinson'; console.warn(`Danger ${name}! Danger!`); // Prints: Danger Will Robinson! Danger!, to stderr Example using the Console class: const out = getStreamSomehow(); const err = getStreamSomehow(); const myConsole = new console.Console(out, err); Console 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 195/739 myConsole.log('hello world'); // Prints: hello world, to out myConsole.log('hello %s', 'world'); // Prints: hello world, to out myConsole.error(new Error('Whoops, something bad happened')); // Prints: [Error: Whoops, something bad happened], to err const name = 'Will Robinson'; myConsole.warn(`Danger ${name}! Danger!`); // Prints: Danger Will Robinson! Danger!, to err The Console class can be used to create a simple logger with configurable output streams and can be accessed using either require('console').Console or console.Console (or their destructured counterparts): const { Console } = require('console'); const { Console } = console; options <Object> stdout <stream.Writable> stderr <stream.Writable> ignoreErrors <boolean> Ignore errors when writing to the underlying streams. Default: true . colorMode <boolean> | <string> Set color support for this Console instance. Setting to true enables coloring while inspecting values, setting to 'auto' will make color support depend on the value of the isTTY property and the value returned by getColorDepth() on the respective stream. Default: 'auto' . Creates a new Console with one or two writable stream instances. stdout is a writable stream to print log or info output. stderr is used for warning or error output. If stderr is not provided, stdout is used for stderr . const output = fs.createWriteStream('./stdout.log'); const errorOutput = fs.createWriteStream('./stderr.log'); // custom simple logger const logger = new Console({ stdout: output, stderr: errorOutput }); // use it like console const count = 5; logger.log('count: %d', count); // in stdout.log: count 5 The global console is a special Console whose output is sent to process.stdout and process.stderr . It is equivalent to calling: new Console({ stdout: process.stdout, stderr: process.stderr }); Class: Console new Console(stdout[, stderr][, ignoreerrors]) new Console(options) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 196/739 value <any> The value tested for being truthy. ...message <any> All arguments besides value are used as error message. A simple assertion test that verifies whether value is truthy. If it is not, Assertion failed is logged. If provided, the error message is formatted using util.format() by passing along all message arguments. The output is used as the error message. console.assert(true, 'does nothing'); // OK console.assert(false, 'Whoops %s work', 'didn\'t'); // Assertion failed: Whoops didn't work Calling console.assert() with a falsy assertion will only cause the message to be printed to the console without interrupting execution of subsequent code. When stdout is a TTY, calling console.clear() will attempt to clear the TTY. When stdout is not a TTY, this method does nothing. The specific operation of console.clear() can vary across operating systems and terminal types. For most Linux operating systems, console.clear() operates similarly to the clear shell command. On Windows, console.clear() will clear only the output in the current terminal viewport for the Node.js binary. label <string> The display label for the counter. Default: 'default' . Maintains an internal counter specific to label and outputs to stdout the number of times console.count() has been called with the given label . > console.count() default: 1 undefined > console.count('default') default: 2 undefined > console.count('abc') abc: 1 undefined > console.count('xyz') xyz: 1 undefined > console.count('abc') abc: 2 undefined > console.count() default: 3 undefined > console.assert(value[, ...message]) console.clear() console.count([label]) console.countReset([label]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 197/739 label <string> The display label for the counter. Default: 'default' . Resets the internal counter specific to label . > console.count('abc'); abc: 1 undefined > console.countReset('abc'); undefined > console.count('abc'); abc: 1 undefined > data <any> ...args <any> The console.debug() function is an alias for console.log() . obj <any> options <Object> showHidden <boolean> If true then the object's non-enumerable and symbol properties will be shown too. Default: false . depth <number> Tells util.inspect() how many times to recurse while formatting the object. This is useful for inspecting large complicated objects. To make it recurse indefinitely, pass null . Default: 2 . colors <boolean> If true , then the output will be styled with ANSI color codes. Colors are customizable; see customizing util.inspect() colors . Default: false . Uses util.inspect() on obj and prints the resulting string to stdout . This function bypasses any custom inspect() function defined on obj . ...data <any> This method calls console.log() passing it the arguments received. Please note that this method does not produce any XML formatting. data <any> ...args <any> Prints to stderr with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to printf(3) (the arguments are all passed to util.format() ). const code = 5; console.error('error #%d', code); // Prints: error #5, to stderr console.error('error', code); // Prints: error 5, to stderr console.debug(data[, ...args]) console.dir(obj[, options]) console.dirxml(...data) console.error([data][, ...args]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 198/739 If formatting elements (e.g. %d ) are not found in the first string then util.inspect() is called on each argument and the resulting string values are concatenated. See util.format() for more information. ...label <any> Increases indentation of subsequent lines by two spaces. If one or more label s are provided, those are printed first without the additional indentation. An alias for console.group() . Decreases indentation of subsequent lines by two spaces. data <any> ...args <any> The console.info() function is an alias for console.log() . data <any> ...args <any> Prints to stdout with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to printf(3) (the arguments are all passed to util.format() ). const count = 5; console.log('count: %d', count); // Prints: count: 5, to stdout console.log('count:', count); // Prints: count: 5, to stdout See util.format() for more information. tabularData <any> properties <string[]> Alternate properties for constructing the table. Try to construct a table with the columns of the properties of tabularData (or use properties ) and rows of tabularData and log it. Falls back to just logging the argument if it can’t be parsed as tabular. // These can't be parsed as tabular data console.table(Symbol()); // Symbol() console.group([...label]) console.groupCollapsed() console.groupEnd() console.info([data][, ...args]) console.log([data][, ...args]) console.table(tabularData[, properties]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 199/739 console.table(undefined); // undefined console.table([{ a: 1, b: 'Y' }, { a: 'Z', b: 2 }]); // ┌─────────┬─────┬─────┐ // │ (index) │ a │ b │ // ├─────────┼─────┼─────┤ // │ 0 │ 1 │ 'Y' │ // │ 1 │ 'Z' │ 2 │ // └─────────┴─────┴─────┘ console.table([{ a: 1, b: 'Y' }, { a: 'Z', b: 2 }], ['a']); // ┌─────────┬─────┐ // │ (index) │ a │ // ├─────────┼─────┤ // │ 0 │ 1 │ // │ 1 │ 'Z' │ // └─────────┴─────┘ label <string> Default: 'default' Starts a timer that can be used to compute the duration of an operation. Timers are identified by a unique label . Use the same label when calling console.timeEnd() to stop the timer and output the elapsed time in milliseconds to stdout . Timer durations are accurate to the sub- millisecond. label <string> Default: 'default' Stops a timer that was previously started by calling console.time() and prints the result to stdout : console.time('100-elements'); for (let i = 0; i < 100; i++) {} console.timeEnd('100-elements'); // prints 100-elements: 225.438ms label <string> Default: 'default' ...data <any> For a timer that was previously started by calling console.time() , prints the elapsed time and other data arguments to stdout : console.time('process'); const value = expensiveProcess1(); // Returns 42 console.timeLog('process', value); // Prints "process: 365.227ms 42". doExpensiveProcess2(value); console.timeEnd('process'); console.time([label]) console.timeEnd([label]) console.timeLog([label][, ...data]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 200/739 message <any> ...args <any> Prints to stderr the string 'Trace: ' , followed by the util.format() formatted message and stack trace to the current position in the code. console.trace('Show me'); // Prints: (stack trace will vary based on where trace is called) // Trace: Show me // at repl:2:9 // at REPLServer.defaultEval (repl.js:248:27) // at bound (domain.js:287:14) // at REPLServer.runBound [as eval] (domain.js:300:12) // at REPLServer.<anonymous> (repl.js:412:12) // at emitOne (events.js:82:20) // at REPLServer.emit (events.js:169:7) // at REPLServer.Interface._onLine (readline.js:210:10) // at REPLServer.Interface._line (readline.js:549:8) // at REPLServer.Interface._ttyWrite (readline.js:826:14) data <any> ...args <any> The console.warn() function is an alias for console.error() . The following methods are exposed by the V8 engine in the general API but do not display anything unless used in conjunction with the inspector ( --inspect flag). label <string> Default: 'default' This method does not display anything unless used in the inspector. The console.markTimeline() method is the deprecated form of console.timeStamp() . label <string> This method does not display anything unless used in the inspector. The console.profile() method starts a JavaScript CPU profile with an optional label until console.profileEnd() is called. The profile is then added to the Profile panel of the inspector. console.profile('MyLabel'); // Some code console.profileEnd('MyLabel'); // Adds the profile 'MyLabel' to the Profiles panel of the inspector. console.trace([message][, ...args]) console.warn([data][, ...args]) Inspector only methods console.markTimeline([label]) console.profile([label]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 201/739 label <string> This method does not display anything unless used in the inspector. Stops the current JavaScript CPU profiling session if one has been started and prints the report to the Profiles panel of the inspector. See console.profile() for an example. If this method is called without a label, the most recently started profile is stopped. label <string> This method does not display anything unless used in the inspector. The console.timeStamp() method adds an event with the label 'label' to the Timeline panel of the inspector. label <string> Default: 'default' This method does not display anything unless used in the inspector. The console.timeline() method is the deprecated form of console.time() . label <string> Default: 'default' This method does not display anything unless used in the inspector. The console.timelineEnd() method is the deprecated form of console.timeEnd() . Stability: 2 - Stable The crypto module provides cryptographic functionality that includes a set of wrappers for OpenSSL's hash, HMAC, cipher, decipher, sign, and verify functions. Use require('crypto') to access this module. const crypto = require('crypto'); const secret = 'abcdefg'; const hash = crypto.createHmac('sha256', secret) .update('I love cupcakes') .digest('hex'); console.log(hash); // Prints: // c0fa1bc00531bd78ef38c628449c5102aeabd49b5dc3a2a516ea6ea959d6658e It is possible for Node.js to be built without including support for the crypto module. In such cases, calling require('crypto') will result in an error being thrown. console.profileEnd([label]) console.timeStamp([label]) console.timeline([label]) console.timelineEnd([label]) Crypto Determining if crypto support is unavailable 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 202/739 let crypto; try { crypto = require('crypto'); } catch (err) { console.log('crypto support is disabled!'); } SPKAC is a Certificate Signing Request mechanism originally implemented by Netscape and was specified formally as part of HTML5's keygen element . Note that <keygen> is deprecated since HTML 5.2 and new projects should not use this element anymore. The crypto module provides the Certificate class for working with SPKAC data. The most common usage is handling output generated by the HTML5 <keygen> element. Node.js uses OpenSSL's SPKAC implementation internally. spkac <string> | <Buffer> | <TypedArray> | <DataView> Returns: <Buffer> The challenge component of the spkac data structure, which includes a public key and a challenge. const { Certificate } = require('crypto'); const spkac = getSpkacSomehow(); const challenge = Certificate.exportChallenge(spkac); console.log(challenge.toString('utf8')); // Prints: the challenge as a UTF8 string spkac <string> | <Buffer> | <TypedArray> | <DataView> encoding <string> The encoding of the spkac string. Returns: <Buffer> The public key component of the spkac data structure, which includes a public key and a challenge. const { Certificate } = require('crypto'); const spkac = getSpkacSomehow(); const publicKey = Certificate.exportPublicKey(spkac); console.log(publicKey); // Prints: the public key as <Buffer ...> spkac <Buffer> | <TypedArray> | <DataView> Returns: <boolean> true if the given spkac data structure is valid, false otherwise. const { Certificate } = require('crypto'); const spkac = getSpkacSomehow(); console.log(Certificate.verifySpkac(Buffer.from(spkac))); // Prints: true or false Class: Certificate Certificate.exportChallenge(spkac) Certificate.exportPublicKey(spkac[, encoding]) Certificate.verifySpkac(spkac) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 203/739 As a still supported legacy interface, it is possible (but not recommended) to create new instances of the crypto.Certificate class as illustrated in the examples below. Instances of the Certificate class can be created using the new keyword or by calling crypto.Certificate() as a function: const crypto = require('crypto'); const cert1 = new crypto.Certificate(); const cert2 = crypto.Certificate(); spkac <string> | <Buffer> | <TypedArray> | <DataView> Returns: <Buffer> The challenge component of the spkac data structure, which includes a public key and a challenge. const cert = require('crypto').Certificate(); const spkac = getSpkacSomehow(); const challenge = cert.exportChallenge(spkac); console.log(challenge.toString('utf8')); // Prints: the challenge as a UTF8 string spkac <string> | <Buffer> | <TypedArray> | <DataView> Returns: <Buffer> The public key component of the spkac data structure, which includes a public key and a challenge. const cert = require('crypto').Certificate(); const spkac = getSpkacSomehow(); const publicKey = cert.exportPublicKey(spkac); console.log(publicKey); // Prints: the public key as <Buffer ...> spkac <Buffer> | <TypedArray> | <DataView> Returns: <boolean> true if the given spkac data structure is valid, false otherwise. const cert = require('crypto').Certificate(); const spkac = getSpkacSomehow(); console.log(cert.verifySpkac(Buffer.from(spkac))); // Prints: true or false Legacy API new crypto.Certificate() certificate.exportChallenge(spkac) certificate.exportPublicKey(spkac) certificate.verifySpkac(spkac) Class: Cipher 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 204/739 Instances of the Cipher class are used to encrypt data. The class can be used in one of two ways: As a stream that is both readable and writable, where plain unencrypted data is written to produce encrypted data on the readable side, or Using the cipher.update() and cipher.final() methods to produce the encrypted data. The crypto.createCipher() or crypto.createCipheriv() methods are used to create Cipher instances. Cipher objects are not to be created directly using the new keyword. Example: Using Cipher objects as streams: const crypto = require('crypto'); const algorithm = 'aes-192-cbc'; const password = 'Password used to generate key'; // Key length is dependent on the algorithm. In this case for aes192, it is // 24 bytes (192 bits). // Use async `crypto.scrypt()` instead. const key = crypto.scryptSync(password, 'salt', 24); // Use `crypto.randomBytes()` to generate a random iv instead of the static iv // shown here. const iv = Buffer.alloc(16, 0); // Initialization vector. const cipher = crypto.createCipheriv(algorithm, key, iv); let encrypted = ''; cipher.on('readable', () => { let chunk; while (null !== (chunk = cipher.read())) { encrypted += chunk.toString('hex'); } }); cipher.on('end', () => { console.log(encrypted); // Prints: e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa }); cipher.write('some clear text data'); cipher.end(); Example: Using Cipher and piped streams: const crypto = require('crypto'); const fs = require('fs'); const algorithm = 'aes-192-cbc'; const password = 'Password used to generate key'; // Use the async `crypto.scrypt()` instead. const key = crypto.scryptSync(password, 'salt', 24); // Use `crypto.randomBytes()` to generate a random iv instead of the static iv // shown here. const iv = Buffer.alloc(16, 0); // Initialization vector. const cipher = crypto.createCipheriv(algorithm, key, iv); 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 205/739 const input = fs.createReadStream('test.js'); const output = fs.createWriteStream('test.enc'); input.pipe(cipher).pipe(output); Example: Using the cipher.update() and cipher.final() methods: const crypto = require('crypto'); const algorithm = 'aes-192-cbc'; const password = 'Password used to generate key'; // Use the async `crypto.scrypt()` instead. const key = crypto.scryptSync(password, 'salt', 24); // Use `crypto.randomBytes` to generate a random iv instead of the static iv // shown here. const iv = Buffer.alloc(16, 0); // Initialization vector. const cipher = crypto.createCipheriv(algorithm, key, iv); let encrypted = cipher.update('some clear text data', 'utf8', 'hex'); encrypted += cipher.final('hex'); console.log(encrypted); // Prints: e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa outputEncoding <string> The encoding of the return value. Returns: <Buffer> | <string> Any remaining enciphered contents. If outputEncoding is specified, a string is returned. If an outputEncoding is not provided, a Buffer is returned. Once the cipher.final() method has been called, the Cipher object can no longer be used to encrypt data. Attempts to call cipher.final() more than once will result in an error being thrown. buffer <Buffer> options <Object> stream.transform options plaintextLength <number> Returns: <Cipher> for method chaining. When using an authenticated encryption mode ( GCM , CCM and OCB are currently supported), the cipher.setAAD() method sets the value used for the additional authenticated data (AAD) input parameter. The options argument is optional for GCM and OCB . When using CCM , the plaintextLength option must be specified and its value must match the length of the plaintext in bytes. See CCM mode . The cipher.setAAD() method must be called before cipher.update() . cipher.final([outputEncoding]) cipher.setAAD(buffer[, options]) cipher.getAuthTag() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 206/739 Returns: <Buffer> When using an authenticated encryption mode ( GCM , CCM and OCB are currently supported), the cipher.getAuthTag() method returns a Buffer containing the authentication tag that has been computed from the given data. The cipher.getAuthTag() method should only be called after encryption has been completed using the cipher.final() method. autoPadding <boolean> Default: true Returns: <Cipher> for method chaining. When using block encryption algorithms, the Cipher class will automatically add padding to the input data to the appropriate block size. To disable the default padding call cipher.setAutoPadding(false) . When autoPadding is false , the length of the entire input data must be a multiple of the cipher's block size or cipher.final() will throw an error. Disabling automatic padding is useful for non-standard padding, for instance using 0x0 instead of PKCS padding. The cipher.setAutoPadding() method must be called before cipher.final() . data <string> | <Buffer> | <TypedArray> | <DataView> inputEncoding <string> The encoding of the data. outputEncoding <string> The encoding of the return value. Returns: <Buffer> | <string> Updates the cipher with data . If the inputEncoding argument is given, the data argument is a string using the specified encoding. If the inputEncoding argument is not given, data must be a Buffer , TypedArray , or DataView . If data is a Buffer , TypedArray , or DataView , then inputEncoding is ignored. The outputEncoding specifies the output format of the enciphered data. If the outputEncoding is specified, a string using the specified encoding is returned. If no outputEncoding is provided, a Buffer is returned. The cipher.update() method can be called multiple times with new data until cipher.final() is called. Calling cipher.update() after cipher.final() will result in an error being thrown. Instances of the Decipher class are used to decrypt data. The class can be used in one of two ways: As a stream that is both readable and writable, where plain encrypted data is written to produce unencrypted data on the readable side, or Using the decipher.update() and decipher.final() methods to produce the unencrypted data. The crypto.createDecipher() or crypto.createDecipheriv() methods are used to create Decipher instances. Decipher objects are not to be created directly using the new keyword. Example: Using Decipher objects as streams: const crypto = require('crypto'); const algorithm = 'aes-192-cbc'; const password = 'Password used to generate key'; // Key length is dependent on the algorithm. In this case for aes192, it is // 24 bytes (192 bits). // Use the async `crypto.scrypt()` instead. const key = crypto.scryptSync(password, 'salt', 24); cipher.setAutoPadding([autoPadding]) cipher.update(data[, inputEncoding][, outputencoding]) Class: Decipher 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 207/739 // The IV is usually passed along with the ciphertext. const iv = Buffer.alloc(16, 0); // Initialization vector. const decipher = crypto.createDecipheriv(algorithm, key, iv); let decrypted = ''; decipher.on('readable', () => { while (null !== (chunk = decipher.read())) { decrypted += chunk.toString('utf8'); } }); decipher.on('end', () => { console.log(decrypted); // Prints: some clear text data }); // Encrypted with same algorithm, key and iv. const encrypted = 'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa'; decipher.write(encrypted, 'hex'); decipher.end(); Example: Using Decipher and piped streams: const crypto = require('crypto'); const fs = require('fs'); const algorithm = 'aes-192-cbc'; const password = 'Password used to generate key'; // Use the async `crypto.scrypt()` instead. const key = crypto.scryptSync(password, 'salt', 24); // The IV is usually passed along with the ciphertext. const iv = Buffer.alloc(16, 0); // Initialization vector. const decipher = crypto.createDecipheriv(algorithm, key, iv); const input = fs.createReadStream('test.enc'); const output = fs.createWriteStream('test.js'); input.pipe(decipher).pipe(output); Example: Using the decipher.update() and decipher.final() methods: const crypto = require('crypto'); const algorithm = 'aes-192-cbc'; const password = 'Password used to generate key'; // Use the async `crypto.scrypt()` instead. const key = crypto.scryptSync(password, 'salt', 24); // The IV is usually passed along with the ciphertext. const iv = Buffer.alloc(16, 0); // Initialization vector. t d i h t t D i h i ( l ith k i ) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 208/739 const decipher = crypto.createDecipheriv(algorithm, key, iv); // Encrypted using same algorithm, key and iv. const encrypted = 'e5f79c5915c02171eec6b212d5520d44480993d7d622a7c4c2da32f6efda0ffa'; let decrypted = decipher.update(encrypted, 'hex', 'utf8'); decrypted += decipher.final('utf8'); console.log(decrypted); // Prints: some clear text data outputEncoding <string> The encoding of the return value. Returns: <Buffer> | <string> Any remaining deciphered contents. If outputEncoding is specified, a string is returned. If an outputEncoding is not provided, a Buffer is returned. Once the decipher.final() method has been called, the Decipher object can no longer be used to decrypt data. Attempts to call decipher.final() more than once will result in an error being thrown. buffer <Buffer> | <TypedArray> | <DataView> options <Object> stream.transform options plaintextLength <number> Returns: <Decipher> for method chaining. When using an authenticated encryption mode ( GCM , CCM and OCB are currently supported), the decipher.setAAD() method sets the value used for the additional authenticated data (AAD) input parameter. The options argument is optional for GCM . When using CCM , the plaintextLength option must be specified and its value must match the length of the plaintext in bytes. See CCM mode . The decipher.setAAD() method must be called before decipher.update() . buffer <Buffer> | <TypedArray> | <DataView> Returns: <Decipher> for method chaining. When using an authenticated encryption mode ( GCM , CCM and OCB are currently supported), the decipher.setAuthTag() method is used to pass in the received authentication tag. If no tag is provided, or if the cipher text has been tampered with, decipher.final() will throw, indicating that the cipher text should be discarded due to failed authentication. Note that this Node.js version does not verify the length of GCM authentication tags. Such a check must be implemented by applications and is crucial to the authenticity of the encrypted data, otherwise, an attacker can use an arbitrarily short authentication tag to increase the chances of successfully passing authentication (up to 0.39%). It is highly recommended to associate one of the values 16, 15, 14, 13, 12, 8 or 4 bytes with each key, and to only permit authentication tags of that length, see NIST SP 800-38D . The decipher.setAuthTag() method must be called before decipher.final() . autoPadding <boolean> Default: true decipher.final([outputEncoding]) decipher.setAAD(buffer[, options]) decipher.setAuthTag(buffer) decipher.setAutoPadding([autoPadding]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 209/739 Returns: <Decipher> for method chaining. When data has been encrypted without standard block padding, calling decipher.setAutoPadding(false) will disable automatic padding to prevent decipher.final() from checking for and removing padding. Turning auto padding off will only work if the input data's length is a multiple of the ciphers block size. The decipher.setAutoPadding() method must be called before decipher.final() . data <string> | <Buffer> | <TypedArray> | <DataView> inputEncoding <string> The encoding of the data string. outputEncoding <string> The encoding of the return value. Returns: <Buffer> | <string> Updates the decipher with data . If the inputEncoding argument is given, the data argument is a string using the specified encoding. If the inputEncoding argument is not given, data must be a Buffer . If data is a Buffer then inputEncoding is ignored. The outputEncoding specifies the output format of the enciphered data. If the outputEncoding is specified, a string using the specified encoding is returned. If no outputEncoding is provided, a Buffer is returned. The decipher.update() method can be called multiple times with new data until decipher.final() is called. Calling decipher.update() after decipher.final() will result in an error being thrown. The DiffieHellman class is a utility for creating Diffie-Hellman key exchanges. Instances of the DiffieHellman class can be created using the crypto.createDiffieHellman() function. const crypto = require('crypto'); const assert = require('assert'); // Generate Alice's keys... const alice = crypto.createDiffieHellman(2048); const aliceKey = alice.generateKeys(); // Generate Bob's keys... const bob = crypto.createDiffieHellman(alice.getPrime(), alice.getGenerator()); const bobKey = bob.generateKeys(); // Exchange and generate the secret... const aliceSecret = alice.computeSecret(bobKey); const bobSecret = bob.computeSecret(aliceKey); // OK assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex')); otherPublicKey <string> | <Buffer> | <TypedArray> | <DataView> inputEncoding <string> The encoding of an otherPublicKey string. decipher.update(data[, inputEncoding][, outputencoding]) Class: DiffieHellman diffieHellman.computeSecret(otherPublicKey[, inputEncoding][, outputencoding]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 210/739 outputEncoding <string> The encoding of the return value. Returns: <Buffer> | <string> Computes the shared secret using otherPublicKey as the other party's public key and returns the computed shared secret. The supplied key is interpreted using the specified inputEncoding , and secret is encoded using specified outputEncoding . If the inputEncoding is not provided, otherPublicKey is expected to be a Buffer , TypedArray , or DataView . If outputEncoding is given a string is returned; otherwise, a Buffer is returned. encoding <string> The encoding of the return value. Returns: <Buffer> | <string> Generates private and public Diffie-Hellman key values, and returns the public key in the specified encoding . This key should be transferred to the other party. If encoding is provided a string is returned; otherwise a Buffer is returned. encoding <string> The encoding of the return value. Returns: <Buffer> | <string> Returns the Diffie-Hellman generator in the specified encoding . If encoding is provided a string is returned; otherwise a Buffer is returned. encoding <string> The encoding of the return value. Returns: <Buffer> | <string> Returns the Diffie-Hellman prime in the specified encoding . If encoding is provided a string is returned; otherwise a Buffer is returned. encoding <string> The encoding of the return value. Returns: <Buffer> | <string> Returns the Diffie-Hellman private key in the specified encoding . If encoding is provided a string is returned; otherwise a Buffer is returned. encoding <string> The encoding of the return value. Returns: <Buffer> | <string> Returns the Diffie-Hellman public key in the specified encoding . If encoding is provided a string is returned; otherwise a Buffer is returned. privateKey <string> | <Buffer> | <TypedArray> | <DataView> encoding <string> The encoding of the privateKey string. Sets the Diffie-Hellman private key. If the encoding argument is provided, privateKey is expected to be a string. If no encoding is provided, privateKey is expected to be a Buffer , TypedArray , or DataView . diffieHellman.generateKeys( encoding ) diffieHellman.getGenerator( encoding ) diffieHellman.getPrime( encoding ) diffieHellman.getPrivateKey( encoding ) diffieHellman.getPublicKey( encoding ) diffieHellman.setPrivateKey(privateKey[, encoding]) diffieHellman.setPublicKey(publicKey[, encoding]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 211/739 publicKey <string> | <Buffer> | <TypedArray> | <DataView> encoding <string> The encoding of the publicKey string. Sets the Diffie-Hellman public key. If the encoding argument is provided, publicKey is expected to be a string. If no encoding is provided, publicKey is expected to be a Buffer , TypedArray , or DataView . A bit field containing any warnings and/or errors resulting from a check performed during initialization of the DiffieHellman object. The following values are valid for this property (as defined in constants module): DH_CHECK_P_NOT_SAFE_PRIME DH_CHECK_P_NOT_PRIME DH_UNABLE_TO_CHECK_GENERATOR DH_NOT_SUITABLE_GENERATOR The ECDH class is a utility for creating Elliptic Curve Diffie-Hellman (ECDH) key exchanges. Instances of the ECDH class can be created using the crypto.createECDH() function. const crypto = require('crypto'); const assert = require('assert'); // Generate Alice's keys... const alice = crypto.createECDH('secp521r1'); const aliceKey = alice.generateKeys(); // Generate Bob's keys... const bob = crypto.createECDH('secp521r1'); const bobKey = bob.generateKeys(); // Exchange and generate the secret... const aliceSecret = alice.computeSecret(bobKey); const bobSecret = bob.computeSecret(aliceKey); assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex')); // OK key <string> | <Buffer> | <TypedArray> | <DataView> curve <string> inputEncoding <string> The encoding of the key string. outputEncoding <string> The encoding of the return value. format <string> Default: 'uncompressed' Returns: <Buffer> | <string> diffieHellman.verifyError Class: ECDH Class Method: ECDH.convertKey(key, curve[, inputEncoding[, outputEncoding[, format]]]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 212/739 Converts the EC Diffie-Hellman public key specified by key and curve to the format specified by format . The format argument specifies point encoding and can be 'compressed' , 'uncompressed' or 'hybrid' . The supplied key is interpreted using the specified inputEncoding , and the returned key is encoded using the specified outputEncoding . Use crypto.getCurves() to obtain a list of available curve names. On recent OpenSSL releases, openssl ecparam -list_curves will also display the name and description of each available elliptic curve. If format is not specified the point will be returned in 'uncompressed' format. If the inputEncoding is not provided, key is expected to be a Buffer , TypedArray , or DataView . Example (uncompressing a key): const { createECDH, ECDH } = require('crypto'); const ecdh = createECDH('secp256k1'); ecdh.generateKeys(); const compressedKey = ecdh.getPublicKey('hex', 'compressed'); const uncompressedKey = ECDH.convertKey(compressedKey, 'secp256k1', 'hex', 'hex', 'uncompressed'); // the converted key and the uncompressed public key should be the same console.log(uncompressedKey === ecdh.getPublicKey('hex')); otherPublicKey <string> | <Buffer> | <TypedArray> | <DataView> inputEncoding <string> The encoding of the otherPublicKey string. outputEncoding <string> The encoding of the return value. Returns: <Buffer> | <string> Computes the shared secret using otherPublicKey as the other party's public key and returns the computed shared secret. The supplied key is interpreted using specified inputEncoding , and the returned secret is encoded using the specified outputEncoding . If the inputEncoding is not provided, otherPublicKey is expected to be a Buffer , TypedArray , or DataView . If outputEncoding is given a string will be returned; otherwise a Buffer is returned. ecdh.computeSecret will throw an ERR_CRYPTO_ECDH_INVALID_PUBLIC_KEY error when otherPublicKey lies outside of the elliptic curve. Since otherPublicKey is usually supplied from a remote user over an insecure network, its recommended for developers to handle this exception accordingly. encoding <string> The encoding of the return value. format <string> Default: 'uncompressed' Returns: <Buffer> | <string> ecdh.computeSecret(otherPublicKey[, inputEncoding][, outputencoding]) ecdh.generateKeys([encoding[, format]]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 213/739 Generates private and public EC Diffie-Hellman key values, and returns the public key in the specified format and encoding . This key should be transferred to the other party. The format argument specifies point encoding and can be 'compressed' or 'uncompressed' . If format is not specified, the point will be returned in 'uncompressed' format. If encoding is provided a string is returned; otherwise a Buffer is returned. encoding <string> The encoding of the return value. Returns: <Buffer> | <string> The EC Diffie-Hellman in the specified encoding . If encoding is specified, a string is returned; otherwise a Buffer is returned. encoding <string> The encoding of the return value. format <string> Default: 'uncompressed' Returns: <Buffer> | <string> The EC Diffie-Hellman public key in the specified encoding and format . The format argument specifies point encoding and can be 'compressed' or 'uncompressed' . If format is not specified the point will be returned in 'uncompressed' format. If encoding is specified, a string is returned; otherwise a Buffer is returned. privateKey <string> | <Buffer> | <TypedArray> | <DataView> encoding <string> The encoding of the privateKey string. Sets the EC Diffie-Hellman private key. If encoding is provided, privateKey is expected to be a string; otherwise privateKey is expected to be a Buffer , TypedArray , or DataView . If privateKey is not valid for the curve specified when the ECDH object was created, an error is thrown. Upon setting the private key, the associated public point (key) is also generated and set in the ECDH object. Stability: 0 - Deprecated publicKey <string> | <Buffer> | <TypedArray> | <DataView> encoding <string> The encoding of the publicKey string. Sets the EC Diffie-Hellman public key. If encoding is provided publicKey is expected to be a string; otherwise a Buffer , TypedArray , or DataView is expected. Note that there is not normally a reason to call this method because ECDH only requires a private key and the other party's public key to compute the shared secret. Typically either ecdh.generateKeys() or ecdh.setPrivateKey() will be called. The ecdh.setPrivateKey() method attempts to generate the public point/key associated with the private key being set. Example (obtaining a shared secret): const crypto = require('crypto'); ecdh.getPrivateKey( encoding ) ecdh.getPublicKey([encoding][, format]) ecdh.setPrivateKey(privateKey[, encoding]) ecdh.setPublicKey(publicKey[, encoding]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 214/739 const alice = crypto.createECDH('secp256k1'); const bob = crypto.createECDH('secp256k1'); // This is a shortcut way of specifying one of Alice's previous private // keys. It would be unwise to use such a predictable private key in a real // application. alice.setPrivateKey( crypto.createHash('sha256').update('alice', 'utf8').digest() ); // Bob uses a newly generated cryptographically strong // pseudorandom key pair bob.generateKeys(); const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex'); const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex'); // aliceSecret and bobSecret should be the same shared secret value console.log(aliceSecret === bobSecret); The Hash class is a utility for creating hash digests of data. It can be used in one of two ways: As a stream that is both readable and writable, where data is written to produce a computed hash digest on the readable side, or Using the hash.update() and hash.digest() methods to produce the computed hash. The crypto.createHash() method is used to create Hash instances. Hash objects are not to be created directly using the new keyword. Example: Using Hash objects as streams: const crypto = require('crypto'); const hash = crypto.createHash('sha256'); hash.on('readable', () => { // Only one element is going to be produced by the // hash stream. const data = hash.read(); if (data) { console.log(data.toString('hex')); // Prints: // 6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50 } }); hash.write('some data to hash'); hash.end(); Example: Using Hash and piped streams: const crypto = require('crypto'); const fs = require('fs'); Class: Hash 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 215/739 const hash = crypto.createHash('sha256'); const input = fs.createReadStream('test.js'); input.pipe(hash).pipe(process.stdout); Example: Using the hash.update() and hash.digest() methods: const crypto = require('crypto'); const hash = crypto.createHash('sha256'); hash.update('some data to hash'); console.log(hash.digest('hex')); // Prints: // 6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50 encoding <string> The encoding of the return value. Returns: <Buffer> | <string> Calculates the digest of all of the data passed to be hashed (using the hash.update() method). If encoding is provided a string will be returned; otherwise a Buffer is returned. The Hash object can not be used again after hash.digest() method has been called. Multiple calls will cause an error to be thrown. data <string> | <Buffer> | <TypedArray> | <DataView> inputEncoding <string> The encoding of the data string. Updates the hash content with the given data , the encoding of which is given in inputEncoding . If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer , TypedArray , or DataView , then inputEncoding is ignored. This can be called many times with new data as it is streamed. The Hmac Class is a utility for creating cryptographic HMAC digests. It can be used in one of two ways: As a stream that is both readable and writable, where data is written to produce a computed HMAC digest on the readable side, or Using the hmac.update() and hmac.digest() methods to produce the computed HMAC digest. The crypto.createHmac() method is used to create Hmac instances. Hmac objects are not to be created directly using the new keyword. Example: Using Hmac objects as streams: const crypto = require('crypto'); const hmac = crypto.createHmac('sha256', 'a secret'); hmac.on('readable', () => { // Only one element is going to be produced by the // hash stream. const data = hmac.read(); if (data) { hash.digest( encoding ) hash.update(data[, inputEncoding]) Class: Hmac 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 216/739 console.log(data.toString('hex')); // Prints: // 7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e } }); hmac.write('some data to hash'); hmac.end(); Example: Using Hmac and piped streams: const crypto = require('crypto'); const fs = require('fs'); const hmac = crypto.createHmac('sha256', 'a secret'); const input = fs.createReadStream('test.js'); input.pipe(hmac).pipe(process.stdout); Example: Using the hmac.update() and hmac.digest() methods: const crypto = require('crypto'); const hmac = crypto.createHmac('sha256', 'a secret'); hmac.update('some data to hash'); console.log(hmac.digest('hex')); // Prints: // 7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e encoding <string> The encoding of the return value. Returns: <Buffer> | <string> Calculates the HMAC digest of all of the data passed using hmac.update() . If encoding is provided a string is returned; otherwise a Buffer is returned; The Hmac object can not be used again after hmac.digest() has been called. Multiple calls to hmac.digest() will result in an error being thrown. data <string> | <Buffer> | <TypedArray> | <DataView> inputEncoding <string> The encoding of the data string. Updates the Hmac content with the given data , the encoding of which is given in inputEncoding . If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer , TypedArray , or DataView , then inputEncoding is ignored. This can be called many times with new data as it is streamed. The Sign Class is a utility for generating signatures. It can be used in one of two ways: hmac.digest( encoding ) hmac.update(data[, inputEncoding]) Class: Sign 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 217/739 As a writable stream , where data to be signed is written and the sign.sign() method is used to generate and return the signature, or Using the sign.update() and sign.sign() methods to produce the signature. The crypto.createSign() method is used to create Sign instances. The argument is the string name of the hash function to use. Sign objects are not to be created directly using the new keyword. Example: Using Sign objects as streams: const crypto = require('crypto'); const sign = crypto.createSign('SHA256'); sign.write('some data to sign'); sign.end(); const privateKey = getPrivateKeySomehow(); console.log(sign.sign(privateKey, 'hex')); // Prints: the calculated signature using the specified private key and // SHA-256. For RSA keys, the algorithm is RSASSA-PKCS1-v1_5 (see padding // parameter below for RSASSA-PSS). For EC keys, the algorithm is ECDSA. Example: Using the sign.update() and sign.sign() methods: const crypto = require('crypto'); const sign = crypto.createSign('SHA256'); sign.update('some data to sign'); const privateKey = getPrivateKeySomehow(); console.log(sign.sign(privateKey, 'hex')); // Prints: the calculated signature In some cases, a Sign instance can also be created by passing in a signature algorithm name, such as 'RSA-SHA256'. This will use the corresponding digest algorithm. This does not work for all signature algorithms, such as 'ecdsa-with-SHA256'. Use digest names instead. Example: signing using legacy signature algorithm name const crypto = require('crypto'); const sign = crypto.createSign('RSA-SHA256'); sign.update('some data to sign'); const privateKey = getPrivateKeySomehow(); console.log(sign.sign(privateKey, 'hex')); // Prints: the calculated signature privateKey <string> | <Object> key <string> passphrase <string> sign.sign(privateKey[, outputEncoding]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 218/739 padding <integer> saltLength <integer> outputEncoding <string> The encoding of the return value. Returns: <Buffer> | <string> Calculates the signature on all the data passed through using either sign.update() or sign.write() . The privateKey argument can be an object or a string. If privateKey is a string, it is treated as a raw key with no passphrase. If privateKey is an object, it must contain one or more of the following properties: key : <string> - PEM encoded private key (required) passphrase : <string> - passphrase for the private key padding : <integer> - Optional padding value for RSA, one of the following: crypto.constants.RSA_PKCS1_PADDING (default) crypto.constants.RSA_PKCS1_PSS_PADDING Note that RSA_PKCS1_PSS_PADDING will use MGF1 with the same hash function used to sign the message as specified in section 3.1 of RFC 4055 . saltLength : <integer> - salt length for when padding is RSA_PKCS1_PSS_PADDING . The special value crypto.constants.RSA_PSS_SALTLEN_DIGEST sets the salt length to the digest size, crypto.constants.RSA_PSS_SALTLEN_MAX_SIGN (default) sets it to the maximum permissible value. If outputEncoding is provided a string is returned; otherwise a Buffer is returned. The Sign object can not be again used after sign.sign() method has been called. Multiple calls to sign.sign() will result in an error being thrown. data <string> | <Buffer> | <TypedArray> | <DataView> inputEncoding <string> The encoding of the data string. Updates the Sign content with the given data , the encoding of which is given in inputEncoding . If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer , TypedArray , or DataView , then inputEncoding is ignored. This can be called many times with new data as it is streamed. The Verify class is a utility for verifying signatures. It can be used in one of two ways: As a writable stream where written data is used to validate against the supplied signature, or Using the verify.update() and verify.verify() methods to verify the signature. The crypto.createVerify() method is used to create Verify instances. Verify objects are not to be created directly using the new keyword. Example: Using Verify objects as streams: const crypto = require('crypto'); const verify = crypto.createVerify('SHA256'); sign.update(data[, inputEncoding]) Class: Verify 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 219/739 verify.write('some data to sign'); verify.end(); const publicKey = getPublicKeySomehow(); const signature = getSignatureToVerify(); console.log(verify.verify(publicKey, signature)); // Prints: true or false Example: Using the verify.update() and verify.verify() methods: const crypto = require('crypto'); const verify = crypto.createVerify('SHA256'); verify.update('some data to sign'); const publicKey = getPublicKeySomehow(); const signature = getSignatureToVerify(); console.log(verify.verify(publicKey, signature)); // Prints: true or false data <string> | <Buffer> | <TypedArray> | <DataView> inputEncoding <string> The encoding of the data string. Updates the Verify content with the given data , the encoding of which is given in inputEncoding . If inputEncoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer , TypedArray , or DataView , then inputEncoding is ignored. This can be called many times with new data as it is streamed. object <string> | <Object> signature <string> | <Buffer> | <TypedArray> | <DataView> signatureEncoding <string> The encoding of the signature string. Returns: <boolean> true or false depending on the validity of the signature for the data and public key. Verifies the provided data using the given object and signature . The object argument can be either a string containing a PEM encoded object, which can be an RSA public key, a DSA public key, or an X.509 certificate, or an object with one or more of the following properties: key : <string> - PEM encoded public key (required) padding : <integer> - Optional padding value for RSA, one of the following: crypto.constants.RSA_PKCS1_PADDING (default) crypto.constants.RSA_PKCS1_PSS_PADDING Note that RSA_PKCS1_PSS_PADDING will use MGF1 with the same hash function used to verify the message as specified in section 3.1 of RFC 4055 . saltLength : <integer> - salt length for when padding is RSA_PKCS1_PSS_PADDING . The special value crypto.constants.RSA_PSS_SALTLEN_DIGEST sets the salt length to the digest size, crypto.constants.RSA_PSS_SALTLEN_AUTO (default) causes it to be determined automatically. verify.update(data[, inputEncoding]) verify.verify(object, signature[, signatureEncoding]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 220/739 The signature argument is the previously calculated signature for the data, in the signatureEncoding . If a signatureEncoding is specified, the signature is expected to be a string; otherwise signature is expected to be a Buffer , TypedArray , or DataView . The verify object can not be used again after verify.verify() has been called. Multiple calls to verify.verify() will result in an error being thrown. Returns: <Object> An object containing commonly used constants for crypto and security related operations. The specific constants currently defined are described in Crypto Constants . Stability: 0 - Deprecated The default encoding to use for functions that can take either strings or buffers . The default value is 'buffer' , which makes methods default to Buffer objects. The crypto.DEFAULT_ENCODING mechanism is provided for backwards compatibility with legacy programs that expect 'latin1' to be the default encoding. New applications should expect the default to be 'buffer' . This property is deprecated. Stability: 0 - Deprecated Property for checking and controlling whether a FIPS compliant crypto provider is currently in use. Setting to true requires a FIPS build of Node.js. This property is deprecated. Please use crypto.setFips() and crypto.getFips() instead. Stability: 0 - Deprecated: Use crypto.createCipheriv() instead. algorithm <string> password <string> | <Buffer> | <TypedArray> | <DataView> options <Object> stream.transform options Returns: <Cipher> Creates and returns a Cipher object that uses the given algorithm and password . The options argument controls stream behavior and is optional except when a cipher in CCM or OCB mode is used (e.g. 'aes-128-ccm' ). In that case, the authTagLength option is required and specifies the length of the authentication tag in bytes, see CCM mode . In GCM mode, the crypto module methods and properties crypto.constants crypto.DEFAULT_ENCODING crypto.fips crypto.createCipher(algorithm, password[, options]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 221/739 authTagLength option is not required but can be used to set the length of the authentication tag that will be returned by getAuthTag() and defaults to 16 bytes. The algorithm is dependent on OpenSSL, examples are 'aes192' , etc. On recent OpenSSL releases, openssl list -cipher-algorithms ( openssl list-cipher-algorithms for older versions of OpenSSL) will display the available cipher algorithms. The password is used to derive the cipher key and initialization vector (IV). The value must be either a 'latin1' encoded string, a Buffer , a TypedArray , or a DataView . The implementation of crypto.createCipher() derives keys using the OpenSSL function EVP_BytesToKey with the digest algorithm set to MD5, one iteration, and no salt. The lack of salt allows dictionary attacks as the same password always creates the same key. The low iteration count and non-cryptographically secure hash algorithm allow passwords to be tested very rapidly. In line with OpenSSL's recommendation to use a more modern algorithm instead of EVP_BytesToKey it is recommended that developers derive a key and IV on their own using crypto.scrypt() and to use crypto.createCipheriv() to create the Cipher object. Users should not use ciphers with counter mode (e.g. CTR, GCM, or CCM) in crypto.createCipher() . A warning is emitted when they are used in order to avoid the risk of IV reuse that causes vulnerabilities. For the case when IV is reused in GCM, see Nonce-Disrespecting Adversaries for details. algorithm <string> key <string> | <Buffer> | <TypedArray> | <DataView> iv <string> | <Buffer> | <TypedArray> | <DataView> options <Object> stream.transform options Returns: <Cipher> Creates and returns a Cipher object, with the given algorithm , key and initialization vector ( iv ). The options argument controls stream behavior and is optional except when a cipher in CCM or OCB mode is used (e.g. 'aes-128-ccm' ). In that case, the authTagLength option is required and specifies the length of the authentication tag in bytes, see CCM mode . In GCM mode, the authTagLength option is not required but can be used to set the length of the authentication tag that will be returned by getAuthTag() and defaults to 16 bytes. The algorithm is dependent on OpenSSL, examples are 'aes192' , etc. On recent OpenSSL releases, openssl list -cipher-algorithms ( openssl list-cipher-algorithms for older versions of OpenSSL) will display the available cipher algorithms. The key is the raw key used by the algorithm and iv is an initialization vector . Both arguments must be 'utf8' encoded strings, Buffers , TypedArray , or DataView s. If the cipher does not need an initialization vector, iv may be null . Initialization vectors should be unpredictable and unique; ideally, they will be cryptographically random. They do not have to be secret: IVs are typically just added to ciphertext messages unencrypted. It may sound contradictory that something has to be unpredictable and unique, but does not have to be secret; it is important to remember that an attacker must not be able to predict ahead of time what a given IV will be. Stability: 0 - Deprecated: Use tls.createSecureContext() instead. details <Object> Identical to tls.createSecureContext() . Returns: <tls.SecureContext> The crypto.createCredentials() method is a deprecated function for creating and returning a tls.SecureContext . It should not be used. Replace it with tls.createSecureContext() which has the exact same arguments and return value. crypto.createCipheriv(algorithm, key, iv[, options]) crypto.createCredentials(details) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 222/739 Returns a tls.SecureContext , as-if tls.createSecureContext() had been called. Stability: 0 - Deprecated: Use crypto.createDecipheriv() instead. algorithm <string> password <string> | <Buffer> | <TypedArray> | <DataView> options <Object> stream.transform options Returns: <Decipher> Creates and returns a Decipher object that uses the given algorithm and password (key). The options argument controls stream behavior and is optional except when a cipher in CCM or OCB mode is used (e.g. 'aes-128-ccm' ). In that case, the authTagLength option is required and specifies the length of the authentication tag in bytes, see CCM mode . The implementation of crypto.createDecipher() derives keys using the OpenSSL function EVP_BytesToKey with the digest algorithm set to MD5, one iteration, and no salt. The lack of salt allows dictionary attacks as the same password always creates the same key. The low iteration count and non-cryptographically secure hash algorithm allow passwords to be tested very rapidly. In line with OpenSSL's recommendation to use a more modern algorithm instead of EVP_BytesToKey it is recommended that developers derive a key and IV on their own using crypto.scrypt() and to use crypto.createDecipheriv() to create the Decipher object. algorithm <string> key <string> | <Buffer> | <TypedArray> | <DataView> iv <string> | <Buffer> | <TypedArray> | <DataView> options <Object> stream.transform options Returns: <Decipher> Creates and returns a Decipher object that uses the given algorithm , key and initialization vector ( iv ). The options argument controls stream behavior and is optional except when a cipher in CCM or OCB mode is used (e.g. 'aes-128-ccm' ). In that case, the authTagLength option is required and specifies the length of the authentication tag in bytes, see CCM mode . In GCM mode, the authTagLength option is not required but can be used to restrict accepted authentication tags to those with the specified length. The algorithm is dependent on OpenSSL, examples are 'aes192' , etc. On recent OpenSSL releases, openssl list -cipher-algorithms ( openssl list-cipher-algorithms for older versions of OpenSSL) will display the available cipher algorithms. The key is the raw key used by the algorithm and iv is an initialization vector . Both arguments must be 'utf8' encoded strings, Buffers , TypedArray , or DataView s. If the cipher does not need an initialization vector, iv may be null . Initialization vectors should be unpredictable and unique; ideally, they will be cryptographically random. They do not have to be secret: IVs are typically just added to ciphertext messages unencrypted. It may sound contradictory that something has to be unpredictable and unique, but does not have to be secret; it is important to remember that an attacker must not be able to predict ahead of time what a given IV will be. prime <string> | <Buffer> | <TypedArray> | <DataView> primeEncoding <string> The encoding of the prime string. generator <number> | <string> | <Buffer> | <TypedArray> | <DataView> Default: 2 crypto.createDecipher(algorithm, password[, options]) crypto.createDecipheriv(algorithm, key, iv[, options]) crypto.createDiffieHellman(prime[, primeEncoding][, generator][, generatorEncoding]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 223/739 generatorEncoding <string> The encoding of the generator string. Returns: <DiffieHellman> Creates a DiffieHellman key exchange object using the supplied prime and an optional specific generator . The generator argument can be a number, string, or Buffer . If generator is not specified, the value 2 is used. If primeEncoding is specified, prime is expected to be a string; otherwise a Buffer , TypedArray , or DataView is expected. If generatorEncoding is specified, generator is expected to be a string; otherwise a number, Buffer , TypedArray , or DataView is expected. primeLength <number> generator <number> | <string> | <Buffer> | <TypedArray> | <DataView> Default: 2 Returns: <DiffieHellman> Creates a DiffieHellman key exchange object and generates a prime of primeLength bits using an optional specific numeric generator . If generator is not specified, the value 2 is used. curveName <string> Returns: <ECDH> Creates an Elliptic Curve Diffie-Hellman ( ECDH ) key exchange object using a predefined curve specified by the curveName string. Use crypto.getCurves() to obtain a list of available curve names. On recent OpenSSL releases, openssl ecparam -list_curves will also display the name and description of each available elliptic curve. algorithm <string> options <Object> stream.transform options Returns: <Hash> Creates and returns a Hash object that can be used to generate hash digests using the given algorithm . Optional options argument controls stream behavior. The algorithm is dependent on the available algorithms supported by the version of OpenSSL on the platform. Examples are 'sha256' , 'sha512' , etc. On recent releases of OpenSSL, openssl list -digest-algorithms ( openssl list-message-digest-algorithms for older versions of OpenSSL) will display the available digest algorithms. Example: generating the sha256 sum of a file const filename = process.argv[2]; const crypto = require('crypto'); const fs = require('fs'); const hash = crypto.createHash('sha256'); const input = fs.createReadStream(filename); input.on('readable', () => { // Only one element is going to be produced by the // hash stream. const data = input.read(); crypto.createDiffieHellman(primeLength[, generator]) crypto.createECDH(curveName) crypto.createHash(algorithm[, options]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 224/739 if (data) hash.update(data); else { console.log(`${hash.digest('hex')} ${filename}`); } }); algorithm <string> key <string> | <Buffer> | <TypedArray> | <DataView> options <Object> stream.transform options Returns: <Hmac> Creates and returns an Hmac object that uses the given algorithm and key . Optional options argument controls stream behavior. The algorithm is dependent on the available algorithms supported by the version of OpenSSL on the platform. Examples are 'sha256' , 'sha512' , etc. On recent releases of OpenSSL, openssl list -digest-algorithms ( openssl list-message-digest-algorithms for older versions of OpenSSL) will display the available digest algorithms. The key is the HMAC key used to generate the cryptographic HMAC hash. Example: generating the sha256 HMAC of a file const filename = process.argv[2]; const crypto = require('crypto'); const fs = require('fs'); const hmac = crypto.createHmac('sha256', 'a secret'); const input = fs.createReadStream(filename); input.on('readable', () => { // Only one element is going to be produced by the // hash stream. const data = input.read(); if (data) hmac.update(data); else { console.log(`${hmac.digest('hex')} ${filename}`); } }); algorithm <string> options <Object> stream.Writable options Returns: <Sign> Creates and returns a Sign object that uses the given algorithm . Use crypto.getHashes() to obtain an array of names of the available signing algorithms. Optional options argument controls the stream.Writable behavior. crypto.createHmac(algorithm, key[, options]) crypto.createSign(algorithm[, options]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 225/739 algorithm <string> options <Object> stream.Writable options Returns: <Verify> Creates and returns a Verify object that uses the given algorithm. Use crypto.getHashes() to obtain an array of names of the available signing algorithms. Optional options argument controls the stream.Writable behavior. type : <string> Must be 'rsa' , 'dsa' or 'ec' . options : <Object> modulusLength : <number> Key size in bits (RSA, DSA). publicExponent : <number> Public exponent (RSA). Default: 0x10001 . divisorLength : <number> Size of q in bits (DSA). namedCurve : <string> Name of the curve to use (EC). publicKeyEncoding : <Object> type : <string> Must be one of 'pkcs1' (RSA only) or 'spki' . format : <string> Must be 'pem' or 'der' . privateKeyEncoding : <Object> type : <string> Must be one of 'pkcs1' (RSA only), 'pkcs8' or 'sec1' (EC only). format : <string> Must be 'pem' or 'der' . cipher : <string> If specified, the private key will be encrypted with the given cipher and passphrase using PKCS#5 v2.0 password based encryption. passphrase : <string> The passphrase to use for encryption, see cipher . callback : <Function> err : <Error> publicKey : <string> | <Buffer> privateKey : <string> | <Buffer> Generates a new asymmetric key pair of the given type . Only RSA, DSA and EC are currently supported. It is recommended to encode public keys as 'spki' and private keys as 'pkcs8' with encryption: const { generateKeyPair } = require('crypto'); generateKeyPair('rsa', { modulusLength: 4096, publicKeyEncoding: { type: 'spki', format: 'pem' }, privateKeyEncoding: { type: 'pkcs8', format: 'pem', cipher: 'aes-256-cbc', passphrase: 'top secret' crypto.createVerify(algorithm[, options]) crypto.generateKeyPair(type, options, callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 226/739 } }, (err, publicKey, privateKey) => { // Handle errors and use the generated key pair. }); On completion, callback will be called with err set to undefined and publicKey / privateKey representing the generated key pair. When PEM encoding was selected, the result will be a string, otherwise it will be a buffer containing the data encoded as DER. Note that Node.js itself does not accept DER, it is supported for interoperability with other libraries such as WebCrypto only. If this method is invoked as its util.promisify() ed version, it returns a Promise for an Object with publicKey and privateKey properties. type : <string> Must be 'rsa' , 'dsa' or 'ec' . options : <Object> modulusLength : <number> Key size in bits (RSA, DSA). publicExponent : <number> Public exponent (RSA). Default: 0x10001 . divisorLength : <number> Size of q in bits (DSA). namedCurve : <string> Name of the curve to use (EC). publicKeyEncoding : <Object> type : <string> Must be one of 'pkcs1' (RSA only) or 'spki' . format : <string> Must be 'pem' or 'der' . privateKeyEncoding : <Object> type : <string> Must be one of 'pkcs1' (RSA only), 'pkcs8' or 'sec1' (EC only). format : <string> Must be 'pem' or 'der' . cipher : <string> If specified, the private key will be encrypted with the given cipher and passphrase using PKCS#5 v2.0 password based encryption. passphrase : <string> The passphrase to use for encryption, see cipher . Returns: <Object> publicKey : <string> | <Buffer> privateKey : <string> | <Buffer> Generates a new asymmetric key pair of the given type . Only RSA, DSA and EC are currently supported. It is recommended to encode public keys as 'spki' and private keys as 'pkcs8' with encryption: const { generateKeyPairSync } = require('crypto'); const { publicKey, privateKey } = generateKeyPairSync('rsa', { modulusLength: 4096, publicKeyEncoding: { type: 'spki', format: 'pem' }, privateKeyEncoding: { type: 'pkcs8', format: 'pem', cipher: 'aes-256-cbc', crypto.generateKeyPairSync(type, options) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 227/739 passphrase: 'top secret' } }); The return value { publicKey, privateKey } represents the generated key pair. When PEM encoding was selected, the respective key will be a string, otherwise it will be a buffer containing the data encoded as DER. Returns: <string[]> An array with the names of the supported cipher algorithms. const ciphers = crypto.getCiphers(); console.log(ciphers); // ['aes-128-cbc', 'aes-128-ccm', ...] Returns: <string[]> An array with the names of the supported elliptic curves. const curves = crypto.getCurves(); console.log(curves); // ['Oakley-EC2N-3', 'Oakley-EC2N-4', ...] groupName <string> Returns: <DiffieHellman> Creates a predefined DiffieHellman key exchange object. The supported groups are: 'modp1' , 'modp2' , 'modp5' (defined in RFC 2412 , but see Caveats ) and 'modp14' , 'modp15' , 'modp16' , 'modp17' , 'modp18' (defined in RFC 3526 ). The returned object mimics the interface of objects created by crypto.createDiffieHellman() , but will not allow changing the keys (with diffieHellman.setPublicKey() , for example). The advantage of using this method is that the parties do not have to generate nor exchange a group modulus beforehand, saving both processor and communication time. Example (obtaining a shared secret): const crypto = require('crypto'); const alice = crypto.getDiffieHellman('modp14'); const bob = crypto.getDiffieHellman('modp14'); alice.generateKeys(); bob.generateKeys(); const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex'); const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex'); /* aliceSecret and bobSecret should be the same */ console.log(aliceSecret === bobSecret); Returns: <boolean> true if and only if a FIPS compliant crypto provider is currently in use. crypto.getCiphers() crypto.getCurves() crypto.getDiffieHellman(groupName) crypto.getFips() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 228/739 Returns: <string[]> An array of the names of the supported hash algorithms, such as 'RSA-SHA256' . const hashes = crypto.getHashes(); console.log(hashes); // ['DSA', 'DSA-SHA', 'DSA-SHA1', ...] password <string> | <Buffer> | <TypedArray> | <DataView> salt <string> | <Buffer> | <TypedArray> | <DataView> iterations <number> keylen <number> digest <string> callback <Function> err <Error> derivedKey <Buffer> Provides an asynchronous Password-Based Key Derivation Function 2 (PBKDF2) implementation. A selected HMAC digest algorithm specified by digest is applied to derive a key of the requested byte length ( keylen ) from the password , salt and iterations . The supplied callback function is called with two arguments: err and derivedKey . If an error occurs while deriving the key, err will be set; otherwise err will be null . By default, the successfully generated derivedKey will be passed to the callback as a Buffer . An error will be thrown if any of the input arguments specify invalid values or types. If digest is null , 'sha1' will be used. This behavior will be deprecated in a future version of Node.js. The iterations argument must be a number set as high as possible. The higher the number of iterations, the more secure the derived key will be, but will take a longer amount of time to complete. The salt should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details. const crypto = require('crypto'); crypto.pbkdf2('secret', 'salt', 100000, 64, 'sha512', (err, derivedKey) => { if (err) throw err; console.log(derivedKey.toString('hex')); // '3745e48...08d59ae' }); The crypto.DEFAULT_ENCODING property can be used to change the way the derivedKey is passed to the callback. This property, however, has been deprecated and use should be avoided. const crypto = require('crypto'); crypto.DEFAULT_ENCODING = 'hex'; crypto.pbkdf2('secret', 'salt', 100000, 512, 'sha512', (err, derivedKey) => { if (err) throw err; console.log(derivedKey); // '3745e48...aa39b34' }); An array of supported digest functions can be retrieved using crypto.getHashes() . crypto.getHashes() crypto.pbkdf2(password, salt, iterations, keylen, digest, callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 229/739 Note that this API uses libuv's threadpool, which can have surprising and negative performance implications for some applications, see the UV_THREADPOOL_SIZE documentation for more information. password <string> | <Buffer> | <TypedArray> | <DataView> salt <string> | <Buffer> | <TypedArray> | <DataView> iterations <number> keylen <number> digest <string> Returns: <Buffer> Provides a synchronous Password-Based Key Derivation Function 2 (PBKDF2) implementation. A selected HMAC digest algorithm specified by digest is applied to derive a key of the requested byte length ( keylen ) from the password , salt and iterations . If an error occurs an Error will be thrown, otherwise the derived key will be returned as a Buffer . If digest is null , 'sha1' will be used. This behavior will be deprecated in a future version of Node.js. The iterations argument must be a number set as high as possible. The higher the number of iterations, the more secure the derived key will be, but will take a longer amount of time to complete. The salt should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details. const crypto = require('crypto'); const key = crypto.pbkdf2Sync('secret', 'salt', 100000, 64, 'sha512'); console.log(key.toString('hex')); // '3745e48...08d59ae' The crypto.DEFAULT_ENCODING property may be used to change the way the derivedKey is returned. This property, however, is deprecated and use should be avoided. const crypto = require('crypto'); crypto.DEFAULT_ENCODING = 'hex'; const key = crypto.pbkdf2Sync('secret', 'salt', 100000, 512, 'sha512'); console.log(key); // '3745e48...aa39b34' An array of supported digest functions can be retrieved using crypto.getHashes() . privateKey <Object> | <string> key <string> A PEM encoded private key. passphrase <string> An optional passphrase for the private key. padding <crypto.constants> An optional padding value defined in crypto.constants , which may be: crypto.constants.RSA_NO_PADDING , crypto.constants.RSA_PKCS1_PADDING , or crypto.constants.RSA_PKCS1_OAEP_PADDING . buffer <Buffer> | <TypedArray> | <DataView> Returns: <Buffer> A new Buffer with the decrypted content. Decrypts buffer with privateKey . buffer was previously encrypted using the corresponding public key, for example using crypto.publicEncrypt() . crypto.pbkdf2Sync(password, salt, iterations, keylen, digest) crypto.privateDecrypt(privateKey, buffer) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 230/739 privateKey can be an object or a string. If privateKey is a string, it is treated as the key with no passphrase and will use RSA_PKCS1_OAEP_PADDING . privateKey <Object> | <string> key <string> A PEM encoded private key. passphrase <string> An optional passphrase for the private key. padding <crypto.constants> An optional padding value defined in crypto.constants , which may be: crypto.constants.RSA_NO_PADDING or crypto.constants.RSA_PKCS1_PADDING . buffer <Buffer> | <TypedArray> | <DataView> Returns: <Buffer> A new Buffer with the encrypted content. Encrypts buffer with privateKey . The returned data can be decrypted using the corresponding public key, for example using crypto.publicDecrypt() . privateKey can be an object or a string. If privateKey is a string, it is treated as the key with no passphrase and will use RSA_PKCS1_PADDING . key <Object> | <string> key <string> A PEM encoded public or private key. passphrase <string> An optional passphrase for the private key. padding <crypto.constants> An optional padding value defined in crypto.constants , which may be: crypto.constants.RSA_NO_PADDING or crypto.constants.RSA_PKCS1_PADDING . buffer <Buffer> | <TypedArray> | <DataView> Returns: <Buffer> A new Buffer with the decrypted content. Decrypts buffer with key . buffer was previously encrypted using the corresponding private key, for example using crypto.privateEncrypt() . key can be an object or a string. If key is a string, it is treated as the key with no passphrase and will use RSA_PKCS1_PADDING . Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key. key <Object> | <string> key <string> A PEM encoded public or private key. passphrase <string> An optional passphrase for the private key. padding <crypto.constants> An optional padding value defined in crypto.constants , which may be: crypto.constants.RSA_NO_PADDING , crypto.constants.RSA_PKCS1_PADDING , or crypto.constants.RSA_PKCS1_OAEP_PADDING . buffer <Buffer> | <TypedArray> | <DataView> Returns: <Buffer> A new Buffer with the encrypted content. Encrypts the content of buffer with key and returns a new Buffer with encrypted content. The returned data can be decrypted using the corresponding private key, for example using crypto.privateDecrypt() . key can be an object or a string. If key is a string, it is treated as the key with no passphrase and will use RSA_PKCS1_OAEP_PADDING . crypto.privateEncrypt(privateKey, buffer) crypto.publicDecrypt(key, buffer) crypto.publicEncrypt(key, buffer) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 231/739 Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key. size <number> callback <Function> err <Error> buf <Buffer> Returns: <Buffer> if the callback function is not provided. Generates cryptographically strong pseudo-random data. The size argument is a number indicating the number of bytes to generate. If a callback function is provided, the bytes are generated asynchronously and the callback function is invoked with two arguments: err and buf . If an error occurs, err will be an Error object; otherwise it is null . The buf argument is a Buffer containing the generated bytes. // Asynchronous const crypto = require('crypto'); crypto.randomBytes(256, (err, buf) => { if (err) throw err; console.log(`${buf.length} bytes of random data: ${buf.toString('hex')}`); }); If the callback function is not provided, the random bytes are generated synchronously and returned as a Buffer . An error will be thrown if there is a problem generating the bytes. // Synchronous const buf = crypto.randomBytes(256); console.log( `${buf.length} bytes of random data: ${buf.toString('hex')}`); The crypto.randomBytes() method will not complete until there is sufficient entropy available. This should normally never take longer than a few milliseconds. The only time when generating the random bytes may conceivably block for a longer period of time is right after boot, when the whole system is still low on entropy. Note that this API uses libuv's threadpool, which can have surprising and negative performance implications for some applications, see the UV_THREADPOOL_SIZE documentation for more information. The asynchronous version of crypto.randomBytes() is carried out in a single threadpool request. To minimize threadpool task length variation, partition large randomBytes requests when doing so as part of fulfilling a client request. buffer <Buffer> | <TypedArray> | <DataView> Must be supplied. offset <number> Default: 0 size <number> Default: buffer.length - offset Returns: <Buffer> | <TypedArray> | <DataView> The object passed as buffer argument. Synchronous version of crypto.randomFill() . const buf = Buffer.alloc(10); console.log(crypto.randomFillSync(buf).toString('hex')); crypto.randomBytes(size[, callback]) crypto.randomFillSync(buffer[, offset][, size]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 232/739 crypto.randomFillSync(buf, 5); console.log(buf.toString('hex')); // The above is equivalent to the following: crypto.randomFillSync(buf, 5, 5); console.log(buf.toString('hex')); Any TypedArray or DataView instance may be passed as buffer . const a = new Uint32Array(10); console.log(Buffer.from(crypto.randomFillSync(a).buffer, a.byteOffset, a.byteLength).toString('hex')); const b = new Float64Array(10); console.log(Buffer.from(crypto.randomFillSync(b).buffer, b.byteOffset, b.byteLength).toString('hex')); const c = new DataView(new ArrayBuffer(10)); console.log(Buffer.from(crypto.randomFillSync(c).buffer, c.byteOffset, c.byteLength).toString('hex')); buffer <Buffer> | <TypedArray> | <DataView> Must be supplied. offset <number> Default: 0 size <number> Default: buffer.length - offset callback <Function> function(err, buf) {} . This function is similar to crypto.randomBytes() but requires the first argument to be a Buffer that will be filled. It also requires that a callback is passed in. If the callback function is not provided, an error will be thrown. const buf = Buffer.alloc(10); crypto.randomFill(buf, (err, buf) => { if (err) throw err; console.log(buf.toString('hex')); }); crypto.randomFill(buf, 5, (err, buf) => { if (err) throw err; console.log(buf.toString('hex')); }); // The above is equivalent to the following: crypto.randomFill(buf, 5, 5, (err, buf) => { if (err) throw err; console.log(buf.toString('hex')); }); crypto.randomFill(buffer[, offset][, size], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 233/739 Any TypedArray or DataView instance may be passed as buffer . const a = new Uint32Array(10); crypto.randomFill(a, (err, buf) => { if (err) throw err; console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength) .toString('hex')); }); const b = new Float64Array(10); crypto.randomFill(b, (err, buf) => { if (err) throw err; console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength) .toString('hex')); }); const c = new DataView(new ArrayBuffer(10)); crypto.randomFill(c, (err, buf) => { if (err) throw err; console.log(Buffer.from(buf.buffer, buf.byteOffset, buf.byteLength) .toString('hex')); }); Note that this API uses libuv's threadpool, which can have surprising and negative performance implications for some applications, see the UV_THREADPOOL_SIZE documentation for more information. The asynchronous version of crypto.randomFill() is carried out in a single threadpool request. To minimize threadpool task length variation, partition large randomFill requests when doing so as part of fulfilling a client request. password <string> | <Buffer> | <TypedArray> | <DataView> salt <string> | <Buffer> | <TypedArray> | <DataView> keylen <number> options <Object> cost <number> CPU/memory cost parameter. Must be a power of two greater N <number> CPU/memory cost parameter. Must be a power of two greater than one. Default: 16384 . blockSize <number> Block size parameter. Default: 8 . parallelization <number> Parallelization parameter. Default: 1 . N <number> Alias for cost . Only one of both may be specified. r <number> Alias for blockSize . Only one of both may be specified. p <number> Alias for parallelization . Only one of both may be specified. maxmem <number> Memory upper bound. It is an error when (approximately) 128 * N * r > maxmem . Default: 32 * 1024 * 1024 . callback <Function> err <Error> derivedKey <Buffer> crypto.scrypt(password, salt, keylen[, options], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 234/739 Provides an asynchronous scrypt implementation. Scrypt is a password-based key derivation function that is designed to be expensive computationally and memory-wise in order to make brute-force attacks unrewarding. The salt should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details. The callback function is called with two arguments: err and derivedKey . err is an exception object when key derivation fails, otherwise err is null . derivedKey is passed to the callback as a Buffer . An exception is thrown when any of the input arguments specify invalid values or types. const crypto = require('crypto'); // Using the factory defaults. crypto.scrypt('secret', 'salt', 64, (err, derivedKey) => { if (err) throw err; console.log(derivedKey.toString('hex')); // '3745e48...08d59ae' }); // Using a custom N parameter. Must be a power of two. crypto.scrypt('secret', 'salt', 64, { N: 1024 }, (err, derivedKey) => { if (err) throw err; console.log(derivedKey.toString('hex')); // '3745e48...aa39b34' }); password <string> | <Buffer> | <TypedArray> | <DataView> salt <string> | <Buffer> | <TypedArray> | <DataView> keylen <number> options <Object> cost <number> CPU/memory cost parameter. Must be a power of two greater N <number> CPU/memory cost parameter. Must be a power of two greater than one. Default: 16384 . blockSize <number> Block size parameter. Default: 8 . parallelization <number> Parallelization parameter. Default: 1 . N <number> Alias for cost . Only one of both may be specified. r <number> Alias for blockSize . Only one of both may be specified. p <number> Alias for parallelization . Only one of both may be specified. maxmem <number> Memory upper bound. It is an error when (approximately) 128 * N * r > maxmem . Default: 32 * 1024 * 1024 . Returns: <Buffer> Provides a synchronous scrypt implementation. Scrypt is a password-based key derivation function that is designed to be expensive computationally and memory-wise in order to make brute-force attacks unrewarding. The salt should be as unique as possible. It is recommended that a salt is random and at least 16 bytes long. See NIST SP 800-132 for details. An exception is thrown when key derivation fails, otherwise the derived key is returned as a Buffer . An exception is thrown when any of the input arguments specify invalid values or types. const crypto = require('crypto'); // Using the factory defaults. const key1 = crypto.scryptSync('secret', 'salt', 64); crypto.scryptSync(password, salt, keylen[, options]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 235/739 console.log(key1.toString('hex')); // '3745e48...08d59ae' // Using a custom N parameter. Must be a power of two. const key2 = crypto.scryptSync('secret', 'salt', 64, { N: 1024 }); console.log(key2.toString('hex')); // '3745e48...aa39b34' engine <string> flags <crypto.constants> Default: crypto.constants.ENGINE_METHOD_ALL Load and set the engine for some or all OpenSSL functions (selected by flags). engine could be either an id or a path to the engine's shared library. The optional flags argument uses ENGINE_METHOD_ALL by default. The flags is a bit field taking one of or a mix of the following flags (defined in crypto.constants ): crypto.constants.ENGINE_METHOD_RSA crypto.constants.ENGINE_METHOD_DSA crypto.constants.ENGINE_METHOD_DH crypto.constants.ENGINE_METHOD_RAND crypto.constants.ENGINE_METHOD_EC crypto.constants.ENGINE_METHOD_CIPHERS crypto.constants.ENGINE_METHOD_DIGESTS crypto.constants.ENGINE_METHOD_PKEY_METHS crypto.constants.ENGINE_METHOD_PKEY_ASN1_METHS crypto.constants.ENGINE_METHOD_ALL crypto.constants.ENGINE_METHOD_NONE The flags below are deprecated in OpenSSL-1.1.0. crypto.constants.ENGINE_METHOD_ECDH crypto.constants.ENGINE_METHOD_ECDSA crypto.constants.ENGINE_METHOD_STORE bool <boolean> true to enable FIPS mode. Enables the FIPS compliant crypto provider in a FIPS-enabled Node.js build. Throws an error if FIPS mode is not available. a <Buffer> | <TypedArray> | <DataView> b <Buffer> | <TypedArray> | <DataView> Returns: <boolean> This function is based on a constant-time algorithm. Returns true if a is equal to b , without leaking timing information that would allow an attacker to guess one of the values. This is suitable for comparing HMAC digests or secret values like authentication cookies or capability urls . a and b must both be Buffer s, TypedArray s, or DataView s, and they must have the same length. crypto.setEngine(engine[, flags]) crypto.setFips(bool) crypto.timingSafeEqual(a, b) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 236/739 Use of crypto.timingSafeEqual does not guarantee that the surrounding code is timing-safe. Care should be taken to ensure that the surrounding code does not introduce timing vulnerabilities. The Crypto module was added to Node.js before there was the concept of a unified Stream API, and before there were Buffer objects for handling binary data. As such, the many of the crypto defined classes have methods not typically found on other Node.js classes that implement the streams API (e.g. update() , final() , or digest() ). Also, many methods accepted and returned 'latin1' encoded strings by default rather than Buffer s. This default was changed after Node.js v0.8 to use Buffer objects by default instead. Usage of ECDH with non-dynamically generated key pairs has been simplified. Now, ecdh.setPrivateKey() can be called with a preselected private key and the associated public point (key) will be computed and stored in the object. This allows code to only store and provide the private part of the EC key pair. ecdh.setPrivateKey() now also validates that the private key is valid for the selected curve. The ecdh.setPublicKey() method is now deprecated as its inclusion in the API is not useful. Either a previously stored private key should be set, which automatically generates the associated public key, or ecdh.generateKeys() should be called. The main drawback of using ecdh.setPublicKey() is that it can be used to put the ECDH key pair into an inconsistent state. The crypto module still supports some algorithms which are already compromised and are not currently recommended for use. The API also allows the use of ciphers and hashes with a small key size that are considered to be too weak for safe use. Users should take full responsibility for selecting the crypto algorithm and key size according to their security requirements. Based on the recommendations of NIST SP 800-131A : MD5 and SHA-1 are no longer acceptable where collision resistance is required such as digital signatures. The key used with RSA, DSA, and DH algorithms is recommended to have at least 2048 bits and that of the curve of ECDSA and ECDH at least 224 bits, to be safe to use for several years. The DH groups of modp1 , modp2 and modp5 have a key size smaller than 2048 bits and are not recommended. See the reference for other recommendations and details. CCM is one of the supported AEAD algorithms . Applications which use this mode must adhere to certain restrictions when using the cipher API: The authentication tag length must be specified during cipher creation by setting the authTagLength option and must be one of 4, 6, 8, 10, 12, 14 or 16 bytes. The length of the initialization vector (nonce) N must be between 7 and 13 bytes ( 7 ≤ N ≤ 13 ). The length of the plaintext is limited to 2 ** (8 * (15 - N)) bytes. When decrypting, the authentication tag must be set via setAuthTag() before specifying additional authenticated data or calling update() . Otherwise, decryption will fail and final() will throw an error in compliance with section 2.6 of RFC 3610 . Using stream methods such as write(data) , end(data) or pipe() in CCM mode might fail as CCM cannot handle more than one chunk of data per instance. When passing additional authenticated data (AAD), the length of the actual message in bytes must be passed to setAAD() via the plaintextLength option. This is not necessary if no AAD is used. Notes Legacy Streams API (pre Node.js v0.10) Recent ECDH Changes Support for weak or compromised algorithms CCM mode 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 237/739 As CCM processes the whole message at once, update() can only be called once. Even though calling update() is sufficient to encrypt/decrypt the message, applications must call final() to compute or verify the authentication tag. const crypto = require('crypto'); const key = 'keykeykeykeykeykeykeykey'; const nonce = crypto.randomBytes(12); const aad = Buffer.from('0123456789', 'hex'); const cipher = crypto.createCipheriv('aes-192-ccm', key, nonce, { authTagLength: 16 }); const plaintext = 'Hello world'; cipher.setAAD(aad, { plaintextLength: Buffer.byteLength(plaintext) }); const ciphertext = cipher.update(plaintext, 'utf8'); cipher.final(); const tag = cipher.getAuthTag(); // Now transmit { ciphertext, nonce, tag }. const decipher = crypto.createDecipheriv('aes-192-ccm', key, nonce, { authTagLength: 16 }); decipher.setAuthTag(tag); decipher.setAAD(aad, { plaintextLength: ciphertext.length }); const receivedPlaintext = decipher.update(ciphertext, null, 'utf8'); try { decipher.final(); } catch (err) { console.error('Authentication failed!'); } console.log(receivedPlaintext); The following constants exported by crypto.constants apply to various uses of the crypto , tls , and https modules and are generally specific to OpenSSL. Constant Description SSL_OP_ALL Applies multiple bug workarounds within OpenSSL. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html for detail. Crypto Constants OpenSSL Options 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 238/739 SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION Allows legacy insecure renegotiation between OpenSSL and unpatched clients or servers. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html . SSL_OP_CIPHER_SERVER_PREFERENCE Attempts to use the server's preferences instead of the client's when selecting a cipher. Behavior depends on protocol version. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html . SSL_OP_CISCO_ANYCONNECT Instructs OpenSSL to use Cisco's "speshul" version of DTLS_BAD_VER. SSL_OP_COOKIE_EXCHANGE Instructs OpenSSL to turn on cookie exchange. SSL_OP_CRYPTOPRO_TLSEXT_BUG Instructs OpenSSL to add server-hello extension from an early version of the cryptopro draft. SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS Instructs OpenSSL to disable a SSL 3.0/TLS 1.0 vulnerability workaround added in OpenSSL 0.9.6d. SSL_OP_EPHEMERAL_RSA Instructs OpenSSL to always use the tmp_rsa key when performing RSA operations. SSL_OP_LEGACY_SERVER_CONNECT Allows initial connection to servers that do not support RI. SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER SSL_OP_MICROSOFT_SESS_ID_BUG SSL_OP_MSIE_SSLV2_RSA_PADDING Instructs OpenSSL to disable the workaround for a man-in-the-middle protocol- version vulnerability in the SSL 2.0 server implementation. SSL_OP_NETSCAPE_CA_DN_BUG SSL_OP_NETSCAPE_CHALLENGE_BUG SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG SSL_OP_NO_COMPRESSION Instructs OpenSSL to disable support for SSL/TLS compression. SSL_OP_NO_QUERY_MTU SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION Instructs OpenSSL to always start a new session when performing renegotiation. SSL_OP_NO_SSLv2 Instructs OpenSSL to turn off SSL v2 SSL_OP_NO_SSLv3 Instructs OpenSSL to turn off SSL v3 SSL_OP_NO_TICKET Instructs OpenSSL to disable use of RFC4507bis tickets. SSL_OP_NO_TLSv1 Instructs OpenSSL to turn off TLS v1 SSL_OP_NO_TLSv1_1 Instructs OpenSSL to turn off TLS v1.1 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 239/739 SSL_OP_NO_TLSv1_2 Instructs OpenSSL to turn off TLS v1.2 SSL_OP_PKCS1_CHECK_1 SSL_OP_PKCS1_CHECK_2 SSL_OP_SINGLE_DH_USE Instructs OpenSSL to always create a new key when using temporary/ephemeral DH parameters. SSL_OP_SINGLE_ECDH_USE Instructs OpenSSL to always create a new key when using temporary/ephemeral ECDH parameters. SSL_OP_SSLEAY_080_CLIENT_DH_BUG SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG SSL_OP_TLS_BLOCK_PADDING_BUG SSL_OP_TLS_D5_BUG SSL_OP_TLS_ROLLBACK_BUG Instructs OpenSSL to disable version rollback attack detection. Constant Description ENGINE_METHOD_RSA Limit engine usage to RSA ENGINE_METHOD_DSA Limit engine usage to DSA ENGINE_METHOD_DH Limit engine usage to DH ENGINE_METHOD_RAND Limit engine usage to RAND ENGINE_METHOD_EC Limit engine usage to EC ENGINE_METHOD_CIPHERS Limit engine usage to CIPHERS ENGINE_METHOD_DIGESTS Limit engine usage to DIGESTS ENGINE_METHOD_PKEY_METHS Limit engine usage to PKEY_METHDS ENGINE_METHOD_PKEY_ASN1_METHS Limit engine usage to PKEY_ASN1_METHS ENGINE_METHOD_ALL ENGINE_METHOD_NONE Constant Description OpenSSL Engine Constants Other OpenSSL Constants 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 240/739 DH_CHECK_P_NOT_SAFE_PRIME DH_CHECK_P_NOT_PRIME DH_UNABLE_TO_CHECK_GENERATOR DH_NOT_SUITABLE_GENERATOR ALPN_ENABLED RSA_PKCS1_PADDING RSA_SSLV23_PADDING RSA_NO_PADDING RSA_PKCS1_OAEP_PADDING RSA_X931_PADDING RSA_PKCS1_PSS_PADDING RSA_PSS_SALTLEN_DIGEST Sets the salt length for RSA_PKCS1_PSS_PADDING to the digest size when signing or verifying. RSA_PSS_SALTLEN_MAX_SIGN Sets the salt length for RSA_PKCS1_PSS_PADDING to the maximum permissible value when signing data. RSA_PSS_SALTLEN_AUTO Causes the salt length for RSA_PKCS1_PSS_PADDING to be determined automatically when verifying a signature. POINT_CONVERSION_COMPRESSED POINT_CONVERSION_UNCOMPRESSED POINT_CONVERSION_HYBRID Constant Description defaultCoreCipherList Specifies the built-in default cipher list used by Node.js. defaultCipherList Specifies the active default cipher list used by the current Node.js process. Stability: 2 - Stable Node.js includes an out-of-process debugging utility accessible via a V8 Inspector and built-in debugging client. To use it, start Node.js with the inspect argument followed by the path to the script to debug; a prompt will be displayed indicating successful launch of the debugger: Node.js Crypto Constants Debugger 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 241/739 $ node inspect myscript.js < Debugger listening on ws://127.0.0.1:9229/80e7a814-7cd3-49fb-921a-2e02228cd5ba < For help, see: https://nodejs.org/en/docs/inspector < Debugger attached. Break on start in myscript.js:1 > 1 (function (exports, require, module, __filename, __dirname) { global.x = 5; 2 setTimeout(() => { 3 console.log('world'); debug> Node.js's debugger client is not a full-featured debugger, but simple step and inspection are possible. Inserting the statement debugger; into the source code of a script will enable a breakpoint at that position in the code: // myscript.js global.x = 5; setTimeout(() => { debugger; console.log('world'); }, 1000); console.log('hello'); Once the debugger is run, a breakpoint will occur at line 3: $ node inspect myscript.js < Debugger listening on ws://127.0.0.1:9229/80e7a814-7cd3-49fb-921a-2e02228cd5ba < For help, see: https://nodejs.org/en/docs/inspector < Debugger attached. Break on start in myscript.js:1 > 1 (function (exports, require, module, __filename, __dirname) { global.x = 5; 2 setTimeout(() => { 3 debugger; debug> cont < hello break in myscript.js:3 1 (function (exports, require, module, __filename, __dirname) { global.x = 5; 2 setTimeout(() => { > 3 debugger; 4 console.log('world'); 5 }, 1000); debug> next break in myscript.js:4 2 setTimeout(() => { 3 debugger; > 4 console.log('world'); 5 }, 1000); 6 console.log('hello'); debug> repl Press Ctrl + C to leave debug repl > x 5 > 2 + 2 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 242/739 > 2 + 2 4 debug> next < world break in myscript.js:5 3 debugger; 4 console.log('world'); > 5 }, 1000); 6 console.log('hello'); 7 debug> .exit The repl command allows code to be evaluated remotely. The next command steps to the next line. Type help to see what other commands are available. Pressing enter without typing a command will repeat the previous debugger command. It is possible to watch expression and variable values while debugging. On every breakpoint, each expression from the watchers list will be evaluated in the current context and displayed immediately before the breakpoint's source code listing. To begin watching an expression, type watch('my_expression') . The command watchers will print the active watchers. To remove a watcher, type unwatch('my_expression') . cont , c - Continue execution next , n - Step next step , s - Step in out , o - Step out pause - Pause running code (like pause button in Developer Tools) setBreakpoint() , sb() - Set breakpoint on current line setBreakpoint(line) , sb(line) - Set breakpoint on specific line setBreakpoint('fn()') , sb(...) - Set breakpoint on a first statement in functions body setBreakpoint('script.js', 1) , sb(...) - Set breakpoint on first line of script.js clearBreakpoint('script.js', 1) , cb(...) - Clear breakpoint in script.js on line 1 It is also possible to set a breakpoint in a file (module) that is not loaded yet: $ node inspect main.js < Debugger listening on ws://127.0.0.1:9229/4e3db158-9791-4274-8909-914f7facf3bd < For help, see: https://nodejs.org/en/docs/inspector < Debugger attached. Break on start in main.js:1 > 1 (function (exports, require, module, __filename, __dirname) { const mod = require('./mod.js'); 2 mod.hello(); 3 d h ll () Watchers Command reference Stepping Breakpoints 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 243/739 3 mod.hello(); debug> setBreakpoint('mod.js', 22) Warning: script 'mod.js' was not loaded yet. debug> c break in mod.js:22 20 // USE OR OTHER DEALINGS IN THE SOFTWARE. 21 >22 exports.hello = function() { 23 return 'hello from module'; 24 }; debug> backtrace , bt - Print backtrace of current execution frame list(5) - List scripts source code with 5 line context (5 lines before and after) watch(expr) - Add expression to watch list unwatch(expr) - Remove expression from watch list watchers - List all watchers and their values (automatically listed on each breakpoint) repl - Open debugger's repl for evaluation in debugging script's context exec expr - Execute an expression in debugging script's context run - Run script (automatically runs on debugger's start) restart - Restart script kill - Kill script scripts - List all loaded scripts version - Display V8's version V8 Inspector integration allows attaching Chrome DevTools to Node.js instances for debugging and profiling. It uses the Chrome DevTools Protocol . V8 Inspector can be enabled by passing the --inspect flag when starting a Node.js application. It is also possible to supply a custom port with that flag, e.g. --inspect=9222 will accept DevTools connections on port 9222. To break on the first line of the application code, pass the --inspect-brk flag instead of --inspect . $ node --inspect index.js Debugger listening on 127.0.0.1:9229. To start debugging, open the following URL in Chrome: chrome-devtools://devtools/bundled/js_app.html?experiments=true&v8only=true&ws=127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d Information Execution control Various Advanced Usage V8 Inspector Integration for Node.js 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 244/739 (In the example above, the UUID dc9010dd-f8b8-4ac5-a510-c1a114ec7d29 at the end of the URL is generated on the fly, it varies in different debugging sessions.) If the Chrome browser is older than 66.0.3345.0, use inspector.html instead of js_app.html in the above URL. Node.js may deprecate APIs when either: (a) use of the API is considered to be unsafe, (b) an improved alternative API is available, or (c) breaking changes to the API are expected in a future major release. Node.js utilizes three kinds of Deprecations: Documentation-only Runtime End-of-Life A Documentation-only deprecation is one that is expressed only within the Node.js API docs. These generate no side-effects while running Node.js. Some Documentation-only deprecations trigger a runtime warning when launched with --pending-deprecation flag (or its alternative, NODE_PENDING_DEPRECATION=1 environment variable), similarly to Runtime deprecations below. Documentation-only deprecations that support that flag are explicitly labeled as such in the list of Deprecated APIs . A Runtime deprecation will, by default, generate a process warning that will be printed to stderr the first time the deprecated API is used. When the --throw-deprecation command-line flag is used, a Runtime deprecation will cause an error to be thrown. An End-of-Life deprecation is used when functionality is or will soon be removed from Node.js. Occasionally, the deprecation of an API may be reversed. In such situations, this document will be updated with information relevant to the decision. However, the deprecation identifier will not be modified. Type: Runtime The OutgoingMessage.prototype.flush() method is deprecated. Use OutgoingMessage.prototype.flushHeaders() instead. Type: End-of-Life The _linklist module is deprecated. Please use a userland alternative. Type: Runtime The _writableState.buffer property is deprecated. Use the _writableState.getBuffer() method instead. Deprecated APIs Revoking deprecations List of Deprecated APIs DEP0001: http.OutgoingMessage.prototype.flush DEP0002: require('_linklist') DEP0003: _writableState.buffer 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 245/739 Type: End-of-Life The CryptoStream.prototype.readyState property was removed. Type: Runtime (supports --pending-deprecation ) The Buffer() function and new Buffer() constructor are deprecated due to API usability issues that can potentially lead to accidental security issues. As an alternative, use of the following methods of constructing Buffer objects is strongly recommended: Buffer.alloc(size[, fill[, encoding]]) - Create a Buffer with initialized memory. Buffer.allocUnsafe(size) - Create a Buffer with uninitialized memory. Buffer.allocUnsafeSlow(size) - Create a Buffer with uninitialized memory. Buffer.from(array) - Create a Buffer with a copy of array Buffer.from(arrayBuffer[, byteOffset[, length]]) - Create a Buffer that wraps the given arrayBuffer . Buffer.from(buffer) - Create a Buffer that copies buffer . Buffer.from(string[, encoding]) - Create a Buffer that copies string . As of v10.0.0, a deprecation warning is printed at runtime when --pending-deprecation is used or when the calling code is outside node_modules in order to better target developers, rather than users. Type: Runtime Within the child_process module's spawn() , fork() , and exec() methods, the options.customFds option is deprecated. The options.stdio option should be used instead. Type: End-of-Life In an earlier version of the Node.js cluster , a boolean property with the name suicide was added to the Worker object. The intent of this property was to provide an indication of how and why the Worker instance exited. In Node.js 6.0.0, the old property was deprecated and replaced with a new worker.exitedAfterDisconnect property. The old property name did not precisely describe the actual semantics and was unnecessarily emotion-laden. Type: Documentation-only DEP0004: CryptoStream.prototype.readyState DEP0005: Buffer() constructor DEP0006: child_process options.customFds DEP0007: Replace cluster worker.suicide with worker.exitedAfterDisconnect DEP0008: require('constants') 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 246/739 The constants module is deprecated. When requiring access to constants relevant to specific Node.js builtin modules, developers should instead refer to the constants property exposed by the relevant module. For instance, require('fs').constants and require('os').constants . Type: End-of-Life Use of the crypto.pbkdf2() API without specifying a digest was deprecated in Node.js 6.0 because the method defaulted to using the non- recommended 'SHA1' digest. Previously, a deprecation warning was printed. Starting in Node.js 8.0.0, calling crypto.pbkdf2() or crypto.pbkdf2Sync() with an undefined digest will throw a TypeError . Type: Runtime The crypto.createCredentials() API is deprecated. Please use tls.createSecureContext() instead. Type: Runtime The crypto.Credentials class is deprecated. Please use tls.SecureContext instead. Type: End-of-Life Domain.dispose() has been removed. Recover from failed I/O actions explicitly via error event handlers set on the domain instead. Type: End-of-Life Calling an asynchronous function without a callback throws a TypeError in Node.js 10.0.0 onwards. (See https://github.com/nodejs/node/pull/12562 .) Type: End-of-Life The fs.read() legacy String interface is deprecated. Use the Buffer API as mentioned in the documentation instead. DEP0009: crypto.pbkdf2 without digest DEP0010: crypto.createCredentials DEP0011: crypto.Credentials DEP0012: Domain.dispose DEP0013: fs asynchronous function without callback DEP0014: fs.read legacy String interface 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 247/739 Type: End-of-Life The fs.readSync() legacy String interface is deprecated. Use the Buffer API as mentioned in the documentation instead. Type: Runtime The GLOBAL and root aliases for the global property are deprecated and should no longer be used. Type: End-of-Life Intl.v8BreakIterator was a non-standard extension and has been removed. See Intl.Segmenter . Type: Runtime Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code. Type: Runtime In certain cases, require('.') may resolve outside the package directory. This behavior is deprecated and will be removed in a future major Node.js release. Type: Runtime The Server.connections property is deprecated. Please use the Server.getConnections() method instead. Type: Runtime The Server.listenFD() method is deprecated. Please use Server.listen({fd: <number>}) instead. DEP0015: fs.readSync legacy String interface DEP0016: GLOBAL/root DEP0017: Intl.v8BreakIterator DEP0018: Unhandled promise rejections DEP0019: require('.') resolved outside directory DEP0020: Server.connections DEP0021: Server.listenFD 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 248/739 Type: Runtime The os.tmpDir() API is deprecated. Please use os.tmpdir() instead. Type: Runtime The os.getNetworkInterfaces() method is deprecated. Please use the os.networkInterfaces property instead. Type: End-of-Life The REPLServer.prototype.convertToContext() API has been removed. Type: Runtime The sys module is deprecated. Please use the util module instead. Type: Runtime The util.print() API is deprecated. Please use console.log() instead. Type: Runtime The util.puts() API is deprecated. Please use console.log() instead. Type: Runtime The util.debug() API is deprecated. Please use console.error() instead. DEP0022: os.tmpDir() DEP0023: os.getNetworkInterfaces() DEP0024: REPLServer.prototype.convertToContext() DEP0025: require('sys') DEP0026: util.print() DEP0027: util.puts() DEP0028: util.debug() DEP0029: util.error() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 249/739 Type: Runtime The util.error() API is deprecated. Please use console.error() instead. Type: Documentation-only The SlowBuffer class is deprecated. Please use Buffer.allocUnsafeSlow(size) instead. Type: Documentation-only The ecdh.setPublicKey() method is now deprecated as its inclusion in the API is not useful. Type: Documentation-only The domain module is deprecated and should not be used. Type: Documentation-only The EventEmitter.listenerCount(emitter, eventName) API is deprecated. Please use emitter.listenerCount(eventName) instead. Type: Documentation-only The fs.exists(path, callback) API is deprecated. Please use fs.stat() or fs.access() instead. Type: Documentation-only The fs.lchmod(path, mode, callback) API is deprecated. Type: Documentation-only DEP0030: SlowBuffer DEP0031: ecdh.setPublicKey() DEP0032: domain module DEP0033: EventEmitter.listenerCount() DEP0034: fs.exists(path, callback) DEP0035: fs.lchmod(path, mode, callback) DEP0036: fs.lchmodSync(path, mode) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 250/739 The fs.lchmodSync(path, mode) API is deprecated. Type: Deprecation revoked The fs.lchown(path, uid, gid, callback) API is deprecated. Type: Deprecation revoked The fs.lchownSync(path, uid, gid) API is deprecated. Type: Documentation-only The require.extensions property is deprecated. Type: Documentation-only The punycode module is deprecated. Please use a userland alternative instead. Type: End-of-Life The NODE_REPL_HISTORY_FILE environment variable was removed. Please use NODE_REPL_HISTORY instead. Type: End-of-Life The tls.CryptoStream class was removed. Please use tls.TLSSocket instead. Type: Documentation-only The tls.SecurePair class is deprecated. Please use tls.TLSSocket instead. DEP0037: fs.lchown(path, uid, gid, callback) DEP0038: fs.lchownSync(path, uid, gid) DEP0039: require.extensions DEP0040: punycode module DEP0041: NODE_REPL_HISTORY_FILE environment variable DEP0042: tls.CryptoStream DEP0043: tls.SecurePair 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 251/739 Type: Documentation-only The util.isArray() API is deprecated. Please use Array.isArray() instead. Type: Documentation-only The util.isBoolean() API is deprecated. Type: Documentation-only The util.isBuffer() API is deprecated. Please use Buffer.isBuffer() instead. Type: Documentation-only The util.isDate() API is deprecated. Type: Documentation-only The util.isError() API is deprecated. Type: Documentation-only The util.isFunction() API is deprecated. Type: Documentation-only The util.isNull() API is deprecated. DEP0044: util.isArray() DEP0045: util.isBoolean() DEP0046: util.isBuffer() DEP0047: util.isDate() DEP0048: util.isError() DEP0049: util.isFunction() DEP0050: util.isNull() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 252/739 Type: Documentation-only The util.isNullOrUndefined() API is deprecated. Type: Documentation-only The util.isNumber() API is deprecated. Type: Documentation-only The util.isObject() API is deprecated. Type: Documentation-only The util.isPrimitive() API is deprecated. Type: Documentation-only The util.isRegExp() API is deprecated. Type: Documentation-only The util.isString() API is deprecated. Type: Documentation-only The util.isSymbol() API is deprecated. DEP0051: util.isNullOrUndefined() DEP0052: util.isNumber() DEP0053 util.isObject() DEP0054: util.isPrimitive() DEP0055: util.isRegExp() DEP0056: util.isString() DEP0057: util.isSymbol() DEP0058: util.isUndefined() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 253/739 Type: Documentation-only The util.isUndefined() API is deprecated. Type: Documentation-only The util.log() API is deprecated. Type: Documentation-only The util._extend() API is deprecated. Type: Runtime The fs.SyncWriteStream class was never intended to be a publicly accessible API. No alternative API is available. Please use a userland alternative. Type: Runtime --debug activates the legacy V8 debugger interface, which was removed as of V8 5.8. It is replaced by Inspector which is activated with -- inspect instead. Type: Documentation-only The http module ServerResponse.prototype.writeHeader() API is deprecated. Please use ServerResponse.prototype.writeHead() instead. The ServerResponse.prototype.writeHeader() method was never documented as an officially supported API. Type: Runtime The tls.createSecurePair() API was deprecated in documentation in Node.js 0.11.3. Users should use tls.Socket instead. DEP0059: util.log() DEP0060: util._extend() DEP0061: fs.SyncWriteStream DEP0062: node --debug DEP0063: ServerResponse.prototype.writeHeader() DEP0064: tls.createSecurePair() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 254/739 Type: End-of-Life The repl module's REPL_MODE_MAGIC constant, used for replMode option, has been removed. Its behavior has been functionally identical to that of REPL_MODE_SLOPPY since Node.js 6.0.0, when V8 5.0 was imported. Please use REPL_MODE_SLOPPY instead. The NODE_REPL_MODE environment variable is used to set the underlying replMode of an interactive node session. Its value, magic , is also removed. Please use sloppy instead. Type: Documentation-only The http module outgoingMessage._headers and outgoingMessage._headerNames properties are deprecated. Use one of the public methods (e.g. outgoingMessage.getHeader() , outgoingMessage.getHeaders() , outgoingMessage.getHeaderNames() , outgoingMessage.hasHeader() , outgoingMessage.removeHeader() , outgoingMessage.setHeader() ) for working with outgoing headers. The outgoingMessage._headers and outgoingMessage._headerNames properties were never documented as officially supported properties. Type: Documentation-only The http module OutgoingMessage.prototype._renderHeaders() API is deprecated. The OutgoingMessage.prototype._renderHeaders property was never documented as an officially supported API. Type: Runtime node debug corresponds to the legacy CLI debugger which has been replaced with a V8-inspector based CLI debugger available through node inspect . Type: End-of-Life DebugContext has been removed in V8 and is not available in Node.js 10+. DebugContext was an experimental API. Type: End-of-Life async_hooks.currentId() was renamed to async_hooks.executionAsyncId() for clarity. DEP0065: repl.REPL_MODE_MAGIC and NODE_REPL_MODE=magic DEP0066: outgoingMessage._headers, outgoingMessage._headerNames DEP0067: OutgoingMessage.prototype._renderHeaders DEP0068: node debug DEP0069: vm.runInDebugContext(string) DEP0070: async_hooks.currentId() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 255/739 This change was made while async_hooks was an experimental API. Type: End-of-Life async_hooks.triggerId() was renamed to async_hooks.triggerAsyncId() for clarity. This change was made while async_hooks was an experimental API. Type: End-of-Life async_hooks.AsyncResource.triggerId() was renamed to async_hooks.AsyncResource.triggerAsyncId() for clarity. This change was made while async_hooks was an experimental API. Type: End-of-Life Accessing several internal, undocumented properties of net.Server instances with inappropriate names is deprecated. As the original API was undocumented and not generally useful for non-internal code, no replacement API is provided. Type: Runtime The REPLServer.bufferedCommand property was deprecated in favor of REPLServer.clearBufferedCommand() . Type: Runtime REPLServer.parseREPLKeyword() was removed from userland visibility. Type: Runtime tls.parseCertString() is a trivial parsing helper that was made public by mistake. This function can usually be replaced with: DEP0071: async_hooks.triggerId() DEP0072: async_hooks.AsyncResource.triggerId() DEP0073: Several internal properties of net.Server DEP0074: REPLServer.bufferedCommand DEP0075: REPLServer.parseREPLKeyword() DEP0076: tls.parseCertString() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 256/739 const querystring = require('querystring'); querystring.parse(str, '\n', '='); This function is not completely equivalent to querystring.parse() . One difference is that querystring.parse() does url decoding: > querystring.parse('%E5%A5%BD=1', '\n', '='); { '好': '1' } > tls.parseCertString('%E5%A5%BD=1'); { '%E5%A5%BD': '1' } Type: Runtime Module._debug() is deprecated. The Module._debug() function was never documented as an officially supported API. Type: Runtime REPLServer.turnOffEditorMode() was removed from userland visibility. Type: Runtime Using a property named inspect on an object to specify a custom inspection function for util.inspect() is deprecated. Use util.inspect.custom instead. For backward compatibility with Node.js prior to version 6.4.0, both may be specified. Type: Documentation-only The internal path._makeLong() was not intended for public use. However, userland modules have found it useful. The internal API is deprecated and replaced with an identical, public path.toNamespacedPath() method. Type: Runtime fs.truncate() fs.truncateSync() usage with a file descriptor is deprecated. Please use fs.ftruncate() or fs.ftruncateSync() to work with file descriptors. DEP0077: Module._debug() DEP0078: REPLServer.turnOffEditorMode() DEP0079: Custom inspection function on Objects via .inspect() DEP0080: path._makeLong() DEP0081: fs.truncate() using a file descriptor 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 257/739 Type: Runtime REPLServer.prototype.memory() is only necessary for the internal mechanics of the REPLServer itself. Do not use this function. Type: End-of-Life. The ecdhCurve option to tls.createSecureContext() and tls.TLSSocket could be set to false to disable ECDH entirely on the server only. This mode was deprecated in preparation for migrating to OpenSSL 1.1.0 and consistency with the client and is now unsupported. Use the ciphers parameter instead. Type: Runtime Since Node.js versions 4.4.0 and 5.2.0, several modules only intended for internal usage are mistakenly exposed to user code through require() . These modules are: v8/tools/codemap v8/tools/consarray v8/tools/csvparser v8/tools/logreader v8/tools/profile_view v8/tools/profile v8/tools/SourceMap v8/tools/splaytree v8/tools/tickprocessor-driver v8/tools/tickprocessor node-inspect/lib/_inspect (from 7.6.0) node-inspect/lib/internal/inspect_client (from 7.6.0) node-inspect/lib/internal/inspect_repl (from 7.6.0) The v8/* modules do not have any exports, and if not imported in a specific order would in fact throw errors. As such there are virtually no legitimate use cases for importing them through require() . On the other hand, node-inspect may be installed locally through a package manager, as it is published on the npm registry under the same name. No source code modification is necessary if that is done. Type: End-of-Life DEP0082: REPLServer.prototype.memory() DEP0083: Disabling ECDH by setting ecdhCurve to false DEP0084: requiring bundled internal dependencies DEP0085: AsyncHooks Sensitive API 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 258/739 The AsyncHooks Sensitive API was never documented and had various minor issues. (See https://github.com/nodejs/node/issues/15572 .) Use the AsyncResource API instead. Type: End-of-Life runInAsyncIdScope doesn't emit the 'before' or 'after' event and can thus cause a lot of issues. See https://github.com/nodejs/node/issues/14328 for more details. Type: Documentation-only Importing assert directly is not recommended as the exposed functions will use loose equality checks. Use require('assert').strict instead. The API is the same as the legacy assert but it will always use strict equality checks. Type: Runtime Node.js supports all GCM authentication tag lengths which are accepted by OpenSSL when calling decipher.setAuthTag() . This behavior will change in a future version at which point only authentication tag lengths of 128, 120, 112, 104, 96, 64, and 32 bits will be allowed. Authentication tags whose length is not included in this list will be considered invalid in compliance with NIST SP 800-38D . Type: Runtime The crypto.DEFAULT_ENCODING property is deprecated. Type: Documentation-only Assigning properties to the top-level this as an alternative to module.exports is deprecated. Developers should use exports or module.exports instead. Type: Documentation-only The crypto.fips property is deprecated. Please use crypto.setFips() and crypto.getFips() instead. DEP0086: Remove runInAsyncIdScope DEP0089: require('assert') DEP0090: Invalid GCM authentication tag lengths DEP0091: crypto.DEFAULT_ENCODING DEP0092: Top-level this bound to module.exports DEP0093: crypto.fips is deprecated and replaced. 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 259/739 Type: Runtime Using assert.fail() with more than one argument is deprecated. Use assert.fail() with only one argument or use a different assert module method. Type: Runtime timers.enroll() is deprecated. Please use the publicly documented setTimeout() or setInterval() instead. Type: Runtime timers.unenroll() is deprecated. Please use the publicly documented clearTimeout() or clearInterval() instead. Type: Runtime Users of MakeCallback that add the domain property to carry context, should start using the async_context variant of MakeCallback or CallbackScope , or the high-level AsyncResource class. Type: Runtime The embedded API provided by AsyncHooks exposes .emitBefore() and .emitAfter() methods which are very easy to use incorrectly which can lead to unrecoverable errors. Use asyncResource.runInAsyncScope() API instead which provides a much safer, and more convenient, alternative. See https://github.com/nodejs/node/pull/18513 for more details. Type: Compile-time Certain versions of node::MakeCallback APIs available to native modules are deprecated. Please use the versions of the API that accept an async_context parameter. DEP0094: Using assert.fail() with more than one argument. DEP0095: timers.enroll() DEP0096: timers.unenroll() DEP0097: MakeCallback with domain property DEP0098: AsyncHooks Embedder AsyncResource.emitBefore and AsyncResource.emitAfter APIs DEP0099: async context-unaware node::MakeCallback C++ APIs DEP0100: process.assert() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 260/739 Type: Runtime process.assert() is deprecated. Please use the assert module instead. This was never a documented feature. Type: End-of-Life The --with-lttng compile-time option has been removed. Type: End-of-Life Using the noAssert argument has no functionality anymore. All input is going to be verified, no matter if it is set to true or not. Skipping the verification could lead to hard to find errors and crashes. Type: Documentation-only (supports --pending-deprecation ) Using process.binding() in general should be avoided. The type checking methods in particular can be replaced by using util.types . This deprecation has been superseded by the deprecation of the process.binding() API ( DEP0111 ). Type: Documentation-only (supports --pending-deprecation ) When assigning a non-string property to process.env , the assigned value is implicitly converted to a string. This behavior is deprecated if the assigned value is not a string, boolean, or number. In the future, such assignment may result in a thrown error. Please convert the property to a string before assigning it to process.env . Type: Runtime decipher.finaltol() has never been documented and is currently an alias for decipher.final() . In the future, this API will likely be removed, and it is recommended to use decipher.final() instead. Type: Documentation-only DEP0101: --with-lttng DEP0102: Using noAssert in Buffer#(read|write) operations. DEP0103: process.binding('util').is[...] typechecks DEP0104: process.env string coercion DEP0105: decipher.finaltol DEP0106: crypto.createCipher and crypto.createDecipher 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 261/739 Using crypto.createCipher() and crypto.createDecipher() should be avoided as they use a weak key derivation function (MD5 with no salt) and static initialization vectors. It is recommended to derive a key using crypto.pbkdf2() or crypto.scrypt() and to use crypto.createCipheriv() and crypto.createDecipheriv() to obtain the Cipher and Decipher objects respectively. Type: Runtime This was an undocumented helper function not intended for use outside Node.js core and obsoleted by the removal of NPN (Next Protocol Negotiation) support. Type: Documentation-only Deprecated alias for zlib.bytesWritten . This original name was chosen because it also made sense to interpret the value as the number of bytes read by the engine, but is inconsistent with other streams in Node.js that expose values under these names. Type: Documentation-only The produceCachedData option is deprecated. Use script.createCachedData() instead. Type: Documentation-only process.binding() is for use by Node.js internal code only. Stability: 2 - Stable The dns module contains functions belonging to two different categories: 1) Functions that use the underlying operating system facilities to perform name resolution, and that do not necessarily perform any network communication. This category contains only one function: dns.lookup() . Developers looking to perform name resolution in the same way that other applications on the same operating system behave should use dns.lookup() . For example, looking up iana.org . const dns = require('dns'); dns.lookup('iana.org', (err, address, family) => { console.log('address: %j family: IPv%s', address, family); DEP0107: tls.convertNPNProtocols() DEP0108: zlib.bytesRead DEP0110: vm.Script cached data DEP0111: process.binding() DNS 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 262/739 }); // address: "192.0.43.8" family: IPv4 2) Functions that connect to an actual DNS server to perform name resolution, and that always use the network to perform DNS queries. This category contains all functions in the dns module except dns.lookup() . These functions do not use the same set of configuration files used by dns.lookup() (e.g. /etc/hosts ). These functions should be used by developers who do not want to use the underlying operating system's facilities for name resolution, and instead want to always perform DNS queries. Below is an example that resolves 'archive.org' then reverse resolves the IP addresses that are returned. const dns = require('dns'); dns.resolve4('archive.org', (err, addresses) => { if (err) throw err; console.log(`addresses: ${JSON.stringify(addresses)}`); addresses.forEach((a) => { dns.reverse(a, (err, hostnames) => { if (err) { throw err; } console.log(`reverse for ${a}: ${JSON.stringify(hostnames)}`); }); }); }); There are subtle consequences in choosing one over the other, please consult the Implementation considerations section for more information. An independent resolver for DNS requests. Note that creating a new resolver uses the default server settings. Setting the servers used for a resolver using resolver.setServers() does not affect other resolvers: const { Resolver } = require('dns'); const resolver = new Resolver(); resolver.setServers(['4.4.4.4']); // This request will use the server at 4.4.4.4, independent of global settings. resolver.resolve4('example.org', (err, addresses) => { // ... }); The following methods from the dns module are available: resolver.getServers() resolver.resolve() resolver.resolve4() resolver.resolve6() Class: dns.Resolver 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 263/739 resolver.resolveAny() resolver.resolveCname() resolver.resolveMx() resolver.resolveNaptr() resolver.resolveNs() resolver.resolvePtr() resolver.resolveSoa() resolver.resolveSrv() resolver.resolveTxt() resolver.reverse() resolver.setServers() Cancel all outstanding DNS queries made by this resolver. The corresponding callbacks will be called with an error with code ECANCELLED . Returns: <string[]> Returns an array of IP address strings, formatted according to rfc5952 , that are currently configured for DNS resolution. A string will include a port section if a custom port is used. [ '4.4.4.4', '2001:4860:4860::8888', '4.4.4.4:1053', '[2001:4860:4860::8888]:1053' ] hostname <string> options <integer> | <Object> family <integer> The record family. Must be 4 or 6 . IPv4 and IPv6 addresses are both returned by default. hints <number> One or more supported getaddrinfo flags . Multiple flags may be passed by bitwise OR ing their values. all <boolean> When true , the callback returns all resolved addresses in an array. Otherwise, returns a single address. Default: false . verbatim <boolean> When true , the callback receives IPv4 and IPv6 addresses in the order the DNS resolver returned them. When false , IPv4 addresses are placed before IPv6 addresses. Default: currently false (addresses are reordered) but this is expected to change in the not too distant future. New code should use { verbatim: true } . callback <Function> err <Error> address <string> A string representation of an IPv4 or IPv6 address. family <integer> 4 or 6 , denoting the family of address . resolver.cancel() dns.getServers() dns.lookup(hostname[, options], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 264/739 Resolves a hostname (e.g. 'nodejs.org' ) into the first found A (IPv4) or AAAA (IPv6) record. All option properties are optional. If options is an integer, then it must be 4 or 6 – if options is not provided, then IPv4 and IPv6 addresses are both returned if found. With the all option set to true , the arguments for callback change to (err, addresses) , with addresses being an array of objects with the properties address and family . On error, err is an Error object, where err.code is the error code. Keep in mind that err.code will be set to 'ENOENT' not only when the hostname does not exist but also when the lookup fails in other ways such as no available file descriptors. dns.lookup() does not necessarily have anything to do with the DNS protocol. The implementation uses an operating system facility that can associate names with addresses, and vice versa. This implementation can have subtle but important consequences on the behavior of any Node.js program. Please take some time to consult the Implementation considerations section before using dns.lookup() . Example usage: const dns = require('dns'); const options = { family: 6, hints: dns.ADDRCONFIG | dns.V4MAPPED, }; dns.lookup('example.com', options, (err, address, family) => console.log('address: %j family: IPv%s', address, family)); // address: "2606:2800:220:1:248:1893:25c8:1946" family: IPv6 // When options.all is true, the result will be an Array. options.all = true; dns.lookup('example.com', options, (err, addresses) => console.log('addresses: %j', addresses)); // addresses: [{"address":"2606:2800:220:1:248:1893:25c8:1946","family":6}] If this method is invoked as its util.promisify() ed version, and all is not set to true , it returns a Promise for an Object with address and family properties. The following flags can be passed as hints to dns.lookup() . dns.ADDRCONFIG : Returned address types are determined by the types of addresses supported by the current system. For example, IPv4 addresses are only returned if the current system has at least one IPv4 address configured. Loopback addresses are not considered. dns.V4MAPPED : If the IPv6 family was specified, but no IPv6 addresses were found, then return IPv4 mapped IPv6 addresses. Note that it is not supported on some operating systems (e.g FreeBSD 10.1). address <string> port <number> callback <Function> err <Error> hostname <string> e.g. example.com service <string> e.g. http Resolves the given address and port into a hostname and service using the operating system's underlying getnameinfo implementation. Supported getaddrinfo flags dns.lookupService(address, port, callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 265/739 If address is not a valid IP address, a TypeError will be thrown. The port will be coerced to a number. If it is not a legal port, a TypeError will be thrown. On an error, err is an Error object, where err.code is the error code. const dns = require('dns'); dns.lookupService('127.0.0.1', 22, (err, hostname, service) => { console.log(hostname, service); // Prints: localhost ssh }); If this method is invoked as its util.promisify() ed version, it returns a Promise for an Object with hostname and service properties. hostname <string> Hostname to resolve. rrtype <string> Resource record type. Default: 'A' . callback <Function> err <Error> records <string[]> | <Object[]> | <Object> Uses the DNS protocol to resolve a hostname (e.g. 'nodejs.org' ) into an array of the resource records. The callback function has arguments (err, records) . When successful, records will be an array of resource records. The type and structure of individual results varies based on rrtype : rrtype records contains Result type Shorthand method 'A' IPv4 addresses (default) <string> dns.resolve4() 'AAAA' IPv6 addresses <string> dns.resolve6() 'ANY' any records <Object> dns.resolveAny() 'CNAME' canonical name records <string> dns.resolveCname() 'MX' mail exchange records <Object> dns.resolveMx() 'NAPTR' name authority pointer records <Object> dns.resolveNaptr() 'NS' name server records <string> dns.resolveNs() 'PTR' pointer records <string> dns.resolvePtr() 'SOA' start of authority records <Object> dns.resolveSoa() 'SRV' service records <Object> dns.resolveSrv() 'TXT' text records <string[]> dns.resolveTxt() On error, err is an Error object, where err.code is one of the DNS error codes . dns.resolve(hostname[, rrtype], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 266/739 hostname <string> Hostname to resolve. options <Object> ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. When true , the callback receives an array of { address: '1.2.3.4', ttl: 60 } objects rather than an array of strings, with the TTL expressed in seconds. callback <Function> err <Error> addresses <string[]> | <Object[]> Uses the DNS protocol to resolve a IPv4 addresses ( A records) for the hostname . The addresses argument passed to the callback function will contain an array of IPv4 addresses (e.g. ['74.125.79.104', '74.125.79.105', '74.125.79.106'] ). hostname <string> Hostname to resolve. options <Object> ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. When true , the callback receives an array of { address: '0:1:2:3:4:5:6:7', ttl: 60 } objects rather than an array of strings, with the TTL expressed in seconds. callback <Function> err <Error> addresses <string[]> | <Object[]> Uses the DNS protocol to resolve a IPv6 addresses ( AAAA records) for the hostname . The addresses argument passed to the callback function will contain an array of IPv6 addresses. hostname <string> callback <Function> err <Error> ret <Object[]> Uses the DNS protocol to resolve all records (also known as ANY or * query). The ret argument passed to the callback function will be an array containing various types of records. Each object has a property type that indicates the type of the current record. And depending on the type , additional properties will be present on the object: Type Properties 'A' address / ttl 'AAAA' address / ttl 'CNAME' value 'MX' Refer to dns.resolveMx() 'NAPTR' Refer to dns.resolveNaptr() dns.resolve4(hostname[, options], callback) dns.resolve6(hostname[, options], callback) dns.resolveAny(hostname, callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 267/739 Type Properties 'NS' value 'PTR' value 'SOA' Refer to dns.resolveSoa() 'SRV' Refer to dns.resolveSrv() 'TXT' This type of record contains an array property called entries which refers to dns.resolveTxt() , e.g. { entries: ['...'], type: 'TXT' } Here is an example of the ret object passed to the callback: [ { type: 'A', address: '127.0.0.1', ttl: 299 }, { type: 'CNAME', value: 'example.com' }, { type: 'MX', exchange: 'alt4.aspmx.l.example.com', priority: 50 }, { type: 'NS', value: 'ns1.example.com' }, { type: 'TXT', entries: [ 'v=spf1 include:_spf.example.com ~all' ] }, { type: 'SOA', nsname: 'ns1.example.com', hostmaster: 'admin.example.com', serial: 156696742, refresh: 900, retry: 900, expire: 1800, minttl: 60 } ] DNS server operators may choose not to respond to ANY queries. It may be better to call individual methods like dns.resolve4() , dns.resolveMx() , and so on. For more details, see RFC 8482 . hostname <string> callback <Function> err <Error> addresses <string[]> Uses the DNS protocol to resolve CNAME records for the hostname . The addresses argument passed to the callback function will contain an array of canonical name records available for the hostname (e.g. ['bar.example.com'] ). hostname <string> callback <Function> err <Error> addresses <Object[]> dns.resolveCname(hostname, callback) dns.resolveMx(hostname, callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 268/739 Uses the DNS protocol to resolve mail exchange records ( MX records) for the hostname . The addresses argument passed to the callback function will contain an array of objects containing both a priority and exchange property (e.g. [{priority: 10, exchange: 'mx.example.com'}, ...] ). hostname <string> callback <Function> err <Error> addresses <Object[]> Uses the DNS protocol to resolve regular expression based records ( NAPTR records) for the hostname . The addresses argument passed to the callback function will contain an array of objects with the following properties: flags service regexp replacement order preference { flags: 's', service: 'SIP+D2U', regexp: '', replacement: '_sip._udp.example.com', order: 30, preference: 100 } hostname <string> callback <Function> err <Error> addresses <string[]> Uses the DNS protocol to resolve name server records ( NS records) for the hostname . The addresses argument passed to the callback function will contain an array of name server records available for hostname (e.g. ['ns1.example.com', 'ns2.example.com'] ). hostname <string> callback <Function> err <Error> addresses <string[]> dns.resolveNaptr(hostname, callback) dns.resolveNs(hostname, callback) dns.resolvePtr(hostname, callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 269/739 Uses the DNS protocol to resolve pointer records ( PTR records) for the hostname . The addresses argument passed to the callback function will be an array of strings containing the reply records. hostname <string> callback <Function> err <Error> address <Object> Uses the DNS protocol to resolve a start of authority record ( SOA record) for the hostname . The address argument passed to the callback function will be an object with the following properties: nsname hostmaster serial refresh retry expire minttl { nsname: 'ns.example.com', hostmaster: 'root.example.com', serial: 2013101809, refresh: 10000, retry: 2400, expire: 604800, minttl: 3600 } hostname <string> callback <Function> err <Error> addresses <Object[]> Uses the DNS protocol to resolve service records ( SRV records) for the hostname . The addresses argument passed to the callback function will be an array of objects with the following properties: priority weight port name { priority: 10, dns.resolveSoa(hostname, callback) dns.resolveSrv(hostname, callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 270/739 weight: 5, port: 21223, name: 'service.example.com' } hostname <string> callback <Function> err <Error> records {string[][]} Uses the DNS protocol to resolve text queries ( TXT records) for the hostname . The records argument passed to the callback function is a two-dimensional array of the text records available for hostname (e.g. [ ['v=spf1 ip4:0.0.0.0 ', '~all' ] ] ). Each sub-array contains TXT chunks of one record. Depending on the use case, these could be either joined together or treated separately. ip <string> callback <Function> err <Error> hostnames <string[]> Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of hostnames. On error, err is an Error object, where err.code is one of the DNS error codes . servers <string[]> array of rfc5952 formatted addresses Sets the IP address and port of servers to be used when performing DNS resolution. The servers argument is an array of rfc5952 formatted addresses. If the port is the IANA default DNS port (53) it can be omitted. dns.setServers([ '4.4.4.4', '[2001:4860:4860::8888]', '4.4.4.4:1053', '[2001:4860:4860::8888]:1053' ]); An error will be thrown if an invalid address is provided. The dns.setServers() method must not be called while a DNS query is in progress. The dns.setServers() method affects only dns.resolve() , [ dns.resolve*() ][] and dns.reverse() (and specifically not dns.lookup() ). Note that this method works much like resolve.conf . That is, if attempting to resolve with the first server provided results in a NOTFOUND error, the resolve() method will not attempt to resolve with subsequent servers provided. Fallback DNS servers will only be used if the earlier ones time out or result in some other error. dns.resolveTxt(hostname, callback) dns.reverse(ip, callback) dns.setServers(servers) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 271/739 Stability: 1 - Experimental The dns.promises API provides an alternative set of asynchronous DNS methods that return Promise objects rather than using callbacks. The API is accessible via require('dns').promises . An independent resolver for DNS requests. Note that creating a new resolver uses the default server settings. Setting the servers used for a resolver using resolver.setServers() does not affect other resolvers: const { Resolver } = require('dns').promises; const resolver = new Resolver(); resolver.setServers(['4.4.4.4']); // This request will use the server at 4.4.4.4, independent of global settings. resolver.resolve4('example.org').then((addresses) => { // ... }); // Alternatively, the same code can be written using async-await style. (async function() { const addresses = await resolver.resolve4('example.org'); })(); The following methods from the dnsPromises API are available: resolver.getServers() resolver.resolve() resolver.resolve4() resolver.resolve6() resolver.resolveAny() resolver.resolveCname() resolver.resolveMx() resolver.resolveNaptr() resolver.resolveNs() resolver.resolvePtr() resolver.resolveSoa() resolver.resolveSrv() resolver.resolveTxt() resolver.reverse() resolver.setServers() DNS Promises API Class: dnsPromises.Resolver dnsPromises.getServers() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 272/739 Returns: <string[]> Returns an array of IP address strings, formatted according to rfc5952 , that are currently configured for DNS resolution. A string will include a port section if a custom port is used. [ '4.4.4.4', '2001:4860:4860::8888', '4.4.4.4:1053', '[2001:4860:4860::8888]:1053' ] hostname <string> options <integer> | <Object> family <integer> The record family. Must be 4 or 6 . IPv4 and IPv6 addresses are both returned by default. hints <number> One or more supported getaddrinfo flags . Multiple flags may be passed by bitwise OR ing their values. all <boolean> When true , the Promise is resolved with all addresses in an array. Otherwise, returns a single address. Default: false . verbatim <boolean> When true , the Promise is resolved with IPv4 and IPv6 addresses in the order the DNS resolver returned them. When false , IPv4 addresses are placed before IPv6 addresses. Default: currently false (addresses are reordered) but this is expected to change in the not too distant future. New code should use { verbatim: true } . Resolves a hostname (e.g. 'nodejs.org' ) into the first found A (IPv4) or AAAA (IPv6) record. All option properties are optional. If options is an integer, then it must be 4 or 6 – if options is not provided, then IPv4 and IPv6 addresses are both returned if found. With the all option set to true , the Promise is resolved with addresses being an array of objects with the properties address and family . On error, the Promise is rejected with an Error object, where err.code is the error code. Keep in mind that err.code will be set to 'ENOENT' not only when the hostname does not exist but also when the lookup fails in other ways such as no available file descriptors. dnsPromises.lookup() does not necessarily have anything to do with the DNS protocol. The implementation uses an operating system facility that can associate names with addresses, and vice versa. This implementation can have subtle but important consequences on the behavior of any Node.js program. Please take some time to consult the Implementation considerations section before using dnsPromises.lookup() . Example usage: const dns = require('dns'); const dnsPromises = dns.promises; const options = { family: 6, hints: dns.ADDRCONFIG | dns.V4MAPPED, }; dnsPromises.lookup('example.com', options).then((result) => { console.log('address: %j family: IPv%s', result.address, result.family); // address: "2606:2800:220:1:248:1893:25c8:1946" family: IPv6 }); // When options.all is true, the result will be an Array. dnsPromises.lookup(hostname[, options]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 273/739 options.all = true; dnsPromises.lookup('example.com', options).then((result) => { console.log('addresses: %j', result); // addresses: [{"address":"2606:2800:220:1:248:1893:25c8:1946","family":6}] }); address <string> port <number> Resolves the given address and port into a hostname and service using the operating system's underlying getnameinfo implementation. If address is not a valid IP address, a TypeError will be thrown. The port will be coerced to a number. If it is not a legal port, a TypeError will be thrown. On error, the Promise is rejected with an Error object, where err.code is the error code. const dnsPromises = require('dns').promises; dnsPromises.lookupService('127.0.0.1', 22).then((result) => { console.log(result.hostname, result.service); // Prints: localhost ssh }); hostname <string> Hostname to resolve. rrtype <string> Resource record type. Default: 'A' . Uses the DNS protocol to resolve a hostname (e.g. 'nodejs.org' ) into an array of the resource records. When successful, the Promise is resolved with an array of resource records. The type and structure of individual results vary based on rrtype : rrtype records contains Result type Shorthand method 'A' IPv4 addresses (default) <string> dnsPromises.resolve4() 'AAAA' IPv6 addresses <string> dnsPromises.resolve6() 'ANY' any records <Object> dnsPromises.resolveAny() 'CNAME' canonical name records <string> dnsPromises.resolveCname() 'MX' mail exchange records <Object> dnsPromises.resolveMx() 'NAPTR' name authority pointer records <Object> dnsPromises.resolveNaptr() 'NS' name server records <string> dnsPromises.resolveNs() 'PTR' pointer records <string> dnsPromises.resolvePtr() 'SOA' start of authority records <Object> dnsPromises.resolveSoa() dnsPromises.lookupService(address, port) dnsPromises.resolve(hostname[, rrtype]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 274/739 rrtype records contains Result type Shorthand method 'SRV' service records <Object> dnsPromises.resolveSrv() 'TXT' text records <string[]> dnsPromises.resolveTxt() On error, the Promise is rejected with an Error object, where err.code is one of the DNS error codes . hostname <string> Hostname to resolve. options <Object> ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. When true , the Promise is resolved with an array of { address: '1.2.3.4', ttl: 60 } objects rather than an array of strings, with the TTL expressed in seconds. Uses the DNS protocol to resolve IPv4 addresses ( A records) for the hostname . On success, the Promise is resolved with an array of IPv4 addresses (e.g. ['74.125.79.104', '74.125.79.105', '74.125.79.106'] ). hostname <string> Hostname to resolve. options <Object> ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. When true , the Promise is resolved with an array of { address: '0:1:2:3:4:5:6:7', ttl: 60 } objects rather than an array of strings, with the TTL expressed in seconds. Uses the DNS protocol to resolve IPv6 addresses ( AAAA records) for the hostname . On success, the Promise is resolved with an array of IPv6 addresses. hostname <string> Uses the DNS protocol to resolve all records (also known as ANY or * query). On success, the Promise is resolved with an array containing various types of records. Each object has a property type that indicates the type of the current record. And depending on the type , additional properties will be present on the object: Type Properties 'A' address / ttl 'AAAA' address / ttl 'CNAME' value 'MX' Refer to dnsPromises.resolveMx() 'NAPTR' Refer to dnsPromises.resolveNaptr() 'NS' value 'PTR' value dnsPromises.resolve4(hostname[, options]) dnsPromises.resolve6(hostname[, options]) dnsPromises.resolveAny(hostname) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 275/739 Type Properties 'SOA' Refer to dnsPromises.resolveSoa() 'SRV' Refer to dnsPromises.resolveSrv() 'TXT' This type of record contains an array property called entries which refers to dnsPromises.resolveTxt() , e.g. { entries: ['...'], type: 'TXT' } Here is an example of the result object: [ { type: 'A', address: '127.0.0.1', ttl: 299 }, { type: 'CNAME', value: 'example.com' }, { type: 'MX', exchange: 'alt4.aspmx.l.example.com', priority: 50 }, { type: 'NS', value: 'ns1.example.com' }, { type: 'TXT', entries: [ 'v=spf1 include:_spf.example.com ~all' ] }, { type: 'SOA', nsname: 'ns1.example.com', hostmaster: 'admin.example.com', serial: 156696742, refresh: 900, retry: 900, expire: 1800, minttl: 60 } ] hostname <string> Uses the DNS protocol to resolve CNAME records for the hostname . On success, the Promise is resolved with an array of canonical name records available for the hostname (e.g. ['bar.example.com'] ). hostname <string> Uses the DNS protocol to resolve mail exchange records ( MX records) for the hostname . On success, the Promise is resolved with an array of objects containing both a priority and exchange property (e.g. [{priority: 10, exchange: 'mx.example.com'}, ...] ). hostname <string> Uses the DNS protocol to resolve regular expression based records ( NAPTR records) for the hostname . On success, the Promise is resolved with an array of objects with the following properties: flags service regexp replacement order preference dnsPromises.resolveCname(hostname) dnsPromises.resolveMx(hostname) dnsPromises.resolveNaptr(hostname) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 276/739 { flags: 's', service: 'SIP+D2U', regexp: '', replacement: '_sip._udp.example.com', order: 30, preference: 100 } hostname <string> Uses the DNS protocol to resolve name server records ( NS records) for the hostname . On success, the Promise is resolved with an array of name server records available for hostname (e.g. ['ns1.example.com', 'ns2.example.com'] ). hostname <string> Uses the DNS protocol to resolve pointer records ( PTR records) for the hostname . On success, the Promise is resolved with an array of strings containing the reply records. hostname <string> Uses the DNS protocol to resolve a start of authority record ( SOA record) for the hostname . On success, the Promise is resolved with an object with the following properties: nsname hostmaster serial refresh retry expire minttl { nsname: 'ns.example.com', hostmaster: 'root.example.com', serial: 2013101809, refresh: 10000, retry: 2400, expire: 604800, minttl: 3600 } hostname <string> dnsPromises.resolveNs(hostname) dnsPromises.resolvePtr(hostname) dnsPromises.resolveSoa(hostname) dnsPromises.resolveSrv(hostname) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 277/739 Uses the DNS protocol to resolve service records ( SRV records) for the hostname . On success, the Promise is resolved with an array of objects with the following properties: priority weight port name { priority: 10, weight: 5, port: 21223, name: 'service.example.com' } hostname <string> Uses the DNS protocol to resolve text queries ( TXT records) for the hostname . On success, the Promise is resolved with a two-dimensional array of the text records available for hostname (e.g. [ ['v=spf1 ip4:0.0.0.0 ', '~all' ] ] ). Each sub-array contains TXT chunks of one record. Depending on the use case, these could be either joined together or treated separately. ip <string> Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of hostnames. On error, the Promise is rejected with an Error object, where err.code is one of the DNS error codes . servers <string[]> array of rfc5952 formatted addresses Sets the IP address and port of servers to be used when performing DNS resolution. The servers argument is an array of rfc5952 formatted addresses. If the port is the IANA default DNS port (53) it can be omitted. dnsPromises.setServers([ '4.4.4.4', '[2001:4860:4860::8888]', '4.4.4.4:1053', '[2001:4860:4860::8888]:1053' ]); An error will be thrown if an invalid address is provided. The dnsPromises.setServers() method must not be called while a DNS query is in progress. Note that this method works much like resolve.conf . That is, if attempting to resolve with the first server provided results in a NOTFOUND error, the resolve() method will not attempt to resolve with subsequent servers provided. Fallback DNS servers will only be used if the earlier ones time out or result in some other error. dnsPromises.resolveTxt(hostname) dnsPromises.reverse(ip) dnsPromises.setServers(servers) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 278/739 Each DNS query can return one of the following error codes: dns.NODATA : DNS server returned answer with no data. dns.FORMERR : DNS server claims query was misformatted. dns.SERVFAIL : DNS server returned general failure. dns.NOTFOUND : Domain name not found. dns.NOTIMP : DNS server does not implement requested operation. dns.REFUSED : DNS server refused query. dns.BADQUERY : Misformatted DNS query. dns.BADNAME : Misformatted hostname. dns.BADFAMILY : Unsupported address family. dns.BADRESP : Misformatted DNS reply. dns.CONNREFUSED : Could not contact DNS servers. dns.TIMEOUT : Timeout while contacting DNS servers. dns.EOF : End of file. dns.FILE : Error reading file. dns.NOMEM : Out of memory. dns.DESTRUCTION : Channel is being destroyed. dns.BADSTR : Misformatted string. dns.BADFLAGS : Illegal flags specified. dns.NONAME : Given hostname is not numeric. dns.BADHINTS : Illegal hints flags specified. dns.NOTINITIALIZED : c-ares library initialization not yet performed. dns.LOADIPHLPAPI : Error loading iphlpapi.dll . dns.ADDRGETNETWORKPARAMS : Could not find GetNetworkParams function. dns.CANCELLED : DNS query cancelled. Although dns.lookup() and the various dns.resolve*()/dns.reverse() functions have the same goal of associating a network name with a network address (or vice versa), their behavior is quite different. These differences can have subtle but significant consequences on the behavior of Node.js programs. Under the hood, dns.lookup() uses the same operating system facilities as most other programs. For instance, dns.lookup() will almost always resolve a given name the same way as the ping command. On most POSIX-like operating systems, the behavior of the dns.lookup() function can be modified by changing settings in nsswitch.conf(5) and/or resolv.conf(5) , but note that changing these files will change the behavior of all other programs running on the same operating system. Though the call to dns.lookup() will be asynchronous from JavaScript's perspective, it is implemented as a synchronous call to getaddrinfo(3) that runs on libuv's threadpool. This can have surprising negative performance implications for some applications, see the UV_THREADPOOL_SIZE documentation for more information. Error codes Implementation considerations dns.lookup() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 279/739 Note that various networking APIs will call dns.lookup() internally to resolve host names. If that is an issue, consider resolving the hostname to an address using dns.resolve() and using the address instead of a host name. Also, some networking APIs (such as socket.connect() and dgram.createSocket() ) allow the default resolver, dns.lookup() , to be replaced. These functions are implemented quite differently than dns.lookup() . They do not use getaddrinfo(3) and they always perform a DNS query on the network. This network communication is always done asynchronously, and does not use libuv's threadpool. As a result, these functions cannot have the same negative impact on other processing that happens on libuv's threadpool that dns.lookup() can have. They do not use the same set of configuration files than what dns.lookup() uses. For instance, they do not use the configuration from /etc/hosts . Stability: 0 - Deprecated This module is pending deprecation. Once a replacement API has been finalized, this module will be fully deprecated. Most end users should not have cause to use this module. Users who absolutely must have the functionality that domains provide may rely on it for the time being but should expect to have to migrate to a different solution in the future. Domains provide a way to handle multiple different IO operations as a single group. If any of the event emitters or callbacks registered to a domain emit an 'error' event, or throw an error, then the domain object will be notified, rather than losing the context of the error in the process.on('uncaughtException') handler, or causing the program to exit immediately with an error code. Domain error handlers are not a substitute for closing down a process when an error occurs. By the very nature of how throw works in JavaScript, there is almost never any way to safely "pick up where it left off", without leaking references, or creating some other sort of undefined brittle state. The safest way to respond to a thrown error is to shut down the process. Of course, in a normal web server, there may be many open connections, and it is not reasonable to abruptly shut those down because an error was triggered by someone else. The better approach is to send an error response to the request that triggered the error, while letting the others finish in their normal time, and stop listening for new requests in that worker. In this way, domain usage goes hand-in-hand with the cluster module, since the master process can fork a new worker when a worker encounters an error. For Node.js programs that scale to multiple machines, the terminating proxy or service registry can take note of the failure, and react accordingly. For example, this is not a good idea: // XXX WARNING! BAD IDEA! const d = require('domain').create(); d.on('error', (er) => { // The error won't crash the process, but what it does is worse! // Though we've prevented abrupt process restarting, we are leaking // resources like crazy if this ever happens. // Thi i b tt th (' htE ti ')! dns.resolve() , dns.resolve*() and dns.reverse() Domain Warning: Don't Ignore Errors! 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 280/739 // This is no better than process.on('uncaughtException')! console.log(`error, but oh well ${er.message}`); }); d.run(() => { require('http').createServer((req, res) => { handleRequest(req, res); }).listen(PORT); }); By using the context of a domain, and the resilience of separating our program into multiple worker processes, we can react more appropriately, and handle errors with much greater safety. // Much better! const cluster = require('cluster'); const PORT = +process.env.PORT || 1337; if (cluster.isMaster) { // A more realistic scenario would have more than 2 workers, // and perhaps not put the master and worker in the same file. // // It is also possible to get a bit fancier about logging, and // implement whatever custom logic is needed to prevent DoS // attacks and other bad behavior. // // See the options in the cluster documentation. // // The important thing is that the master does very little, // increasing our resilience to unexpected errors. cluster.fork(); cluster.fork(); cluster.on('disconnect', (worker) => { console.error('disconnect!'); cluster.fork(); }); } else { // the worker // // This is where we put our bugs! const domain = require('domain'); // See the cluster documentation for more details about using // worker processes to serve requests. How it works, caveats, etc. const server = require('http').createServer((req, res) => { const d = domain.create(); d.on('error', (er) => { console.error(`error ${er.stack}`); // We're in dangerous territory! 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 281/739 // g y // By definition, something unexpected occurred, // which we probably didn't want. // Anything can happen now! Be very careful! try { // make sure we close down within 30 seconds const killtimer = setTimeout(() => { process.exit(1); }, 30000); // But don't keep the process open just for that! killtimer.unref(); // stop taking new requests. server.close(); // Let the master know we're dead. This will trigger a // 'disconnect' in the cluster master, and then it will fork // a new worker. cluster.worker.disconnect(); // try to send an error to the request that triggered the problem res.statusCode = 500; res.setHeader('content-type', 'text/plain'); res.end('Oops, there was a problem!\n'); } catch (er2) { // oh well, not much we can do at this point. console.error(`Error sending 500! ${er2.stack}`); } }); // Because req and res were created before this domain existed, // we need to explicitly add them. // See the explanation of implicit vs explicit binding below. d.add(req); d.add(res); // Now run the handler function in the domain. d.run(() => { handleRequest(req, res); }); }); server.listen(PORT); } // This part is not important. Just an example routing thing. // Put fancy application logic here. function handleRequest(req, res) { switch (req.url) { case '/error': // We do some async stuff, and then... setTimeout(() => { // Whoops! flerb.bark(); }, timeout); 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 282/739 }, ); break; default: res.end('ok'); } } Any time an Error object is routed through a domain, a few extra fields are added to it. error.domain The domain that first handled the error. error.domainEmitter The event emitter that emitted an 'error' event with the error object. error.domainBound The callback function which was bound to the domain, and passed an error as its first argument. error.domainThrown A boolean indicating whether the error was thrown, emitted, or passed to a bound callback function. If domains are in use, then all new EventEmitter objects (including Stream objects, requests, responses, etc.) will be implicitly bound to the active domain at the time of their creation. Additionally, callbacks passed to lowlevel event loop requests (such as to fs.open() , or other callback-taking methods) will automatically be bound to the active domain. If they throw, then the domain will catch the error. In order to prevent excessive memory usage, Domain objects themselves are not implicitly added as children of the active domain. If they were, then it would be too easy to prevent request and response objects from being properly garbage collected. To nest Domain objects as children of a parent Domain they must be explicitly added. Implicit binding routes thrown errors and 'error' events to the Domain 's 'error' event, but does not register the EventEmitter on the Domain . Implicit binding only takes care of thrown errors and 'error' events. Sometimes, the domain in use is not the one that ought to be used for a specific event emitter. Or, the event emitter could have been created in the context of one domain, but ought to instead be bound to some other domain. For example, there could be one domain in use for an HTTP server, but perhaps we would like to have a separate domain to use for each request. That is possible via explicit binding. // create a top-level domain for the server const domain = require('domain'); const http = require('http'); const serverDomain = domain.create(); serverDomain.run(() => { // server is created in the scope of serverDomain http.createServer((req, res) => { // req and res are also created in the scope of serverDomain // however, we'd prefer to have a separate domain for each request. // create it first thing, and add req and res to it. const reqd = domain.create(); ( ) Additions to Error objects Implicit Binding Explicit Binding 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 283/739 reqd.add(req); reqd.add(res); reqd.on('error', (er) => { console.error('Error', er, req.url); try { res.writeHead(500); res.end('Error occurred, sorry.'); } catch (er2) { console.error('Error sending 500', er2, req.url); } }); }).listen(1337); }); Returns: <Domain> The Domain class encapsulates the functionality of routing errors and uncaught exceptions to the active Domain object. Domain is a child class of EventEmitter . To handle the errors that it catches, listen to its 'error' event. <Array> An array of timers and event emitters that have been explicitly added to the domain. emitter <EventEmitter> | <Timer> emitter or timer to be added to the domain Explicitly adds an emitter to the domain. If any event handlers called by the emitter throw an error, or if the emitter emits an 'error' event, it will be routed to the domain's 'error' event, just like with implicit binding. This also works with timers that are returned from setInterval() and setTimeout() . If their callback function throws, it will be caught by the domain 'error' handler. If the Timer or EventEmitter was already bound to a domain, it is removed from that one, and bound to this one instead. callback <Function> The callback function Returns: <Function> The bound function The returned function will be a wrapper around the supplied callback function. When the returned function is called, any errors that are thrown will be routed to the domain's 'error' event. const d = domain.create(); function readSomeFile(filename, cb) { fs.readFile(filename, 'utf8', d.bind((er, data) => { // if this throws, it will also be passed to the domain domain.create() Class: Domain domain.members domain.add(emitter) domain.bind(callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 284/739 return cb(er, data ? JSON.parse(data) : null); })); } d.on('error', (er) => { // an error occurred somewhere. // if we throw it now, it will crash the program // with the normal line number and stack message. }); The enter() method is plumbing used by the run() , bind() , and intercept() methods to set the active domain. It sets domain.active and process.domain to the domain, and implicitly pushes the domain onto the domain stack managed by the domain module (see domain.exit() for details on the domain stack). The call to enter() delimits the beginning of a chain of asynchronous calls and I/O operations bound to a domain. Calling enter() changes only the active domain, and does not alter the domain itself. enter() and exit() can be called an arbitrary number of times on a single domain. The exit() method exits the current domain, popping it off the domain stack. Any time execution is going to switch to the context of a different chain of asynchronous calls, it's important to ensure that the current domain is exited. The call to exit() delimits either the end of or an interruption to the chain of asynchronous calls and I/O operations bound to a domain. If there are multiple, nested domains bound to the current execution context, exit() will exit any domains nested within this domain. Calling exit() changes only the active domain, and does not alter the domain itself. enter() and exit() can be called an arbitrary number of times on a single domain. callback <Function> The callback function Returns: <Function> The intercepted function This method is almost identical to domain.bind(callback) . However, in addition to catching thrown errors, it will also intercept Error objects sent as the first argument to the function. In this way, the common if (err) return callback(err); pattern can be replaced with a single error handler in a single place. const d = domain.create(); function readSomeFile(filename, cb) { fs.readFile(filename, 'utf8', d.intercept((data) => { // note, the first argument is never passed to the // callback since it is assumed to be the 'Error' argument // and thus intercepted by the domain. // if this throws, it will also be passed to the domain // so the error-handling logic can be moved to the 'error' // event on the domain instead of being repeated throughout // the program. domain.enter() domain.exit() domain.intercept(callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 285/739 return cb(null, JSON.parse(data)); })); } d.on('error', (er) => { // an error occurred somewhere. // if we throw it now, it will crash the program // with the normal line number and stack message. }); emitter <EventEmitter> | <Timer> emitter or timer to be removed from the domain The opposite of domain.add(emitter) . Removes domain handling from the specified emitter. fn <Function> ...args <any> Run the supplied function in the context of the domain, implicitly binding all event emitters, timers, and lowlevel requests that are created in that context. Optionally, arguments can be passed to the function. This is the most basic way to use a domain. const domain = require('domain'); const fs = require('fs'); const d = domain.create(); d.on('error', (er) => { console.error('Caught error!', er); }); d.run(() => { process.nextTick(() => { setTimeout(() => { // simulating some various async stuff fs.open('non-existent file', 'r', (er, fd) => { if (er) throw er; // proceed... }); }, 100); }); }); In this example, the d.on('error') handler will be triggered, rather than crashing the program. As of Node.js 8.0.0, the handlers of Promises are run inside the domain in which the call to .then() or .catch() itself was made: const d1 = domain.create(); const d2 = domain.create(); let p; domain.remove(emitter) domain.run(fn[, ...args]) Domains and Promises 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 286/739 d1.run(() => { p = Promise.resolve(42); }); d2.run(() => { p.then((v) => { // running in d2 }); }); A callback may be bound to a specific domain using domain.bind(callback) : const d1 = domain.create(); const d2 = domain.create(); let p; d1.run(() => { p = Promise.resolve(42); }); d2.run(() => { p.then(p.domain.bind((v) => { // running in d1 })); }); Note that domains will not interfere with the error handling mechanisms for Promises, i.e. no 'error' event will be emitted for unhandled Promise rejections. Stability: 1 - Experimental Node.js contains support for ES Modules based upon the Node.js EP for ES Modules . Not all features of the EP are complete and will be landing as both VM support and implementation is ready. Error messages are still being polished. The --experimental-modules flag can be used to enable features for loading ESM modules. Once this has been set, files ending with .mjs will be able to be loaded as ES Modules. node --experimental-modules my-app.mjs ECMAScript Modules Enabling Features Supported 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 287/739 Only the CLI argument for the main entry point to the program can be an entry point into an ESM graph. Dynamic import can also be used to create entry points into ESM graphs at runtime. <Object> The import.meta metaproperty is an Object that contains the following property: url <string> The absolute file: URL of the module. Feature Reason require('./foo.mjs') ES Modules have differing resolution and timing, use dynamic import NODE_PATH is not part of resolving import specifiers. Please use symlinks if this behavior is desired. require.extensions is not used by import . The expectation is that loader hooks can provide this workflow in the future. require.cache is not used by import . It has a separate cache. ESM are resolved and cached based upon URL semantics. This means that files containing special characters such as # and ? need to be escaped. Modules will be loaded multiple times if the import specifier used to resolve them have a different query or fragment. import './foo?query=1'; // loads ./foo with query of "?query=1" import './foo?query=2'; // loads ./foo with query of "?query=2" For now, only modules using the file: protocol can be loaded. All CommonJS, JSON, and C++ modules can be used with import . Modules loaded this way will only be loaded once, even if their query or fragment string differs between import statements. When loaded via import these modules will provide a single default export representing the value of module.exports at the time they finished evaluating. import.meta Unsupported Notable differences between import and require No NODE_PATH No require.extensions No require.cache URL based paths Interop with existing modules 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 288/739 // foo.js module.exports = { one: 1 }; // bar.mjs import foo from './foo.js'; foo.one === 1; // true Builtin modules will provide named exports of their public API, as well as a default export which can be used for, among other things, modifying the named exports. Named exports of builtin modules are updated when the corresponding exports property is accessed, redefined, or deleted. import EventEmitter from 'events'; const e = new EventEmitter(); import { readFile } from 'fs'; readFile('./foo.txt', (err, source) => { if (err) { console.error(err); } else { console.log(source); } }); import fs, { readFileSync } from 'fs'; fs.readFileSync = () => Buffer.from('Hello, ESM'); fs.readFileSync === readFileSync; To customize the default module resolution, loader hooks can optionally be provided via a --loader ./loader-name.mjs argument to Node.js. When hooks are used they only apply to ES module loading and not to any CommonJS modules loaded. The resolve hook returns the resolved file URL and module format for a given module specifier and parent file URL: const baseURL = new URL('file://'); baseURL.pathname = `${process.cwd()}/`; export async function resolve(specifier, parentModuleURL = baseURL, defaultResolver) { return { url: new URL(specifier, parentModuleURL).href, format: 'esm' }; } Loader hooks Resolve hook 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 289/739 The parentModuleURL is provided as undefined when performing main Node.js load itself. The default Node.js ES module resolution function is provided as a third argument to the resolver for easy compatibility workflows. In addition to returning the resolved file URL value, the resolve hook also returns a format property specifying the module format of the resolved module. This can be one of the following: format Description 'esm' Load a standard JavaScript module 'cjs' Load a node-style CommonJS module 'builtin' Load a node builtin CommonJS module 'json' Load a JSON file 'addon' Load a C++ Addon 'dynamic' Use a dynamic instantiate hook For example, a dummy loader to load JavaScript restricted to browser resolution rules with only JS file extension and Node.js builtin modules support could be written: import path from 'path'; import process from 'process'; import Module from 'module'; const builtins = Module.builtinModules; const JS_EXTENSIONS = new Set(['.js', '.mjs']); const baseURL = new URL('file://'); baseURL.pathname = `${process.cwd()}/`; export function resolve(specifier, parentModuleURL = baseURL, defaultResolve) { if (builtins.includes(specifier)) { return { url: specifier, format: 'builtin' }; } if (/^\.{0,2}[/]/.test(specifier) !== true && !specifier.startsWith('file:')) { // For node_modules support: // return defaultResolve(specifier, parentModuleURL); throw new Error( `imports must begin with '/', './', or '../'; '${specifier}' does not`); } const resolved = new URL(specifier, parentModuleURL); const ext = path.extname(resolved.pathname); if (!JS_EXTENSIONS.has(ext)) { throw new Error( `Cannot load file with non-JavaScript file extension ${ext}.`); 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 290/739 Ca ot oad e t o Ja aSc pt e e te s o ${e t}. ); } return { url: resolved.href, format: 'esm' }; } With this loader, running: NODE_OPTIONS='--experimental-modules --loader ./custom-loader.mjs' node x.js would load the module x.js as an ES module with relative resolution support (with node_modules loading skipped in this example). To create a custom dynamic module that doesn't correspond to one of the existing format interpretations, the dynamicInstantiate hook can be used. This hook is called only for modules that return format: 'dynamic' from the resolve hook. export async function dynamicInstantiate(url) { return { exports: ['customExportName'], execute: (exports) => { // get and set functions provided for pre-allocated export names exports.customExportName.set('value'); } }; } With the list of module exports provided upfront, the execute function will then be called at the exact point of module evaluation order for that module in the import tree. Applications running in Node.js will generally experience four categories of errors: Standard JavaScript errors such as <EvalError> , <SyntaxError> , <RangeError> , <ReferenceError> , <TypeError> , and <URIError> . System errors triggered by underlying operating system constraints such as attempting to open a file that does not exist or attempting to send data over a closed socket. User-specified errors triggered by application code. AssertionError s are a special class of error that can be triggered when Node.js detects an exceptional logic violation that should never occur. These are raised typically by the assert module. All JavaScript and System errors raised by Node.js inherit from, or are instances of, the standard JavaScript <Error> class and are guaranteed to provide at least the properties available on that class. Node.js supports several mechanisms for propagating and handling errors that occur while an application is running. How these errors are reported and handled depends entirely on the type of Error and the style of the API that is called. All JavaScript errors are handled as exceptions that immediately generate and throw an error using the standard JavaScript throw mechanism. These are handled using the try…catch construct provided by the JavaScript language. Dynamic instantiate hook Errors Error Propagation and Interception 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 291/739 // Throws with a ReferenceError because z is not defined. try { const m = 1; const n = m + z; } catch (err) { // Handle the error here. } Any use of the JavaScript throw mechanism will raise an exception that must be handled using try…catch or the Node.js process will exit immediately. With few exceptions, Synchronous APIs (any blocking method that does not accept a callback function, such as fs.readFileSync ), will use throw to report errors. Errors that occur within Asynchronous APIs may be reported in multiple ways: Most asynchronous methods that accept a callback function will accept an Error object passed as the first argument to that function. If that first argument is not null and is an instance of Error , then an error occurred that should be handled. const fs = require('fs'); fs.readFile('a file that does not exist', (err, data) => { if (err) { console.error('There was an error reading the file!', err); return; } // Otherwise handle the data }); When an asynchronous method is called on an object that is an EventEmitter , errors can be routed to that object's 'error' event. const net = require('net'); const connection = net.connect('localhost'); // Adding an 'error' event handler to a stream: connection.on('error', (err) => { // If the connection is reset by the server, or if it can't // connect at all, or on any sort of error encountered by // the connection, the error will be sent here. console.error(err); }); connection.pipe(process.stdout); A handful of typically asynchronous methods in the Node.js API may still use the throw mechanism to raise exceptions that must be handled using try…catch . There is no comprehensive list of such methods; please refer to the documentation of each method to determine the appropriate error handling mechanism required. The use of the 'error' event mechanism is most common for stream-based and event emitter-based APIs, which themselves represent a series of asynchronous operations over time (as opposed to a single operation that may pass or fail). For all EventEmitter objects, if an 'error' event handler is not provided, the error will be thrown, causing the Node.js process to report an uncaught exception and crash unless either: The domain module is used appropriately or a handler has been registered for the 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 292/739 'uncaughtException' event. const EventEmitter = require('events'); const ee = new EventEmitter(); setImmediate(() => { // This will crash the process because no 'error' event // handler has been added. ee.emit('error', new Error('This will crash')); }); Errors generated in this way cannot be intercepted using try…catch as they are thrown after the calling code has already exited. Developers must refer to the documentation for each method to determine exactly how errors raised by those methods are propagated. Most asynchronous methods exposed by the Node.js core API follow an idiomatic pattern referred to as an error-first callback. With this pattern, a callback function is passed to the method as an argument. When the operation either completes or an error is raised, the callback function is called with the Error object (if any) passed as the first argument. If no error was raised, the first argument will be passed as null . const fs = require('fs'); function errorFirstCallback(err, data) { if (err) { console.error('There was an error', err); return; } console.log(data); } fs.readFile('/some/file/that/does-not-exist', errorFirstCallback); fs.readFile('/some/file/that/does-exist', errorFirstCallback); The JavaScript try…catch mechanism cannot be used to intercept errors generated by asynchronous APIs. A common mistake for beginners is to try to use throw inside an error-first callback: // THIS WILL NOT WORK: const fs = require('fs'); try { fs.readFile('/some/file/that/does-not-exist', (err, data) => { // mistaken assumption: throwing here... if (err) { throw err; } }); } catch (err) { // This will not catch the throw! console.error(err); } Error-first callbacks 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 293/739 This will not work because the callback function passed to fs.readFile() is called asynchronously. By the time the callback has been called, the surrounding code (including the try { } catch (err) { } block will have already exited. Throwing an error inside the callback can crash the Node.js process in most cases. If domains are enabled, or a handler has been registered with process.on('uncaughtException') , such errors can be intercepted. A generic JavaScript Error object that does not denote any specific circumstance of why the error occurred. Error objects capture a "stack trace" detailing the point in the code at which the Error was instantiated, and may provide a text description of the error. For crypto only, Error objects will include the OpenSSL error stack in a separate property called opensslErrorStack if it is available when the error is thrown. All errors generated by Node.js, including all System and JavaScript errors, will either be instances of, or inherit from, the Error class. message <string> Creates a new Error object and sets the error.message property to the provided text message. If an object is passed as message , the text message is generated by calling message.toString() . The error.stack property will represent the point in the code at which new Error() was called. Stack traces are dependent on V8's stack trace API . Stack traces extend only to either (a) the beginning of synchronous code execution, or (b) the number of frames given by the property Error.stackTraceLimit , whichever is smaller. targetObject <Object> constructorOpt <Function> Creates a .stack property on targetObject , which when accessed returns a string representing the location in the code at which Error.captureStackTrace() was called. const myObject = {}; Error.captureStackTrace(myObject); myObject.stack; // similar to `new Error().stack` The first line of the trace will be prefixed with ${myObject.name}: ${myObject.message} . The optional constructorOpt argument accepts a function. If given, all frames above constructorOpt , including constructorOpt , will be omitted from the generated stack trace. The constructorOpt argument is useful for hiding implementation details of error generation from an end user. For instance: function MyError() { Error.captureStackTrace(this, MyError); } // Without passing MyError to captureStackTrace, the MyError // frame would show up in the .stack property. By passing // the constructor, we omit that frame, and retain all frames below it. new MyError().stack; Class: Error new Error(message) Error.captureStackTrace(targetObject[, constructorOpt]) Error.stackTraceLimit 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 294/739 <number> The Error.stackTraceLimit property specifies the number of stack frames collected by a stack trace (whether generated by new Error().stack or Error.captureStackTrace(obj) ). The default value is 10 but may be set to any valid JavaScript number. Changes will affect any stack trace captured after the value has been changed. If set to a non-number value, or set to a negative number, stack traces will not capture any frames. <string> The error.code property is a string label that identifies the kind of error. error.code is the most stable way to identify an error. It will only change between major versions of Node.js. In contrast, error.message strings may change between any versions of Node.js. See Node.js Error Codes for details about specific codes. <string> The error.message property is the string description of the error as set by calling new Error(message) . The message passed to the constructor will also appear in the first line of the stack trace of the Error , however changing this property after the Error object is created may not change the first line of the stack trace (for example, when error.stack is read before this property is changed). const err = new Error('The message'); console.error(err.message); // Prints: The message <string> The error.stack property is a string describing the point in the code at which the Error was instantiated. Error: Things keep happening! at /home/gbusey/file.js:525:2 at Frobnicator.refrobulate (/home/gbusey/business-logic.js:424:21) at Actor.<anonymous> (/home/gbusey/actors.js:400:8) at increaseSynergy (/home/gbusey/actors.js:701:6) The first line is formatted as <error class name>: <error message> , and is followed by a series of stack frames (each line beginning with "at "). Each frame describes a call site within the code that lead to the error being generated. V8 attempts to display a name for each function (by variable name, function name, or object method name), but occasionally it will not be able to find a suitable name. If V8 cannot determine a name for the function, only location information will be displayed for that frame. Otherwise, the determined function name will be displayed with location information appended in parentheses. Frames are only generated for JavaScript functions. If, for example, execution synchronously passes through a C++ addon function called cheetahify which itself calls a JavaScript function, the frame representing the cheetahify call will not be present in the stack traces: const cheetahify = require('./native-binding.node'); function makeFaster() { error.code error.message error.stack 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 295/739 // cheetahify *synchronously* calls speedy. cheetahify(function speedy() { throw new Error('oh no!'); }); } makeFaster(); // will throw: // /home/gbusey/file.js:6 // throw new Error('oh no!'); // ^ // Error: oh no! // at speedy (/home/gbusey/file.js:6:11) // at makeFaster (/home/gbusey/file.js:5:3) // at Object.<anonymous> (/home/gbusey/file.js:10:1) // at Module._compile (module.js:456:26) // at Object.Module._extensions..js (module.js:474:10) // at Module.load (module.js:356:32) // at Function.Module._load (module.js:312:12) // at Function.Module.runMain (module.js:497:10) // at startup (node.js:119:16) // at node.js:906:3 The location information will be one of: native , if the frame represents a call internal to V8 (as in [].forEach ). plain-filename.js:line:column , if the frame represents a call internal to Node.js. /absolute/path/to/file.js:line:column , if the frame represents a call in a user program, or its dependencies. The string representing the stack trace is lazily generated when the error.stack property is accessed. The number of frames captured by the stack trace is bounded by the smaller of Error.stackTraceLimit or the number of available frames on the current event loop tick. System-level errors are generated as augmented Error instances, which are detailed here . A subclass of Error that indicates the failure of an assertion. For details, see Class: assert.AssertionError . A subclass of Error that indicates that a provided argument was not within the set or range of acceptable values for a function; whether that is a numeric range, or outside the set of options for a given function parameter. require('net').connect(-1); // throws "RangeError: "port" option should be >= 0 and < 65536: -1" Node.js will generate and throw RangeError instances immediately as a form of argument validation. Class: AssertionError Class: RangeError Class: ReferenceError 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 296/739 A subclass of Error that indicates that an attempt is being made to access a variable that is not defined. Such errors commonly indicate typos in code, or an otherwise broken program. While client code may generate and propagate these errors, in practice, only V8 will do so. doesNotExist; // throws ReferenceError, doesNotExist is not a variable in this program. Unless an application is dynamically generating and running code, ReferenceError instances should always be considered a bug in the code or its dependencies. A subclass of Error that indicates that a program is not valid JavaScript. These errors may only be generated and propagated as a result of code evaluation. Code evaluation may happen as a result of eval , Function , require , or vm . These errors are almost always indicative of a broken program. try { require('vm').runInThisContext('binary ! isNotOk'); } catch (err) { // err will be a SyntaxError } SyntaxError instances are unrecoverable in the context that created them – they may only be caught by other contexts. A subclass of Error that indicates that a provided argument is not an allowable type. For example, passing a function to a parameter which expects a string would be considered a TypeError . require('url').parse(() => { }); // throws TypeError, since it expected a string Node.js will generate and throw TypeError instances immediately as a form of argument validation. A JavaScript exception is a value that is thrown as a result of an invalid operation or as the target of a throw statement. While it is not required that these values are instances of Error or classes which inherit from Error , all exceptions thrown by Node.js or the JavaScript runtime will be instances of Error . Some exceptions are unrecoverable at the JavaScript layer. Such exceptions will always cause the Node.js process to crash. Examples include assert() checks or abort() calls in the C++ layer. Node.js generates system errors when exceptions occur within its runtime environment. These usually occur when an application violates an operating system constraint. For example, a system error will occur if an application attempts to read a file that does not exist. System errors are usually generated at the syscall level. For a comprehensive list, see the errno(3) man page . In Node.js, system errors are Error objects with extra properties. Class: SyntaxError Class: TypeError Exceptions vs. Errors System Errors 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 297/739 address <string> If present, the address to which a network connection failed code <string> The string error code dest <string> If present, the file path destination when reporting a file system error errno <number> | <string> The system-provided error number info <Object> If present, extra details about the error condition message <string> A system-provided human-readable description of the error path <string> If present, the file path when reporting a file system error port <number> If present, the network connection port that is not available syscall <string> The name of the system call that triggered the error <string> If present, error.address is a string describing the address to which a network connection failed. <string> The error.code property is a string representing the error code. <string> If present, error.dest is the file path destination when reporting a file system error. <string> | <number> The error.errno property is a number or a string. If it is a number, it is a negative value which corresponds to the error code defined in libuv Error handling . See the libuv errno.h header file ( deps/uv/include/uv/errno.h in the Node.js source tree) for details. In case of a string, it is the same as error.code . <Object> If present, error.info is an object with details about the error condition. <string> error.message is a system-provided human-readable description of the error. <string> If present, error.path is a string containing a relevant invalid pathname. Class: SystemError error.address error.code error.dest error.errno error.info error.message error.path error.port 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 298/739 <number> If present, error.port is the network connection port that is not available. <string> The error.syscall property is a string describing the syscall that failed. This is a list of system errors commonly-encountered when writing a Node.js program. For a comprehensive list, see the errno(3) man page . EACCES (Permission denied): An attempt was made to access a file in a way forbidden by its file access permissions. EADDRINUSE (Address already in use): An attempt to bind a server ( net , http , or https ) to a local address failed due to another server on the local system already occupying that address. ECONNREFUSED (Connection refused): No connection could be made because the target machine actively refused it. This usually results from trying to connect to a service that is inactive on the foreign host. ECONNRESET (Connection reset by peer): A connection was forcibly closed by a peer. This normally results from a loss of the connection on the remote socket due to a timeout or reboot. Commonly encountered via the http and net modules. EEXIST (File exists): An existing file was the target of an operation that required that the target not exist. EISDIR (Is a directory): An operation expected a file, but the given pathname was a directory. EMFILE (Too many open files in system): Maximum number of file descriptors allowable on the system has been reached, and requests for another descriptor cannot be fulfilled until at least one has been closed. This is encountered when opening many files at once in parallel, especially on systems (in particular, macOS) where there is a low file descriptor limit for processes. To remedy a low limit, run ulimit -n 2048 in the same shell that will run the Node.js process. ENOENT (No such file or directory): Commonly raised by fs operations to indicate that a component of the specified pathname does not exist — no entity (file or directory) could be found by the given path. ENOTDIR (Not a directory): A component of the given pathname existed, but was not a directory as expected. Commonly raised by fs.readdir . ENOTEMPTY (Directory not empty): A directory with entries was the target of an operation that requires an empty directory — usually fs.unlink . EPERM (Operation not permitted): An attempt was made to perform an operation that requires elevated privileges. EPIPE (Broken pipe): A write on a pipe, socket, or FIFO for which there is no process to read the data. Commonly encountered at the net and http layers, indicative that the remote side of the stream being written to has been closed. ETIMEDOUT (Operation timed out): A connect or send request failed because the connected party did not properly respond after a period of time. Usually encountered by http or net — often a sign that a socket.end() was not properly called. error.syscall Common System Errors Node.js Error Codes ERR_AMBIGUOUS_ARGUMENT 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 299/739 A function argument is being used in a way that suggests that the function signature may be misunderstood. This is thrown by the assert module when the message parameter in assert.throws(block, message) matches the error message thrown by block because that usage suggests that the user believes message is the expected message rather than the message the AssertionError will display if block does not throw. An iterable argument (i.e. a value that works with for...of loops) was required, but not provided to a Node.js API. A special type of error that can be triggered whenever Node.js detects an exceptional logic violation that should never occur. These are raised typically by the assert module. An attempt was made to register something that is not a function as an AsyncHooks callback. The type of an asynchronous resource was invalid. Note that users are also able to define their own types if using the public embedder API. Data passed to a Brotli stream was not successfully compressed. An invalid parameter key was passed during construction of a Brotli stream. An operation outside the bounds of a Buffer was attempted. An attempt has been made to create a Buffer larger than the maximum allowed size. ERR_ARG_NOT_ITERABLE ERR_ASSERTION ERR_ASYNC_CALLBACK ERR_ASYNC_TYPE ERR_BROTLI_COMPRESSION_FAILED ERR_BROTLI_INVALID_PARAM ERR_BUFFER_OUT_OF_BOUNDS ERR_BUFFER_TOO_LARGE 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 300/739 The value passed to postMessage() contained an object that is not supported for transferring. Node.js was unable to watch for the SIGINT signal. A child process was closed before the parent received a reply. Used when a child process is being forked without specifying an IPC channel. Used when the main process is trying to read data from the child process's STDERR/STDOUT, and the data's length is longer than the maxBuffer option. There was an attempt to use a MessagePort instance in a closed state, usually after .close() has been called. Console was instantiated without stdout stream, or Console has a non-writable stdout or stderr stream. A constructor for a class was called without new . The native call from process.cpuUsage could not be processed. ERR_CANNOT_TRANSFER_OBJECT ERR_CANNOT_WATCH_SIGINT ERR_CHILD_CLOSED_BEFORE_REPLY ERR_CHILD_PROCESS_IPC_REQUIRED ERR_CHILD_PROCESS_STDIO_MAXBUFFER ERR_CLOSED_MESSAGE_PORT ERR_CONSOLE_WRITABLE_STREAM ERR_CONSTRUCT_CALL_REQUIRED ERR_CPU_USAGE ERR_CRYPTO_CUSTOM_ENGINE_NOT_SUPPORTED 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 301/739 A client certificate engine was requested that is not supported by the version of OpenSSL being used. An invalid value for the format argument was passed to the crypto.ECDH() class getPublicKey() method. An invalid value for the key argument has been passed to the crypto.ECDH() class computeSecret() method. It means that the public key lies outside of the elliptic curve. An invalid crypto engine identifier was passed to require('crypto').setEngine() . The --force-fips command-line argument was used but there was an attempt to enable or disable FIPS mode in the crypto module. An attempt was made to enable or disable FIPS mode, but FIPS mode was not available. The UTF-16 encoding was used with hash.digest() . While the hash.digest() method does allow an encoding argument to be passed in, causing the method to return a string rather than a Buffer , the UTF-16 encoding (e.g. ucs or utf16le ) is not supported. hash.digest() was called multiple times. The hash.digest() method must be called no more than one time per instance of a Hash object. hash.update() failed for any reason. This should rarely, if ever, happen. The selected public or private key encoding is incompatible with other options. ERR_CRYPTO_ECDH_INVALID_FORMAT ERR_CRYPTO_ECDH_INVALID_PUBLIC_KEY ERR_CRYPTO_ENGINE_UNKNOWN ERR_CRYPTO_FIPS_FORCED ERR_CRYPTO_FIPS_UNAVAILABLE ERR_CRYPTO_HASH_DIGEST_NO_UTF16 ERR_CRYPTO_HASH_FINALIZED ERR_CRYPTO_HASH_UPDATE_FAILED ERR_CRYPTO_INCOMPATIBLE_KEY_OPTIONS 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 302/739 An invalid crypto digest algorithm was specified. A crypto method was used on an object that was in an invalid state. For instance, calling cipher.getAuthTag() before calling cipher.final() . The PBKDF2 algorithm failed for unspecified reasons. OpenSSL does not provide more details and therefore neither does Node.js. One or more crypto.scrypt() or crypto.scryptSync() parameters are outside their legal range. Node.js was compiled without scrypt support. Not possible with the official release binaries but can happen with custom builds, including distro builds. A signing key was not provided to the sign.sign() method. crypto.timingSafeEqual() was called with Buffer , TypedArray , or DataView arguments of different lengths. c-ares failed to set the DNS server. The domain module was not usable since it could not establish the required error handling hooks, because process.setUncaughtExceptionCaptureCallback() had been called at an earlier point in time. ERR_CRYPTO_INVALID_DIGEST ERR_CRYPTO_INVALID_STATE ERR_CRYPTO_PBKDF2_ERROR ERR_CRYPTO_SCRYPT_INVALID_PARAMETER ERR_CRYPTO_SCRYPT_NOT_SUPPORTED ERR_CRYPTO_SIGN_KEY_REQUIRED ERR_CRYPTO_TIMING_SAFE_EQUAL_LENGTH ERR_DNS_SET_SERVERS_FAILED ERR_DOMAIN_CALLBACK_NOT_AVAILABLE 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 303/739 process.setUncaughtExceptionCaptureCallback() could not be called because the domain module has been loaded at an earlier point in time. The stack trace is extended to include the point in time at which the domain module had been loaded. Data provided to util.TextDecoder() API was invalid according to the encoding provided. Encoding provided to util.TextDecoder() API was not one of the WHATWG Supported Encodings . A Promise that was callbackified via util.callbackify() was rejected with a falsy value. An attempt has been made to read a file whose size is larger than the maximum allowed size for a Buffer . An invalid symlink type was passed to the fs.symlink() or fs.symlinkSync() methods. An attempt was made to add more headers after the headers had already been sent. An invalid HTTP header value was specified. Status code was outside the regular status code range (100-999). ERR_DOMAIN_CANNOT_SET_UNCAUGHT_EXCEPTION_CAPTURE ERR_ENCODING_INVALID_ENCODED_DATA ERR_ENCODING_NOT_SUPPORTED ERR_FALSY_VALUE_REJECTION ERR_FS_FILE_TOO_LARGE ERR_FS_INVALID_SYMLINK_TYPE ERR_HTTP_HEADERS_SENT ERR_HTTP_INVALID_HEADER_VALUE ERR_HTTP_INVALID_STATUS_CODE 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 304/739 The Trailer header was set even though the transfer encoding does not support that. HTTP/2 ALTSVC frames require a valid origin. HTTP/2 ALTSVC frames are limited to a maximum of 16,382 payload bytes. For HTTP/2 requests using the CONNECT method, the :authority pseudo-header is required. For HTTP/2 requests using the CONNECT method, the :path pseudo-header is forbidden. For HTTP/2 requests using the CONNECT method, the :scheme pseudo-header is forbidden. A non-specific HTTP/2 error has occurred. New HTTP/2 Streams may not be opened after the Http2Session has received a GOAWAY frame from the connected peer. An additional headers was specified after an HTTP/2 response was initiated. An attempt was made to send multiple response headers. ERR_HTTP_TRAILER_INVALID ERR_HTTP2_ALTSVC_INVALID_ORIGIN ERR_HTTP2_ALTSVC_LENGTH ERR_HTTP2_CONNECT_AUTHORITY ERR_HTTP2_CONNECT_PATH ERR_HTTP2_CONNECT_SCHEME ERR_HTTP2_ERROR ERR_HTTP2_GOAWAY_SESSION ERR_HTTP2_HEADERS_AFTER_RESPOND ERR_HTTP2_HEADERS_SENT 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 305/739 Multiple values were provided for an HTTP/2 header field that was required to have only a single value. Informational HTTP status codes ( 1xx ) may not be set as the response status code on HTTP/2 responses. HTTP/1 connection specific headers are forbidden to be used in HTTP/2 requests and responses. An invalid HTTP/2 header value was specified. An invalid HTTP informational status code has been specified. Informational status codes must be an integer between 100 and 199 (inclusive). HTTP/2 ORIGIN frames require a valid origin. Input Buffer and Uint8Array instances passed to the http2.getUnpackedSettings() API must have a length that is a multiple of six. Only valid HTTP/2 pseudoheaders ( :status , :path , :authority , :scheme , and :method ) may be used. An action was performed on an Http2Session object that had already been destroyed. ERR_HTTP2_HEADER_SINGLE_VALUE ERR_HTTP2_INFO_STATUS_NOT_ALLOWED ERR_HTTP2_INVALID_CONNECTION_HEADERS ERR_HTTP2_INVALID_HEADER_VALUE ERR_HTTP2_INVALID_INFO_STATUS ERR_HTTP2_INVALID_ORIGIN ERR_HTTP2_INVALID_PACKED_SETTINGS_LENGTH ERR_HTTP2_INVALID_PSEUDOHEADER ERR_HTTP2_INVALID_SESSION 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 306/739 An invalid value has been specified for an HTTP/2 setting. An operation was performed on a stream that had already been destroyed. Whenever an HTTP/2 SETTINGS frame is sent to a connected peer, the peer is required to send an acknowledgment that it has received and applied the new SETTINGS . By default, a maximum number of unacknowledged SETTINGS frames may be sent at any given time. This error code is used when that limit has been reached. An attempt was made to initiate a new push stream from within a push stream. Nested push streams are not permitted. An attempt was made to directly manipulate (read, write, pause, resume, etc.) a socket attached to an Http2Session . HTTP/2 ORIGIN frames are limited to a length of 16382 bytes. The number of streams created on a single HTTP/2 session reached the maximum limit. A message payload was specified for an HTTP response code for which a payload is forbidden. An HTTP/2 ping was canceled. ERR_HTTP2_INVALID_SETTING_VALUE ERR_HTTP2_INVALID_STREAM ERR_HTTP2_MAX_PENDING_SETTINGS_ACK ERR_HTTP2_NESTED_PUSH ERR_HTTP2_NO_SOCKET_MANIPULATION ERR_HTTP2_ORIGIN_LENGTH ERR_HTTP2_OUT_OF_STREAMS ERR_HTTP2_PAYLOAD_FORBIDDEN ERR_HTTP2_PING_CANCEL ERR_HTTP2_PING_LENGTH 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 307/739 HTTP/2 ping payloads must be exactly 8 bytes in length. An HTTP/2 pseudo-header has been used inappropriately. Pseudo-headers are header key names that begin with the : prefix. An attempt was made to create a push stream, which had been disabled by the client. An attempt was made to use the Http2Stream.prototype.responseWithFile() API to send a directory. An attempt was made to use the Http2Stream.prototype.responseWithFile() API to send something other than a regular file, but offset or length options were provided. The Http2Session closed with a non-zero error code. The Http2Session settings canceled. An attempt was made to connect a Http2Session object to a net.Socket or tls.TLSSocket that had already been bound to another Http2Session object. An attempt was made to use the socket property of an Http2Session that has already been closed. Use of the 101 Informational status code is forbidden in HTTP/2. ERR_HTTP2_PSEUDOHEADER_NOT_ALLOWED ERR_HTTP2_PUSH_DISABLED ERR_HTTP2_SEND_FILE ERR_HTTP2_SEND_FILE_NOSEEK ERR_HTTP2_SESSION_ERROR ERR_HTTP2_SETTINGS_CANCEL ERR_HTTP2_SOCKET_BOUND ERR_HTTP2_SOCKET_UNBOUND ERR_HTTP2_STATUS_101 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 308/739 An invalid HTTP status code has been specified. Status codes must be an integer between 100 and 599 (inclusive). An Http2Stream was destroyed before any data was transmitted to the connected peer. A non-zero error code was been specified in an RST_STREAM frame. When setting the priority for an HTTP/2 stream, the stream may be marked as a dependency for a parent stream. This error code is used when an attempt is made to mark a stream and dependent of itself. Trailing headers have already been sent on the Http2Stream . The http2stream.sendTrailers() method cannot be called until after the 'wantTrailers' event is emitted on an Http2Stream object. The 'wantTrailers' event will only be emitted if the waitForTrailers option is set for the Http2Stream . http2.connect() was passed a URL that uses any protocol other than http: or https: . A given index was out of the accepted range (e.g. negative offsets). While using the inspector module, an attempt was made to connect when the inspector was already connected. ERR_HTTP2_STATUS_INVALID ERR_HTTP2_STREAM_CANCEL ERR_HTTP2_STREAM_ERROR ERR_HTTP2_STREAM_SELF_DEPENDENCY ERR_HTTP2_TRAILERS_ALREADY_SENT ERR_HTTP2_TRAILERS_NOT_READY ERR_HTTP2_UNSUPPORTED_PROTOCOL ERR_INDEX_OUT_OF_RANGE ERR_INSPECTOR_ALREADY_CONNECTED 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 309/739 While using the inspector module, an attempt was made to use the inspector after the session had already closed. The inspector module is not available for use. While using the inspector module, an attempt was made to use the inspector before it was connected. The provided address family is not understood by the Node.js API. An argument of the wrong type was passed to a Node.js API. An invalid or unsupported value was passed for a given argument. An array was not of the expected length or in a valid range. An invalid asyncId or triggerAsyncId was passed using AsyncHooks . An id less than -1 should never happen. A swap was performed on a Buffer but its size was not compatible with the operation. A callback function was required but was not been provided to a Node.js API. ERR_INSPECTOR_CLOSED ERR_INSPECTOR_NOT_AVAILABLE ERR_INSPECTOR_NOT_CONNECTED ERR_INVALID_ADDRESS_FAMILY ERR_INVALID_ARG_TYPE ERR_INVALID_ARG_VALUE ERR_INVALID_ARRAY_LENGTH ERR_INVALID_ASYNC_ID ERR_INVALID_BUFFER_SIZE ERR_INVALID_CALLBACK 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 310/739 Invalid characters were detected in headers. A cursor on a given stream cannot be moved to a specified row without a specified column. hostname can not be parsed from a provided URL. A file descriptor ('fd') was not valid (e.g. it was a negative value). A file descriptor ('fd') type was not valid. A Node.js API that consumes file: URLs (such as certain functions in the fs module) encountered a file URL with an incompatible host. This situation can only occur on Unix-like systems where only localhost or an empty host is supported. A Node.js API that consumes file: URLs (such as certain functions in the fs module) encountered a file URL with an incompatible path. The exact semantics for determining whether a path can be used is platform-dependent. An attempt was made to send an unsupported "handle" over an IPC communication channel to a child process. See subprocess.send() and process.send() for more information. An invalid HTTP token was supplied. ERR_INVALID_CHAR ERR_INVALID_CURSOR_POS ERR_INVALID_DOMAIN_NAME ERR_INVALID_FD ERR_INVALID_FD_TYPE ERR_INVALID_FILE_URL_HOST ERR_INVALID_FILE_URL_PATH ERR_INVALID_HANDLE_TYPE ERR_INVALID_HTTP_TOKEN 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 311/739 An IP address is not valid. An invalid or unexpected value was passed in an options object. An invalid or unknown file encoding was passed. While using the Performance Timing API ( perf_hooks ), a performance mark is invalid. An invalid options.protocol was passed. Both breakEvalOnSigint and eval options were set in the REPL config, which is not supported. Thrown in case a function option does not provide a valid value for one of its returned object properties on execution. Thrown in case a function option does not provide an expected value type for one of its returned object properties on execution. Thrown in case a function option does not return an expected value type on execution, such as when a function is expected to return a promise. ERR_INVALID_IP_ADDRESS ERR_INVALID_OPT_VALUE ERR_INVALID_OPT_VALUE_ENCODING ERR_INVALID_PERFORMANCE_MARK ERR_INVALID_PROTOCOL ERR_INVALID_REPL_EVAL_CONFIG ERR_INVALID_RETURN_PROPERTY ERR_INVALID_RETURN_PROPERTY_VALUE ERR_INVALID_RETURN_VALUE ERR_INVALID_SYNC_FORK_INPUT 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 312/739 A Buffer , TypedArray , DataView or string was provided as stdio input to an asynchronous fork. See the documentation for the child_process module for more information. A Node.js API function was called with an incompatible this value. const urlSearchParams = new URLSearchParams('foo=bar&baz=new'); const buf = Buffer.alloc(1); urlSearchParams.has.call(buf, 'foo'); // Throws a TypeError with code 'ERR_INVALID_THIS' An invalid transfer object was passed to postMessage() . An element in the iterable provided to the WHATWG URLSearchParams constructor did not represent a [name, value] tuple – that is, if an element is not iterable, or does not consist of exactly two elements. An invalid URI was passed. An invalid URL was passed to the WHATWG URL constructor to be parsed. The thrown error object typically has an additional property 'input' that contains the URL that failed to parse. An attempt was made to use a URL of an incompatible scheme (protocol) for a specific purpose. It is only used in the WHATWG URL API support in the fs module (which only accepts URLs with 'file' scheme), but may be used in other Node.js APIs as well in the future. An attempt was made to use an IPC communication channel that was already closed. ERR_INVALID_THIS ERR_INVALID_TRANSFER_OBJECT ERR_INVALID_TUPLE ERR_INVALID_URI ERR_INVALID_URL ERR_INVALID_URL_SCHEME ERR_IPC_CHANNEL_CLOSED 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 313/739 An attempt was made to disconnect an IPC communication channel that was already disconnected. See the documentation for the child_process module for more information. An attempt was made to create a child Node.js process using more than one IPC communication channel. See the documentation for the child_process module for more information. An attempt was made to open an IPC communication channel with a synchronously forked Node.js process. See the documentation for the child_process module for more information. An attempt was made to allocate memory (usually in the C++ layer) but it failed. A method is required but not implemented. A required argument of a Node.js API was not passed. This is only used for strict compliance with the API specification (which in some cases may accept func(undefined) but not func() ). In most native Node.js APIs, func(undefined) and func() are treated identically, and the ERR_INVALID_ARG_TYPE error code may be used instead. Stability: 1 - Experimental An ES6 module loader hook specified format: 'dynamic' but did not provide a dynamicInstantiate hook. A MessagePort was found in the object passed to a postMessage() call, but not provided in the transferList for that call. ERR_IPC_DISCONNECTED ERR_IPC_ONE_PIPE ERR_IPC_SYNC_FORK ERR_MEMORY_ALLOCATION_FAILED ERR_METHOD_NOT_IMPLEMENTED ERR_MISSING_ARGS ERR_MISSING_DYNAMIC_INSTANTIATE_HOOK ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 314/739 Stability: 1 - Experimental An ES6 module could not be resolved. The V8 platform used by this instance of Node.js does not support creating Workers. This is caused by lack of embedder support for Workers. In particular, this error will not occur with standard builds of Node.js. Stability: 1 - Experimental A failure occurred resolving imports in an ES6 module . A callback was called more than once. A callback is almost always meant to only be called once as the query can either be fulfilled or rejected but not both at the same time. The latter would be possible by calling a callback more than once. While using N-API , a constructor passed was not a function. While calling napi_create_dataview() , a given offset was outside the bounds of the dataview or offset + length was larger than a length of given buffer . While calling napi_create_typedarray() , the provided offset was not a multiple of the element size. ERR_MISSING_MODULE ERR_MISSING_PLATFORM_FOR_WORKER ERR_MODULE_RESOLUTION_LEGACY ERR_MULTIPLE_CALLBACK ERR_NAPI_CONS_FUNCTION ERR_NAPI_INVALID_DATAVIEW_ARGS ERR_NAPI_INVALID_TYPEDARRAY_ALIGNMENT ERR_NAPI_INVALID_TYPEDARRAY_LENGTH 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 315/739 While calling napi_create_typedarray() , (length * size_of_element) + byte_offset was larger than the length of given buffer . An error occurred while invoking the JavaScript portion of the thread-safe function. An error occurred while attempting to retrieve the JavaScript undefined value. On the main thread, values are removed from the queue associated with the thread-safe function in an idle loop. This error indicates that an error has occurred when attempting to start the loop. Once no more items are left in the queue, the idle loop must be suspended. This error indicates that the idle loop has failed to stop. An attempt was made to use crypto features while Node.js was not compiled with OpenSSL crypto support. An attempt was made to use features that require ICU , but Node.js was not compiled with ICU support. A Node.js API was called in an unsupported manner, such as Buffer.write(string, encoding, offset[, length]) . A given value is out of the accepted range. ERR_NAPI_TSFN_CALL_JS ERR_NAPI_TSFN_GET_UNDEFINED ERR_NAPI_TSFN_START_IDLE_LOOP ERR_NAPI_TSFN_STOP_IDLE_LOOP ERR_NO_CRYPTO ERR_NO_ICU ERR_NO_LONGER_SUPPORTED ERR_OUT_OF_RANGE ERR_REQUIRE_ESM 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 316/739 Stability: 1 - Experimental An attempt was made to require() an ES6 module . Script execution was interrupted by SIGINT (For example, when Ctrl+C was pressed). The server.listen() method was called while a net.Server was already listening. This applies to all instances of net.Server , including HTTP, HTTPS, and HTTP/2 Server instances. The server.close() method was called when a net.Server was not running. This applies to all instances of net.Server , including HTTP, HTTPS, and HTTP/2 Server instances. An attempt was made to bind a socket that has already been bound. An invalid (negative) size was passed for either the recvBufferSize or sendBufferSize options in dgram.createSocket() . An API function expecting a port >= 0 and < 65536 received an invalid value. An API function expecting a socket type ( udp4 or udp6 ) received an invalid value. While using dgram.createSocket() , the size of the receive or send Buffer could not be determined. ERR_SCRIPT_EXECUTION_INTERRUPTED ERR_SERVER_ALREADY_LISTEN ERR_SERVER_NOT_RUNNING ERR_SOCKET_ALREADY_BOUND ERR_SOCKET_BAD_BUFFER_SIZE ERR_SOCKET_BAD_PORT ERR_SOCKET_BAD_TYPE ERR_SOCKET_BUFFER_SIZE 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 317/739 Data could be sent on a socket. An attempt was made to operate on an already closed socket. A call was made and the UDP subsystem was not running. An attempt was made to call stream.pipe() on a Writable stream. A stream method was called that cannot complete because the stream was destroyed using stream.destroy() . An attempt was made to call stream.write() with a null chunk. An error returned by stream.finished() and stream.pipeline() , when a stream or a pipeline ends non gracefully with no explicit error. An attempt was made to call stream.push() after a null (EOF) had been pushed to the stream. An attempt was made to call stream.unshift() after the 'end' event was emitted. Prevents an abort if a string decoder was set on the Socket or if the decoder is in objectMode . ERR_SOCKET_CANNOT_SEND ERR_SOCKET_CLOSED ERR_SOCKET_DGRAM_NOT_RUNNING ERR_STREAM_CANNOT_PIPE ERR_STREAM_DESTROYED ERR_STREAM_NULL_VALUES ERR_STREAM_PREMATURE_CLOSE ERR_STREAM_PUSH_AFTER_EOF ERR_STREAM_UNSHIFT_AFTER_END_EVENT ERR_STREAM_WRAP 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 318/739 const Socket = require('net').Socket; const instance = new Socket(); instance.setEncoding('utf8'); An attempt was made to call stream.write() after stream.end() has been called. An attempt has been made to create a string longer than the maximum allowed length. An unspecified or non-specific system error has occurred within the Node.js process. The error object will have an err.info object property with additional details. While using TLS, the hostname/IP of the peer did not match any of the subjectAltNames in its certificate. While using TLS, the parameter offered for the Diffie-Hellman ( DH ) key-agreement protocol is too small. By default, the key length must be greater than or equal to 1024 bits to avoid vulnerabilities, even though it is strongly recommended to use 2048 bits or larger for stronger security. A TLS/SSL handshake timed out. In this case, the server must also abort the connection. Valid TLS protocol versions are 'TLSv1' , 'TLSv1.1' , or 'TLSv1.2' . ERR_STREAM_WRITE_AFTER_END ERR_STRING_TOO_LONG ERR_SYSTEM_ERROR ERR_TLS_CERT_ALTNAME_INVALID ERR_TLS_DH_PARAM_SIZE ERR_TLS_HANDSHAKE_TIMEOUT ERR_TLS_INVALID_PROTOCOL_VERSION ERR_TLS_PROTOCOL_VERSION_CONFLICT 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 319/739 Attempting to set a TLS protocol minVersion or maxVersion conflicts with an attempt to set the secureProtocol explicitly. Use one mechanism or the other. An attempt to renegotiate the TLS session failed. An attempt was made to renegotiate TLS on a socket instance with TLS disabled. While using TLS, the server.addContext() method was called without providing a hostname in the first parameter. An excessive amount of TLS renegotiations is detected, which is a potential vector for denial-of-service attacks. An attempt was made to issue Server Name Indication from a TLS server-side socket, which is only valid from a client. The trace_events.createTracing() method requires at least one trace event category. The trace_events module could not be loaded because Node.js was compiled with the --without-v8-platform flag. A SharedArrayBuffer whose memory is not managed by the JavaScript engine or by Node.js was encountered during serialization. Such a SharedArrayBuffer cannot be serialized. This can only happen when native addons create SharedArrayBuffer s in "externalized" mode, or put existing SharedArrayBuffer into externalized mode. ERR_TLS_RENEGOTIATE ERR_TLS_RENEGOTIATION_DISABLED ERR_TLS_REQUIRED_SERVER_NAME ERR_TLS_SESSION_ATTACK ERR_TLS_SNI_FROM_SERVER ERR_TRACE_EVENTS_CATEGORY_REQUIRED ERR_TRACE_EVENTS_UNAVAILABLE ERR_TRANSFERRING_EXTERNALIZED_SHAREDARRAYBUFFER 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 320/739 A Transform stream finished while it was still transforming. A Transform stream finished with data still in the write buffer. The initialization of a TTY failed due to a system error. process.setUncaughtExceptionCaptureCallback() was called twice, without first resetting the callback to null . This error is designed to prevent accidentally overwriting a callback registered from another module. A string that contained unescaped characters was received. An unhandled error occurred (for instance, when an 'error' event is emitted by an EventEmitter but an 'error' handler is not registered). Used to identify a specific kind of internal Node.js error that should not typically be triggered by user code. Instances of this error point to an internal bug within the Node.js binary itself. An invalid or unknown encoding option was passed to an API. Stability: 1 - Experimental ERR_TRANSFORM_ALREADY_TRANSFORMING ERR_TRANSFORM_WITH_LENGTH_0 ERR_TTY_INIT_FAILED ERR_UNCAUGHT_EXCEPTION_CAPTURE_ALREADY_SET ERR_UNESCAPED_CHARACTERS ERR_UNHANDLED_ERROR ERR_UNKNOWN_BUILTIN_MODULE ERR_UNKNOWN_ENCODING ERR_UNKNOWN_FILE_EXTENSION 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 321/739 An attempt was made to load a module with an unknown or unsupported file extension. Stability: 1 - Experimental An attempt was made to load a module with an unknown or unsupported format. An invalid or unknown process signal was passed to an API expecting a valid signal (such as subprocess.kill() ). An attempt was made to launch a Node.js process with an unknown stdin file type. This error is usually an indication of a bug within Node.js itself, although it is possible for user code to trigger it. An attempt was made to launch a Node.js process with an unknown stdout or stderr file type. This error is usually an indication of a bug within Node.js itself, although it is possible for user code to trigger it. The V8 BreakIterator API was used but the full ICU data set is not installed. While using the Performance Timing API ( perf_hooks ), no valid performance entry types were found. A dynamic import callback was not specified. The module attempted to be linked is not eligible for linking, because of one of the following reasons: It has already been linked ( linkingStatus is 'linked' ) ERR_UNKNOWN_MODULE_FORMAT ERR_UNKNOWN_SIGNAL ERR_UNKNOWN_STDIN_TYPE ERR_UNKNOWN_STREAM_TYPE ERR_V8BREAKITERATOR ERR_VALID_PERFORMANCE_ENTRY_TYPE ERR_VM_DYNAMIC_IMPORT_CALLBACK_MISSING ERR_VM_MODULE_ALREADY_LINKED 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 322/739 It is being linked ( linkingStatus is 'linking' ) Linking has failed for this module ( linkingStatus is 'errored' ) The module being returned from the linker function is from a different context than the parent module. Linked modules must share the same context. The linker function returned a module for which linking has failed. The module must be successfully linked before instantiation. The fulfilled value of a linking promise is not a vm.SourceTextModule object. The current module's status does not allow for this operation. The specific meaning of the error depends on the specific function. The path for the main script of a worker is neither an absolute path nor a relative path starting with ./ or ../ . All attempts at serializing an uncaught exception from a worker thread failed. The pathname used for the main script of a worker has an unknown file extension. Creation of a zlib object failed due to incorrect configuration. ERR_VM_MODULE_DIFFERENT_CONTEXT ERR_VM_MODULE_LINKING_ERRORED ERR_VM_MODULE_NOT_LINKED ERR_VM_MODULE_NOT_MODULE ERR_VM_MODULE_STATUS ERR_WORKER_PATH ERR_WORKER_UNSERIALIZABLE_ERROR ERR_WORKER_UNSUPPORTED_EXTENSION ERR_ZLIB_INITIALIZATION_FAILED 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 323/739 Too much HTTP header data was received. In order to protect against malicious or malconfigured clients, if more than 8KB of HTTP header data is received then HTTP parsing will abort without a request or response object being created, and an Error with this code will be emitted. A module file could not be resolved while attempting a require() or import operation. Stability: 0 - Deprecated. These error codes are either inconsistent, or have been removed. Used when a failure occurs sending an individual frame on the HTTP/2 session. Used when an HTTP/2 Headers Object is expected. Used when a required header is missing in an HTTP/2 message. HTTP/2 informational headers must only be sent prior to calling the Http2Stream.prototype.respond() method. Used when an action has been performed on an HTTP/2 Stream that has already been closed. Used when an invalid character is found in an HTTP response status message (reason phrase). HPE_HEADER_OVERFLOW MODULE_NOT_FOUND Legacy Node.js Error Codes ERR_HTTP2_FRAME_ERROR ERR_HTTP2_HEADERS_OBJECT ERR_HTTP2_HEADER_REQUIRED ERR_HTTP2_INFO_HEADERS_AFTER_RESPOND ERR_HTTP2_STREAM_CLOSED ERR_HTTP_INVALID_CHAR 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 324/739 Used by the N-API when Constructor.prototype is not an object. Used generically to identify that an operation caused an out of memory condition. The repl module was unable to parse data from the REPL history file. An attempt was made to close the process.stderr stream. By design, Node.js does not allow stdout or stderr streams to be closed by user code. An attempt was made to close the process.stdout stream. By design, Node.js does not allow stdout or stderr streams to be closed by user code. Used when an attempt is made to use a readable stream that has not implemented readable._read() . Used when a TLS renegotiation request has failed in a non-specific way. The 'ERR_UNKNOWN_BUILTIN_MODULE' error code is used to identify a specific kind of internal Node.js error that should not typically be triggered by user code. Instances of this error point to an internal bug within the Node.js binary itself. Used when a given value is out of the accepted range. ERR_NAPI_CONS_PROTOTYPE_OBJECT ERR_OUTOFMEMORY ERR_PARSE_HISTORY_DATA ERR_STDERR_CLOSE ERR_STDOUT_CLOSE ERR_STREAM_READ_NOT_IMPLEMENTED ERR_TLS_RENEGOTIATION_FAILED ERR_UNKNOWN_BUILTIN_MODULE ERR_VALUE_OUT_OF_RANGE 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 325/739 Used when an attempt is made to use a zlib object after it has already been closed. These errors have never been released, but had been present on master between releases. An attempt was made to start a watcher returned by fs.watch() that has already been started. An attempt was made to initiate operations on a watcher returned by fs.watch() that has not yet been started. Occurs with multiple attempts to shutdown an HTTP/2 session. A non-specific HTTP/2 error has occurred. Used in the repl in case the old history file is used and an error occurred while trying to read and parse it. Used when an ES6 module loader hook specifies format: 'dynamic' but does not provide a dynamicInstantiate hook. Used to prevent an abort if a string decoder was set on the Socket. const Socket = require('net').Socket; const instance = new Socket(); instance.setEncoding('utf8'); ERR_ZLIB_BINDING_CLOSED Other error codes ERR_FS_WATCHER_ALREADY_STARTED ERR_FS_WATCHER_NOT_STARTED ERR_HTTP2_ALREADY_SHUTDOWN ERR_HTTP2_ERROR ERR_INVALID_REPL_HISTORY ERR_MISSING_DYNAMIC_INSTANTIATE_HOOK ERR_STREAM_HAS_STRINGDECODER 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 326/739 An attempt has been made to create a string larger than the maximum allowed size. Stability: 2 - Stable Much of the Node.js core API is built around an idiomatic asynchronous event-driven architecture in which certain kinds of objects (called "emitters") emit named events that cause Function objects ("listeners") to be called. For instance: a net.Server object emits an event each time a peer connects to it; a fs.ReadStream emits an event when the file is opened; a stream emits an event whenever data is available to be read. All objects that emit events are instances of the EventEmitter class. These objects expose an eventEmitter.on() function that allows one or more functions to be attached to named events emitted by the object. Typically, event names are camel-cased strings but any valid JavaScript property key can be used. When the EventEmitter object emits an event, all of the functions attached to that specific event are called synchronously. Any values returned by the called listeners are ignored and will be discarded. The following example shows a simple EventEmitter instance with a single listener. The eventEmitter.on() method is used to register listeners, while the eventEmitter.emit() method is used to trigger the event. const EventEmitter = require('events'); class MyEmitter extends EventEmitter {} const myEmitter = new MyEmitter(); myEmitter.on('event', () => { console.log('an event occurred!'); }); myEmitter.emit('event'); The eventEmitter.emit() method allows an arbitrary set of arguments to be passed to the listener functions. It is important to keep in mind that when an ordinary listener function is called, the standard this keyword is intentionally set to reference the EventEmitter instance to which the listener is attached. const myEmitter = new MyEmitter(); myEmitter.on('event', function(a, b) { console.log(a, b, this, this === myEmitter); // Prints: // a b MyEmitter { // domain: null, // _events: { event: [Function] }, // _eventsCount: 1, // _maxListeners: undefined } true }); myEmitter.emit('event', 'a', 'b'); ERR_STRING_TOO_LARGE Events Passing arguments and this to listeners 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 327/739 It is possible to use ES6 Arrow Functions as listeners, however, when doing so, the this keyword will no longer reference the EventEmitter instance: const myEmitter = new MyEmitter(); myEmitter.on('event', (a, b) => { console.log(a, b, this); // Prints: a b {} }); myEmitter.emit('event', 'a', 'b'); The EventEmitter calls all listeners synchronously in the order in which they were registered. This is important to ensure the proper sequencing of events and to avoid race conditions or logic errors. When appropriate, listener functions can switch to an asynchronous mode of operation using the setImmediate() or process.nextTick() methods: const myEmitter = new MyEmitter(); myEmitter.on('event', (a, b) => { setImmediate(() => { console.log('this happens asynchronously'); }); }); myEmitter.emit('event', 'a', 'b'); When a listener is registered using the eventEmitter.on() method, that listener will be invoked every time the named event is emitted. const myEmitter = new MyEmitter(); let m = 0; myEmitter.on('event', () => { console.log(++m); }); myEmitter.emit('event'); // Prints: 1 myEmitter.emit('event'); // Prints: 2 Using the eventEmitter.once() method, it is possible to register a listener that is called at most once for a particular event. Once the event is emitted, the listener is unregistered and then called. const myEmitter = new MyEmitter(); let m = 0; myEmitter.once('event', () => { console.log(++m); }); myEmitter.emit('event'); // Prints: 1 myEmitter.emit('event'); // Ignored Asynchronous vs. Synchronous Handling events only once 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 328/739 When an error occurs within an EventEmitter instance, the typical action is for an 'error' event to be emitted. These are treated as special cases within Node.js. If an EventEmitter does not have at least one listener registered for the 'error' event, and an 'error' event is emitted, the error is thrown, a stack trace is printed, and the Node.js process exits. const myEmitter = new MyEmitter(); myEmitter.emit('error', new Error('whoops!')); // Throws and crashes Node.js To guard against crashing the Node.js process the domain module can be used. (Note, however, that the domain module is deprecated.) As a best practice, listeners should always be added for the 'error' events. const myEmitter = new MyEmitter(); myEmitter.on('error', (err) => { console.error('whoops! there was an error'); }); myEmitter.emit('error', new Error('whoops!')); // Prints: whoops! there was an error The EventEmitter class is defined and exposed by the events module: const EventEmitter = require('events'); All EventEmitter s emit the event 'newListener' when new listeners are added and 'removeListener' when existing listeners are removed. eventName <string> | <symbol> The name of the event being listened for listener <Function> The event handler function The EventEmitter instance will emit its own 'newListener' event before a listener is added to its internal array of listeners. Listeners registered for the 'newListener' event will be passed the event name and a reference to the listener being added. The fact that the event is triggered before adding the listener has a subtle but important side effect: any additional listeners registered to the same name within the 'newListener' callback will be inserted before the listener that is in the process of being added. const myEmitter = new MyEmitter(); // Only do this once so we don't loop forever myEmitter.once('newListener', (event, listener) => { if (event === 'event') { // Insert a new listener in front myEmitter.on('event', () => { console.log('B'); }); } Error events Class: EventEmitter Event: 'newListener' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 329/739 }); myEmitter.on('event', () => { console.log('A'); }); myEmitter.emit('event'); // Prints: // B // A eventName <string> | <symbol> The event name listener <Function> The event handler function The 'removeListener' event is emitted after the listener is removed. emitter <EventEmitter> The emitter to query eventName <string> | <symbol> The event name Stability: 0 - Deprecated: Use emitter.listenerCount() instead. A class method that returns the number of listeners for the given eventName registered on the given emitter . const myEmitter = new MyEmitter(); myEmitter.on('event', () => {}); myEmitter.on('event', () => {}); console.log(EventEmitter.listenerCount(myEmitter, 'event')); // Prints: 2 By default, a maximum of 10 listeners can be registered for any single event. This limit can be changed for individual EventEmitter instances using the emitter.setMaxListeners(n) method. To change the default for all EventEmitter instances, the EventEmitter.defaultMaxListeners property can be used. If this value is not a positive number, a TypeError will be thrown. Take caution when setting the EventEmitter.defaultMaxListeners because the change affects all EventEmitter instances, including those created before the change is made. However, calling emitter.setMaxListeners(n) still has precedence over EventEmitter.defaultMaxListeners . Note that this is not a hard limit. The EventEmitter instance will allow more listeners to be added but will output a trace warning to stderr indicating that a "possible EventEmitter memory leak" has been detected. For any single EventEmitter , the emitter.getMaxListeners() and emitter.setMaxListeners() methods can be used to temporarily avoid this warning: emitter.setMaxListeners(emitter.getMaxListeners() + 1); emitter.once('event', () => { // do stuff emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0)); }); Event: 'removeListener' EventEmitter.listenerCount(emitter, eventName) EventEmitter.defaultMaxListeners 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 330/739 The --trace-warnings command line flag can be used to display the stack trace for such warnings. The emitted warning can be inspected with process.on('warning') and will have the additional emitter , type and count properties, referring to the event emitter instance, the event’s name and the number of attached listeners, respectively. Its name property is set to 'MaxListenersExceededWarning' . eventName <string> | <symbol> listener <Function> Alias for emitter.on(eventName, listener) . eventName <string> | <symbol> ...args <any> Returns: <boolean> Synchronously calls each of the listeners registered for the event named eventName , in the order they were registered, passing the supplied arguments to each. Returns true if the event had listeners, false otherwise. Returns: <Array> Returns an array listing the events for which the emitter has registered listeners. The values in the array will be strings or Symbol s. const EventEmitter = require('events'); const myEE = new EventEmitter(); myEE.on('foo', () => {}); myEE.on('bar', () => {}); const sym = Symbol('symbol'); myEE.on(sym, () => {}); console.log(myEE.eventNames()); // Prints: [ 'foo', 'bar', Symbol(symbol) ] Returns: <integer> Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to EventEmitter.defaultMaxListeners . eventName <string> | <symbol> The name of the event being listened for Returns: <integer> emitter.addListener(eventName, listener) emitter.emit(eventName[, ...args]) emitter.eventNames() emitter.getMaxListeners() emitter.listenerCount(eventName) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 331/739 Returns the number of listeners listening to the event named eventName . eventName <string> | <symbol> Returns: <Function[]> Returns a copy of the array of listeners for the event named eventName . server.on('connection', (stream) => { console.log('someone connected!'); }); console.log(util.inspect(server.listeners('connection'))); // Prints: [ [Function] ] eventName <string> | <symbol> listener <Function> Returns: <EventEmitter> Alias for emitter.removeListener() . eventName <string> | <symbol> The name of the event. listener <Function> The callback function Returns: <EventEmitter> Adds the listener function to the end of the listeners array for the event named eventName . No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times. server.on('connection', (stream) => { console.log('someone connected!'); }); Returns a reference to the EventEmitter , so that calls can be chained. By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array. const myEE = new EventEmitter(); myEE.on('foo', () => console.log('a')); myEE.prependListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a emitter.listeners(eventName) emitter.off(eventName, listener) emitter.on(eventName, listener) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 332/739 eventName <string> | <symbol> The name of the event. listener <Function> The callback function Returns: <EventEmitter> Adds a one-time listener function for the event named eventName . The next time eventName is triggered, this listener is removed and then invoked. server.once('connection', (stream) => { console.log('Ah, we have our first user!'); }); Returns a reference to the EventEmitter , so that calls can be chained. By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array. const myEE = new EventEmitter(); myEE.once('foo', () => console.log('a')); myEE.prependOnceListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a eventName <string> | <symbol> The name of the event. listener <Function> The callback function Returns: <EventEmitter> Adds the listener function to the beginning of the listeners array for the event named eventName . No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times. server.prependListener('connection', (stream) => { console.log('someone connected!'); }); Returns a reference to the EventEmitter , so that calls can be chained. eventName <string> | <symbol> The name of the event. listener <Function> The callback function Returns: <EventEmitter> Adds a one-time listener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked. emitter.once(eventName, listener) emitter.prependListener(eventName, listener) emitter.prependOnceListener(eventName, listener) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 333/739 server.prependOnceListener('connection', (stream) => { console.log('Ah, we have our first user!'); }); Returns a reference to the EventEmitter , so that calls can be chained. eventName <string> | <symbol> Returns: <EventEmitter> Removes all listeners, or those of the specified eventName . Note that it is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams). Returns a reference to the EventEmitter , so that calls can be chained. eventName <string> | <symbol> listener <Function> Returns: <EventEmitter> Removes the specified listener from the listener array for the event named eventName . const callback = (stream) => { console.log('someone connected!'); }; server.on('connection', callback); // ... server.removeListener('connection', callback); removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName , then removeListener() must be called multiple times to remove each instance. Note that once an event has been emitted, all listeners attached to it at the time of emitting will be called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them from emit() in progress. Subsequent events will behave as expected. const myEmitter = new MyEmitter(); const callbackA = () => { console.log('A'); myEmitter.removeListener('event', callbackB); }; const callbackB = () => { console.log('B'); }; myEmitter.on('event', callbackA); emitter.removeAllListeners([eventName]) emitter.removeListener(eventName, listener) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 334/739 myEmitter.on('event', callbackB); // callbackA removes listener callbackB but it will still be called. // Internal listener array at time of emit [callbackA, callbackB] myEmitter.emit('event'); // Prints: // A // B // callbackB is now removed. // Internal listener array [callbackA] myEmitter.emit('event'); // Prints: // A Because listeners are managed using an internal array, calling this will change the position indices of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated. When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping') listener is removed: const ee = new EventEmitter(); function pong() { console.log('pong'); } ee.on('ping', pong); ee.once('ping', pong); ee.removeListener('ping', pong); ee.emit('ping'); ee.emit('ping'); Returns a reference to the EventEmitter , so that calls can be chained. n <integer> Returns: <EventEmitter> By default EventEmitter s will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. Obviously, not all events should be limited to just 10 listeners. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0 ) to indicate an unlimited number of listeners. Returns a reference to the EventEmitter , so that calls can be chained. eventName <string> | <symbol> Returns: <Function[]> Returns a copy of the array of listeners for the event named eventName , including any wrappers (such as those created by .once() ). emitter.setMaxListeners(n) emitter.rawListeners(eventName) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 335/739 const emitter = new EventEmitter(); emitter.once('log', () => console.log('log once')); // Returns a new Array with a function `onceWrapper` which has a property // `listener` which contains the original listener bound above const listeners = emitter.rawListeners('log'); const logFnWrapper = listeners[0]; // logs "log once" to the console and does not unbind the `once` event logFnWrapper.listener(); // logs "log once" to the console and removes the listener logFnWrapper(); emitter.on('log', () => console.log('log persistently')); // will return a new Array with a single function bound by `.on()` above const newListeners = emitter.rawListeners('log'); // logs "log persistently" twice newListeners[0](); emitter.emit('log'); emitter <EventEmitter> name <string> Returns: <Promise> Creates a Promise that is resolved when the EventEmitter emits the given event or that is rejected when the EventEmitter emits 'error' . The Promise will resolve with an array of all the arguments emitted to the given event. const { once, EventEmitter } = require('events'); async function run() { const ee = new EventEmitter(); process.nextTick(() => { ee.emit('myevent', 42); }); const [value] = await once(ee, 'myevent'); console.log(value); const err = new Error('kaboom'); process.nextTick(() => { ee.emit('error', err); }); try { await once(ee, 'myevent'); } catch (err) { console.log('error happened', err); events.once(emitter, name) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 336/739 } } run(); Stability: 2 - Stable The fs module provides an API for interacting with the file system in a manner closely modeled around standard POSIX functions. To use this module: const fs = require('fs'); All file system operations have synchronous and asynchronous forms. The asynchronous form always takes a completion callback as its last argument. The arguments passed to the completion callback depend on the method, but the first argument is always reserved for an exception. If the operation was completed successfully, then the first argument will be null or undefined . const fs = require('fs'); fs.unlink('/tmp/hello', (err) => { if (err) throw err; console.log('successfully deleted /tmp/hello'); }); Exceptions that occur using synchronous operations are thrown immediately and may be handled using try / catch , or may be allowed to bubble up. const fs = require('fs'); try { fs.unlinkSync('/tmp/hello'); console.log('successfully deleted /tmp/hello'); } catch (err) { // handle the error } There is no guaranteed ordering when using asynchronous methods. So the following is prone to error because the fs.stat() operation may complete before the fs.rename() operation: fs.rename('/tmp/hello', '/tmp/world', (err) => { if (err) throw err; console.log('renamed complete'); }); fs.stat('/tmp/world', (err, stats) => { if (err) throw err; File System 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 337/739 console.log(`stats: ${JSON.stringify(stats)}`); }); To correctly order the operations, move the fs.stat() call into the callback of the fs.rename() operation: fs.rename('/tmp/hello', '/tmp/world', (err) => { if (err) throw err; fs.stat('/tmp/world', (err, stats) => { if (err) throw err; console.log(`stats: ${JSON.stringify(stats)}`); }); }); In busy processes, the programmer is strongly encouraged to use the asynchronous versions of these calls. The synchronous versions will block the entire process until they complete — halting all connections. While it is not recommended, most fs functions allow the callback argument to be omitted, in which case a default callback is used that rethrows errors. To get a trace to the original call site, set the NODE_DEBUG environment variable: Omitting the callback function on asynchronous fs functions is deprecated and may result in an error being thrown in the future. $ cat script.js function bad() { require('fs').readFile('/'); } bad(); $ env NODE_DEBUG=fs node script.js fs.js:88 throw backtrace; ^ Error: EISDIR: illegal operation on a directory, read <stack trace.> Most fs operations accept filepaths that may be specified in the form of a string, a Buffer , or a URL object using the file: protocol. String form paths are interpreted as UTF-8 character sequences identifying the absolute or relative filename. Relative paths will be resolved relative to the current working directory as specified by process.cwd() . Example using an absolute path on POSIX: const fs = require('fs'); fs.open('/open/some/file.txt', 'r', (err, fd) => { if (err) throw err; fs.close(fd, (err) => { if (err) throw err; }); }); File paths 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 338/739 Example using a relative path on POSIX (relative to process.cwd() ): fs.open('file.txt', 'r', (err, fd) => { if (err) throw err; fs.close(fd, (err) => { if (err) throw err; }); }); Paths specified using a Buffer are useful primarily on certain POSIX operating systems that treat file paths as opaque byte sequences. On such systems, it is possible for a single file path to contain sub-sequences that use multiple character encodings. As with string paths, Buffer paths may be relative or absolute: Example using an absolute path on POSIX: fs.open(Buffer.from('/open/some/file.txt'), 'r', (err, fd) => { if (err) throw err; fs.close(fd, (err) => { if (err) throw err; }); }); On Windows, Node.js follows the concept of per-drive working directory. This behavior can be observed when using a drive path without a backslash. For example fs.readdirSync('c:\\') can potentially return a different result than fs.readdirSync('c:') . For more information, see this MSDN page . For most fs module functions, the path or filename argument may be passed as a WHATWG URL object. Only URL objects using the file: protocol are supported. const fs = require('fs'); const fileUrl = new URL('file:///tmp/hello'); fs.readFileSync(fileUrl); file: URLs are always absolute paths. Using WHATWG URL objects might introduce platform-specific behaviors. On Windows, file: URLs with a hostname convert to UNC paths, while file: URLs with drive letters convert to local absolute paths. file: URLs without a hostname nor a drive letter will result in a throw: // On Windows : // - WHATWG file URLs with hostname convert to UNC path // file://hostname/p/a/t/h/file => \\hostname\p\a\t\h\file fs.readFileSync(new URL('file://hostname/p/a/t/h/file')); // - WHATWG file URLs with drive letters convert to absolute path // file:///C:/tmp/hello => C:\tmp\hello fs.readFileSync(new URL('file:///C:/tmp/hello')); URL object support 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 339/739 // - WHATWG file URLs without hostname must have a drive letters fs.readFileSync(new URL('file:///notdriveletter/p/a/t/h/file')); fs.readFileSync(new URL('file:///c/p/a/t/h/file')); // TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must be absolute file: URLs with drive letters must use : as a separator just after the drive letter. Using another separator will result in a throw. On all other platforms, file: URLs with a hostname are unsupported and will result in a throw: // On other platforms: // - WHATWG file URLs with hostname are unsupported // file://hostname/p/a/t/h/file => throw! fs.readFileSync(new URL('file://hostname/p/a/t/h/file')); // TypeError [ERR_INVALID_FILE_URL_PATH]: must be absolute // - WHATWG file URLs convert to absolute path // file:///tmp/hello => /tmp/hello fs.readFileSync(new URL('file:///tmp/hello')); A file: URL having encoded slash characters will result in a throw on all platforms: // On Windows fs.readFileSync(new URL('file:///C:/p/a/t/h/%2F')); fs.readFileSync(new URL('file:///C:/p/a/t/h/%2f')); /* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded \ or / characters */ // On POSIX fs.readFileSync(new URL('file:///p/a/t/h/%2F')); fs.readFileSync(new URL('file:///p/a/t/h/%2f')); /* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded / characters */ On Windows, file: URLs having encoded backslash will result in a throw: // On Windows fs.readFileSync(new URL('file:///C:/path/%5C')); fs.readFileSync(new URL('file:///C:/path/%5c')); /* TypeError [ERR_INVALID_FILE_URL_PATH]: File URL path must not include encoded \ or / characters */ On POSIX systems, for every process, the kernel maintains a table of currently open files and resources. Each open file is assigned a simple numeric identifier called a file descriptor. At the system-level, all file system operations use these file descriptors to identify and track each specific file. Windows systems use a different but conceptually similar mechanism for tracking resources. To simplify things for users, Node.js abstracts away the specific differences between operating systems and assigns all open files a numeric file descriptor. File Descriptors 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 340/739 The fs.open() method is used to allocate a new file descriptor. Once allocated, the file descriptor may be used to read data from, write data to, or request information about the file. fs.open('/open/some/file.txt', 'r', (err, fd) => { if (err) throw err; fs.fstat(fd, (err, stat) => { if (err) throw err; // use stat // always close the file descriptor! fs.close(fd, (err) => { if (err) throw err; }); }); }); Most operating systems limit the number of file descriptors that may be open at any given time so it is critical to close the descriptor when operations are completed. Failure to do so will result in a memory leak that will eventually cause an application to crash. All file system APIs except fs.FSWatcher() and those that are explicitly synchronous use libuv's threadpool, which can have surprising and negative performance implications for some applications. See the UV_THREADPOOL_SIZE documentation for more information. When fs.readdir() or fs.readdirSync() is called with the withFileTypes option set to true , the resulting array is filled with fs.Dirent objects, rather than strings or Buffers . Returns: <boolean> Returns true if the fs.Dirent object describes a block device. Returns: <boolean> Returns true if the fs.Dirent object describes a character device. Returns: <boolean> Returns true if the fs.Dirent object describes a file system directory. Returns: <boolean> Returns true if the fs.Dirent object describes a first-in-first-out (FIFO) pipe. Returns: <boolean> Threadpool Usage Class: fs.Dirent dirent.isBlockDevice() dirent.isCharacterDevice() dirent.isDirectory() dirent.isFIFO() dirent.isFile() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 341/739 Returns true if the fs.Dirent object describes a regular file. Returns: <boolean> Returns true if the fs.Dirent object describes a socket. Returns: <boolean> Returns true if the fs.Dirent object describes a symbolic link. <string> | <Buffer> The file name that this fs.Dirent object refers to. The type of this value is determined by the options.encoding passed to fs.readdir() or fs.readdirSync() . A successful call to fs.watch() method will return a new fs.FSWatcher object. All fs.FSWatcher objects are EventEmitter 's that will emit a 'change' event whenever a specific watched file is modified. eventType <string> The type of change event that has occurred filename <string> | <Buffer> The filename that changed (if relevant/available) Emitted when something changes in a watched directory or file. See more details in fs.watch() . The filename argument may not be provided depending on operating system support. If filename is provided, it will be provided as a Buffer if fs.watch() is called with its encoding option set to 'buffer' , otherwise filename will be a UTF-8 string. // Example when handled through fs.watch() listener fs.watch('./tmp', { encoding: 'buffer' }, (eventType, filename) => { if (filename) { console.log(filename); // Prints: <Buffer ...> } }); Emitted when the watcher stops watching for changes. The closed fs.FSWatcher object is no longer usable in the event handler. error <Error> Emitted when an error occurs while watching the file. The errored fs.FSWatcher object is no longer usable in the event handler. dirent.isSocket() dirent.isSymbolicLink() dirent.name Class: fs.FSWatcher Event: 'change' Event: 'close' Event: 'error' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 342/739 Stop watching for changes on the given fs.FSWatcher . Once stopped, the fs.FSWatcher object is no longer usable. A successful call to fs.createReadStream() will return a new fs.ReadStream object. All fs.ReadStream objects are Readable Streams . Emitted when the fs.ReadStream 's underlying file descriptor has been closed. fd <integer> Integer file descriptor used by the ReadStream . Emitted when the fs.ReadStream 's file descriptor has been opened. Emitted when the fs.ReadStream is ready to be used. Fires immediately after 'open' . <number> The number of bytes that have been read so far. <string> | <Buffer> The path to the file the stream is reading from as specified in the first argument to fs.createReadStream() . If path is passed as a string, then readStream.path will be a string. If path is passed as a Buffer , then readStream.path will be a Buffer . <boolean> This property is true if the underlying file has not been opened yet, i.e. before the 'ready' event is emitted. A fs.Stats object provides information about a file. Objects returned from fs.stat() , fs.lstat() and fs.fstat() and their synchronous counterparts are of this type. If bigint in the options passed to those methods is true, the numeric values will be bigint instead of number . Stats { dev: 2114, ino: 48064969, mode: 33188, nlink: 1, watcher.close() Class: fs.ReadStream Event: 'close' Event: 'open' Event: 'ready' readStream.bytesRead readStream.path readStream.pending Class: fs.Stats 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 343/739 uid: 85, gid: 100, rdev: 0, size: 527, blksize: 4096, blocks: 8, atimeMs: 1318289051000.1, mtimeMs: 1318289051000.1, ctimeMs: 1318289051000.1, birthtimeMs: 1318289051000.1, atime: Mon, 10 Oct 2011 23:24:11 GMT, mtime: Mon, 10 Oct 2011 23:24:11 GMT, ctime: Mon, 10 Oct 2011 23:24:11 GMT, birthtime: Mon, 10 Oct 2011 23:24:11 GMT } bigint version: Stats { dev: 2114n, ino: 48064969n, mode: 33188n, nlink: 1n, uid: 85n, gid: 100n, rdev: 0n, size: 527n, blksize: 4096n, blocks: 8n, atimeMs: 1318289051000n, mtimeMs: 1318289051000n, ctimeMs: 1318289051000n, birthtimeMs: 1318289051000n, atime: Mon, 10 Oct 2011 23:24:11 GMT, mtime: Mon, 10 Oct 2011 23:24:11 GMT, ctime: Mon, 10 Oct 2011 23:24:11 GMT, birthtime: Mon, 10 Oct 2011 23:24:11 GMT } Returns: <boolean> Returns true if the fs.Stats object describes a block device. Returns: <boolean> Returns true if the fs.Stats object describes a character device. Returns: <boolean> Returns true if the fs.Stats object describes a file system directory. stats.isBlockDevice() stats.isCharacterDevice() stats.isDirectory() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 344/739 Returns: <boolean> Returns true if the fs.Stats object describes a first-in-first-out (FIFO) pipe. Returns: <boolean> Returns true if the fs.Stats object describes a regular file. Returns: <boolean> Returns true if the fs.Stats object describes a socket. Returns: <boolean> Returns true if the fs.Stats object describes a symbolic link. This method is only valid when using fs.lstat() . <number> | <bigint> The numeric identifier of the device containing the file. <number> | <bigint> The file system specific "Inode" number for the file. <number> | <bigint> A bit-field describing the file type and mode. <number> | <bigint> The number of hard-links that exist for the file. <number> | <bigint> The numeric user identifier of the user that owns the file (POSIX). <number> | <bigint> The numeric group identifier of the group that owns the file (POSIX). stats.isFIFO() stats.isFile() stats.isSocket() stats.isSymbolicLink() stats.dev stats.ino stats.mode stats.nlink stats.uid stats.gid 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 345/739 <number> | <bigint> A numeric device identifier if the file is considered "special". <number> | <bigint> The size of the file in bytes. <number> | <bigint> The file system block size for i/o operations. <number> | <bigint> The number of blocks allocated for this file. <number> | <bigint> The timestamp indicating the last time this file was accessed expressed in milliseconds since the POSIX Epoch. <number> | <bigint> The timestamp indicating the last time this file was modified expressed in milliseconds since the POSIX Epoch. <number> | <bigint> The timestamp indicating the last time the file status was changed expressed in milliseconds since the POSIX Epoch. <number> | <bigint> The timestamp indicating the creation time of this file expressed in milliseconds since the POSIX Epoch. <Date> The timestamp indicating the last time this file was accessed. <Date> The timestamp indicating the last time this file was modified. stats.rdev stats.size stats.blksize stats.blocks stats.atimeMs stats.mtimeMs stats.ctimeMs stats.birthtimeMs stats.atime stats.mtime stats.ctime 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 346/739 <Date> The timestamp indicating the last time the file status was changed. <Date> The timestamp indicating the creation time of this file. The atimeMs , mtimeMs , ctimeMs , birthtimeMs properties are numbers that hold the corresponding times in milliseconds. Their precision is platform specific. atime , mtime , ctime , and birthtime are Date object alternate representations of the various times. The Date and number values are not connected. Assigning a new number value, or mutating the Date value, will not be reflected in the corresponding alternate representation. The times in the stat object have the following semantics: atime "Access Time" - Time when file data last accessed. Changed by the mknod(2) , utimes(2) , and read(2) system calls. mtime "Modified Time" - Time when file data last modified. Changed by the mknod(2) , utimes(2) , and write(2) system calls. ctime "Change Time" - Time when file status was last changed (inode data modification). Changed by the chmod(2) , chown(2) , link(2) , mknod(2) , rename(2) , unlink(2) , utimes(2) , read(2) , and write(2) system calls. birthtime "Birth Time" - Time of file creation. Set once when the file is created. On filesystems where birthtime is not available, this field may instead hold either the ctime or 1970-01-01T00:00Z (ie, unix epoch timestamp 0 ). This value may be greater than atime or mtime in this case. On Darwin and other FreeBSD variants, also set if the atime is explicitly set to an earlier value than the current birthtime using the utimes(2) system call. Prior to Node.js 0.12, the ctime held the birthtime on Windows systems. As of 0.12, ctime is not "creation time", and on Unix systems, it never was. WriteStream is a Writable Stream . Emitted when the WriteStream 's underlying file descriptor has been closed. fd <integer> Integer file descriptor used by the WriteStream . Emitted when the WriteStream 's file is opened. Emitted when the fs.WriteStream is ready to be used. Fires immediately after 'open' . The number of bytes written so far. Does not include data that is still queued for writing. stats.birthtime Stat Time Values Class: fs.WriteStream Event: 'close' Event: 'open' Event: 'ready' writeStream.bytesWritten writeStream.path 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 347/739 The path to the file the stream is writing to as specified in the first argument to fs.createWriteStream() . If path is passed as a string, then writeStream.path will be a string. If path is passed as a Buffer , then writeStream.path will be a Buffer . <boolean> This property is true if the underlying file has not been opened yet, i.e. before the 'ready' event is emitted. path <string> | <Buffer> | <URL> mode <integer> Default: fs.constants.F_OK callback <Function> err <Error> Tests a user's permissions for the file or directory specified by path . The mode argument is an optional integer that specifies the accessibility checks to be performed. Check File Access Constants for possible values of mode . It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.W_OK | fs.constants.R_OK ). The final argument, callback , is a callback function that is invoked with a possible error argument. If any of the accessibility checks fail, the error argument will be an Error object. The following examples check if package.json exists, and if it is readable or writable. const file = 'package.json'; // Check if the file exists in the current directory. fs.access(file, fs.constants.F_OK, (err) => { console.log(`${file} ${err ? 'does not exist' : 'exists'}`); }); // Check if the file is readable. fs.access(file, fs.constants.R_OK, (err) => { console.log(`${file} ${err ? 'is not readable' : 'is readable'}`); }); // Check if the file is writable. fs.access(file, fs.constants.W_OK, (err) => { console.log(`${file} ${err ? 'is not writable' : 'is writable'}`); }); // Check if the file exists in the current directory, and if it is writable. fs.access(file, fs.constants.F_OK | fs.constants.W_OK, (err) => { if (err) { console.error( `${file} ${err.code === 'ENOENT' ? 'does not exist' : 'is read-only'}`); } else { console.log(`${file} exists, and it is writable`); } }); Using fs.access() to check for the accessibility of a file before calling fs.open() , fs.readFile() or fs.writeFile() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should writeStream.pending fs.access(path[, mode], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 348/739 open/read/write the file directly and handle the error raised if the file is not accessible. write (NOT RECOMMENDED) fs.access('myfile', (err) => { if (!err) { console.error('myfile already exists'); return; } fs.open('myfile', 'wx', (err, fd) => { if (err) throw err; writeMyData(fd); }); }); write (RECOMMENDED) fs.open('myfile', 'wx', (err, fd) => { if (err) { if (err.code === 'EEXIST') { console.error('myfile already exists'); return; } throw err; } writeMyData(fd); }); read (NOT RECOMMENDED) fs.access('myfile', (err) => { if (err) { if (err.code === 'ENOENT') { console.error('myfile does not exist'); return; } throw err; } fs.open('myfile', 'r', (err, fd) => { if (err) throw err; readMyData(fd); }); }); read (RECOMMENDED) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 349/739 fs.open('myfile', 'r', (err, fd) => { if (err) { if (err.code === 'ENOENT') { console.error('myfile does not exist'); return; } throw err; } readMyData(fd); }); The "not recommended" examples above check for accessibility and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any. In general, check for the accessibility of a file only if the file will not be used directly, for example when its accessibility is a signal from another process. On Windows, access-control policies (ACLs) on a directory may limit access to a file or directory. The fs.access() function, however, does not check the ACL and therefore may report that a path is accessible even if the ACL restricts the user from reading or writing to it. path <string> | <Buffer> | <URL> mode <integer> Default: fs.constants.F_OK Synchronously tests a user's permissions for the file or directory specified by path . The mode argument is an optional integer that specifies the accessibility checks to be performed. Check File Access Constants for possible values of mode . It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.W_OK | fs.constants.R_OK ). If any of the accessibility checks fail, an Error will be thrown. Otherwise, the method will return undefined . try { fs.accessSync('etc/passwd', fs.constants.R_OK | fs.constants.W_OK); console.log('can read/write'); } catch (err) { console.error('no access!'); } path <string> | <Buffer> | <URL> | <number> filename or file descriptor data <string> | <Buffer> options <Object> | <string> encoding <string> | <null> Default: 'utf8' mode <integer> Default: 0o666 flag <string> See support of file system flags . Default: 'a' . callback <Function> fs.accessSync(path[, mode]) fs.appendFile(path, data[, options], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 350/739 err <Error> Asynchronously append data to a file, creating the file if it does not yet exist. data can be a string or a Buffer . fs.appendFile('message.txt', 'data to append', (err) => { if (err) throw err; console.log('The "data to append" was appended to file!'); }); If options is a string, then it specifies the encoding: fs.appendFile('message.txt', 'data to append', 'utf8', callback); The path may be specified as a numeric file descriptor that has been opened for appending (using fs.open() or fs.openSync() ). The file descriptor will not be closed automatically. fs.open('message.txt', 'a', (err, fd) => { if (err) throw err; fs.appendFile(fd, 'data to append', 'utf8', (err) => { fs.close(fd, (err) => { if (err) throw err; }); if (err) throw err; }); }); path <string> | <Buffer> | <URL> | <number> filename or file descriptor data <string> | <Buffer> options <Object> | <string> encoding <string> | <null> Default: 'utf8' mode <integer> Default: 0o666 flag <string> See support of file system flags . Default: 'a' . Synchronously append data to a file, creating the file if it does not yet exist. data can be a string or a Buffer . try { fs.appendFileSync('message.txt', 'data to append'); console.log('The "data to append" was appended to file!'); } catch (err) { /* Handle the error */ } If options is a string, then it specifies the encoding: fs.appendFileSync('message.txt', 'data to append', 'utf8'); fs.appendFileSync(path, data[, options]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 351/739 The path may be specified as a numeric file descriptor that has been opened for appending (using fs.open() or fs.openSync() ). The file descriptor will not be closed automatically. let fd; try { fd = fs.openSync('message.txt', 'a'); fs.appendFileSync(fd, 'data to append', 'utf8'); } catch (err) { /* Handle the error */ } finally { if (fd !== undefined) fs.closeSync(fd); } path <string> | <Buffer> | <URL> mode <integer> callback <Function> err <Error> Asynchronously changes the permissions of a file. No arguments other than a possible exception are given to the completion callback. See also: chmod(2) . The mode argument used in both the fs.chmod() and fs.chmodSync() methods is a numeric bitmask created using a logical OR of the following constants: Constant Octal Description fs.constants.S_IRUSR 0o400 read by owner fs.constants.S_IWUSR 0o200 write by owner fs.constants.S_IXUSR 0o100 execute/search by owner fs.constants.S_IRGRP 0o40 read by group fs.constants.S_IWGRP 0o20 write by group fs.constants.S_IXGRP 0o10 execute/search by group fs.constants.S_IROTH 0o4 read by others fs.constants.S_IWOTH 0o2 write by others fs.constants.S_IXOTH 0o1 execute/search by others fs.chmod(path, mode, callback) File modes 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 352/739 An easier method of constructing the mode is to use a sequence of three octal digits (e.g. 765 ). The left-most digit ( 7 in the example), specifies the permissions for the file owner. The middle digit ( 6 in the example), specifies permissions for the group. The right-most digit ( 5 in the example), specifies the permissions for others. Number Description 7 read, write, and execute 6 read and write 5 read and execute 4 read only 3 write and execute 2 write only 1 execute only 0 no permission For example, the octal value 0o765 means: The owner may read, write and execute the file. The group may read and write the file. Others may read and execute the file. When using raw numbers where file modes are expected, any value larger than 0o777 may result in platform-specific behaviors that are not supported to work consistently. Therefore constants like S_ISVTX , S_ISGID or S_ISUID are not exposed in fs.constants . Caveats: on Windows only the write permission can be changed, and the distinction among the permissions of group, owner or others is not implemented. path <string> | <Buffer> | <URL> mode <integer> For detailed information, see the documentation of the asynchronous version of this API: fs.chmod() . See also: chmod(2) . path <string> | <Buffer> | <URL> uid <integer> gid <integer> callback <Function> err <Error> Asynchronously changes owner and group of a file. No arguments other than a possible exception are given to the completion callback. fs.chmodSync(path, mode) fs.chown(path, uid, gid, callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 353/739 See also: chown(2) . path <string> | <Buffer> | <URL> uid <integer> gid <integer> Synchronously changes owner and group of a file. Returns undefined . This is the synchronous version of fs.chown() . See also: chown(2) . fd <integer> callback <Function> err <Error> Asynchronous close(2) . No arguments other than a possible exception are given to the completion callback. fd <integer> Synchronous close(2) . Returns undefined . <Object> Returns an object containing commonly used constants for file system operations. The specific constants currently defined are described in FS Constants . src <string> | <Buffer> | <URL> source filename to copy dest <string> | <Buffer> | <URL> destination filename of the copy operation flags <number> modifiers for copy operation. Default: 0 . callback <Function> Asynchronously copies src to dest . By default, dest is overwritten if it already exists. No arguments other than a possible exception are given to the callback function. Node.js makes no guarantees about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, Node.js will attempt to remove the destination. flags is an optional integer that specifies the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE ). fs.constants.COPYFILE_EXCL - The copy operation will fail if dest already exists. fs.constants.COPYFILE_FICLONE - The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used. fs.constants.COPYFILE_FICLONE_FORCE - The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail. fs.chownSync(path, uid, gid) fs.close(fd, callback) fs.closeSync(fd) fs.constants fs.copyFile(src, dest[, flags], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 354/739 const fs = require('fs'); // destination.txt will be created or overwritten by default. fs.copyFile('source.txt', 'destination.txt', (err) => { if (err) throw err; console.log('source.txt was copied to destination.txt'); }); If the third argument is a number, then it specifies flags : const fs = require('fs'); const { COPYFILE_EXCL } = fs.constants; // By using COPYFILE_EXCL, the operation will fail if destination.txt exists. fs.copyFile('source.txt', 'destination.txt', COPYFILE_EXCL, callback); src <string> | <Buffer> | <URL> source filename to copy dest <string> | <Buffer> | <URL> destination filename of the copy operation flags <number> modifiers for copy operation. Default: 0 . Synchronously copies src to dest . By default, dest is overwritten if it already exists. Returns undefined . Node.js makes no guarantees about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, Node.js will attempt to remove the destination. flags is an optional integer that specifies the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE ). fs.constants.COPYFILE_EXCL - The copy operation will fail if dest already exists. fs.constants.COPYFILE_FICLONE - The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used. fs.constants.COPYFILE_FICLONE_FORCE - The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail. const fs = require('fs'); // destination.txt will be created or overwritten by default. fs.copyFileSync('source.txt', 'destination.txt'); console.log('source.txt was copied to destination.txt'); If the third argument is a number, then it specifies flags : const fs = require('fs'); const { COPYFILE_EXCL } = fs.constants; // By using COPYFILE_EXCL, the operation will fail if destination.txt exists. fs.copyFileSync('source.txt', 'destination.txt', COPYFILE_EXCL); fs.copyFileSync(src, dest[, flags]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 355/739 path <string> | <Buffer> | <URL> options <string> | <Object> flags <string> See support of file system flags . Default: 'r' . encoding <string> Default: null fd <integer> Default: null mode <integer> Default: 0o666 autoClose <boolean> Default: true start <integer> end <integer> Default: Infinity highWaterMark <integer> Default: 64 * 1024 Returns: <fs.ReadStream> See Readable Streams . Unlike the 16 kb default highWaterMark for a readable stream, the stream returned by this method has a default highWaterMark of 64 kb. options can include start and end values to read a range of bytes from the file instead of the entire file. Both start and end are inclusive and start counting at 0. If fd is specified and start is omitted or undefined , fs.createReadStream() reads sequentially from the current file position. The encoding can be any one of those accepted by Buffer . If fd is specified, ReadStream will ignore the path argument and will use the specified file descriptor. This means that no 'open' event will be emitted. fd should be blocking; non-blocking fd s should be passed to net.Socket . If fd points to a character device that only supports blocking reads (such as keyboard or sound card), read operations do not finish until data is available. This can prevent the process from exiting and the stream from closing naturally. const fs = require('fs'); // Create a stream from some character device. const stream = fs.createReadStream('/dev/input/event0'); setTimeout(() => { stream.close(); // This may not close the stream. // Artificially marking end-of-stream, as if the underlying resource had // indicated end-of-file by itself, allows the stream to close. // This does not cancel pending read operations, and if there is such an // operation, the process may still not be able to exit successfully // until it finishes. stream.push(null); stream.read(0); }, 100); If autoClose is false, then the file descriptor won't be closed, even if there's an error. It is the application's responsibility to close it and make sure there's no file descriptor leak. If autoClose is set to true (default behavior), on 'error' or 'end' the file descriptor will be closed automatically. mode sets the file mode (permission and sticky bits), but only if the file was created. An example to read the last 10 bytes of a file which is 100 bytes long: fs.createReadStream('sample.txt', { start: 90, end: 99 }); fs.createReadStream(path[, options]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 356/739 If options is a string, then it specifies the encoding. path <string> | <Buffer> | <URL> options <string> | <Object> flags <string> See support of file system flags . Default: 'w' . encoding <string> Default: 'utf8' fd <integer> Default: null mode <integer> Default: 0o666 autoClose <boolean> Default: true start <integer> Returns: <fs.WriteStream> See Writable Stream . options may also include a start option to allow writing data at some position past the beginning of the file. Modifying a file rather than replacing it may require a flags mode of r+ rather than the default mode w . The encoding can be any one of those accepted by Buffer . If autoClose is set to true (default behavior) on 'error' or 'finish' the file descriptor will be closed automatically. If autoClose is false, then the file descriptor won't be closed, even if there's an error. It is the application's responsibility to close it and make sure there's no file descriptor leak. Like ReadStream , if fd is specified, WriteStream will ignore the path argument and will use the specified file descriptor. This means that no 'open' event will be emitted. fd should be blocking; non-blocking fd s should be passed to net.Socket . If options is a string, then it specifies the encoding. Stability: 0 - Deprecated: Use fs.stat() or fs.access() instead. path <string> | <Buffer> | <URL> callback <Function> exists <boolean> Test whether or not the given path exists by checking with the file system. Then call the callback argument with either true or false: fs.exists('/etc/passwd', (exists) => { console.log(exists ? 'it\'s there' : 'no passwd!'); }); The parameters for this callback are not consistent with other Node.js callbacks. Normally, the first parameter to a Node.js callback is an err parameter, optionally followed by other parameters. The fs.exists() callback has only one boolean parameter. This is one reason fs.access() is recommended instead of fs.exists() . Using fs.exists() to check for the existence of a file before calling fs.open() , fs.readFile() or fs.writeFile() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file does not exist. fs.createWriteStream(path[, options]) fs.exists(path, callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 357/739 write (NOT RECOMMENDED) fs.exists('myfile', (exists) => { if (exists) { console.error('myfile already exists'); } else { fs.open('myfile', 'wx', (err, fd) => { if (err) throw err; writeMyData(fd); }); } }); write (RECOMMENDED) fs.open('myfile', 'wx', (err, fd) => { if (err) { if (err.code === 'EEXIST') { console.error('myfile already exists'); return; } throw err; } writeMyData(fd); }); read (NOT RECOMMENDED) fs.exists('myfile', (exists) => { if (exists) { fs.open('myfile', 'r', (err, fd) => { if (err) throw err; readMyData(fd); }); } else { console.error('myfile does not exist'); } }); read (RECOMMENDED) fs.open('myfile', 'r', (err, fd) => { if (err) { if (err.code === 'ENOENT') { console.error('myfile does not exist'); return; } throw err; 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 358/739 } readMyData(fd); }); The "not recommended" examples above check for existence and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any. In general, check for the existence of a file only if the file won’t be used directly, for example when its existence is a signal from another process. path <string> | <Buffer> | <URL> Returns: <boolean> Returns true if the path exists, false otherwise. For detailed information, see the documentation of the asynchronous version of this API: fs.exists() . fs.exists() is deprecated, but fs.existsSync() is not. The callback parameter to fs.exists() accepts parameters that are inconsistent with other Node.js callbacks. fs.existsSync() does not use a callback. fd <integer> mode <integer> callback <Function> err <Error> Asynchronous fchmod(2) . No arguments other than a possible exception are given to the completion callback. fd <integer> mode <integer> Synchronous fchmod(2) . Returns undefined . fd <integer> uid <integer> gid <integer> callback <Function> err <Error> Asynchronous fchown(2) . No arguments other than a possible exception are given to the completion callback. fd <integer> uid <integer> fs.existsSync(path) fs.fchmod(fd, mode, callback) fs.fchmodSync(fd, mode) fs.fchown(fd, uid, gid, callback) fs.fchownSync(fd, uid, gid) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 359/739 gid <integer> Synchronous fchown(2) . Returns undefined . fd <integer> callback <Function> err <Error> Asynchronous fdatasync(2) . No arguments other than a possible exception are given to the completion callback. fd <integer> Synchronous fdatasync(2) . Returns undefined . fd <integer> options <Object> bigint <boolean> Whether the numeric values in the returned fs.Stats object should be bigint . Default: false . callback <Function> err <Error> stats <fs.Stats> Asynchronous fstat(2) . The callback gets two arguments (err, stats) where stats is an fs.Stats object. fstat() is identical to stat() , except that the file to be stat-ed is specified by the file descriptor fd . fd <integer> options <Object> bigint <boolean> Whether the numeric values in the returned fs.Stats object should be bigint . Default: false . Returns: <fs.Stats> Synchronous fstat(2) . fd <integer> callback <Function> err <Error> Asynchronous fsync(2) . No arguments other than a possible exception are given to the completion callback. fd <integer> fs.fdatasync(fd, callback) fs.fdatasyncSync(fd) fs.fstat(fd[, options], callback) fs.fstatSync(fd[, options]) fs.fsync(fd, callback) fs.fsyncSync(fd) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 360/739 Synchronous fsync(2) . Returns undefined . fd <integer> len <integer> Default: 0 callback <Function> err <Error> Asynchronous ftruncate(2) . No arguments other than a possible exception are given to the completion callback. If the file referred to by the file descriptor was larger than len bytes, only the first len bytes will be retained in the file. For example, the following program retains only the first four bytes of the file: console.log(fs.readFileSync('temp.txt', 'utf8')); // Prints: Node.js // get the file descriptor of the file to be truncated const fd = fs.openSync('temp.txt', 'r+'); // truncate the file to first four bytes fs.ftruncate(fd, 4, (err) => { assert.ifError(err); console.log(fs.readFileSync('temp.txt', 'utf8')); }); // Prints: Node If the file previously was shorter than len bytes, it is extended, and the extended part is filled with null bytes ( '\0' ): console.log(fs.readFileSync('temp.txt', 'utf8')); // Prints: Node.js // get the file descriptor of the file to be truncated const fd = fs.openSync('temp.txt', 'r+'); // truncate the file to 10 bytes, whereas the actual size is 7 bytes fs.ftruncate(fd, 10, (err) => { assert.ifError(err); console.log(fs.readFileSync('temp.txt')); }); // Prints: <Buffer 4e 6f 64 65 2e 6a 73 00 00 00> // ('Node.js\0\0\0' in UTF8) The last three bytes are null bytes ( '\0' ), to compensate the over-truncation. fd <integer> len <integer> Default: 0 Returns undefined . fs.ftruncate(fd[, len], callback) fs.ftruncateSync(fd[, len]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 361/739 For detailed information, see the documentation of the asynchronous version of this API: fs.ftruncate() . fd <integer> atime <number> | <string> | <Date> mtime <number> | <string> | <Date> callback <Function> err <Error> Change the file system timestamps of the object referenced by the supplied file descriptor. See fs.utimes() . This function does not work on AIX versions before 7.1, it will return the error UV_ENOSYS . fd <integer> atime <integer> mtime <integer> Synchronous version of fs.futimes() . Returns undefined . path <string> | <Buffer> | <URL> mode <integer> callback <Function> err <Error> Asynchronous lchmod(2) . No arguments other than a possible exception are given to the completion callback. Only available on macOS. path <string> | <Buffer> | <URL> mode <integer> Synchronous lchmod(2) . Returns undefined . path <string> | <Buffer> | <URL> uid <integer> gid <integer> callback <Function> err <Error> Asynchronous lchown(2) . No arguments other than a possible exception are given to the completion callback. fs.futimes(fd, atime, mtime, callback) fs.futimesSync(fd, atime, mtime) fs.lchmod(path, mode, callback) fs.lchmodSync(path, mode) fs.lchown(path, uid, gid, callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 362/739 path <string> | <Buffer> | <URL> uid <integer> gid <integer> Synchronous lchown(2) . Returns undefined . existingPath <string> | <Buffer> | <URL> newPath <string> | <Buffer> | <URL> callback <Function> err <Error> Asynchronous link(2) . No arguments other than a possible exception are given to the completion callback. existingPath <string> | <Buffer> | <URL> newPath <string> | <Buffer> | <URL> Synchronous link(2) . Returns undefined . path <string> | <Buffer> | <URL> options <Object> bigint <boolean> Whether the numeric values in the returned fs.Stats object should be bigint . Default: false . callback <Function> err <Error> stats <fs.Stats> Asynchronous lstat(2) . The callback gets two arguments (err, stats) where stats is a fs.Stats object. lstat() is identical to stat() , except that if path is a symbolic link, then the link itself is stat-ed, not the file that it refers to. path <string> | <Buffer> | <URL> options <Object> bigint <boolean> Whether the numeric values in the returned fs.Stats object should be bigint . Default: false . Returns: <fs.Stats> Synchronous lstat(2) . path <string> | <Buffer> | <URL> options <Object> | <integer> fs.lchownSync(path, uid, gid) fs.link(existingPath, newPath, callback) fs.linkSync(existingPath, newPath) fs.lstat(path[, options], callback) fs.lstatSync(path[, options]) fs.mkdir(path[, options], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 363/739 recursive <boolean> Default: false mode <integer> Not supported on Windows. Default: 0o777 . callback <Function> err <Error> Asynchronously creates a directory. No arguments other than a possible exception are given to the completion callback. The optional options argument can be an integer specifying mode (permission and sticky bits), or an object with a mode property and a recursive property indicating whether parent folders should be created. // Creates /tmp/a/apple, regardless of whether `/tmp` and /tmp/a exist. fs.mkdir('/tmp/a/apple', { recursive: true }, (err) => { if (err) throw err; }); On Windows, using fs.mkdir() on the root directory even with recursion will result in an error: fs.mkdir('/', { recursive: true }, (err) => { // => [Error: EPERM: operation not permitted, mkdir 'C:\'] }); See also: mkdir(2) . path <string> | <Buffer> | <URL> options <Object> | <integer> recursive <boolean> Default: false mode <integer> Not supported on Windows. Default: 0o777 . Synchronously creates a directory. Returns undefined . This is the synchronous version of fs.mkdir() . See also: mkdir(2) . prefix <string> options <string> | <Object> encoding <string> Default: 'utf8' callback <Function> err <Error> folder <string> Creates a unique temporary directory. Generates six random characters to be appended behind a required prefix to create a unique temporary directory. The created folder path is passed as a string to the callback's second parameter. fs.mkdirSync(path[, options]) fs.mkdtemp(prefix[, options], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 364/739 The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use. fs.mkdtemp(path.join(os.tmpdir(), 'foo-'), (err, folder) => { if (err) throw err; console.log(folder); // Prints: /tmp/foo-itXde2 or C:\Users\...\AppData\Local\Temp\foo-itXde2 }); The fs.mkdtemp() method will append the six randomly selected characters directly to the prefix string. For instance, given a directory /tmp , if the intention is to create a temporary directory within /tmp , the prefix must end with a trailing platform-specific path separator ( require('path').sep ). // The parent directory for the new temporary directory const tmpDir = os.tmpdir(); // This method is *INCORRECT*: fs.mkdtemp(tmpDir, (err, folder) => { if (err) throw err; console.log(folder); // Will print something similar to `/tmpabc123`. // A new temporary directory is created at the file system root // rather than *within* the /tmp directory. }); // This method is *CORRECT*: const { sep } = require('path'); fs.mkdtemp(`${tmpDir}${sep}`, (err, folder) => { if (err) throw err; console.log(folder); // Will print something similar to `/tmp/abc123`. // A new temporary directory is created within // the /tmp directory. }); prefix <string> options <string> | <Object> encoding <string> Default: 'utf8' Returns: <string> Returns the created folder path. For detailed information, see the documentation of the asynchronous version of this API: fs.mkdtemp() . The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use. fs.mkdtempSync(prefix[, options]) fs.open(path[, flags[, mode]], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 365/739 path <string> | <Buffer> | <URL> flags <string> | <number> See support of file system flags . Default: 'r' . mode <integer> Default: 0o666 (readable and writable) callback <Function> err <Error> fd <integer> Asynchronous file open. See open(2) . mode sets the file mode (permission and sticky bits), but only if the file was created. On Windows, only the write permission can be manipulated; see fs.chmod() . The callback gets two arguments (err, fd) . Some characters ( < > : " / \ | ? * ) are reserved under Windows as documented by Naming Files, Paths, and Namespaces . Under NTFS, if the filename contains a colon, Node.js will open a file system stream, as described by this MSDN page . Functions based on fs.open() exhibit this behavior as well: fs.writeFile() , fs.readFile() , etc. path <string> | <Buffer> | <URL> flags <string> | <number> Default: 'r' . See support of file system flags . mode <integer> Default: 0o666 Returns: <number> Returns an integer representing the file descriptor. For detailed information, see the documentation of the asynchronous version of this API: fs.open() . fd <integer> buffer <Buffer> | <TypedArray> | <DataView> offset <integer> length <integer> position <integer> callback <Function> err <Error> bytesRead <integer> buffer <Buffer> Read data from the file specified by fd . buffer is the buffer that the data will be written to. offset is the offset in the buffer to start writing at. length is an integer specifying the number of bytes to read. fs.openSync(path[, flags, mode]) fs.read(fd, buffer, offset, length, position, callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 366/739 position is an argument specifying where to begin reading from in the file. If position is null , data will be read from the current file position, and the file position will be updated. If position is an integer, the file position will remain unchanged. The callback is given the three arguments, (err, bytesRead, buffer) . If this method is invoked as its util.promisify() ed version, it returns a Promise for an Object with bytesRead and buffer properties. path <string> | <Buffer> | <URL> options <string> | <Object> encoding <string> Default: 'utf8' withFileTypes <boolean> Default: false callback <Function> err <Error> files <string[]> | <Buffer[]> | <fs.Dirent[]> Asynchronous readdir(3) . Reads the contents of a directory. The callback gets two arguments (err, files) where files is an array of the names of the files in the directory excluding '.' and '..' . The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the filenames passed to the callback. If the encoding is set to 'buffer' , the filenames returned will be passed as Buffer objects. If options.withFileTypes is set to true , the files array will contain fs.Dirent objects. path <string> | <Buffer> | <URL> options <string> | <Object> encoding <string> Default: 'utf8' withFileTypes <boolean> Default: false Returns: <string[]> | <Buffer[]> | <fs.Dirent[]> Synchronous readdir(3) . The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the filenames returned. If the encoding is set to 'buffer' , the filenames returned will be passed as Buffer objects. If options.withFileTypes is set to true , the result will contain fs.Dirent objects. path <string> | <Buffer> | <URL> | <integer> filename or file descriptor options <Object> | <string> encoding <string> | <null> Default: null flag <string> See support of file system flags . Default: 'r' . callback <Function> fs.readdir(path[, options], callback) fs.readdirSync(path[, options]) fs.readFile(path[, options], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 367/739 err <Error> data <string> | <Buffer> Asynchronously reads the entire contents of a file. fs.readFile('/etc/passwd', (err, data) => { if (err) throw err; console.log(data); }); The callback is passed two arguments (err, data) , where data is the contents of the file. If no encoding is specified, then the raw buffer is returned. If options is a string, then it specifies the encoding: fs.readFile('/etc/passwd', 'utf8', callback); When the path is a directory, the behavior of fs.readFile() and fs.readFileSync() is platform-specific. On macOS, Linux, and Windows, an error will be returned. On FreeBSD, a representation of the directory's contents will be returned. // macOS, Linux, and Windows fs.readFile('<directory>', (err, data) => { // => [Error: EISDIR: illegal operation on a directory, read <directory>] }); // FreeBSD fs.readFile('<directory>', (err, data) => { // => null, <data> }); The fs.readFile() function buffers the entire file. To minimize memory costs, when possible prefer streaming via fs.createReadStream() . 1. Any specified file descriptor has to support reading. 2. If a file descriptor is specified as the path , it will not be closed automatically. 3. The reading will begin at the current position. For example, if the file already had 'Hello World ' and six bytes are read with the file descriptor, the call to fs.readFile() with the same file descriptor, would give 'World' , rather than 'Hello World' . path <string> | <Buffer> | <URL> | <integer> filename or file descriptor options <Object> | <string> encoding <string> | <null> Default: null flag <string> See support of file system flags . Default: 'r' . Returns: <string> | <Buffer> Returns the contents of the path . For detailed information, see the documentation of the asynchronous version of this API: fs.readFile() . File Descriptors fs.readFileSync(path[, options]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 368/739 If the encoding option is specified then this function returns a string. Otherwise it returns a buffer. Similar to fs.readFile() , when the path is a directory, the behavior of fs.readFileSync() is platform-specific. // macOS, Linux, and Windows fs.readFileSync('<directory>'); // => [Error: EISDIR: illegal operation on a directory, read <directory>] // FreeBSD fs.readFileSync('<directory>'); // => <data> path <string> | <Buffer> | <URL> options <string> | <Object> encoding <string> Default: 'utf8' callback <Function> err <Error> linkString <string> | <Buffer> Asynchronous readlink(2) . The callback gets two arguments (err, linkString) . The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path passed to the callback. If the encoding is set to 'buffer' , the link path returned will be passed as a Buffer object. path <string> | <Buffer> | <URL> options <string> | <Object> encoding <string> Default: 'utf8' Returns: <string> | <Buffer> Synchronous readlink(2) . Returns the symbolic link's string value. The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path returned. If the encoding is set to 'buffer' , the link path returned will be passed as a Buffer object. fd <integer> buffer <Buffer> | <TypedArray> | <DataView> offset <integer> length <integer> position <integer> Returns: <number> Returns the number of bytesRead . fs.readlink(path[, options], callback) fs.readlinkSync(path[, options]) fs.readSync(fd, buffer, offset, length, position) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 369/739 For detailed information, see the documentation of the asynchronous version of this API: fs.read() . path <string> | <Buffer> | <URL> options <string> | <Object> encoding <string> Default: 'utf8' callback <Function> err <Error> resolvedPath <string> | <Buffer> Asynchronously computes the canonical pathname by resolving . , .. and symbolic links. A canonical pathname is not necessarily unique. Hard links and bind mounts can expose a file system entity through many pathnames. This function behaves like realpath(3) , with some exceptions: 1. No case conversion is performed on case-insensitive file systems. 2. The maximum number of symbolic links is platform-independent and generally (much) higher than what the native realpath(3) implementation supports. The callback gets two arguments (err, resolvedPath) . May use process.cwd to resolve relative paths. Only paths that can be converted to UTF8 strings are supported. The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path passed to the callback. If the encoding is set to 'buffer' , the path returned will be passed as a Buffer object. If path resolves to a socket or a pipe, the function will return a system dependent name for that object. path <string> | <Buffer> | <URL> options <string> | <Object> encoding <string> Default: 'utf8' callback <Function> err <Error> resolvedPath <string> | <Buffer> Asynchronous realpath(3) . The callback gets two arguments (err, resolvedPath) . Only paths that can be converted to UTF8 strings are supported. The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path passed to the callback. If the encoding is set to 'buffer' , the path returned will be passed as a Buffer object. On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on /proc in order for this function to work. Glibc does not have this restriction. fs.realpath(path[, options], callback) fs.realpath.native(path[, options], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 370/739 path <string> | <Buffer> | <URL> options <string> | <Object> encoding <string> Default: 'utf8' Returns: <string> | <Buffer> Returns the resolved pathname. For detailed information, see the documentation of the asynchronous version of this API: fs.realpath() . path <string> | <Buffer> | <URL> options <string> | <Object> encoding <string> Default: 'utf8' Returns: <string> | <Buffer> Synchronous realpath(3) . Only paths that can be converted to UTF8 strings are supported. The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path returned. If the encoding is set to 'buffer' , the path returned will be passed as a Buffer object. On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on /proc in order for this function to work. Glibc does not have this restriction. oldPath <string> | <Buffer> | <URL> newPath <string> | <Buffer> | <URL> callback <Function> err <Error> Asynchronously rename file at oldPath to the pathname provided as newPath . In the case that newPath already exists, it will be overwritten. No arguments other than a possible exception are given to the completion callback. See also: rename(2) . fs.rename('oldFile.txt', 'newFile.txt', (err) => { if (err) throw err; console.log('Rename complete!'); }); oldPath <string> | <Buffer> | <URL> newPath <string> | <Buffer> | <URL> Synchronous rename(2) . Returns undefined . fs.realpathSync(path[, options]) fs.realpathSync.native(path[, options]) fs.rename(oldPath, newPath, callback) fs.renameSync(oldPath, newPath) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 371/739 path <string> | <Buffer> | <URL> callback <Function> err <Error> Asynchronous rmdir(2) . No arguments other than a possible exception are given to the completion callback. Using fs.rmdir() on a file (not a directory) results in an ENOENT error on Windows and an ENOTDIR error on POSIX. path <string> | <Buffer> | <URL> Synchronous rmdir(2) . Returns undefined . Using fs.rmdirSync() on a file (not a directory) results in an ENOENT error on Windows and an ENOTDIR error on POSIX. path <string> | <Buffer> | <URL> options <Object> bigint <boolean> Whether the numeric values in the returned fs.Stats object should be bigint . Default: false . callback <Function> err <Error> stats <fs.Stats> Asynchronous stat(2) . The callback gets two arguments (err, stats) where stats is an fs.Stats object. In case of an error, the err.code will be one of Common System Errors . Using fs.stat() to check for the existence of a file before calling fs.open() , fs.readFile() or fs.writeFile() is not recommended. Instead, user code should open/read/write the file directly and handle the error raised if the file is not available. To check if a file exists without manipulating it afterwards, fs.access() is recommended. path <string> | <Buffer> | <URL> options <Object> bigint <boolean> Whether the numeric values in the returned fs.Stats object should be bigint . Default: false . Returns: <fs.Stats> Synchronous stat(2) . target <string> | <Buffer> | <URL> path <string> | <Buffer> | <URL> type <string> Default: 'file' callback <Function> fs.rmdir(path, callback) fs.rmdirSync(path) fs.stat(path[, options], callback) fs.statSync(path[, options]) fs.symlink(target, path[, type], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 372/739 err <Error> Asynchronous symlink(2) . No arguments other than a possible exception are given to the completion callback. The type argument can be set to 'dir' , 'file' , or 'junction' and is only available on Windows (ignored on other platforms). Windows junction points require the destination path to be absolute. When using 'junction' , the target argument will automatically be normalized to absolute path. Here is an example below: fs.symlink('./foo', './new-port', callback); It creates a symbolic link named "new-port" that points to "foo". target <string> | <Buffer> | <URL> path <string> | <Buffer> | <URL> type <string> Default: 'file' Returns undefined . For detailed information, see the documentation of the asynchronous version of this API: fs.symlink() . path <string> | <Buffer> | <URL> len <integer> Default: 0 callback <Function> err <Error> Asynchronous truncate(2) . No arguments other than a possible exception are given to the completion callback. A file descriptor can also be passed as the first argument. In this case, fs.ftruncate() is called. Passing a file descriptor is deprecated and may result in an error being thrown in the future. path <string> | <Buffer> | <URL> len <integer> Default: 0 Synchronous truncate(2) . Returns undefined . A file descriptor can also be passed as the first argument. In this case, fs.ftruncateSync() is called. Passing a file descriptor is deprecated and may result in an error being thrown in the future. path <string> | <Buffer> | <URL> callback <Function> err <Error> Asynchronously removes a file or symbolic link. No arguments other than a possible exception are given to the completion callback. fs.symlinkSync(target, path[, type]) fs.truncate(path[, len], callback) fs.truncateSync(path[, len]) fs.unlink(path, callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 373/739 // Assuming that 'path/file.txt' is a regular file. fs.unlink('path/file.txt', (err) => { if (err) throw err; console.log('path/file.txt was deleted'); }); fs.unlink() will not work on a directory, empty or otherwise. To remove a directory, use fs.rmdir() . See also: unlink(2) . path <string> | <Buffer> | <URL> Synchronous unlink(2) . Returns undefined . filename <string> | <Buffer> | <URL> listener <Function> Optional, a listener previously attached using fs.watchFile() Stop watching for changes on filename . If listener is specified, only that particular listener is removed. Otherwise, all listeners are removed, effectively stopping watching of filename . Calling fs.unwatchFile() with a filename that is not being watched is a no-op, not an error. Using fs.watch() is more efficient than fs.watchFile() and fs.unwatchFile() . fs.watch() should be used instead of fs.watchFile() and fs.unwatchFile() when possible. path <string> | <Buffer> | <URL> atime <number> | <string> | <Date> mtime <number> | <string> | <Date> callback <Function> err <Error> Change the file system timestamps of the object referenced by path . The atime and mtime arguments follow these rules: Values can be either numbers representing Unix epoch time, Date s, or a numeric string like '123456789.0' . If the value can not be converted to a number, or is NaN , Infinity or -Infinity , an Error will be thrown. path <string> | <Buffer> | <URL> atime <integer> mtime <integer> Returns undefined . For detailed information, see the documentation of the asynchronous version of this API: fs.utimes() . fs.unlinkSync(path) fs.unwatchFile(filename[, listener]) fs.utimes(path, atime, mtime, callback) fs.utimesSync(path, atime, mtime) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 374/739 filename <string> | <Buffer> | <URL> options <string> | <Object> persistent <boolean> Indicates whether the process should continue to run as long as files are being watched. Default: true . recursive <boolean> Indicates whether all subdirectories should be watched, or only the current directory. This applies when a directory is specified, and only on supported platforms (See Caveats ). Default: false . encoding <string> Specifies the character encoding to be used for the filename passed to the listener. Default: 'utf8' . listener <Function> | <undefined> Default: undefined eventType <string> filename <string> | <Buffer> Returns: <fs.FSWatcher> Watch for changes on filename , where filename is either a file or a directory. The second argument is optional. If options is provided as a string, it specifies the encoding . Otherwise options should be passed as an object. The listener callback gets two arguments (eventType, filename) . eventType is either 'rename' or 'change' , and filename is the name of the file which triggered the event. On most platforms, 'rename' is emitted whenever a filename appears or disappears in the directory. The listener callback is attached to the 'change' event fired by fs.FSWatcher , but it is not the same thing as the 'change' value of eventType . The fs.watch API is not 100% consistent across platforms, and is unavailable in some situations. The recursive option is only supported on macOS and Windows. This feature depends on the underlying operating system providing a way to be notified of filesystem changes. On Linux systems, this uses inotify(7) . On BSD systems, this uses kqueue(2) . On macOS, this uses kqueue(2) for files and FSEvents for directories. On SunOS systems (including Solaris and SmartOS), this uses event ports . On Windows systems, this feature depends on ReadDirectoryChangesW . On Aix systems, this feature depends on AHAFS , which must be enabled. If the underlying functionality is not available for some reason, then fs.watch will not be able to function. For example, watching files or directories can be unreliable, and in some cases impossible, on network file systems (NFS, SMB, etc), or host file systems when using virtualization software such as Vagrant, Docker, etc. It is still possible to use fs.watchFile() , which uses stat polling, but this method is slower and less reliable. fs.watch(filename[, options][, listener]) Caveats Availability Inodes 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 375/739 On Linux and macOS systems, fs.watch() resolves the path to an inode and watches the inode. If the watched path is deleted and recreated, it is assigned a new inode. The watch will emit an event for the delete but will continue watching the original inode. Events for the new inode will not be emitted. This is expected behavior. AIX files retain the same inode for the lifetime of a file. Saving and closing a watched file on AIX will result in two notifications (one for adding new content, and one for truncation). Providing filename argument in the callback is only supported on Linux, macOS, Windows, and AIX. Even on supported platforms, filename is not always guaranteed to be provided. Therefore, don't assume that filename argument is always provided in the callback, and have some fallback logic if it is null . fs.watch('somedir', (eventType, filename) => { console.log(`event type is: ${eventType}`); if (filename) { console.log(`filename provided: ${filename}`); } else { console.log('filename not provided'); } }); filename <string> | <Buffer> | <URL> options <Object> persistent <boolean> Default: true interval <integer> Default: 5007 listener <Function> current <fs.Stats> previous <fs.Stats> Watch for changes on filename . The callback listener will be called each time the file is accessed. The options argument may be omitted. If provided, it should be an object. The options object may contain a boolean named persistent that indicates whether the process should continue to run as long as files are being watched. The options object may specify an interval property indicating how often the target should be polled in milliseconds. The listener gets two arguments the current stat object and the previous stat object: fs.watchFile('message.text', (curr, prev) => { console.log(`the current mtime is: ${curr.mtime}`); console.log(`the previous mtime was: ${prev.mtime}`); }); These stat objects are instances of fs.Stat . To be notified when the file was modified, not just accessed, it is necessary to compare curr.mtime and prev.mtime . When an fs.watchFile operation results in an ENOENT error, it will invoke the listener once, with all the fields zeroed (or, for dates, the Unix Epoch). In Windows, blksize and blocks fields will be undefined , instead of zero. If the file is created later on, the listener will be called Filename Argument fs.watchFile(filename[, options], listener) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 376/739 again, with the latest stat objects. This is a change in functionality since v0.10. Using fs.watch() is more efficient than fs.watchFile and fs.unwatchFile . fs.watch should be used instead of fs.watchFile and fs.unwatchFile when possible. When a file being watched by fs.watchFile() disappears and reappears, then the previousStat reported in the second callback event (the file's reappearance) will be the same as the previousStat of the first callback event (its disappearance). This happens when: the file is deleted, followed by a restore the file is renamed twice - the second time back to its original name fd <integer> buffer <Buffer> | <TypedArray> | <DataView> offset <integer> length <integer> position <integer> callback <Function> err <Error> bytesWritten <integer> buffer <Buffer> | <TypedArray> | <DataView> Write buffer to the file specified by fd . offset determines the part of the buffer to be written, and length is an integer specifying the number of bytes to write. position refers to the offset from the beginning of the file where this data should be written. If typeof position !== 'number' , the data will be written at the current position. See pwrite(2) . The callback will be given three arguments (err, bytesWritten, buffer) where bytesWritten specifies how many bytes were written from buffer . If this method is invoked as its util.promisify() ed version, it returns a Promise for an Object with bytesWritten and buffer properties. It is unsafe to use fs.write() multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream() is recommended. On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file. fd <integer> string <string> position <integer> encoding <string> Default: 'utf8' callback <Function> err <Error> fs.write(fd, buffer[, offset[, length[, position]]], callback) fs.write(fd, string[, position[, encoding]], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 377/739 written <integer> string <string> Write string to the file specified by fd . If string is not a string, then the value will be coerced to one. position refers to the offset from the beginning of the file where this data should be written. If typeof position !== 'number' the data will be written at the current position. See pwrite(2) . encoding is the expected string encoding. The callback will receive the arguments (err, written, string) where written specifies how many bytes the passed string required to be written. Bytes written is not necessarily the same as string characters written. See Buffer.byteLength . It is unsafe to use fs.write() multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream() is recommended. On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file. On Windows, if the file descriptor is connected to the console (e.g. fd == 1 or stdout ) a string containing non-ASCII characters will not be rendered properly by default, regardless of the encoding used. It is possible to configure the console to render UTF-8 properly by changing the active codepage with the chcp 65001 command. See the chcp docs for more details. file <string> | <Buffer> | <URL> | <integer> filename or file descriptor data <string> | <Buffer> | <TypedArray> | <DataView> options <Object> | <string> encoding <string> | <null> Default: 'utf8' mode <integer> Default: 0o666 flag <string> See support of file system flags . Default: 'w' . callback <Function> err <Error> Asynchronously writes data to a file, replacing the file if it already exists. data can be a string or a buffer. The encoding option is ignored if data is a buffer. const data = new Uint8Array(Buffer.from('Hello Node.js')); fs.writeFile('message.txt', data, (err) => { if (err) throw err; console.log('The file has been saved!'); }); If options is a string, then it specifies the encoding: fs.writeFile('message.txt', 'Hello Node.js', 'utf8', callback); It is unsafe to use fs.writeFile() multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream() is recommended. fs.writeFile(file, data[, options], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 378/739 1. Any specified file descriptor has to support writing. 2. If a file descriptor is specified as the file , it will not be closed automatically. 3. The writing will begin at the beginning of the file. For example, if the file already had 'Hello World' and the newly written content is 'Aloha' , then the contents of the file would be 'Aloha World' , rather than just 'Aloha' . file <string> | <Buffer> | <URL> | <integer> filename or file descriptor data <string> | <Buffer> | <TypedArray> | <DataView> options <Object> | <string> encoding <string> | <null> Default: 'utf8' mode <integer> Default: 0o666 flag <string> See support of file system flags . Default: 'w' . Returns undefined . For detailed information, see the documentation of the asynchronous version of this API: fs.writeFile() . fd <integer> buffer <Buffer> | <TypedArray> | <DataView> offset <integer> length <integer> position <integer> Returns: <number> The number of bytes written. For detailed information, see the documentation of the asynchronous version of this API: fs.write(fd, buffer...) . fd <integer> string <string> position <integer> encoding <string> Returns: <number> The number of bytes written. For detailed information, see the documentation of the asynchronous version of this API: fs.write(fd, string...) . Stability: 1 - Experimental The fs.promises API provides an alternative set of asynchronous file system methods that return Promise objects rather than using callbacks. The API is accessible via require('fs').promises . File Descriptors fs.writeFileSync(file, data[, options]) fs.writeSync(fd, buffer[, offset[, length[, position]]]) fs.writeSync(fd, string[, position[, encoding]]) fs Promises API 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 379/739 A FileHandle object is a wrapper for a numeric file descriptor. Instances of FileHandle are distinct from numeric file descriptors in that, if the FileHandle is not explicitly closed using the filehandle.close() method, they will automatically close the file descriptor and will emit a process warning, thereby helping to prevent memory leaks. Instances of the FileHandle object are created internally by the fsPromises.open() method. Unlike the callback-based API ( fs.fstat() , fs.fchown() , fs.fchmod() , and so on), a numeric file descriptor is not used by the promise- based API. Instead, the promise-based API uses the FileHandle class in order to help avoid accidental leaking of unclosed file descriptors after a Promise is resolved or rejected. data <string> | <Buffer> options <Object> | <string> encoding <string> | <null> Default: 'utf8' mode <integer> Default: 0o666 flag <string> See support of file system flags . Default: 'a' . Returns: <Promise> Asynchronously append data to this file, creating the file if it does not yet exist. data can be a string or a Buffer . The Promise will be resolved with no arguments upon success. If options is a string, then it specifies the encoding. The FileHandle must have been opened for appending. mode <integer> Returns: <Promise> Modifies the permissions on the file. The Promise is resolved with no arguments upon success. uid <integer> gid <integer> Returns: <Promise> Changes the ownership of the file then resolves the Promise with no arguments upon success. Returns: <Promise> A Promise that will be resolved once the underlying file descriptor is closed, or will be rejected if an error occurs while closing. Closes the file descriptor. const fsPromises = require('fs').promises; async function openAndClose() { let filehandle; try { filehandle = await fsPromises.open('thefile.txt', 'r'); class: FileHandle filehandle.appendFile(data, options) filehandle.chmod(mode) filehandle.chown(uid, gid) filehandle.close() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 380/739 } finally { if (filehandle !== undefined) await filehandle.close(); } } Returns: <Promise> Asynchronous fdatasync(2) . The Promise is resolved with no arguments upon success. <number> The numeric file descriptor managed by the FileHandle object. buffer <Buffer> | <Uint8Array> offset <integer> length <integer> position <integer> Returns: <Promise> Read data from the file. buffer is the buffer that the data will be written to. offset is the offset in the buffer to start writing at. length is an integer specifying the number of bytes to read. position is an argument specifying where to begin reading from in the file. If position is null , data will be read from the current file position, and the file position will be updated. If position is an integer, the file position will remain unchanged. Following successful read, the Promise is resolved with an object with a bytesRead property specifying the number of bytes read, and a buffer property that is a reference to the passed in buffer argument. options <Object> | <string> encoding <string> | <null> Default: null flag <string> See support of file system flags . Default: 'r' . Returns: <Promise> Asynchronously reads the entire contents of a file. The Promise is resolved with the contents of the file. If no encoding is specified (using options.encoding ), the data is returned as a Buffer object. Otherwise, the data will be a string. If options is a string, then it specifies the encoding. When the path is a directory, the behavior of fsPromises.readFile() is platform-specific. On macOS, Linux, and Windows, the promise will be rejected with an error. On FreeBSD, a representation of the directory's contents will be returned. filehandle.datasync() filehandle.fd filehandle.read(buffer, offset, length, position) filehandle.readFile(options) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 381/739 The FileHandle has to support reading. If one or more filehandle.read() calls are made on a file handle and then a filehandle.readFile() call is made, the data will be read from the current position till the end of the file. It doesn't always read from the beginning of the file. options <Object> bigint <boolean> Whether the numeric values in the returned fs.Stats object should be bigint . Default: false . Returns: <Promise> Retrieves the fs.Stats for the file. Returns: <Promise> Asynchronous fsync(2) . The Promise is resolved with no arguments upon success. len <integer> Default: 0 Returns: <Promise> Truncates the file then resolves the Promise with no arguments upon success. If the file was larger than len bytes, only the first len bytes will be retained in the file. For example, the following program retains only the first four bytes of the file: const fs = require('fs'); const fsPromises = fs.promises; console.log(fs.readFileSync('temp.txt', 'utf8')); // Prints: Node.js async function doTruncate() { let filehandle = null; try { filehandle = await fsPromises.open('temp.txt', 'r+'); await filehandle.truncate(4); } finally { if (filehandle) { // close the file if it is opened. await filehandle.close(); } } console.log(fs.readFileSync('temp.txt', 'utf8')); // Prints: Node } doTruncate().catch(console.error); filehandle.stat([options]) filehandle.sync() filehandle.truncate(len) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 382/739 If the file previously was shorter than len bytes, it is extended, and the extended part is filled with null bytes ( '\0' ): const fs = require('fs'); const fsPromises = fs.promises; console.log(fs.readFileSync('temp.txt', 'utf8')); // Prints: Node.js async function doTruncate() { let filehandle = null; try { filehandle = await fsPromises.open('temp.txt', 'r+'); await filehandle.truncate(10); } finally { if (filehandle) { // close the file if it is opened. await filehandle.close(); } } console.log(fs.readFileSync('temp.txt', 'utf8')); // Prints Node.js\0\0\0 } doTruncate().catch(console.error); The last three bytes are null bytes ( '\0' ), to compensate the over-truncation. atime <number> | <string> | <Date> mtime <number> | <string> | <Date> Returns: <Promise> Change the file system timestamps of the object referenced by the FileHandle then resolves the Promise with no arguments upon success. This function does not work on AIX versions before 7.1, it will resolve the Promise with an error using code UV_ENOSYS . buffer <Buffer> | <Uint8Array> offset <integer> length <integer> position <integer> Returns: <Promise> Write buffer to the file. The Promise is resolved with an object containing a bytesWritten property identifying the number of bytes written, and a buffer property containing a reference to the buffer written. offset determines the part of the buffer to be written, and length is an integer specifying the number of bytes to write. position refers to the offset from the beginning of the file where this data should be written. If typeof position !== 'number' , the data will be written at the current position. See pwrite(2) . filehandle.utimes(atime, mtime) filehandle.write(buffer, offset, length, position) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 383/739 It is unsafe to use filehandle.write() multiple times on the same file without waiting for the Promise to be resolved (or rejected). For this scenario, fs.createWriteStream() is strongly recommended. On Linux, positional writes do not work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file. string <string> position <integer> encoding <string> Default: 'utf8' Returns: <Promise> Write string to the file. If string is not a string, then the value will be coerced to one. The Promise is resolved with an object containing a bytesWritten property identifying the number of bytes written, and a buffer property containing a reference to the string written. position refers to the offset from the beginning of the file where this data should be written. If the type of position is not a number the data will be written at the current position. See pwrite(2) . encoding is the expected string encoding. It is unsafe to use filehandle.write() multiple times on the same file without waiting for the Promise to be resolved (or rejected). For this scenario, fs.createWriteStream() is strongly recommended. On Linux, positional writes do not work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file. data <string> | <Buffer> | <Uint8Array> options <Object> | <string> encoding <string> | <null> Default: 'utf8' mode <integer> Default: 0o666 flag <string> See support of file system flags . Default: 'w' . Returns: <Promise> Asynchronously writes data to a file, replacing the file if it already exists. data can be a string or a buffer. The Promise will be resolved with no arguments upon success. The encoding option is ignored if data is a buffer. If options is a string, then it specifies the encoding. The FileHandle has to support writing. It is unsafe to use filehandle.writeFile() multiple times on the same file without waiting for the Promise to be resolved (or rejected). If one or more filehandle.write() calls are made on a file handle and then a filehandle.writeFile() call is made, the data will be written from the current position till the end of the file. It doesn't always write from the beginning of the file. path <string> | <Buffer> | <URL> filehandle.write(string[, position[, encoding]]) filehandle.writeFile(data, options) fsPromises.access(path[, mode]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 384/739 mode <integer> Default: fs.constants.F_OK Returns: <Promise> Tests a user's permissions for the file or directory specified by path . The mode argument is an optional integer that specifies the accessibility checks to be performed. Check File Access Constants for possible values of mode . It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.W_OK | fs.constants.R_OK ). If the accessibility check is successful, the Promise is resolved with no value. If any of the accessibility checks fail, the Promise is rejected with an Error object. The following example checks if the file /etc/passwd can be read and written by the current process. const fs = require('fs'); const fsPromises = fs.promises; fsPromises.access('/etc/passwd', fs.constants.R_OK | fs.constants.W_OK) .then(() => console.log('can access')) .catch(() => console.error('cannot access')); Using fsPromises.access() to check for the accessibility of a file before calling fsPromises.open() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file is not accessible. path <string> | <Buffer> | <URL> | <FileHandle> filename or FileHandle data <string> | <Buffer> options <Object> | <string> encoding <string> | <null> Default: 'utf8' mode <integer> Default: 0o666 flag <string> See support of file system flags . Default: 'a' . Returns: <Promise> Asynchronously append data to a file, creating the file if it does not yet exist. data can be a string or a Buffer . The Promise will be resolved with no arguments upon success. If options is a string, then it specifies the encoding. The path may be specified as a FileHandle that has been opened for appending (using fsPromises.open() ). path <string> | <Buffer> | <URL> mode <integer> Returns: <Promise> Changes the permissions of a file then resolves the Promise with no arguments upon succces. path <string> | <Buffer> | <URL> uid <integer> gid <integer> fsPromises.appendFile(path, data[, options]) fsPromises.chmod(path, mode) fsPromises.chown(path, uid, gid) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 385/739 Returns: <Promise> Changes the ownership of a file then resolves the Promise with no arguments upon success. src <string> | <Buffer> | <URL> source filename to copy dest <string> | <Buffer> | <URL> destination filename of the copy operation flags <number> modifiers for copy operation. Default: 0 . Returns: <Promise> Asynchronously copies src to dest . By default, dest is overwritten if it already exists. The Promise will be resolved with no arguments upon success. Node.js makes no guarantees about the atomicity of the copy operation. If an error occurs after the destination file has been opened for writing, Node.js will attempt to remove the destination. flags is an optional integer that specifies the behavior of the copy operation. It is possible to create a mask consisting of the bitwise OR of two or more values (e.g. fs.constants.COPYFILE_EXCL | fs.constants.COPYFILE_FICLONE ). fs.constants.COPYFILE_EXCL - The copy operation will fail if dest already exists. fs.constants.COPYFILE_FICLONE - The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then a fallback copy mechanism is used. fs.constants.COPYFILE_FICLONE_FORCE - The copy operation will attempt to create a copy-on-write reflink. If the platform does not support copy-on-write, then the operation will fail. const fsPromises = require('fs').promises; // destination.txt will be created or overwritten by default. fsPromises.copyFile('source.txt', 'destination.txt') .then(() => console.log('source.txt was copied to destination.txt')) .catch(() => console.log('The file could not be copied')); If the third argument is a number, then it specifies flags : const fs = require('fs'); const fsPromises = fs.promises; const { COPYFILE_EXCL } = fs.constants; // By using COPYFILE_EXCL, the operation will fail if destination.txt exists. fsPromises.copyFile('source.txt', 'destination.txt', COPYFILE_EXCL) .then(() => console.log('source.txt was copied to destination.txt')) .catch(() => console.log('The file could not be copied')); path <string> | <Buffer> | <URL> mode <integer> Returns: <Promise> Changes the permissions on a symbolic link then resolves the Promise with no arguments upon success. This method is only implemented on macOS. fsPromises.copyFile(src, dest[, flags]) fsPromises.lchmod(path, mode) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 386/739 path <string> | <Buffer> | <URL> uid <integer> gid <integer> Returns: <Promise> Changes the ownership on a symbolic link then resolves the Promise with no arguments upon success. existingPath <string> | <Buffer> | <URL> newPath <string> | <Buffer> | <URL> Returns: <Promise> Asynchronous link(2) . The Promise is resolved with no arguments upon success. path <string> | <Buffer> | <URL> options <Object> bigint <boolean> Whether the numeric values in the returned fs.Stats object should be bigint . Default: false . Returns: <Promise> Asynchronous lstat(2) . The Promise is resolved with the fs.Stats object for the given symbolic link path . path <string> | <Buffer> | <URL> options <Object> | <integer> recursive <boolean> Default: false mode <integer> Not supported on Windows. Default: 0o777 . Returns: <Promise> Asynchronously creates a directory then resolves the Promise with no arguments upon success. The optional options argument can be an integer specifying mode (permission and sticky bits), or an object with a mode property and a recursive property indicating whether parent folders should be created. prefix <string> options <string> | <Object> encoding <string> Default: 'utf8' Returns: <Promise> Creates a unique temporary directory and resolves the Promise with the created folder path. A unique directory name is generated by appending six random characters to the end of the provided prefix . The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use. fsPromises.lchown(path, uid, gid) fsPromises.link(existingPath, newPath) fsPromises.lstat(path[, options]) fsPromises.mkdir(path[, options]) fsPromises.mkdtemp(prefix[, options]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 387/739 fsPromises.mkdtemp(path.join(os.tmpdir(), 'foo-')) .catch(console.error); The fsPromises.mkdtemp() method will append the six randomly selected characters directly to the prefix string. For instance, given a directory /tmp , if the intention is to create a temporary directory within /tmp , the prefix must end with a trailing platform-specific path separator ( require('path').sep ). path <string> | <Buffer> | <URL> flags <string> | <number> See support of file system flags . Default: 'r' . mode <integer> Default: 0o666 (readable and writable) Returns: <Promise> Asynchronous file open that returns a Promise that, when resolved, yields a FileHandle object. See open(2) . mode sets the file mode (permission and sticky bits), but only if the file was created. Some characters ( < > : " / \ | ? * ) are reserved under Windows as documented by Naming Files, Paths, and Namespaces . Under NTFS, if the filename contains a colon, Node.js will open a file system stream, as described by this MSDN page . path <string> | <Buffer> | <URL> options <string> | <Object> encoding <string> Default: 'utf8' withFileTypes <boolean> Default: false Returns: <Promise> Reads the contents of a directory then resolves the Promise with an array of the names of the files in the directory excluding '.' and '..' . The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the filenames. If the encoding is set to 'buffer' , the filenames returned will be passed as Buffer objects. If options.withFileTypes is set to true , the resolved array will contain fs.Dirent objects. path <string> | <Buffer> | <URL> | <FileHandle> filename or FileHandle options <Object> | <string> encoding <string> | <null> Default: null flag <string> See support of file system flags . Default: 'r' . Returns: <Promise> Asynchronously reads the entire contents of a file. The Promise is resolved with the contents of the file. If no encoding is specified (using options.encoding ), the data is returned as a Buffer object. Otherwise, the data will be a string. If options is a string, then it specifies the encoding. fsPromises.open(path, flags[, mode]) fsPromises.readdir(path[, options]) fsPromises.readFile(path[, options]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 388/739 When the path is a directory, the behavior of fsPromises.readFile() is platform-specific. On macOS, Linux, and Windows, the promise will be rejected with an error. On FreeBSD, a representation of the directory's contents will be returned. Any specified FileHandle has to support reading. path <string> | <Buffer> | <URL> options <string> | <Object> encoding <string> Default: 'utf8' Returns: <Promise> Asynchronous readlink(2) . The Promise is resolved with the linkString upon success. The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path returned. If the encoding is set to 'buffer' , the link path returned will be passed as a Buffer object. path <string> | <Buffer> | <URL> options <string> | <Object> encoding <string> Default: 'utf8' Returns: <Promise> Determines the actual location of path using the same semantics as the fs.realpath.native() function then resolves the Promise with the resolved path. Only paths that can be converted to UTF8 strings are supported. The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path. If the encoding is set to 'buffer' , the path returned will be passed as a Buffer object. On Linux, when Node.js is linked against musl libc, the procfs file system must be mounted on /proc in order for this function to work. Glibc does not have this restriction. oldPath <string> | <Buffer> | <URL> newPath <string> | <Buffer> | <URL> Returns: <Promise> Renames oldPath to newPath and resolves the Promise with no arguments upon success. path <string> | <Buffer> | <URL> Returns: <Promise> Removes the directory identified by path then resolves the Promise with no arguments upon success. Using fsPromises.rmdir() on a file (not a directory) results in the Promise being rejected with an ENOENT error on Windows and an ENOTDIR error on POSIX. fsPromises.readlink(path[, options]) fsPromises.realpath(path[, options]) fsPromises.rename(oldPath, newPath) fsPromises.rmdir(path) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 389/739 path <string> | <Buffer> | <URL> options <Object> bigint <boolean> Whether the numeric values in the returned fs.Stats object should be bigint . Default: false . Returns: <Promise> The Promise is resolved with the fs.Stats object for the given path . target <string> | <Buffer> | <URL> path <string> | <Buffer> | <URL> type <string> Default: 'file' Returns: <Promise> Creates a symbolic link then resolves the Promise with no arguments upon success. The type argument is only used on Windows platforms and can be one of 'dir' , 'file' , or 'junction' . Windows junction points require the destination path to be absolute. When using 'junction' , the target argument will automatically be normalized to absolute path. path <string> | <Buffer> | <URL> len <integer> Default: 0 Returns: <Promise> Truncates the path then resolves the Promise with no arguments upon success. The path must be a string or Buffer . path <string> | <Buffer> | <URL> Returns: <Promise> Asynchronous unlink(2) . The Promise is resolved with no arguments upon success. path <string> | <Buffer> | <URL> atime <number> | <string> | <Date> mtime <number> | <string> | <Date> Returns: <Promise> Change the file system timestamps of the object referenced by path then resolves the Promise with no arguments upon success. The atime and mtime arguments follow these rules: Values can be either numbers representing Unix epoch time, Date s, or a numeric string like '123456789.0' . If the value can not be converted to a number, or is NaN , Infinity or -Infinity , an Error will be thrown. file <string> | <Buffer> | <URL> | <FileHandle> filename or FileHandle fsPromises.stat(path[, options]) fsPromises.symlink(target, path[, type]) fsPromises.truncate(path[, len]) fsPromises.unlink(path) fsPromises.utimes(path, atime, mtime) fsPromises.writeFile(file, data[, options]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 390/739 data <string> | <Buffer> | <Uint8Array> options <Object> | <string> encoding <string> | <null> Default: 'utf8' mode <integer> Default: 0o666 flag <string> See support of file system flags . Default: 'w' . Returns: <Promise> Asynchronously writes data to a file, replacing the file if it already exists. data can be a string or a buffer. The Promise will be resolved with no arguments upon success. The encoding option is ignored if data is a buffer. If options is a string, then it specifies the encoding. Any specified FileHandle has to support writing. It is unsafe to use fsPromises.writeFile() multiple times on the same file without waiting for the Promise to be resolved (or rejected). The following constants are exported by fs.constants . Not every constant will be available on every operating system. The following constants are meant for use with fs.access() . Constant Description F_OK Flag indicating that the file is visible to the calling process. This is useful for determining if a file exists, but says nothing about rwx permissions. Default if no mode is specified. R_OK Flag indicating that the file can be read by the calling process. W_OK Flag indicating that the file can be written by the calling process. X_OK Flag indicating that the file can be executed by the calling process. This has no effect on Windows (will behave like fs.constants.F_OK ). The following constants are meant for use with fs.copyFile() . Constant Description COPYFILE_EXCL If present, the copy operation will fail with an error if the destination path already exists. COPYFILE_FICLONE If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then a fallback copy mechanism is used. COPYFILE_FICLONE_FORCE If present, the copy operation will attempt to create a copy-on-write reflink. If the underlying platform does not support copy-on-write, then the operation will fail with an error. FS Constants File Access Constants File Copy Constants 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 391/739 The following constants are meant for use with fs.open() . Constant Description O_RDONLY Flag indicating to open a file for read-only access. O_WRONLY Flag indicating to open a file for write-only access. O_RDWR Flag indicating to open a file for read-write access. O_CREAT Flag indicating to create the file if it does not already exist. O_EXCL Flag indicating that opening a file should fail if the O_CREAT flag is set and the file already exists. O_NOCTTY Flag indicating that if path identifies a terminal device, opening the path shall not cause that terminal to become the controlling terminal for the process (if the process does not already have one). O_TRUNC Flag indicating that if the file exists and is a regular file, and the file is opened successfully for write access, its length shall be truncated to zero. O_APPEND Flag indicating that data will be appended to the end of the file. O_DIRECTORY Flag indicating that the open should fail if the path is not a directory. O_NOATIME Flag indicating reading accesses to the file system will no longer result in an update to the atime information associated with the file. This flag is available on Linux operating systems only. O_NOFOLLOW Flag indicating that the open should fail if the path is a symbolic link. O_SYNC Flag indicating that the file is opened for synchronized I/O with write operations waiting for file integrity. O_DSYNC Flag indicating that the file is opened for synchronized I/O with write operations waiting for data integrity. O_SYMLINK Flag indicating to open the symbolic link itself rather than the resource it is pointing to. O_DIRECT When set, an attempt will be made to minimize caching effects of file I/O. O_NONBLOCK Flag indicating to open the file in nonblocking mode when possible. The following constants are meant for use with the fs.Stats object's mode property for determining a file's type. Constant Description S_IFMT Bit mask used to extract the file type code. S_IFREG File type constant for a regular file. S_IFDIR File type constant for a directory. File Open Constants File Type Constants 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 392/739 S_IFCHR File type constant for a character-oriented device file. S_IFBLK File type constant for a block-oriented device file. S_IFIFO File type constant for a FIFO/pipe. S_IFLNK File type constant for a symbolic link. S_IFSOCK File type constant for a socket. The following constants are meant for use with the fs.Stats object's mode property for determining the access permissions for a file. Constant Description S_IRWXU File mode indicating readable, writable, and executable by owner. S_IRUSR File mode indicating readable by owner. S_IWUSR File mode indicating writable by owner. S_IXUSR File mode indicating executable by owner. S_IRWXG File mode indicating readable, writable, and executable by group. S_IRGRP File mode indicating readable by group. S_IWGRP File mode indicating writable by group. S_IXGRP File mode indicating executable by group. S_IRWXO File mode indicating readable, writable, and executable by others. S_IROTH File mode indicating readable by others. S_IWOTH File mode indicating writable by others. S_IXOTH File mode indicating executable by others. The following flags are available wherever the flag option takes a string: 'a' - Open file for appending. The file is created if it does not exist. 'ax' - Like 'a' but fails if the path exists. 'a+' - Open file for reading and appending. The file is created if it does not exist. 'ax+' - Like 'a+' but fails if the path exists. 'as' - Open file for appending in synchronous mode. The file is created if it does not exist. File Mode Constants File System Flags 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 393/739 'as+' - Open file for reading and appending in synchronous mode. The file is created if it does not exist. 'r' - Open file for reading. An exception occurs if the file does not exist. 'r+' - Open file for reading and writing. An exception occurs if the file does not exist. 'rs+' - Open file for reading and writing in synchronous mode. Instructs the operating system to bypass the local file system cache. This is primarily useful for opening files on NFS mounts as it allows skipping the potentially stale local cache. It has a very real impact on I/O performance so using this flag is not recommended unless it is needed. This doesn't turn fs.open() or fsPromises.open() into a synchronous blocking call. If synchronous operation is desired, something like fs.openSync() should be used. 'w' - Open file for writing. The file is created (if it does not exist) or truncated (if it exists). 'wx' - Like 'w' but fails if the path exists. 'w+' - Open file for reading and writing. The file is created (if it does not exist) or truncated (if it exists). 'wx+' - Like 'w+' but fails if the path exists. flag can also be a number as documented by open(2) ; commonly used constants are available from fs.constants . On Windows, flags are translated to their equivalent ones where applicable, e.g. O_WRONLY to FILE_GENERIC_WRITE , or O_EXCL|O_CREAT to CREATE_NEW , as accepted by CreateFileW . The exclusive flag 'x' ( O_EXCL flag in open(2) ) ensures that path is newly created. On POSIX systems, path is considered to exist even if it is a symlink to a non-existent file. The exclusive flag may or may not work with network file systems. On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file. Modifying a file rather than replacing it may require a flags mode of 'r+' rather than the default mode 'w' . The behavior of some flags are platform-specific. As such, opening a directory on macOS and Linux with the 'a+' flag - see example below - will return an error. In contrast, on Windows and FreeBSD, a file descriptor or a FileHandle will be returned. // macOS and Linux fs.open('<directory>', 'a+', (err, fd) => { // => [Error: EISDIR: illegal operation on a directory, open <directory>] }); // Windows and FreeBSD fs.open('<directory>', 'a+', (err, fd) => { // => null, <fd> }); On Windows, opening an existing hidden file using the 'w' flag (either through fs.open() or fs.writeFile() or fsPromises.open() ) will fail with EPERM . Existing hidden files can be opened for writing with the 'r+' flag. A call to fs.ftruncate() or filehandle.truncate() can be used to reset the file contents. These objects are available in all modules. The following variables may appear to be global but are not. They exist only in the scope of modules, see the module system documentation : Global Objects 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 394/739 __dirname __filename exports module require() The objects listed here are specific to Node.js. There are a number of built-in objects that are part of the JavaScript language itself, which are also globally accessible. <Function> Used to handle binary data. See the buffer section . This variable may appear to be global but is not. See __dirname . This variable may appear to be global but is not. See __filename . clearImmediate is described in the timers section. clearInterval is described in the timers section. clearTimeout is described in the timers section. <Object> Used to print to stdout and stderr. See the console section. This variable may appear to be global but is not. See exports . <Object> The global namespace object. In browsers, the top-level scope is the global scope. This means that within the browser var something will define a new global variable. In Node.js this is different. The top-level scope is not the global scope; var something inside a Node.js module will be local to that module. Class: Buffer __dirname __filename clearImmediate(immediateObject) clearInterval(intervalObject) clearTimeout(timeoutObject) console exports global module 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 395/739 This variable may appear to be global but is not. See module . <Object> The process object. See the process object section. This variable may appear to be global but is not. See require() . setImmediate is described in the timers section. setInterval is described in the timers section. setTimeout is described in the timers section. The WHATWG URL class. See the URL section. The WHATWG URLSearchParams class. See the URLSearchParams section. <Object> The object that acts as the namespace for all W3C WebAssembly related functionality. See the Mozilla Developer Network for usage and compatibility. Stability: 2 - Stable To use the HTTP server and client one must require('http') . The HTTP interfaces in Node.js are designed to support many features of the protocol which have been traditionally difficult to use. In particular, large, possibly chunk-encoded, messages. The interface is careful to never buffer entire requests or responses — the user is able to stream data. HTTP message headers are represented by an object like this: process require() setImmediate(callback[, ...args]) setInterval(callback, delay[, ...args]) setTimeout(callback, delay[, ...args]) URL URLSearchParams WebAssembly HTTP 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 396/739 { 'content-length': '123', 'content-type': 'text/plain', 'connection': 'keep-alive', 'host': 'mysite.com', 'accept': '*/*' } Keys are lowercased. Values are not modified. In order to support the full spectrum of possible HTTP applications, Node.js's HTTP API is very low-level. It deals with stream handling and message parsing only. It parses a message into headers and body but it does not parse the actual headers or the body. See message.headers for details on how duplicate headers are handled. The raw headers as they were received are retained in the rawHeaders property, which is an array of [key, value, key2, value2, ...] . For example, the previous message header object might have a rawHeaders list like the following: [ 'ConTent-Length', '123456', 'content-LENGTH', '123', 'content-type', 'text/plain', 'CONNECTION', 'keep-alive', 'Host', 'mysite.com', 'accepT', '*/*' ] An Agent is responsible for managing connection persistence and reuse for HTTP clients. It maintains a queue of pending requests for a given host and port, reusing a single socket connection for each until the queue is empty, at which time the socket is either destroyed or put into a pool where it is kept to be used again for requests to the same host and port. Whether it is destroyed or pooled depends on the keepAlive option . Pooled connections have TCP Keep-Alive enabled for them, but servers may still close idle connections, in which case they will be removed from the pool and a new connection will be made when a new HTTP request is made for that host and port. Servers may also refuse to allow multiple requests over the same connection, in which case the connection will have to be remade for every request and cannot be pooled. The Agent will still make the requests to that server, but each one will occur over a new connection. When a connection is closed by the client or the server, it is removed from the pool. Any unused sockets in the pool will be unrefed so as not to keep the Node.js process running when there are no outstanding requests. (see socket.unref() ). It is good practice, to destroy() an Agent instance when it is no longer in use, because unused sockets consume OS resources. Sockets are removed from an agent when the socket emits either a 'close' event or an 'agentRemove' event. When intending to keep one HTTP request open for a long time without keeping it in the agent, something like the following may be done: http.get(options, (res) => { // Do stuff }).on('socket', (socket) => { socket.emit('agentRemove'); }); An agent may also be used for an individual request. By providing {agent: false} as an option to the http.get() or http.request() functions, a one-time use Agent with default options will be used for the client connection. Class: http.Agent 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 397/739 agent:false : http.get({ hostname: 'localhost', port: 80, path: '/', agent: false // create a new agent just for this one request }, (res) => { // Do stuff with response }); options <Object> Set of configurable options to set on the agent. Can have the following fields: keepAlive <boolean> Keep sockets around even when there are no outstanding requests, so they can be used for future requests without having to reestablish a TCP connection. Not to be confused with the keep-alive value of the Connection header. The Connection: keep-alive header is always sent when using an agent except when the Connection header is explicitly specified or when the keepAlive and maxSockets options are respectively set to false and Infinity , in which case Connection: close will be used. Default: false . keepAliveMsecs <number> When using the keepAlive option, specifies the initial delay for TCP Keep-Alive packets. Ignored when the keepAlive option is false or undefined . Default: 1000 . maxSockets <number> Maximum number of sockets to allow per host. Each request will use a new socket until the maximum is reached. Default: Infinity . maxFreeSockets <number> Maximum number of sockets to leave open in a free state. Only relevant if keepAlive is set to true . Default: 256 . timeout <number> Socket timeout in milliseconds. This will set the timeout when the socket is created. options in socket.connect() are also supported. The default http.globalAgent that is used by http.request() has all of these values set to their respective defaults. To configure any of them, a custom http.Agent instance must be created. const http = require('http'); const keepAliveAgent = new http.Agent({ keepAlive: true }); options.agent = keepAliveAgent; http.request(options, onResponseCallback); options <Object> Options containing connection details. Check net.createConnection() for the format of the options callback <Function> Callback function that receives the created socket Returns: <net.Socket> Produces a socket/stream to be used for HTTP requests. By default, this function is the same as net.createConnection() . However, custom agents may override this method in case greater flexibility is desired. new Agent([options]) agent.createConnection(options[, callback]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 398/739 A socket/stream can be supplied in one of two ways: by returning the socket/stream from this function, or by passing the socket/stream to callback . callback has a signature of (err, stream) . socket <net.Socket> Called when socket is detached from a request and could be persisted by the Agent . Default behavior is to: socket.setKeepAlive(true, this.keepAliveMsecs); socket.unref(); return true; This method can be overridden by a particular Agent subclass. If this method returns a falsy value, the socket will be destroyed instead of persisting it for use with the next request. socket <net.Socket> request <http.ClientRequest> Called when socket is attached to request after being persisted because of the keep-alive options. Default behavior is to: socket.ref(); This method can be overridden by a particular Agent subclass. Destroy any sockets that are currently in use by the agent. It is usually not necessary to do this. However, if using an agent with keepAlive enabled, then it is best to explicitly shut down the agent when it will no longer be used. Otherwise, sockets may hang open for quite a long time before the server terminates them. <Object> An object which contains arrays of sockets currently awaiting use by the agent when keepAlive is enabled. Do not modify. options <Object> A set of options providing information for name generation host <string> A domain name or IP address of the server to issue the request to port <number> Port of remote server localAddress <string> Local interface to bind for network connections when issuing the request family <integer> Must be 4 or 6 if this doesn't equal undefined . Returns: <string> Get a unique name for a set of request options, to determine whether a connection can be reused. For an HTTP agent, this returns host:port:localAddress or host:port:localAddress:family . For an HTTPS agent, the name includes the CA, cert, ciphers, and other agent.keepSocketAlive(socket) agent.reuseSocket(socket, request) agent.destroy() agent.freeSockets agent.getName(options) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 399/739 HTTPS/TLS-specific options that determine socket reusability. <number> By default set to 256. For agents with keepAlive enabled, this sets the maximum number of sockets that will be left open in the free state. <number> By default set to Infinity . Determines how many concurrent sockets the agent can have open per origin. Origin is the returned value of agent.getName() . <Object> An object which contains queues of requests that have not yet been assigned to sockets. Do not modify. <Object> An object which contains arrays of sockets currently in use by the agent. Do not modify. This object is created internally and returned from http.request() . It represents an in-progress request whose header has already been queued. The header is still mutable using the setHeader(name, value) , getHeader(name) , removeHeader(name) API. The actual header will be sent along with the first data chunk or when calling request.end() . To get the response, add a listener for 'response' to the request object. 'response' will be emitted from the request object when the response headers have been received. The 'response' event is executed with one argument which is an instance of http.IncomingMessage . During the 'response' event, one can add listeners to the response object; particularly to listen for the 'data' event. If no 'response' handler is added, then the response will be entirely discarded. However, if a 'response' event handler is added, then the data from the response object must be consumed, either by calling response.read() whenever there is a 'readable' event, or by adding a 'data' handler, or by calling the .resume() method. Until the data is consumed, the 'end' event will not fire. Also, until the data is read it will consume memory that can eventually lead to a 'process out of memory' error. Node.js does not check whether Content-Length and the length of the body which has been transmitted are equal or not. The request inherits from Stream , and additionally implements the following: Emitted when the request has been aborted by the client. This event is only emitted on the first call to abort() . response <http.IncomingMessage> socket <net.Socket> head <Buffer> agent.maxFreeSockets agent.maxSockets agent.requests agent.sockets Class: http.ClientRequest Event: 'abort' Event: 'connect' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 400/739 Emitted each time a server responds to a request with a CONNECT method. If this event is not being listened for, clients receiving a CONNECT method will have their connections closed. A client and server pair demonstrating how to listen for the 'connect' event: const http = require('http'); const net = require('net'); const url = require('url'); // Create an HTTP tunneling proxy const proxy = http.createServer((req, res) => { res.writeHead(200, { 'Content-Type': 'text/plain' }); res.end('okay'); }); proxy.on('connect', (req, cltSocket, head) => { // connect to an origin server const srvUrl = url.parse(`http://${req.url}`); const srvSocket = net.connect(srvUrl.port, srvUrl.hostname, () => { cltSocket.write('HTTP/1.1 200 Connection Established\r\n' + 'Proxy-agent: Node.js-Proxy\r\n' + '\r\n'); srvSocket.write(head); srvSocket.pipe(cltSocket); cltSocket.pipe(srvSocket); }); }); // now that proxy is running proxy.listen(1337, '127.0.0.1', () => { // make a request to a tunneling proxy const options = { port: 1337, host: '127.0.0.1', method: 'CONNECT', path: 'www.google.com:80' }; const req = http.request(options); req.end(); req.on('connect', (res, socket, head) => { console.log('got connected!'); // make a request over an HTTP tunnel socket.write('GET / HTTP/1.1\r\n' + 'Host: www.google.com:80\r\n' + 'Connection: close\r\n' + '\r\n'); socket.on('data', (chunk) => { console.log(chunk.toString()); }); socket.on('end', () => { proxy.close(); 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 401/739 proxy.close(); }); }); }); Emitted when the server sends a '100 Continue' HTTP response, usually because the request contained 'Expect: 100-continue'. This is an instruction that the client should send the request body. Emitted when the server sends a 1xx response (excluding 101 Upgrade). This event is emitted with a callback containing an object with a status code. const http = require('http'); const options = { host: '127.0.0.1', port: 8080, path: '/length_request' }; // Make a request const req = http.request(options); req.end(); req.on('information', (res) => { console.log(`Got information prior to main response: ${res.statusCode}`); }); 101 Upgrade statuses do not fire this event due to their break from the traditional HTTP request/response chain, such as web sockets, in-place TLS upgrades, or HTTP 2.0. To be notified of 101 Upgrade notices, listen for the 'upgrade' event instead. response <http.IncomingMessage> Emitted when a response is received to this request. This event is emitted only once. socket <net.Socket> Emitted after a socket is assigned to this request. Emitted when the underlying socket times out from inactivity. This only notifies that the socket has been idle. The request must be aborted manually. See also: request.setTimeout() . Event: 'continue' Event: 'information' Event: 'response' Event: 'socket' Event: 'timeout' Event: 'upgrade' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 402/739 response <http.IncomingMessage> socket <net.Socket> head <Buffer> Emitted each time a server responds to a request with an upgrade. If this event is not being listened for and the response status code is 101 Switching Protocols, clients receiving an upgrade header will have their connections closed. A client server pair demonstrating how to listen for the 'upgrade' event. const http = require('http'); // Create an HTTP server const srv = http.createServer((req, res) => { res.writeHead(200, { 'Content-Type': 'text/plain' }); res.end('okay'); }); srv.on('upgrade', (req, socket, head) => { socket.write('HTTP/1.1 101 Web Socket Protocol Handshake\r\n' + 'Upgrade: WebSocket\r\n' + 'Connection: Upgrade\r\n' + '\r\n'); socket.pipe(socket); // echo back }); // now that server is running srv.listen(1337, '127.0.0.1', () => { // make a request const options = { port: 1337, host: '127.0.0.1', headers: { 'Connection': 'Upgrade', 'Upgrade': 'websocket' } }; const req = http.request(options); req.end(); req.on('upgrade', (res, socket, upgradeHead) => { console.log('got upgraded!'); socket.end(); process.exit(0); }); }); Marks the request as aborting. Calling this will cause remaining data in the response to be dropped and the socket to be destroyed. request.abort() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 403/739 If a request has been aborted, this value is the time when the request was aborted, in milliseconds since 1 January 1970 00:00:00 UTC. <net.Socket> See request.socket . data <string> | <Buffer> encoding <string> callback <Function> Returns: <this> Finishes sending the request. If any parts of the body are unsent, it will flush them to the stream. If the request is chunked, this will send the terminating '0\r\n\r\n' . If data is specified, it is equivalent to calling request.write(data, encoding) followed by request.end(callback) . If callback is specified, it will be called when the request stream is finished. <boolean> The request.finished property will be true if request.end() has been called. request.end() will automatically be called if the request was initiated via http.get() . Flush the request headers. For efficiency reasons, Node.js normally buffers the request headers until request.end() is called or the first chunk of request data is written. It then tries to pack the request headers and data into a single TCP packet. That's usually desired (it saves a TCP round-trip), but not when the first data is not sent until possibly much later. request.flushHeaders() bypasses the optimization and kickstarts the request. name <string> Returns: <any> Reads out a header on the request. Note that the name is case insensitive. The type of the return value depends on the arguments provided to request.setHeader() . request.setHeader('content-type', 'text/html'); request.setHeader('Content-Length', Buffer.byteLength(body)); request.setHeader('Cookie', ['type=ninja', 'language=javascript']); const contentType = request.getHeader('Content-Type'); // contentType is 'text/html' const contentLength = request.getHeader('Content-Length'); // contentLength is of type number ( ) request.aborted request.connection request.end([data[, encoding]][, callback]) request.finished request.flushHeaders() request.getHeader(name) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 404/739 const cookie = request.getHeader('Cookie'); // cookie is of type string[] <number> Default: 2000 Limits maximum response headers count. If set to 0, no limit will be applied. name <string> Removes a header that's already defined into headers object. request.removeHeader('Content-Type'); name <string> value <any> Sets a single header value for headers object. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here to send multiple headers with the same name. Non-string values will be stored without modification. Therefore, request.getHeader() may return non-string values. However, the non-string values will be converted to strings for network transmission. request.setHeader('Content-Type', 'application/json'); or request.setHeader('Cookie', ['type=ninja', 'language=javascript']); noDelay <boolean> Once a socket is assigned to this request and is connected socket.setNoDelay() will be called. enable <boolean> initialDelay <number> Once a socket is assigned to this request and is connected socket.setKeepAlive() will be called. timeout <number> Milliseconds before a request times out. callback <Function> Optional function to be called when a timeout occurs. Same as binding to the 'timeout' event. Returns: <http.ClientRequest> Once a socket is assigned to this request and is connected socket.setTimeout() will be called. request.maxHeadersCount request.removeHeader(name) request.setHeader(name, value) request.setNoDelay([noDelay]) request.setSocketKeepAlive([enable][, initialdelay]) request.setTimeout(timeout[, callback]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 405/739 <net.Socket> Reference to the underlying socket. Usually users will not want to access this property. In particular, the socket will not emit 'readable' events because of how the protocol parser attaches to the socket. The socket may also be accessed via request.connection . const http = require('http'); const options = { host: 'www.google.com', }; const req = http.get(options); req.end(); req.once('response', (res) => { const ip = req.socket.localAddress; const port = req.socket.localPort; console.log(`Your IP address is ${ip} and your source port is ${port}.`); // consume response object }); chunk <string> | <Buffer> encoding <string> callback <Function> Returns: <boolean> Sends a chunk of the body. By calling this method many times, a request body can be sent to a server — in that case it is suggested to use the ['Transfer-Encoding', 'chunked'] header line when creating the request. The encoding argument is optional and only applies when chunk is a string. Defaults to 'utf8' . The callback argument is optional and will be called when this chunk of data is flushed, but only if the chunk is non-empty. Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in user memory. 'drain' will be emitted when the buffer is free again. When write function is called with empty string or buffer, it does nothing and waits for more input. This class inherits from net.Server and has the following additional events: request <http.IncomingMessage> response <http.ServerResponse> Emitted each time a request with an HTTP Expect: 100-continue is received. If this event is not listened for, the server will automatically respond with a 100 Continue as appropriate. Handling this event involves calling response.writeContinue() if the client should continue to send the request body, or generating an appropriate HTTP response (e.g. 400 Bad Request) if the client should not continue to send the request body. request.socket request.write(chunk[, encoding][, callback]) Class: http.Server Event: 'checkContinue' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 406/739 Note that when this event is emitted and handled, the 'request' event will not be emitted. request <http.IncomingMessage> response <http.ServerResponse> Emitted each time a request with an HTTP Expect header is received, where the value is not 100-continue . If this event is not listened for, the server will automatically respond with a 417 Expectation Failed as appropriate. Note that when this event is emitted and handled, the 'request' event will not be emitted. exception <Error> socket <net.Socket> If a client connection emits an 'error' event, it will be forwarded here. Listener of this event is responsible for closing/destroying the underlying socket. For example, one may wish to more gracefully close the socket with a custom HTTP response instead of abruptly severing the connection. Default behavior is to close the socket with an HTTP '400 Bad Request' response if possible, otherwise the socket is immediately destroyed. socket is the net.Socket object that the error originated from. const http = require('http'); const server = http.createServer((req, res) => { res.end(); }); server.on('clientError', (err, socket) => { socket.end('HTTP/1.1 400 Bad Request\r\n\r\n'); }); server.listen(8000); When the 'clientError' event occurs, there is no request or response object, so any HTTP response sent, including response headers and payload, must be written directly to the socket object. Care must be taken to ensure the response is a properly formatted HTTP response message. err is an instance of Error with two extra columns: bytesParsed : the bytes count of request packet that Node.js may have parsed correctly; rawPacket : the raw packet of current request. Emitted when the server closes. request <http.IncomingMessage> Arguments for the HTTP request, as it is in the 'request' event socket <net.Socket> Network socket between the server and client head <Buffer> The first packet of the tunneling stream (may be empty) Event: 'checkExpectation' Event: 'clientError' Event: 'close' Event: 'connect' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 407/739 Emitted each time a client requests an HTTP CONNECT method. If this event is not listened for, then clients requesting a CONNECT method will have their connections closed. After this event is emitted, the request's socket will not have a 'data' event listener, meaning it will need to be bound in order to handle data sent to the server on that socket. socket <net.Socket> This event is emitted when a new TCP stream is established. socket is typically an object of type net.Socket . Usually users will not want to access this event. In particular, the socket will not emit 'readable' events because of how the protocol parser attaches to the socket. The socket can also be accessed at request.connection . This event can also be explicitly emitted by users to inject connections into the HTTP server. In that case, any Duplex stream can be passed. request <http.IncomingMessage> response <http.ServerResponse> Emitted each time there is a request. Note that there may be multiple requests per connection (in the case of HTTP Keep-Alive connections). request <http.IncomingMessage> Arguments for the HTTP request, as it is in the 'request' event socket <net.Socket> Network socket between the server and client head <Buffer> The first packet of the upgraded stream (may be empty) Emitted each time a client requests an HTTP upgrade. Listening to this event is optional and clients cannot insist on a protocol change. After this event is emitted, the request's socket will not have a 'data' event listener, meaning it will need to be bound in order to handle data sent to the server on that socket. callback <Function> Stops the server from accepting new connections. See net.Server.close() . Starts the HTTP server listening for connections. This method is identical to server.listen() from net.Server . <boolean> Indicates whether or not the server is listening for connections. <number> Default: 2000 Limits maximum incoming headers count. If set to 0, no limit will be applied. <number> Default: 40000 Event: 'connection' Event: 'request' Event: 'upgrade' server.close([callback]) server.listen() server.listening server.maxHeadersCount server.headersTimeout 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 408/739 Limit the amount of time the parser will wait to receive the complete HTTP headers. In case of inactivity, the rules defined in [server.timeout][] apply. However, that inactivity based timeout would still allow the connection to be kept open if the headers are being sent very slowly (by default, up to a byte per 2 minutes). In order to prevent this, whenever header data arrives an additional check is made that more than server.headersTimeout milliseconds has not passed since the connection was established. If the check fails, a 'timeout' event is emitted on the server object, and (by default) the socket is destroyed. See [server.timeout][] for more information on how timeout behaviour can be customised. msecs <number> Default: 120000 (2 minutes) callback <Function> Returns: <http.Server> Sets the timeout value for sockets, and emits a 'timeout' event on the Server object, passing the socket as an argument, if a timeout occurs. If there is a 'timeout' event listener on the Server object, then it will be called with the timed-out socket as an argument. By default, the Server's timeout value is 2 minutes, and sockets are destroyed automatically if they time out. However, if a callback is assigned to the Server's 'timeout' event, timeouts must be handled explicitly. <number> Timeout in milliseconds. Default: 120000 (2 minutes). The number of milliseconds of inactivity before a socket is presumed to have timed out. A value of 0 will disable the timeout behavior on incoming connections. The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections. <number> Timeout in milliseconds. Default: 5000 (5 seconds). The number of milliseconds of inactivity a server needs to wait for additional incoming data, after it has finished writing the last response, before a socket will be destroyed. If the server receives new data before the keep-alive timeout has fired, it will reset the regular inactivity timeout, i.e., server.timeout . A value of 0 will disable the keep-alive timeout behavior on incoming connections. A value of 0 makes the http server behave similarly to Node.js versions prior to 8.0.0, which did not have a keep-alive timeout. The socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections. This object is created internally by an HTTP server — not by the user. It is passed as the second parameter to the 'request' event. The response inherits from Stream , and additionally implements the following: Indicates that the underlying connection was terminated before response.end() was called or able to flush. server.setTimeout([msecs][, callback]) server.timeout server.keepAliveTimeout Class: http.ServerResponse Event: 'close' Event: 'finish' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 409/739 Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the operating system for transmission over the network. It does not imply that the client has received anything yet. headers <Object> This method adds HTTP trailing headers (a header but at the end of the message) to the response. Trailers will only be emitted if chunked encoding is used for the response; if it is not (e.g. if the request was HTTP/1.0), they will be silently discarded. Note that HTTP requires the Trailer header to be sent in order to emit trailers, with a list of the header fields in its value. E.g., response.writeHead(200, { 'Content-Type': 'text/plain', 'Trailer': 'Content-MD5' }); response.write(fileData); response.addTrailers({ 'Content-MD5': '7895bf4b8828b55ceaf47747b4bca667' }); response.end(); Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown. <net.Socket> See response.socket . data <string> | <Buffer> encoding <string> callback <Function> Returns: <this> This method signals to the server that all of the response headers and body have been sent; that server should consider this message complete. The method, response.end() , MUST be called on each response. If data is specified, it is similar in effect to calling response.write(data, encoding) followed by response.end(callback) . If callback is specified, it will be called when the response stream is finished. <boolean> Boolean value that indicates whether the response has completed. Starts as false . After response.end() executes, the value will be true . name <string> Returns: <any> Reads out a header that's already been queued but not sent to the client. Note that the name is case insensitive. The type of the return value depends on the arguments provided to response.setHeader() . response.addTrailers(headers) response.connection response.end([data][, encoding][, callback]) response.finished response.getHeader(name) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 410/739 response.setHeader('Content-Type', 'text/html'); response.setHeader('Content-Length', Buffer.byteLength(body)); response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']); const contentType = response.getHeader('content-type'); // contentType is 'text/html' const contentLength = response.getHeader('Content-Length'); // contentLength is of type number const setCookie = response.getHeader('set-cookie'); // setCookie is of type string[] Returns: <string[]> Returns an array containing the unique names of the current outgoing headers. All header names are lowercase. response.setHeader('Foo', 'bar'); response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']); const headerNames = response.getHeaderNames(); // headerNames === ['foo', 'set-cookie'] Returns: <Object> Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related http module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase. The object returned by the response.getHeaders() method does not prototypically inherit from the JavaScript Object . This means that typical Object methods such as obj.toString() , obj.hasOwnProperty() , and others are not defined and will not work. response.setHeader('Foo', 'bar'); response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']); const headers = response.getHeaders(); // headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] } name <string> Returns: <boolean> Returns true if the header identified by name is currently set in the outgoing headers. Note that the header name matching is case- insensitive. const hasContentType = response.hasHeader('content-type'); response.getHeaderNames() response.getHeaders() response.hasHeader(name) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 411/739 <boolean> Boolean (read-only). True if headers were sent, false otherwise. name <string> Removes a header that's queued for implicit sending. response.removeHeader('Content-Encoding'); <boolean> When true, the Date header will be automatically generated and sent in the response if it is not already present in the headers. Defaults to true. This should only be disabled for testing; HTTP requires the Date header in responses. name <string> value <any> Sets a single header value for implicit headers. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here to send multiple headers with the same name. Non-string values will be stored without modification. Therefore, response.getHeader() may return non-string values. However, the non-string values will be converted to strings for network transmission. response.setHeader('Content-Type', 'text/html'); or response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']); Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown. When headers have been set with response.setHeader() , they will be merged with any headers passed to response.writeHead() , with the headers passed to response.writeHead() given precedence. // returns content-type = text/plain const server = http.createServer((req, res) => { res.setHeader('Content-Type', 'text/html'); res.setHeader('X-Foo', 'bar'); res.writeHead(200, { 'Content-Type': 'text/plain' }); res.end('ok'); }); If response.writeHead() method is called and this method has not been called, it will directly write the supplied header values onto the network channel without caching internally, and the response.getHeader() on the header will not yield the expected result. If progressive response.headersSent response.removeHeader(name) response.sendDate response.setHeader(name, value) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 412/739 population of headers is desired with potential future retrieval and modification, use response.setHeader() instead of response.writeHead() . msecs <number> callback <Function> Returns: <http.ServerResponse> Sets the Socket's timeout value to msecs . If a callback is provided, then it is added as a listener on the 'timeout' event on the response object. If no 'timeout' listener is added to the request, the response, or the server, then sockets are destroyed when they time out. If a handler is assigned to the request, the response, or the server's 'timeout' events, timed out sockets must be handled explicitly. <net.Socket> Reference to the underlying socket. Usually users will not want to access this property. In particular, the socket will not emit 'readable' events because of how the protocol parser attaches to the socket. After response.end() , the property is nulled. The socket may also be accessed via response.connection . const http = require('http'); const server = http.createServer((req, res) => { const ip = res.socket.remoteAddress; const port = res.socket.remotePort; res.end(`Your IP address is ${ip} and your source port is ${port}.`); }).listen(3000); <number> When using implicit headers (not calling response.writeHead() explicitly), this property controls the status code that will be sent to the client when the headers get flushed. response.statusCode = 404; After response header was sent to the client, this property indicates the status code which was sent out. <string> When using implicit headers (not calling response.writeHead() explicitly), this property controls the status message that will be sent to the client when the headers get flushed. If this is left as undefined then the standard message for the status code will be used. response.statusMessage = 'Not found'; After response header was sent to the client, this property indicates the status message which was sent out. response.setTimeout(msecs[, callback]) response.socket response.statusCode response.statusMessage response.write(chunk[, encoding][, callback]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 413/739 chunk <string> | <Buffer> encoding <string> Default: 'utf8' callback <Function> Returns: <boolean> If this method is called and response.writeHead() has not been called, it will switch to implicit header mode and flush the implicit headers. This sends a chunk of the response body. This method may be called multiple times to provide successive parts of the body. Note that in the http module, the response body is omitted when the request is a HEAD request. Similarly, the 204 and 304 responses must not include a message body. chunk can be a string or a buffer. If chunk is a string, the second parameter specifies how to encode it into a byte stream. callback will be called when this chunk of data is flushed. This is the raw HTTP body and has nothing to do with higher-level multi-part body encodings that may be used. The first time response.write() is called, it will send the buffered header information and the first chunk of the body to the client. The second time response.write() is called, Node.js assumes data will be streamed, and sends the new data separately. That is, the response is buffered up to the first chunk of the body. Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in user memory. 'drain' will be emitted when the buffer is free again. Sends a HTTP/1.1 100 Continue message to the client, indicating that the request body should be sent. See the 'checkContinue' event on Server . statusCode <number> statusMessage <string> headers <Object> Sends a response header to the request. The status code is a 3-digit HTTP status code, like 404 . The last argument, headers , are the response headers. Optionally one can give a human-readable statusMessage as the second argument. const body = 'hello world'; response.writeHead(200, { 'Content-Length': Buffer.byteLength(body), 'Content-Type': 'text/plain' }); This method must only be called once on a message and it must be called before response.end() is called. If response.write() or response.end() are called before calling this, the implicit/mutable headers will be calculated and call this function. When headers have been set with response.setHeader() , they will be merged with any headers passed to response.writeHead() , with the headers passed to response.writeHead() given precedence. If this method is called and response.setHeader() has not been called, it will directly write the supplied header values onto the network channel without caching internally, and the response.getHeader() on the header will not yield the expected result. If progressive population of headers is desired with potential future retrieval and modification, use response.setHeader() instead. response.writeContinue() response.writeHead(statusCode[, statusMessage][, headers]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 414/739 // returns content-type = text/plain const server = http.createServer((req, res) => { res.setHeader('Content-Type', 'text/html'); res.setHeader('X-Foo', 'bar'); res.writeHead(200, { 'Content-Type': 'text/plain' }); res.end('ok'); }); Note that Content-Length is given in bytes not characters. The above example works because the string 'hello world' contains only single byte characters. If the body contains higher coded characters then Buffer.byteLength() should be used to determine the number of bytes in a given encoding. And Node.js does not check whether Content-Length and the length of the body which has been transmitted are equal or not. Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown. Sends a HTTP/1.1 102 Processing message to the client, indicating that the request body should be sent. An IncomingMessage object is created by http.Server or http.ClientRequest and passed as the first argument to the 'request' and 'response' event respectively. It may be used to access response status, headers and data. It implements the Readable Stream interface, as well as the following additional events, methods, and properties. Emitted when the request has been aborted. Indicates that the underlying connection was closed. Just like 'end' , this event occurs only once per response. <boolean> The message.aborted property will be true if the request has been aborted. <boolean> The message.complete property will be true if a complete HTTP message has been received and successfully parsed. This property is particularly useful as a means of determining if a client or server fully transmitted a message before a connection was terminated: const req = http.request({ host: '127.0.0.1', port: 8080, method: 'POST' }, (res) => { res.resume(); res.on('end', () => { response.writeProcessing() Class: http.IncomingMessage Event: 'aborted' Event: 'close' message.aborted message.complete 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 415/739 if (!res.complete) console.error( 'The connection was terminated while the message was still being sent'); }); }); error <Error> Calls destroy() on the socket that received the IncomingMessage . If error is provided, an 'error' event is emitted and error is passed as an argument to any listeners on the event. <Object> The request/response headers object. Key-value pairs of header names and values. Header names are lower-cased. // Prints something like: // // { 'user-agent': 'curl/7.22.0', // host: '127.0.0.1:8000', // accept: '*/*' } console.log(request.headers); Duplicates in raw headers are handled in the following ways, depending on the header name: Duplicates of age , authorization , content-length , content-type , etag , expires , from , host , if-modified-since , if- unmodified-since , last-modified , location , max-forwards , proxy-authorization , referer , retry-after , or user-agent are discarded. set-cookie is always an array. Duplicates are added to the array. For duplicate cookie headers, the values are joined together with '; '. For all other headers, the values are joined together with ', '. <string> In case of server request, the HTTP version sent by the client. In the case of client response, the HTTP version of the connected-to server. Probably either '1.1' or '1.0' . Also message.httpVersionMajor is the first integer and message.httpVersionMinor is the second. <string> Only valid for request obtained from http.Server . The request method as a string. Read only. Examples: 'GET' , 'DELETE' . message.destroy([error]) message.headers message.httpVersion message.method message.rawHeaders 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 416/739 <string[]> The raw request/response headers list exactly as they were received. Note that the keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values. Header names are not lowercased, and duplicates are not merged. // Prints something like: // // [ 'user-agent', // 'this is invalid because there can be only one', // 'User-Agent', // 'curl/7.22.0', // 'Host', // '127.0.0.1:8000', // 'ACCEPT', // '*/*' ] console.log(request.rawHeaders); <string[]> The raw request/response trailer keys and values exactly as they were received. Only populated at the 'end' event. msecs <number> callback <Function> Returns: <http.IncomingMessage> Calls message.connection.setTimeout(msecs, callback) . <net.Socket> The net.Socket object associated with the connection. With HTTPS support, use request.socket.getPeerCertificate() to obtain the client's authentication details. <number> Only valid for response obtained from http.ClientRequest . The 3-digit HTTP response status code. E.G. 404 . <string> Only valid for response obtained from http.ClientRequest . message.rawTrailers message.setTimeout(msecs, callback) message.socket message.statusCode message.statusMessage 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 417/739 The HTTP response status message (reason phrase). E.G. OK or Internal Server Error . <Object> The request/response trailers object. Only populated at the 'end' event. <string> Only valid for request obtained from http.Server . Request URL string. This contains only the URL that is present in the actual HTTP request. If the request is: GET /status?name=ryan HTTP/1.1\r\n Accept: text/plain\r\n \r\n Then request.url will be: '/status?name=ryan' To parse the url into its parts require('url').parse(request.url) can be used: $ node > require('url').parse('/status?name=ryan') Url { protocol: null, slashes: null, auth: null, host: null, port: null, hostname: null, hash: null, search: '?name=ryan', query: 'name=ryan', pathname: '/status', path: '/status?name=ryan', href: '/status?name=ryan' } To extract the parameters from the query string, the require('querystring').parse function can be used, or true can be passed as the second argument to require('url').parse : $ node > require('url').parse('/status?name=ryan', true) Url { protocol: null, slashes: null, auth: null, host: null, port: null, message.trailers message.url 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 418/739 hostname: null, hash: null, search: '?name=ryan', query: { name: 'ryan' }, pathname: '/status', path: '/status?name=ryan', href: '/status?name=ryan' } <string[]> A list of the HTTP methods that are supported by the parser. <Object> A collection of all the standard HTTP response status codes, and the short description of each. For example, http.STATUS_CODES[404] === 'Not Found' . options <Object> IncomingMessage <http.IncomingMessage> Specifies the IncomingMessage class to be used. Useful for extending the original IncomingMessage . Default: IncomingMessage . ServerResponse <http.ServerResponse> Specifies the ServerResponse class to be used. Useful for extending the original ServerResponse . Default: ServerResponse . requestListener <Function> Returns: <http.Server> Returns a new instance of http.Server . The requestListener is a function which is automatically added to the 'request' event. url <string> | <URL> options <Object> Accepts the same options as http.request() , with the method always set to GET . Properties that are inherited from the prototype are ignored. callback <Function> Returns: <http.ClientRequest> Since most requests are GET requests without bodies, Node.js provides this convenience method. The only difference between this method and http.request() is that it sets the method to GET and calls req.end() automatically. Note that the callback must take care to consume the response data for reasons stated in http.ClientRequest section. The callback is invoked with a single argument that is an instance of http.IncomingMessage . JSON fetching example: http.METHODS http.STATUS_CODES http.createServer([options][, requestlistener]) http.get(options[, callback]) http.get(url[, options][, callback]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 419/739 http.get('http://nodejs.org/dist/index.json', (res) => { const { statusCode } = res; const contentType = res.headers['content-type']; let error; if (statusCode !== 200) { error = new Error('Request Failed.\n' + `Status Code: ${statusCode}`); } else if (!/^application\/json/.test(contentType)) { error = new Error('Invalid content-type.\n' + `Expected application/json but received ${contentType}`); } if (error) { console.error(error.message); // consume response data to free up memory res.resume(); return; } res.setEncoding('utf8'); let rawData = ''; res.on('data', (chunk) => { rawData += chunk; }); res.on('end', () => { try { const parsedData = JSON.parse(rawData); console.log(parsedData); } catch (e) { console.error(e.message); } }); }).on('error', (e) => { console.error(`Got error: ${e.message}`); }); <http.Agent> Global instance of Agent which is used as the default for all HTTP client requests. <number> Read-only property specifying the maximum allowed size of HTTP headers in bytes. Defaults to 8KB. Configurable using the --max-http- header-size CLI option. url <string> | <URL> options <Object> http.globalAgent http.maxHeaderSize http.request(options[, callback]) http.request(url[, options][, callback]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 420/739 protocol <string> Protocol to use. Default: 'http:' . host <string> A domain name or IP address of the server to issue the request to. Default: 'localhost' . hostname <string> Alias for host . To support url.parse() , hostname will be used if both host and hostname are specified. family <number> IP address family to use when resolving host or hostname . Valid values are 4 or 6 . When unspecified, both IP v4 and v6 will be used. port <number> Port of remote server. Default: 80 . localAddress <string> Local interface to bind for network connections. socketPath <string> Unix Domain Socket (cannot be used if one of host or port is specified, those specify a TCP Socket). method <string> A string specifying the HTTP request method. Default: 'GET' . path <string> Request path. Should include query string if any. E.G. '/index.html?page=12' . An exception is thrown when the request path contains illegal characters. Currently, only spaces are rejected but that may change in the future. Default: '/' . headers <Object> An object containing request headers. auth <string> Basic authentication i.e. 'user:password' to compute an Authorization header. agent <http.Agent> | <boolean> Controls Agent behavior. Possible values: undefined (default): use http.globalAgent for this host and port. Agent object: explicitly use the passed in Agent . false : causes a new Agent with default values to be used. createConnection <Function> A function that produces a socket/stream to use for the request when the agent option is not used. This can be used to avoid creating a custom Agent class just to override the default createConnection function. See agent.createConnection() for more details. Any Duplex stream is a valid return value. timeout <number> : A number specifying the socket timeout in milliseconds. This will set the timeout before the socket is connected. setHost <boolean> : Specifies whether or not to automatically add the Host header. Defaults to true . callback <Function> Returns: <http.ClientRequest> Node.js maintains several connections per server to make HTTP requests. This function allows one to transparently issue requests. url can be a string or a URL object. If url is a string, it is automatically parsed with url.parse() . If it is a URL object, it will be automatically converted to an ordinary options object. If both url and options are specified, the objects are merged, with the options properties taking precedence. The optional callback parameter will be added as a one-time listener for the 'response' event. http.request() returns an instance of the http.ClientRequest class. The ClientRequest instance is a writable stream. If one needs to upload a file with a POST request, then write to the ClientRequest object. const postData = querystring.stringify({ 'msg': 'Hello World!' }); const options = { hostname: 'www.google.com', port: 80, path: '/upload', method: 'POST', headers: { 'Content-Type': 'application/x-www-form-urlencoded', 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 421/739 'Content-Length': Buffer.byteLength(postData) } }; const req = http.request(options, (res) => { console.log(`STATUS: ${res.statusCode}`); console.log(`HEADERS: ${JSON.stringify(res.headers)}`); res.setEncoding('utf8'); res.on('data', (chunk) => { console.log(`BODY: ${chunk}`); }); res.on('end', () => { console.log('No more data in response.'); }); }); req.on('error', (e) => { console.error(`problem with request: ${e.message}`); }); // write data to request body req.write(postData); req.end(); Note that in the example req.end() was called. With http.request() one must always call req.end() to signify the end of the request - even if there is no data being written to the request body. If any error is encountered during the request (be that with DNS resolution, TCP level errors, or actual HTTP parse errors) an 'error' event is emitted on the returned request object. As with all 'error' events, if no listeners are registered the error will be thrown. There are a few special headers that should be noted. Sending a 'Connection: keep-alive' will notify Node.js that the connection to the server should be persisted until the next request. Sending a 'Content-Length' header will disable the default chunked encoding. Sending an 'Expect' header will immediately send the request headers. Usually, when sending 'Expect: 100-continue', both a timeout and a listener for the 'continue' event should be set. See RFC2616 Section 8.2.3 for more information. Sending an Authorization header will override using the auth option to compute basic authentication. Example using a URL as options : const options = new URL('http://abc:xyz@example.com'); const req = http.request(options, (res) => { // ... }); In a successful request, the following events will be emitted in the following order: 'socket' 'response' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 422/739 'data' any number of times, on the res object ( 'data' will not be emitted at all if the response body is empty, for instance, in most redirects) 'end' on the res object 'close' In the case of a connection error, the following events will be emitted: 'socket' 'error' 'close' If req.abort() is called before the connection succeeds, the following events will be emitted in the following order: 'socket' ( req.abort() called here) 'abort' 'error' with an error with message 'Error: socket hang up' and code 'ECONNRESET' 'close' If req.abort() is called after the response is received, the following events will be emitted in the following order: 'socket' 'response' 'data' any number of times, on the res object ( req.abort() called here) 'abort' 'aborted' on the res object 'close' 'end' on the res object 'close' on the res object Note that setting the timeout option or using the setTimeout() function will not abort the request or do anything besides add a 'timeout' event. Stability: 2 - Stable The http2 module provides an implementation of the HTTP/2 protocol. It can be accessed using: const http2 = require('http2'); The Core API provides a low-level interface designed specifically around support for HTTP/2 protocol features. It is specifically not designed for compatibility with the existing HTTP/1 module API. However, the Compatibility API is. HTTP/2 Core API 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 423/739 The http2 Core API is much more symmetric between client and server than the http API. For instance, most events, like 'error' , 'connect' and 'stream' , can be emitted either by client-side code or server-side code. The following illustrates a simple HTTP/2 server using the Core API. Since there are no browsers known that support unencrypted HTTP/2 , the use of http2.createSecureServer() is necessary when communicating with browser clients. const http2 = require('http2'); const fs = require('fs'); const server = http2.createSecureServer({ key: fs.readFileSync('localhost-privkey.pem'), cert: fs.readFileSync('localhost-cert.pem') }); server.on('error', (err) => console.error(err)); server.on('stream', (stream, headers) => { // stream is a Duplex stream.respond({ 'content-type': 'text/html', ':status': 200 }); stream.end('<h1>Hello World</h1>'); }); server.listen(8443); To generate the certificate and key for this example, run: openssl req -x509 -newkey rsa:2048 -nodes -sha256 -subj '/CN=localhost' \ -keyout localhost-privkey.pem -out localhost-cert.pem The following illustrates an HTTP/2 client: const http2 = require('http2'); const fs = require('fs'); const client = http2.connect('https://localhost:8443', { ca: fs.readFileSync('localhost-cert.pem') }); client.on('error', (err) => console.error(err)); const req = client.request({ ':path': '/' }); req.on('response', (headers, flags) => { for (const name in headers) { console.log(`${name}: ${headers[name]}`); } }); Server-side example Client-side example 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 424/739 req.setEncoding('utf8'); let data = ''; req.on('data', (chunk) => { data += chunk; }); req.on('end', () => { console.log(`\n${data}`); client.close(); }); req.end(); Extends: <EventEmitter> Instances of the http2.Http2Session class represent an active communications session between an HTTP/2 client and server. Instances of this class are not intended to be constructed directly by user code. Each Http2Session instance will exhibit slightly different behaviors depending on whether it is operating as a server or a client. The http2session.type property can be used to determine the mode in which an Http2Session is operating. On the server side, user code should rarely have occasion to work with the Http2Session object directly, with most actions typically taken through interactions with either the Http2Server or Http2Stream objects. User code will not create Http2Session instances directly. Server-side Http2Session instances are created by the Http2Server instance when a new HTTP/2 connection is received. Client-side Http2Session instances are created using the http2.connect() method. Every Http2Session instance is associated with exactly one net.Socket or tls.TLSSocket when it is created. When either the Socket or the Http2Session are destroyed, both will be destroyed. Because of the specific serialization and processing requirements imposed by the HTTP/2 protocol, it is not recommended for user code to read data from or write data to a Socket instance bound to a Http2Session . Doing so can put the HTTP/2 session into an indeterminate state causing the session and the socket to become unusable. Once a Socket has been bound to an Http2Session , user code should rely solely on the API of the Http2Session . The 'close' event is emitted once the Http2Session has been destroyed. Its listener does not expect any arguments. session <Http2Session> socket <net.Socket> The 'connect' event is emitted once the Http2Session has been successfully connected to the remote peer and communication may begin. User code will typically not listen for this event directly. error <Error> The 'error' event is emitted when an error occurs during the processing of an Http2Session . Class: Http2Session Http2Session and Sockets Event: 'close' Event: 'connect' Event: 'error' Event: 'frameError' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 425/739 type <integer> The frame type. code <integer> The error code. id <integer> The stream id (or 0 if the frame isn't associated with a stream). The 'frameError' event is emitted when an error occurs while attempting to send a frame on the session. If the frame that could not be sent is associated with a specific Http2Stream , an attempt to emit 'frameError' event on the Http2Stream is made. If the 'frameError' event is associated with a stream, the stream will be closed and destroyed immediately following the 'frameError' event. If the event is not associated with a stream, the Http2Session will be shut down immediately following the 'frameError' event. errorCode <number> The HTTP/2 error code specified in the GOAWAY frame. lastStreamID <number> The ID of the last stream the remote peer successfully processed (or 0 if no ID is specified). opaqueData <Buffer> If additional opaque data was included in the GOAWAY frame, a Buffer instance will be passed containing that data. The 'goaway' event is emitted when a GOAWAY frame is received. The Http2Session instance will be shut down automatically when the 'goaway' event is emitted. settings <HTTP/2 Settings Object> A copy of the SETTINGS frame received. The 'localSettings' event is emitted when an acknowledgment SETTINGS frame has been received. When using http2session.settings() to submit new settings, the modified settings do not take effect until the 'localSettings' event is emitted. session.settings({ enablePush: false }); session.on('localSettings', (settings) => { /* Use the new settings */ }); payload <Buffer> The PING frame 8-byte payload The 'ping' event is emitted whenever a PING frame is received from the connected peer. settings <HTTP/2 Settings Object> A copy of the SETTINGS frame received. The 'remoteSettings' event is emitted when a new SETTINGS frame is received from the connected peer. session.on('remoteSettings', (settings) => { /* Use the new settings */ }); stream <Http2Stream> A reference to the stream Event: 'goaway' Event: 'localSettings' Event: 'ping' Event: 'remoteSettings' Event: 'stream' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 426/739 headers <HTTP/2 Headers Object> An object describing the headers flags <number> The associated numeric flags rawHeaders <Array> An array containing the raw header names followed by their respective values. The 'stream' event is emitted when a new Http2Stream is created. const http2 = require('http2'); session.on('stream', (stream, headers, flags) => { const method = headers[':method']; const path = headers[':path']; // ... stream.respond({ ':status': 200, 'content-type': 'text/plain' }); stream.write('hello '); stream.end('world'); }); On the server side, user code will typically not listen for this event directly, and would instead register a handler for the 'stream' event emitted by the net.Server or tls.Server instances returned by http2.createServer() and http2.createSecureServer() , respectively, as in the example below: const http2 = require('http2'); // Create an unencrypted HTTP/2 server const server = http2.createServer(); server.on('stream', (stream, headers) => { stream.respond({ 'content-type': 'text/html', ':status': 200 }); stream.on('error', (error) => console.error(error)); stream.end('<h1>Hello World</h1>'); }); server.listen(80); Even though HTTP/2 streams and network sockets are not in a 1:1 correspondence, a network error will destroy each individual stream and must be handled on the stream level, as shown above. After the http2session.setTimeout() method is used to set the timeout period for this Http2Session , the 'timeout' event is emitted if there is no activity on the Http2Session after the configured number of milliseconds. session.setTimeout(2000); session.on('timeout', () => { /* .. */ }); Event: 'timeout' http2session.alpnProtocol 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 427/739 <string> | <undefined> Value will be undefined if the Http2Session is not yet connected to a socket, h2c if the Http2Session is not connected to a TLSSocket , or will return the value of the connected TLSSocket 's own alpnProtocol property. callback <Function> Gracefully closes the Http2Session , allowing any existing streams to complete on their own and preventing new Http2Stream instances from being created. Once closed, http2session.destroy() might be called if there are no open Http2Stream instances. If specified, the callback function is registered as a handler for the 'close' event. <boolean> Will be true if this Http2Session instance has been closed, otherwise false . <boolean> Will be true if this Http2Session instance is still connecting, will be set to false before emitting connect event and/or calling the http2.connect callback. error <Error> An Error object if the Http2Session is being destroyed due to an error. code <number> The HTTP/2 error code to send in the final GOAWAY frame. If unspecified, and error is not undefined, the default is INTERNAL_ERROR , otherwise defaults to NO_ERROR . Immediately terminates the Http2Session and the associated net.Socket or tls.TLSSocket . Once destroyed, the Http2Session will emit the 'close' event. If error is not undefined, an 'error' event will be emitted immediately before the 'close' event. If there are any remaining open Http2Streams associated with the Http2Session , those will also be destroyed. <boolean> Will be true if this Http2Session instance has been destroyed and must no longer be used, otherwise false . <boolean> | <undefined> Value is undefined if the Http2Session session socket has not yet been connected, true if the Http2Session is connected with a TLSSocket , and false if the Http2Session is connected to any other kind of socket or stream. code <number> An HTTP/2 error code lastStreamID <number> The numeric ID of the last processed Http2Stream opaqueData <Buffer> | <TypedArray> | <DataView> A TypedArray or DataView instance containing additional data to be carried within the GOAWAY frame. http2session.close([callback]) http2session.closed http2session.connecting http2session.destroy([error][, code]) http2session.destroyed http2session.encrypted http2session.goaway([code[, lastStreamID[, opaqueData]]]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 428/739 Transmits a GOAWAY frame to the connected peer without shutting down the Http2Session . <HTTP/2 Settings Object> A prototype-less object describing the current local settings of this Http2Session . The local settings are local to this Http2Session instance. <string[]> | <undefined> If the Http2Session is connected to a TLSSocket , the originSet property will return an Array of origins for which the Http2Session may be considered authoritative. The originSet property is only available when using a secure TLS connection. <boolean> Indicates whether or not the Http2Session is currently waiting for an acknowledgment for a sent SETTINGS frame. Will be true after calling the http2session.settings() method. Will be false once all sent SETTINGS frames have been acknowledged. payload <Buffer> | <TypedArray> | <DataView> Optional ping payload. callback <Function> Returns: <boolean> Sends a PING frame to the connected HTTP/2 peer. A callback function must be provided. The method will return true if the PING was sent, false otherwise. The maximum number of outstanding (unacknowledged) pings is determined by the maxOutstandingPings configuration option. The default maximum is 10. If provided, the payload must be a Buffer , TypedArray , or DataView containing 8 bytes of data that will be transmitted with the PING and returned with the ping acknowledgment. The callback will be invoked with three arguments: an error argument that will be null if the PING was successfully acknowledged, a duration argument that reports the number of milliseconds elapsed since the ping was sent and the acknowledgment was received, and a Buffer containing the 8-byte PING payload. session.ping(Buffer.from('abcdefgh'), (err, duration, payload) => { if (!err) { console.log(`Ping acknowledged in ${duration} milliseconds`); console.log(`With payload '${payload.toString()}'`); } }); If the payload argument is not specified, the default payload will be the 64-bit timestamp (little endian) marking the start of the PING duration. Calls ref() on this Http2Session instance's underlying net.Socket . http2session.localSettings http2session.originSet http2session.pendingSettingsAck http2session.ping([payload, ]callback) http2session.ref() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 429/739 <HTTP/2 Settings Object> A prototype-less object describing the current remote settings of this Http2Session . The remote settings are set by the connected HTTP/2 peer. msecs <number> callback <Function> Used to set a callback function that is called when there is no activity on the Http2Session after msecs milliseconds. The given callback is registered as a listener on the 'timeout' event. <net.Socket> | <tls.TLSSocket> Returns a Proxy object that acts as a net.Socket (or tls.TLSSocket ) but limits available methods to ones safe to use with HTTP/2. destroy , emit , end , pause , read , resume , and write will throw an error with code ERR_HTTP2_NO_SOCKET_MANIPULATION . See Http2Session and Sockets for more information. setTimeout method will be called on this Http2Session . All other interactions will be routed directly to the socket. Provides miscellaneous information about the current state of the Http2Session . <Object> effectiveLocalWindowSize <number> The current local (receive) flow control window size for the Http2Session . effectiveRecvDataLength <number> The current number of bytes that have been received since the last flow control WINDOW_UPDATE . nextStreamID <number> The numeric identifier to be used the next time a new Http2Stream is created by this Http2Session . localWindowSize <number> The number of bytes that the remote peer can send without receiving a WINDOW_UPDATE . lastProcStreamID <number> The numeric id of the Http2Stream for which a HEADERS or DATA frame was most recently received. remoteWindowSize <number> The number of bytes that this Http2Session may send without receiving a WINDOW_UPDATE . outboundQueueSize <number> The number of frames currently within the outbound queue for this Http2Session . deflateDynamicTableSize <number> The current size in bytes of the outbound header compression state table. inflateDynamicTableSize <number> The current size in bytes of the inbound header compression state table. An object describing the current status of this Http2Session . settings <HTTP/2 Settings Object> Updates the current local settings for this Http2Session and sends a new SETTINGS frame to the connected HTTP/2 peer. Once called, the http2session.pendingSettingsAck property will be true while the session is waiting for the remote peer to acknowledge the new settings. http2session.remoteSettings http2session.setTimeout(msecs, callback) http2session.socket http2session.state http2session.settings(settings) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 430/739 The new settings will not become effective until the SETTINGS acknowledgment is received and the 'localSettings' event is emitted. It is possible to send multiple SETTINGS frames while acknowledgment is still pending. <number> The http2session.type will be equal to http2.constants.NGHTTP2_SESSION_SERVER if this Http2Session instance is a server, and http2.constants.NGHTTP2_SESSION_CLIENT if the instance is a client. Calls unref() on this Http2Session instance's underlying net.Socket . alt <string> A description of the alternative service configuration as defined by RFC 7838 . originOrStream <number> | <string> | <URL> | <Object> Either a URL string specifying the origin (or an Object with an origin property) or the numeric identifier of an active Http2Stream as given by the http2stream.id property. Submits an ALTSVC frame (as defined by RFC 7838 ) to the connected client. const http2 = require('http2'); const server = http2.createServer(); server.on('session', (session) => { // Set altsvc for origin https://example.org:80 session.altsvc('h2=":8000"', 'https://example.org:80'); }); server.on('stream', (stream) => { // Set altsvc for a specific stream stream.session.altsvc('h2=":8000"', stream.id); }); Sending an ALTSVC frame with a specific stream ID indicates that the alternate service is associated with the origin of the given Http2Stream . The alt and origin string must contain only ASCII bytes and are strictly interpreted as a sequence of ASCII bytes. The special value 'clear' may be passed to clear any previously set alternative service for a given domain. When a string is passed for the originOrStream argument, it will be parsed as a URL and the origin will be derived. For instance, the origin for the HTTP URL 'https://example.org/foo/bar' is the ASCII string 'https://example.org' . An error will be thrown if either the given string cannot be parsed as a URL or if a valid origin cannot be derived. A URL object, or any object with an origin property, may be passed as originOrStream , in which case the value of the origin property will be used. The value of the origin property must be a properly serialized ASCII origin. The format of the alt parameter is strictly defined by RFC 7838 as an ASCII string containing a comma-delimited list of "alternative" protocols associated with a specific host and port. http2session.type http2session.unref() Class: ServerHttp2Session serverhttp2session.altsvc(alt, originOrStream) Specifying alternative services 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 431/739 For example, the value 'h2="example.org:81"' indicates that the HTTP/2 protocol is available on the host 'example.org' on TCP/IP port 81. The host and port must be contained within the quote ( " ) characters. Multiple alternatives may be specified, for instance: 'h2="example.org:81", h2=":82"' . The protocol identifier ( 'h2' in the examples) may be any valid ALPN Protocol ID . The syntax of these values is not validated by the Node.js implementation and are passed through as provided by the user or received from the peer. origins <string> | <URL> | <Object> One or more URL Strings passed as separate arguments. Submits an ORIGIN frame (as defined by RFC 8336 ) to the connected client to advertise the set of origins for which the server is capable of providing authoritative responses. const http2 = require('http2'); const options = getSecureOptionsSomehow(); const server = http2.createSecureServer(options); server.on('stream', (stream) => { stream.respond(); stream.end('ok'); }); server.on('session', (session) => { session.origin('https://example.com', 'https://example.org'); }); When a string is passed as an origin , it will be parsed as a URL and the origin will be derived. For instance, the origin for the HTTP URL 'https://example.org/foo/bar' is the ASCII string 'https://example.org' . An error will be thrown if either the given string cannot be parsed as a URL or if a valid origin cannot be derived. A URL object, or any object with an origin property, may be passed as an origin , in which case the value of the origin property will be used. The value of the origin property must be a properly serialized ASCII origin. Alternatively, the origins option may be used when creating a new HTTP/2 server using the http2.createSecureServer() method: const http2 = require('http2'); const options = getSecureOptionsSomehow(); options.origins = ['https://example.com', 'https://example.org']; const server = http2.createSecureServer(options); server.on('stream', (stream) => { stream.respond(); stream.end('ok'); }); alt <string> origin <string> streamId <number> serverhttp2session.origin(...origins) Class: ClientHttp2Session Event: 'altsvc' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 432/739 The 'altsvc' event is emitted whenever an ALTSVC frame is received by the client. The event is emitted with the ALTSVC value, origin, and stream ID. If no origin is provided in the ALTSVC frame, origin will be an empty string. const http2 = require('http2'); const client = http2.connect('https://example.org'); client.on('altsvc', (alt, origin, streamId) => { console.log(alt); console.log(origin); console.log(streamId); }); origins <string[]> The 'origin' event is emitted whenever an ORIGIN frame is received by the client. The event is emitted with an array of origin strings. The http2session.originSet will be updated to include the received origins. const http2 = require('http2'); const client = http2.connect('https://example.org'); client.on('origin', (origins) => { for (let n = 0; n < origins.length; n++) console.log(origins[n]); }); The 'origin' event is only emitted when using a secure TLS connection. headers <HTTP/2 Headers Object> options <Object> endStream <boolean> true if the Http2Stream writable side should be closed initially, such as when sending a GET request that should not expect a payload body. exclusive <boolean> When true and parent identifies a parent Stream, the created stream is made the sole direct dependency of the parent, with all other existing dependents made a dependent of the newly created stream. Default: false . parent <number> Specifies the numeric identifier of a stream the newly created stream is dependent on. weight <number> Specifies the relative dependency of a stream in relation to other streams with the same parent . The value is a number between 1 and 256 (inclusive). waitForTrailers <boolean> When true , the Http2Stream will emit the 'wantTrailers' event after the final DATA frame has been sent. Returns: <ClientHttp2Stream> For HTTP/2 Client Http2Session instances only, the http2session.request() creates and returns an Http2Stream instance that can be used to send an HTTP/2 request to the connected server. This method is only available if http2session.type is equal to http2.constants.NGHTTP2_SESSION_CLIENT . Event: 'origin' clienthttp2session.request(headers[, options]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 433/739 const http2 = require('http2'); const clientSession = http2.connect('https://localhost:1234'); const { HTTP2_HEADER_PATH, HTTP2_HEADER_STATUS } = http2.constants; const req = clientSession.request({ [HTTP2_HEADER_PATH]: '/' }); req.on('response', (headers) => { console.log(headers[HTTP2_HEADER_STATUS]); req.on('data', (chunk) => { /* .. */ }); req.on('end', () => { /* .. */ }); }); When the options.waitForTrailers option is set, the 'wantTrailers' event is emitted immediately after queuing the last chunk of payload data to be sent. The http2stream.sendTrailers() method can then be called to send trailing headers to the peer. When options.waitForTrailers is set, the Http2Stream will not automatically close when the final DATA frame is transmitted. User code must call either http2stream.sendTrailers() or http2stream.close() to close the Http2Stream . The :method and :path pseudo-headers are not specified within headers , they respectively default to: :method = 'GET' :path = / Extends: <stream.Duplex> Each instance of the Http2Stream class represents a bidirectional HTTP/2 communications stream over an Http2Session instance. Any single Http2Session may have up to 231-1 Http2Stream instances over its lifetime. User code will not construct Http2Stream instances directly. Rather, these are created, managed, and provided to user code through the Http2Session instance. On the server, Http2Stream instances are created either in response to an incoming HTTP request (and handed off to user code via the 'stream' event), or in response to a call to the http2stream.pushStream() method. On the client, Http2Stream instances are created and returned when either the http2session.request() method is called, or in response to an incoming 'push' event. The Http2Stream class is a base for the ServerHttp2Stream and ClientHttp2Stream classes, each of which is used specifically by either the Server or Client side, respectively. All Http2Stream instances are Duplex streams. The Writable side of the Duplex is used to send data to the connected peer, while the Readable side is used to receive data sent by the connected peer. On the server side, instances of ServerHttp2Stream are created either when: A new HTTP/2 HEADERS frame with a previously unused stream ID is received; The http2stream.pushStream() method is called. On the client side, instances of ClientHttp2Stream are created when the http2session.request() method is called. Class: Http2Stream Http2Stream Lifecycle Creation 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 434/739 On the client, the Http2Stream instance returned by http2session.request() may not be immediately ready for use if the parent Http2Session has not yet been fully established. In such cases, operations called on the Http2Stream will be buffered until the 'ready' event is emitted. User code should rarely, if ever, need to handle the 'ready' event directly. The ready status of an Http2Stream can be determined by checking the value of http2stream.id . If the value is undefined , the stream is not yet ready for use. All Http2Stream instances are destroyed either when: An RST_STREAM frame for the stream is received by the connected peer. The http2stream.close() method is called. The http2stream.destroy() or http2session.destroy() methods are called. When an Http2Stream instance is destroyed, an attempt will be made to send an RST_STREAM frame will be sent to the connected peer. When the Http2Stream instance is destroyed, the 'close' event will be emitted. Because Http2Stream is an instance of stream.Duplex , the 'end' event will also be emitted if the stream data is currently flowing. The 'error' event may also be emitted if http2stream.destroy() was called with an Error passed as the first argument. After the Http2Stream has been destroyed, the http2stream.destroyed property will be true and the http2stream.rstCode property will specify the RST_STREAM error code. The Http2Stream instance is no longer usable once destroyed. The 'aborted' event is emitted whenever a Http2Stream instance is abnormally aborted in mid-communication. The 'aborted' event will only be emitted if the Http2Stream writable side has not been ended. The 'close' event is emitted when the Http2Stream is destroyed. Once this event is emitted, the Http2Stream instance is no longer usable. The HTTP/2 error code used when closing the stream can be retrieved using the http2stream.rstCode property. If the code is any value other than NGHTTP2_NO_ERROR ( 0 ), an 'error' event will have also been emitted. error <Error> The 'error' event is emitted when an error occurs during the processing of an Http2Stream . The 'frameError' event is emitted when an error occurs while attempting to send a frame. When invoked, the handler function will receive an integer argument identifying the frame type, and an integer argument identifying the error code. The Http2Stream instance will be destroyed immediately after the 'frameError' event is emitted. The 'timeout' event is emitted after no activity is received for this Http2Stream within the number of milliseconds set using http2stream.setTimeout() . The 'trailers' event is emitted when a block of headers associated with trailing header fields is received. The listener callback is passed the HTTP/2 Headers Object and flags associated with the headers. Destruction Event: 'aborted' Event: 'close' Event: 'error' Event: 'frameError' Event: 'timeout' Event: 'trailers' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 435/739 Note that this event might not be emitted if http2stream.end() is called before trailers are received and the incoming data is not being read or listened for. stream.on('trailers', (headers, flags) => { console.log(headers); }); The 'wantTrailers' event is emitted when the Http2Stream has queued the final DATA frame to be sent on a frame and the Http2Stream is ready to send trailing headers. When initiating a request or response, the waitForTrailers option must be set for this event to be emitted. <boolean> Set to true if the Http2Stream instance was aborted abnormally. When set, the 'aborted' event will have been emitted. <number> This property shows the number of characters currently buffered to be written. See net.Socket.bufferSize for details. code <number> Unsigned 32-bit integer identifying the error code. Default: http2.constants.NGHTTP2_NO_ERROR ( 0x00 ). callback <Function> An optional function registered to listen for the 'close' event. Closes the Http2Stream instance by sending an RST_STREAM frame to the connected HTTP/2 peer. <boolean> Set to true if the Http2Stream instance has been closed. <boolean> Set to true if the Http2Stream instance has been destroyed and is no longer usable. <boolean> Set the true if the END_STREAM flag was set in the request or response HEADERS frame received, indicating that no additional data should be received and the readable side of the Http2Stream will be closed. <boolean> Set to true if the Http2Stream instance has not yet been assigned a numeric stream identifier. options <Object> Event: 'wantTrailers' http2stream.aborted http2stream.bufferSize http2stream.close(code[, callback]) http2stream.closed http2stream.destroyed http2stream.endAfterHeaders http2stream.pending http2stream.priority(options) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 436/739 exclusive <boolean> When true and parent identifies a parent Stream, this stream is made the sole direct dependency of the parent, with all other existing dependents made a dependent of this stream. Default: false . parent <number> Specifies the numeric identifier of a stream this stream is dependent on. weight <number> Specifies the relative dependency of a stream in relation to other streams with the same parent . The value is a number between 1 and 256 (inclusive). silent <boolean> When true , changes the priority locally without sending a PRIORITY frame to the connected peer. Updates the priority for this Http2Stream instance. <number> Set to the RST_STREAM error code reported when the Http2Stream is destroyed after either receiving an RST_STREAM frame from the connected peer, calling http2stream.close() , or http2stream.destroy() . Will be undefined if the Http2Stream has not been closed. <HTTP/2 Headers Object> An object containing the outbound headers sent for this Http2Stream . <HTTP/2 Headers Object[]> An array of objects containing the outbound informational (additional) headers sent for this Http2Stream . <HTTP/2 Headers Object> An object containing the outbound trailers sent for this HttpStream . <Http2Session> A reference to the Http2Session instance that owns this Http2Stream . The value will be undefined after the Http2Stream instance is destroyed. msecs <number> callback <Function> const http2 = require('http2'); const client = http2.connect('http://example.org:8000'); const { NGHTTP2_CANCEL } = http2.constants; const req = client.request({ ':path': '/' }); // Cancel the stream if there's no activity after 5 seconds req.setTimeout(5000, () => req.close(NGHTTP2_CANCEL)); Provides miscellaneous information about the current state of the Http2Stream . http2stream.rstCode http2stream.sentHeaders http2stream.sentInfoHeaders http2stream.sentTrailers http2stream.session http2stream.setTimeout(msecs, callback) http2stream.state 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 437/739 <Object> localWindowSize <number> The number of bytes the connected peer may send for this Http2Stream without receiving a WINDOW_UPDATE . state <number> A flag indicating the low-level current state of the Http2Stream as determined by nghttp2 . localClose <number> true if this Http2Stream has been closed locally. remoteClose <number> true if this Http2Stream has been closed remotely. sumDependencyWeight <number> The sum weight of all Http2Stream instances that depend on this Http2Stream as specified using PRIORITY frames. weight <number> The priority weight of this Http2Stream . A current state of this Http2Stream . headers <HTTP/2 Headers Object> Sends a trailing HEADERS frame to the connected HTTP/2 peer. This method will cause the Http2Stream to be immediately closed and must only be called after the 'wantTrailers' event has been emitted. When sending a request or sending a response, the options.waitForTrailers option must be set in order to keep the Http2Stream open after the final DATA frame so that trailers can be sent. const http2 = require('http2'); const server = http2.createServer(); server.on('stream', (stream) => { stream.respond(undefined, { waitForTrailers: true }); stream.on('wantTrailers', () => { stream.sendTrailers({ xyz: 'abc' }); }); stream.end('Hello World'); }); The HTTP/1 specification forbids trailers from containing HTTP/2 pseudo-header fields (e.g. ':method' , ':path' , etc). Extends <Http2Stream> The ClientHttp2Stream class is an extension of Http2Stream that is used exclusively on HTTP/2 Clients. Http2Stream instances on the client provide events such as 'response' and 'push' that are only relevant on the client. Emitted when the server sends a 100 Continue status, usually because the request contained Expect: 100-continue . This is an instruction that the client should send the request body. The 'headers' event is emitted when an additional block of headers is received for a stream, such as when a block of 1xx informational headers is received. The listener callback is passed the HTTP/2 Headers Object and flags associated with the headers. stream.on('headers', (headers, flags) => { console.log(headers); }); http2stream.sendTrailers(headers) Class: ClientHttp2Stream Event: 'continue' Event: 'headers' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 438/739 The 'push' event is emitted when response headers for a Server Push stream are received. The listener callback is passed the HTTP/2 Headers Object and flags associated with the headers. stream.on('push', (headers, flags) => { console.log(headers); }); The 'response' event is emitted when a response HEADERS frame has been received for this stream from the connected HTTP/2 server. The listener is invoked with two arguments: an Object containing the received HTTP/2 Headers Object , and flags associated with the headers. const http2 = require('http2'); const client = http2.connect('https://localhost'); const req = client.request({ ':path': '/' }); req.on('response', (headers, flags) => { console.log(headers[':status']); }); Extends: <Http2Stream> The ServerHttp2Stream class is an extension of Http2Stream that is used exclusively on HTTP/2 Servers. Http2Stream instances on the server provide additional methods such as http2stream.pushStream() and http2stream.respond() that are only relevant on the server. headers <HTTP/2 Headers Object> Sends an additional informational HEADERS frame to the connected HTTP/2 peer. <boolean> True if headers were sent, false otherwise (read-only). <boolean> Read-only property mapped to the SETTINGS_ENABLE_PUSH flag of the remote client's most recent SETTINGS frame. Will be true if the remote peer accepts push streams, false otherwise. Settings are the same for every Http2Stream in the same Http2Session . headers <HTTP/2 Headers Object> options <Object> exclusive <boolean> When true and parent identifies a parent Stream, the created stream is made the sole direct dependency of the parent, with all other existing dependents made a dependent of the newly created stream. Default: false . parent <number> Specifies the numeric identifier of a stream the newly created stream is dependent on. Event: 'push' Event: 'response' Class: ServerHttp2Stream http2stream.additionalHeaders(headers) http2stream.headersSent http2stream.pushAllowed http2stream.pushStream(headers[, options], callback) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 439/739 callback <Function> Callback that is called once the push stream has been initiated. err <Error> pushStream <ServerHttp2Stream> The returned pushStream object. headers <HTTP/2 Headers Object> Headers object the pushStream was initiated with. Initiates a push stream. The callback is invoked with the new Http2Stream instance created for the push stream passed as the second argument, or an Error passed as the first argument. const http2 = require('http2'); const server = http2.createServer(); server.on('stream', (stream) => { stream.respond({ ':status': 200 }); stream.pushStream({ ':path': '/' }, (err, pushStream, headers) => { if (err) throw err; pushStream.respond({ ':status': 200 }); pushStream.end('some pushed data'); }); stream.end('some data'); }); Setting the weight of a push stream is not allowed in the HEADERS frame. Pass a weight value to http2stream.priority with the silent option set to true to enable server-side bandwidth balancing between concurrent streams. Calling http2stream.pushStream() from within a pushed stream is not permitted and will throw an error. headers <HTTP/2 Headers Object> options <Object> endStream <boolean> Set to true to indicate that the response will not include payload data. waitForTrailers <boolean> When true , the Http2Stream will emit the 'wantTrailers' event after the final DATA frame has been sent. const http2 = require('http2'); const server = http2.createServer(); server.on('stream', (stream) => { stream.respond({ ':status': 200 }); stream.end('some data'); }); When the options.waitForTrailers option is set, the 'wantTrailers' event will be emitted immediately after queuing the last chunk of payload data to be sent. The http2stream.sendTrailers() method can then be used to sent trailing header fields to the peer. When options.waitForTrailers is set, the Http2Stream will not automatically close when the final DATA frame is transmitted. User code must call either http2stream.sendTrailers() or http2stream.close() to close the Http2Stream . const http2 = require('http2'); const server = http2.createServer(); server.on('stream', (stream) => { stream.respond({ ':status': 200 }, { waitForTrailers: true }); http2stream.respond([headers[, options]]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 440/739 stream.on('wantTrailers', () => { stream.sendTrailers({ ABC: 'some value to send' }); }); stream.end('some data'); }); fd <number> A readable file descriptor. headers <HTTP/2 Headers Object> options <Object> statCheck <Function> waitForTrailers <boolean> When true , the Http2Stream will emit the 'wantTrailers' event after the final DATA frame has been sent. offset <number> The offset position at which to begin reading. length <number> The amount of data from the fd to send. Initiates a response whose data is read from the given file descriptor. No validation is performed on the given file descriptor. If an error occurs while attempting to read data using the file descriptor, the Http2Stream will be closed using an RST_STREAM frame using the standard INTERNAL_ERROR code. When used, the Http2Stream object's Duplex interface will be closed automatically. const http2 = require('http2'); const fs = require('fs'); const server = http2.createServer(); server.on('stream', (stream) => { const fd = fs.openSync('/some/file', 'r'); const stat = fs.fstatSync(fd); const headers = { 'content-length': stat.size, 'last-modified': stat.mtime.toUTCString(), 'content-type': 'text/plain' }; stream.respondWithFD(fd, headers); stream.on('close', () => fs.closeSync(fd)); }); The optional options.statCheck function may be specified to give user code an opportunity to set additional content headers based on the fs.Stat details of the given fd. If the statCheck function is provided, the http2stream.respondWithFD() method will perform an fs.fstat() call to collect details on the provided file descriptor. The offset and length options may be used to limit the response to a specific range subset. This can be used, for instance, to support HTTP Range requests. The file descriptor is not closed when the stream is closed, so it will need to be closed manually once it is no longer needed. Note that using the same file descriptor concurrently for multiple streams is not supported and may result in data loss. Re-using a file descriptor after a stream has finished is supported. http2stream.respondWithFD(fd[, headers[, options]]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 441/739 When the options.waitForTrailers option is set, the 'wantTrailers' event will be emitted immediately after queuing the last chunk of payload data to be sent. The http2stream.sendTrailers() method can then be used to sent trailing header fields to the peer. When options.waitForTrailers is set, the Http2Stream will not automatically close when the final DATA frame is transmitted. User code must call either http2stream.sendTrailers() or http2stream.close() to close the Http2Stream . const http2 = require('http2'); const fs = require('fs'); const server = http2.createServer(); server.on('stream', (stream) => { const fd = fs.openSync('/some/file', 'r'); const stat = fs.fstatSync(fd); const headers = { 'content-length': stat.size, 'last-modified': stat.mtime.toUTCString(), 'content-type': 'text/plain' }; stream.respondWithFD(fd, headers, { waitForTrailers: true }); stream.on('wantTrailers', () => { stream.sendTrailers({ ABC: 'some value to send' }); }); stream.on('close', () => fs.closeSync(fd)); }); path <string> | <Buffer> | <URL> headers <HTTP/2 Headers Object> options <Object> statCheck <Function> onError <Function> Callback function invoked in the case of an error before send. waitForTrailers <boolean> When true , the Http2Stream will emit the 'wantTrailers' event after the final DATA frame has been sent. offset <number> The offset position at which to begin reading. length <number> The amount of data from the fd to send. Sends a regular file as the response. The path must specify a regular file or an 'error' event will be emitted on the Http2Stream object. When used, the Http2Stream object's Duplex interface will be closed automatically. The optional options.statCheck function may be specified to give user code an opportunity to set additional content headers based on the fs.Stat details of the given file: If an error occurs while attempting to read the file data, the Http2Stream will be closed using an RST_STREAM frame using the standard INTERNAL_ERROR code. If the onError callback is defined, then it will be called. Otherwise the stream will be destroyed. Example using a file path: http2stream.respondWithFile(path[, headers[, options]]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 442/739 const http2 = require('http2'); const server = http2.createServer(); server.on('stream', (stream) => { function statCheck(stat, headers) { headers['last-modified'] = stat.mtime.toUTCString(); } function onError(err) { if (err.code === 'ENOENT') { stream.respond({ ':status': 404 }); } else { stream.respond({ ':status': 500 }); } stream.end(); } stream.respondWithFile('/some/file', { 'content-type': 'text/plain' }, { statCheck, onError }); }); The options.statCheck function may also be used to cancel the send operation by returning false . For instance, a conditional request may check the stat results to determine if the file has been modified to return an appropriate 304 response: const http2 = require('http2'); const server = http2.createServer(); server.on('stream', (stream) => { function statCheck(stat, headers) { // Check the stat here... stream.respond({ ':status': 304 }); return false; // Cancel the send operation } stream.respondWithFile('/some/file', { 'content-type': 'text/plain' }, { statCheck }); }); The content-length header field will be automatically set. The offset and length options may be used to limit the response to a specific range subset. This can be used, for instance, to support HTTP Range requests. The options.onError function may also be used to handle all the errors that could happen before the delivery of the file is initiated. The default behavior is to destroy the stream. When the options.waitForTrailers option is set, the 'wantTrailers' event will be emitted immediately after queuing the last chunk of payload data to be sent. The http2stream.sendTrailers() method can then be used to sent trailing header fields to the peer. When options.waitForTrailers is set, the Http2Stream will not automatically close when the final DATA frame is transmitted. User code must call either http2stream.sendTrailers() or http2stream.close() to close the Http2Stream . 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 443/739 const http2 = require('http2'); const server = http2.createServer(); server.on('stream', (stream) => { stream.respondWithFile('/some/file', { 'content-type': 'text/plain' }, { waitForTrailers: true }); stream.on('wantTrailers', () => { stream.sendTrailers({ ABC: 'some value to send' }); }); }); Extends: <net.Server> Instances of Http2Server are created using the http2.createServer() function. The Http2Server class is not exported directly by the http2 module. request <http2.Http2ServerRequest> response <http2.Http2ServerResponse> If a 'request' listener is registered or http2.createServer() is supplied a callback function, the 'checkContinue' event is emitted each time a request with an HTTP Expect: 100-continue is received. If this event is not listened for, the server will automatically respond with a status 100 Continue as appropriate. Handling this event involves calling response.writeContinue() if the client should continue to send the request body, or generating an appropriate HTTP response (e.g. 400 Bad Request) if the client should not continue to send the request body. Note that when this event is emitted and handled, the 'request' event will not be emitted. request <http2.Http2ServerRequest> response <http2.Http2ServerResponse> Emitted each time there is a request. Note that there may be multiple requests per session. See the Compatibility API . The 'session' event is emitted when a new Http2Session is created by the Http2Server . The 'sessionError' event is emitted when an 'error' event is emitted by an Http2Session object associated with the Http2Server . The 'stream' event is emitted when a 'stream' event has been emitted by an Http2Session associated with the server. const http2 = require('http2'); const { HTTP2_HEADER_METHOD, HTTP2_HEADER_PATH, Class: Http2Server Event: 'checkContinue' Event: 'request' Event: 'session' Event: 'sessionError' Event: 'stream' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 444/739 HTTP2_HEADER_STATUS, HTTP2_HEADER_CONTENT_TYPE } = http2.constants; const server = http2.createServer(); server.on('stream', (stream, headers, flags) => { const method = headers[HTTP2_HEADER_METHOD]; const path = headers[HTTP2_HEADER_PATH]; // ... stream.respond({ [HTTP2_HEADER_STATUS]: 200, [HTTP2_HEADER_CONTENT_TYPE]: 'text/plain' }); stream.write('hello '); stream.end('world'); }); The 'timeout' event is emitted when there is no activity on the Server for a given number of milliseconds set using http2server.setTimeout() . Default: 2 minutes. callback <Function> Stops the server from accepting new connections. See net.Server.close() . Note that this is not analogous to restricting new requests since HTTP/2 connections are persistent. To achieve a similar graceful shutdown behavior, consider also using http2session.close() on active sessions. msecs <number> Default: 120000 (2 minutes) callback <Function> Returns: <Http2Server> Used to set the timeout value for http2 server requests, and sets a callback function that is called when there is no activity on the Http2Server after msecs milliseconds. The given callback is registered as a listener on the 'timeout' event. In case of no callback function were assigned, a new ERR_INVALID_CALLBACK error will be thrown. Extends: <tls.Server> Instances of Http2SecureServer are created using the http2.createSecureServer() function. The Http2SecureServer class is not exported directly by the http2 module. request <http2.Http2ServerRequest> response <http2.Http2ServerResponse> Event: 'timeout' server.close([callback]) server.setTimeout([msecs][, callback]) Class: Http2SecureServer Event: 'checkContinue' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 445/739 If a 'request' listener is registered or http2.createSecureServer() is supplied a callback function, the 'checkContinue' event is emitted each time a request with an HTTP Expect: 100-continue is received. If this event is not listened for, the server will automatically respond with a status 100 Continue as appropriate. Handling this event involves calling response.writeContinue() if the client should continue to send the request body, or generating an appropriate HTTP response (e.g. 400 Bad Request) if the client should not continue to send the request body. Note that when this event is emitted and handled, the 'request' event will not be emitted. request <http2.Http2ServerRequest> response <http2.Http2ServerResponse> Emitted each time there is a request. Note that there may be multiple requests per session. See the Compatibility API . The 'session' event is emitted when a new Http2Session is created by the Http2SecureServer . The 'sessionError' event is emitted when an 'error' event is emitted by an Http2Session object associated with the Http2SecureServer . The 'stream' event is emitted when a 'stream' event has been emitted by an Http2Session associated with the server. const http2 = require('http2'); const { HTTP2_HEADER_METHOD, HTTP2_HEADER_PATH, HTTP2_HEADER_STATUS, HTTP2_HEADER_CONTENT_TYPE } = http2.constants; const options = getOptionsSomehow(); const server = http2.createSecureServer(options); server.on('stream', (stream, headers, flags) => { const method = headers[HTTP2_HEADER_METHOD]; const path = headers[HTTP2_HEADER_PATH]; // ... stream.respond({ [HTTP2_HEADER_STATUS]: 200, [HTTP2_HEADER_CONTENT_TYPE]: 'text/plain' }); stream.write('hello '); stream.end('world'); }); The 'timeout' event is emitted when there is no activity on the Server for a given number of milliseconds set using http2secureServer.setTimeout() . Default: 2 minutes. Event: 'request' Event: 'session' Event: 'sessionError' Event: 'stream' Event: 'timeout' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 446/739 The 'unknownProtocol' event is emitted when a connecting client fails to negotiate an allowed protocol (i.e. HTTP/2 or HTTP/1.1). The event handler receives the socket for handling. If no listener is registered for this event, the connection is terminated. See the Compatibility API . callback <Function> Stops the server from accepting new connections. See tls.Server.close() . Note that this is not analogous to restricting new requests since HTTP/2 connections are persistent. To achieve a similar graceful shutdown behavior, consider also using http2session.close() on active sessions. msecs <number> Default: 120000 (2 minutes) callback <Function> Returns: <Http2SecureServer> Used to set the timeout value for http2 secure server requests, and sets a callback function that is called when there is no activity on the Http2SecureServer after msecs milliseconds. The given callback is registered as a listener on the 'timeout' event. In case of no callback function were assigned, a new ERR_INVALID_CALLBACK error will be thrown. options <Object> maxDeflateDynamicTableSize <number> Sets the maximum dynamic table size for deflating header fields. Default: 4Kib . maxSessionMemory <number> Sets the maximum memory that the Http2Session is permitted to use. The value is expressed in terms of number of megabytes, e.g. 1 equal 1 megabyte. The minimum value allowed is 1 . This is a credit based limit, existing Http2Stream s may cause this limit to be exceeded, but new Http2Stream instances will be rejected while this limit is exceeded. The current number of Http2Stream sessions, the current memory use of the header compression tables, current data queued to be sent, and unacknowledged PING and SETTINGS frames are all counted towards the current limit. Default: 10 . maxHeaderListPairs <number> Sets the maximum number of header entries. The minimum value is 4 . Default: 128 . maxOutstandingPings <number> Sets the maximum number of outstanding, unacknowledged pings. Default: 10 . maxSendHeaderBlockLength <number> Sets the maximum allowed size for a serialized, compressed block of headers. Attempts to send headers that exceed this limit will result in a 'frameError' event being emitted and the stream being closed and destroyed. paddingStrategy <number> Identifies the strategy used for determining the amount of padding to use for HEADERS and DATA frames. Default: http2.constants.PADDING_STRATEGY_NONE . Value may be one of: http2.constants.PADDING_STRATEGY_NONE - Specifies that no padding is to be applied. http2.constants.PADDING_STRATEGY_MAX - Specifies that the maximum amount of padding, as determined by the internal implementation, is to be applied. http2.constants.PADDING_STRATEGY_CALLBACK - Specifies that the user provided options.selectPadding() callback is to be used to determine the amount of padding. http2.constants.PADDING_STRATEGY_ALIGNED - Will attempt to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, however, there is a maximum allowed number of padding bytes that is determined by current flow control state and settings. If this maximum is less than the calculated amount needed to ensure alignment, the maximum will be used and the total frame length will not necessarily be aligned at 8 bytes. Event: 'unknownProtocol' server.close([callback]) server.setTimeout([msecs][, callback]) http2.createServer(options[, onRequestHandler]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 447/739 peerMaxConcurrentStreams <number> Sets the maximum number of concurrent streams for the remote peer as if a SETTINGS frame had been received. Will be overridden if the remote peer sets its own value for maxConcurrentStreams . Default: 100 . selectPadding <Function> When options.paddingStrategy is equal to http2.constants.PADDING_STRATEGY_CALLBACK , provides the callback function used to determine the padding. See Using options.selectPadding() . settings <HTTP/2 Settings Object> The initial settings to send to the remote peer upon connection. Http1IncomingMessage <http.IncomingMessage> Specifies the IncomingMessage class to used for HTTP/1 fallback. Useful for extending the original http.IncomingMessage . Default: http.IncomingMessage . Http1ServerResponse <http.ServerResponse> Specifies the ServerResponse class to used for HTTP/1 fallback. Useful for extending the original http.ServerResponse . Default: http.ServerResponse . Http2ServerRequest <http2.Http2ServerRequest> Specifies the Http2ServerRequest class to use. Useful for extending the original Http2ServerRequest . Default: Http2ServerRequest . Http2ServerResponse <http2.Http2ServerResponse> Specifies the Http2ServerResponse class to use. Useful for extending the original Http2ServerResponse . Default: Http2ServerResponse . onRequestHandler <Function> See Compatibility API Returns: <Http2Server> Returns a net.Server instance that creates and manages Http2Session instances. Since there are no browsers known that support unencrypted HTTP/2 , the use of http2.createSecureServer() is necessary when communicating with browser clients. const http2 = require('http2'); // Create an unencrypted HTTP/2 server. // Since there are no browsers known that support // unencrypted HTTP/2, the use of `http2.createSecureServer()` // is necessary when communicating with browser clients. const server = http2.createServer(); server.on('stream', (stream, headers) => { stream.respond({ 'content-type': 'text/html', ':status': 200 }); stream.end('<h1>Hello World</h1>'); }); server.listen(80); options <Object> allowHTTP1 <boolean> Incoming client connections that do not support HTTP/2 will be downgraded to HTTP/1.x when set to true . See the 'unknownProtocol' event. See ALPN negotiation . Default: false . maxDeflateDynamicTableSize <number> Sets the maximum dynamic table size for deflating header fields. Default: 4Kib . maxSessionMemory <number> Sets the maximum memory that the Http2Session is permitted to use. The value is expressed in terms of number of megabytes, e.g. 1 equal 1 megabyte. The minimum value allowed is 1 . This is a credit based limit, existing Http2Stream s may cause this limit to be exceeded, but new Http2Stream instances will be rejected while this limit is exceeded. The http2.createSecureServer(options[, onRequestHandler]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 448/739 current number of Http2Stream sessions, the current memory use of the header compression tables, current data queued to be sent, and unacknowledged PING and SETTINGS frames are all counted towards the current limit. Default: 10 . maxHeaderListPairs <number> Sets the maximum number of header entries. The minimum value is 4 . Default: 128 . maxOutstandingPings <number> Sets the maximum number of outstanding, unacknowledged pings. Default: 10 . maxSendHeaderBlockLength <number> Sets the maximum allowed size for a serialized, compressed block of headers. Attempts to send headers that exceed this limit will result in a 'frameError' event being emitted and the stream being closed and destroyed. paddingStrategy <number> Identifies the strategy used for determining the amount of padding to use for HEADERS and DATA frames. Default: http2.constants.PADDING_STRATEGY_NONE . Value may be one of: http2.constants.PADDING_STRATEGY_NONE - Specifies that no padding is to be applied. http2.constants.PADDING_STRATEGY_MAX - Specifies that the maximum amount of padding, as determined by the internal implementation, is to be applied. http2.constants.PADDING_STRATEGY_CALLBACK - Specifies that the user provided options.selectPadding() callback is to be used to determine the amount of padding. http2.constants.PADDING_STRATEGY_ALIGNED - Will attempt to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, however, there is a maximum allowed number of padding bytes that is determined by current flow control state and settings. If this maximum is less than the calculated amount needed to ensure alignment, the maximum will be used and the total frame length will not necessarily be aligned at 8 bytes. peerMaxConcurrentStreams <number> Sets the maximum number of concurrent streams for the remote peer as if a SETTINGS frame had been received. Will be overridden if the remote peer sets its own value for maxConcurrentStreams . Default: 100 . selectPadding <Function> When options.paddingStrategy is equal to http2.constants.PADDING_STRATEGY_CALLBACK , provides the callback function used to determine the padding. See Using options.selectPadding() . settings <HTTP/2 Settings Object> The initial settings to send to the remote peer upon connection. ...: Any tls.createServer() options can be provided. For servers, the identity options ( pfx or key / cert ) are usually required. origins <string[]> An array of origin strings to send within an ORIGIN frame immediately following creation of a new server Http2Session . onRequestHandler <Function> See Compatibility API Returns: <Http2SecureServer> Returns a tls.Server instance that creates and manages Http2Session instances. const http2 = require('http2'); const fs = require('fs'); const options = { key: fs.readFileSync('server-key.pem'), cert: fs.readFileSync('server-cert.pem') }; // Create a secure HTTP/2 server const server = http2.createSecureServer(options); server.on('stream', (stream, headers) => { stream.respond({ 'content-type': 'text/html', ':status': 200 }); stream.end('<h1>Hello World</h1>'); }); 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 449/739 server.listen(80); authority <string> | <URL> options <Object> maxDeflateDynamicTableSize <number> Sets the maximum dynamic table size for deflating header fields. Default: 4Kib . maxSessionMemory <number> Sets the maximum memory that the Http2Session is permitted to use. The value is expressed in terms of number of megabytes, e.g. 1 equal 1 megabyte. The minimum value allowed is 1 . This is a credit based limit, existing Http2Stream s may cause this limit to be exceeded, but new Http2Stream instances will be rejected while this limit is exceeded. The current number of Http2Stream sessions, the current memory use of the header compression tables, current data queued to be sent, and unacknowledged PING and SETTINGS frames are all counted towards the current limit. Default: 10 . maxHeaderListPairs <number> Sets the maximum number of header entries. The minimum value is 1 . Default: 128 . maxOutstandingPings <number> Sets the maximum number of outstanding, unacknowledged pings. Default: 10 . maxReservedRemoteStreams <number> Sets the maximum number of reserved push streams the client will accept at any given time. Once the current number of currently reserved push streams exceeds reaches this limit, new push streams sent by the server will be automatically rejected. maxSendHeaderBlockLength <number> Sets the maximum allowed size for a serialized, compressed block of headers. Attempts to send headers that exceed this limit will result in a 'frameError' event being emitted and the stream being closed and destroyed. paddingStrategy <number> Identifies the strategy used for determining the amount of padding to use for HEADERS and DATA frames. Default: http2.constants.PADDING_STRATEGY_NONE . Value may be one of: http2.constants.PADDING_STRATEGY_NONE - Specifies that no padding is to be applied. http2.constants.PADDING_STRATEGY_MAX - Specifies that the maximum amount of padding, as determined by the internal implementation, is to be applied. http2.constants.PADDING_STRATEGY_CALLBACK - Specifies that the user provided options.selectPadding() callback is to be used to determine the amount of padding. http2.constants.PADDING_STRATEGY_ALIGNED - Will attempt to apply enough padding to ensure that the total frame length, including the 9-byte header, is a multiple of 8. For each frame, however, there is a maximum allowed number of padding bytes that is determined by current flow control state and settings. If this maximum is less than the calculated amount needed to ensure alignment, the maximum will be used and the total frame length will not necessarily be aligned at 8 bytes. peerMaxConcurrentStreams <number> Sets the maximum number of concurrent streams for the remote peer as if a SETTINGS frame had been received. Will be overridden if the remote peer sets its own value for maxConcurrentStreams . Default: 100 . selectPadding <Function> When options.paddingStrategy is equal to http2.constants.PADDING_STRATEGY_CALLBACK , provides the callback function used to determine the padding. See Using options.selectPadding() . settings <HTTP/2 Settings Object> The initial settings to send to the remote peer upon connection. createConnection <Function> An optional callback that receives the URL instance passed to connect and the options object, and returns any Duplex stream that is to be used as the connection for this session. ...: Any net.connect() or tls.connect() options can be provided. listener <Function> Returns: <ClientHttp2Session> Returns a ClientHttp2Session instance. const http2 = require('http2'); const client = http2.connect('https://localhost:1234'); http2.connect(authority[, options][, listener]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 450/739 /* Use the client */ client.close(); Value Name Constant 0x00 No Error http2.constants.NGHTTP2_NO_ERROR 0x01 Protocol Error http2.constants.NGHTTP2_PROTOCOL_ERROR 0x02 Internal Error http2.constants.NGHTTP2_INTERNAL_ERROR 0x03 Flow Control Error http2.constants.NGHTTP2_FLOW_CONTROL_ERROR 0x04 Settings Timeout http2.constants.NGHTTP2_SETTINGS_TIMEOUT 0x05 Stream Closed http2.constants.NGHTTP2_STREAM_CLOSED 0x06 Frame Size Error http2.constants.NGHTTP2_FRAME_SIZE_ERROR 0x07 Refused Stream http2.constants.NGHTTP2_REFUSED_STREAM 0x08 Cancel http2.constants.NGHTTP2_CANCEL 0x09 Compression Error http2.constants.NGHTTP2_COMPRESSION_ERROR 0x0a Connect Error http2.constants.NGHTTP2_CONNECT_ERROR 0x0b Enhance Your Calm http2.constants.NGHTTP2_ENHANCE_YOUR_CALM 0x0c Inadequate Security http2.constants.NGHTTP2_INADEQUATE_SECURITY 0x0d HTTP/1.1 Required http2.constants.NGHTTP2_HTTP_1_1_REQUIRED The 'timeout' event is emitted when there is no activity on the Server for a given number of milliseconds set using http2server.setTimeout() . Returns: <HTTP/2 Settings Object> Returns an object containing the default settings for an Http2Session instance. This method returns a new object instance every time it is called so instances returned may be safely modified for use. http2.constants Error Codes for RST_STREAM and GOAWAY http2.getDefaultSettings() http2.getPackedSettings(settings) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 451/739 settings <HTTP/2 Settings Object> Returns: <Buffer> Returns a Buffer instance containing serialized representation of the given HTTP/2 settings as specified in the HTTP/2 specification. This is intended for use with the HTTP2-Settings header field. const http2 = require('http2'); const packed = http2.getPackedSettings({ enablePush: false }); console.log(packed.toString('base64')); // Prints: AAIAAAAA buf <Buffer> | <Uint8Array> The packed settings. Returns: <HTTP/2 Settings Object> Returns a HTTP/2 Settings Object containing the deserialized settings from the given Buffer as generated by http2.getPackedSettings() . Headers are represented as own-properties on JavaScript objects. The property keys will be serialized to lower-case. Property values should be strings (if they are not they will be coerced to strings) or an Array of strings (in order to send more than one value per header field). const headers = { ':status': '200', 'content-type': 'text-plain', 'ABC': ['has', 'more', 'than', 'one', 'value'] }; stream.respond(headers); Header objects passed to callback functions will have a null prototype. This means that normal JavaScript object methods such as Object.prototype.toString() and Object.prototype.hasOwnProperty() will not work. For incoming headers: The :status header is converted to number . Duplicates of :status , :method , :authority , :scheme , :path , :protocol , age , authorization , access-control-allow- credentials , access-control-max-age , access-control-request-method , content-encoding , content-language , content-length , content-location , content-md5 , content-range , content-type , date , dnt , etag , expires , from , if-match , if-modified-since , if-none-match , if-range , if-unmodified-since , last-modified , location , max-forwards , proxy-authorization , range , referer , retry-after , tk , upgrade-insecure-requests , user-agent or x-content-type-options are discarded. set-cookie is always an array. Duplicates are added to the array. For duplicate cookie headers, the values are joined together with '; '. For all other headers, the values are joined together with ', '. const http2 = require('http2'); const server = http2.createServer(); server.on('stream', (stream, headers) => { http2.getUnpackedSettings(buf) Headers Object 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 452/739 console.log(headers[':path']); console.log(headers.ABC); }); The http2.getDefaultSettings() , http2.getPackedSettings() , http2.createServer() , http2.createSecureServer() , http2session.settings() , http2session.localSettings , and http2session.remoteSettings APIs either return or receive as input an object that defines configuration settings for an Http2Session object. These objects are ordinary JavaScript objects containing the following properties. headerTableSize <number> Specifies the maximum number of bytes used for header compression. The minimum allowed value is 0. The maximum allowed value is 232-1. Default: 4,096 octets . enablePush <boolean> Specifies true if HTTP/2 Push Streams are to be permitted on the Http2Session instances. initialWindowSize <number> Specifies the senders initial window size for stream-level flow control. The minimum allowed value is 0. The maximum allowed value is 232-1. Default: 65,535 bytes . maxFrameSize <number> Specifies the size of the largest frame payload. The minimum allowed value is 16,384. The maximum allowed value is 224-1. Default: 16,384 bytes . maxConcurrentStreams <number> Specifies the maximum number of concurrent streams permitted on an Http2Session . There is no default value which implies, at least theoretically, 231-1 streams may be open concurrently at any given time in an Http2Session . The minimum value is 0. The maximum allowed value is 231-1. maxHeaderListSize <number> Specifies the maximum size (uncompressed octets) of header list that will be accepted. The minimum allowed value is 0. The maximum allowed value is 232-1. Default: 65535 . enableConnectProtocol <boolean> Specifies true if the "Extended Connect Protocol" defined by RFC 8441 is to be enabled. This setting is only meaningful if sent by the server. Once the enableConnectProtocol setting has been enabled for a given Http2Session , it cannot be disabled. All additional properties on the settings object are ignored. When options.paddingStrategy is equal to http2.constants.PADDING_STRATEGY_CALLBACK , the HTTP/2 implementation will consult the options.selectPadding() callback function, if provided, to determine the specific amount of padding to use per HEADERS and DATA frame. The options.selectPadding() function receives two numeric arguments, frameLen and maxFrameLen and must return a number N such that frameLen <= N <= maxFrameLen . const http2 = require('http2'); const server = http2.createServer({ paddingStrategy: http2.constants.PADDING_STRATEGY_CALLBACK, selectPadding(frameLen, maxFrameLen) { return maxFrameLen; } }); The options.selectPadding() function is invoked once for every HEADERS and DATA frame. This has a definite noticeable impact on performance. Settings Object Using options.selectPadding() Error Handling 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 453/739 There are several types of error conditions that may arise when using the http2 module: Validation errors occur when an incorrect argument, option, or setting value is passed in. These will always be reported by a synchronous throw . State errors occur when an action is attempted at an incorrect time (for instance, attempting to send data on a stream after it has closed). These will be reported using either a synchronous throw or via an 'error' event on the Http2Stream , Http2Session or HTTP/2 Server objects, depending on where and when the error occurs. Internal errors occur when an HTTP/2 session fails unexpectedly. These will be reported via an 'error' event on the Http2Session or HTTP/2 Server objects. Protocol errors occur when various HTTP/2 protocol constraints are violated. These will be reported using either a synchronous throw or via an 'error' event on the Http2Stream , Http2Session or HTTP/2 Server objects, depending on where and when the error occurs. The HTTP/2 implementation applies stricter handling of invalid characters in HTTP header names and values than the HTTP/1 implementation. Header field names are case-insensitive and are transmitted over the wire strictly as lower-case strings. The API provided by Node.js allows header names to be set as mixed-case strings (e.g. Content-Type ) but will convert those to lower-case (e.g. content-type ) upon transmission. Header field-names must only contain one or more of the following ASCII characters: a - z , A - Z , 0 - 9 , ! , # , $ , % , & , ' , * , + , - , . , ^ , _ , ` (backtick), | , and ~ . Using invalid characters within an HTTP header field name will cause the stream to be closed with a protocol error being reported. Header field values are handled with more leniency but should not contain new-line or carriage return characters and should be limited to US- ASCII characters, per the requirements of the HTTP specification. To receive pushed streams on the client, set a listener for the 'stream' event on the ClientHttp2Session : const http2 = require('http2'); const client = http2.connect('http://localhost'); client.on('stream', (pushedStream, requestHeaders) => { pushedStream.on('push', (responseHeaders) => { // process response headers }); pushedStream.on('data', (chunk) => { /* handle pushed data */ }); }); const req = client.request({ ':path': '/' }); The CONNECT method is used to allow an HTTP/2 server to be used as a proxy for TCP/IP connections. A simple TCP Server: Invalid character handling in header names and values Push streams on the client Supporting the CONNECT method 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 454/739 const net = require('net'); const server = net.createServer((socket) => { let name = ''; socket.setEncoding('utf8'); socket.on('data', (chunk) => name += chunk); socket.on('end', () => socket.end(`hello ${name}`)); }); server.listen(8000); An HTTP/2 CONNECT proxy: const http2 = require('http2'); const { NGHTTP2_REFUSED_STREAM } = http2.constants; const net = require('net'); const proxy = http2.createServer(); proxy.on('stream', (stream, headers) => { if (headers[':method'] !== 'CONNECT') { // Only accept CONNECT requests stream.close(NGHTTP2_REFUSED_STREAM); return; } const auth = new URL(`tcp://${headers[':authority']}`); // It's a very good idea to verify that hostname and port are // things this proxy should be connecting to. const socket = net.connect(auth.port, auth.hostname, () => { stream.respond(); socket.pipe(stream); stream.pipe(socket); }); socket.on('error', (error) => { stream.close(http2.constants.NGHTTP2_CONNECT_ERROR); }); }); proxy.listen(8001); An HTTP/2 CONNECT client: const http2 = require('http2'); const client = http2.connect('http://localhost:8001'); // Must not specify the ':path' and ':scheme' headers // for CONNECT requests or an error will be thrown. const req = client.request({ ':method': 'CONNECT', ':authority': `localhost:${port}` }); 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 455/739 req.on('response', (headers) => { console.log(headers[http2.constants.HTTP2_HEADER_STATUS]); }); let data = ''; req.setEncoding('utf8'); req.on('data', (chunk) => data += chunk); req.on('end', () => { console.log(`The server says: ${data}`); client.close(); }); req.end('Jane'); RFC 8441 defines an "Extended CONNECT Protocol" extension to HTTP/2 that may be used to bootstrap the use of an Http2Stream using the CONNECT method as a tunnel for other communication protocols (such as WebSockets). The use of the Extended CONNECT Protocol is enabled by HTTP/2 servers by using the enableConnectProtocol setting: const http2 = require('http2'); const settings = { enableConnectProtocol: true }; const server = http2.createServer({ settings }); Once the client receives the SETTINGS frame from the server indicating that the extended CONNECT may be used, it may send CONNECT requests that use the ':protocol' HTTP/2 pseudo-header: const http2 = require('http2'); const client = http2.connect('http://localhost:8080'); client.on('remoteSettings', (settings) => { if (settings.enableConnectProtocol) { const req = client.request({ ':method': 'CONNECT', ':protocol': 'foo' }); // ... } }); The Compatibility API has the goal of providing a similar developer experience of HTTP/1 when using HTTP/2, making it possible to develop applications that support both HTTP/1 and HTTP/2. This API targets only the public API of the HTTP/1 . However many modules use internal methods or state, and those are not supported as it is a completely different implementation. The following example creates an HTTP/2 server using the compatibility API: const http2 = require('http2'); const server = http2.createServer((req, res) => { res.setHeader('Content-Type', 'text/html'); res.setHeader('X-Foo', 'bar'); res.writeHead(200, { 'Content-Type': 'text/plain' }); res.end('ok'); }); The Extended CONNECT Protocol Compatibility API 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 456/739 In order to create a mixed HTTPS and HTTP/2 server, refer to the ALPN negotiation section. Upgrading from non-tls HTTP/1 servers is not supported. The HTTP/2 compatibility API is composed of Http2ServerRequest and Http2ServerResponse . They aim at API compatibility with HTTP/1, but they do not hide the differences between the protocols. As an example, the status message for HTTP codes is ignored. ALPN negotiation allows supporting both HTTPS and HTTP/2 over the same socket. The req and res objects can be either HTTP/1 or HTTP/2, and an application must restrict itself to the public API of HTTP/1 , and detect if it is possible to use the more advanced features of HTTP/2. The following example creates a server that supports both protocols: const { createSecureServer } = require('http2'); const { readFileSync } = require('fs'); const cert = readFileSync('./cert.pem'); const key = readFileSync('./key.pem'); const server = createSecureServer( { cert, key, allowHTTP1: true }, onRequest ).listen(4443); function onRequest(req, res) { // detects if it is a HTTPS request or HTTP/2 const { socket: { alpnProtocol } } = req.httpVersion === '2.0' ? req.stream.session : req; res.writeHead(200, { 'content-type': 'application/json' }); res.end(JSON.stringify({ alpnProtocol, httpVersion: req.httpVersion })); } The 'request' event works identically on both HTTPS and HTTP/2. A Http2ServerRequest object is created by http2.Server or http2.SecureServer and passed as the first argument to the 'request' event. It may be used to access a request status, headers, and data. It implements the Readable Stream interface, as well as the following additional events, methods, and properties. The 'aborted' event is emitted whenever a Http2ServerRequest instance is abnormally aborted in mid-communication. The 'aborted' event will only be emitted if the Http2ServerRequest writable side has not been ended. Indicates that the underlying Http2Stream was closed. Just like 'end' , this event occurs only once per response. ALPN negotiation Class: http2.Http2ServerRequest Event: 'aborted' Event: 'close' 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 457/739 <boolean> The request.aborted property will be true if the request has been aborted. <string> The request authority pseudo header field. It can also be accessed via req.headers[':authority'] . error <Error> Calls destroy() on the Http2Stream that received the Http2ServerRequest . If error is provided, an 'error' event is emitted and error is passed as an argument to any listeners on the event. It does nothing if the stream was already destroyed. <Object> The request/response headers object. Key-value pairs of header names and values. Header names are lower-cased. // Prints something like: // // { 'user-agent': 'curl/7.22.0', // host: '127.0.0.1:8000', // accept: '*/*' } console.log(request.headers); See HTTP/2 Headers Object . In HTTP/2, the request path, hostname, protocol, and method are represented as special headers prefixed with the : character (e.g. ':path' ). These special headers will be included in the request.headers object. Care must be taken not to inadvertently modify these special headers or errors may occur. For instance, removing all headers from the request will cause errors to occur: removeAllHeaders(request.headers); assert(request.url); // Fails because the :path header has been removed <string> In case of server request, the HTTP version sent by the client. In the case of client response, the HTTP version of the connected-to server. Returns '2.0' . Also message.httpVersionMajor is the first integer and message.httpVersionMinor is the second. <string> request.aborted request.authority request.destroy([error]) request.headers request.httpVersion request.method 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 458/739 The request method as a string. Read-only. Examples: 'GET' , 'DELETE' . <string[]> The raw request/response headers list exactly as they were received. Note that the keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values. Header names are not lowercased, and duplicates are not merged. // Prints something like: // // [ 'user-agent', // 'this is invalid because there can be only one', // 'User-Agent', // 'curl/7.22.0', // 'Host', // '127.0.0.1:8000', // 'ACCEPT', // '*/*' ] console.log(request.rawHeaders); <string[]> The raw request/response trailer keys and values exactly as they were received. Only populated at the 'end' event. <string> The request scheme pseudo header field indicating the scheme portion of the target URL. msecs <number> callback <Function> Returns: <http2.Http2ServerRequest> Sets the Http2Stream 's timeout value to msecs . If a callback is provided, then it is added as a listener on the 'timeout' event on the response object. If no 'timeout' listener is added to the request, the response, or the server, then Http2Stream s are destroyed when they time out. If a handler is assigned to the request, the response, or the server's 'timeout' events, timed out sockets must be handled explicitly. <net.Socket> | <tls.TLSSocket> Returns a Proxy object that acts as a net.Socket (or tls.TLSSocket ) but applies getters, setters, and methods based on HTTP/2 logic. destroyed , readable , and writable properties will be retrieved from and set on request.stream . destroy , emit , end , on and once methods will be called on request.stream . request.rawHeaders request.rawTrailers request.scheme request.setTimeout(msecs, callback) request.socket 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 459/739 setTimeout method will be called on request.stream.session . pause , read , resume , and write will throw an error with code ERR_HTTP2_NO_SOCKET_MANIPULATION . See Http2Session and Sockets for more information. All other interactions will be routed directly to the socket. With TLS support, use request.socket.getPeerCertificate() to obtain the client's authentication details. <Http2Stream> The Http2Stream object backing the request. <Object> The request/response trailers object. Only populated at the 'end' event. <string> Request URL string. This contains only the URL that is present in the actual HTTP request. If the request is: GET /status?name=ryan HTTP/1.1\r\n Accept: text/plain\r\n \r\n Then request.url will be: '/status?name=ryan' To parse the url into its parts require('url').parse(request.url) can be used: $ node > require('url').parse('/status?name=ryan') Url { protocol: null, slashes: null, auth: null, host: null, port: null, hostname: null, hash: null, search: '?name=ryan', query: 'name=ryan', pathname: '/status', path: '/status?name=ryan', href: '/status?name=ryan' } To extract the parameters from the query string, the require('querystring').parse function can be used, or true can be passed as the second argument to require('url').parse . request.stream request.trailers request.url 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 460/739 $ node > require('url').parse('/status?name=ryan', true) Url { protocol: null, slashes: null, auth: null, host: null, port: null, hostname: null, hash: null, search: '?name=ryan', query: { name: 'ryan' }, pathname: '/status', path: '/status?name=ryan', href: '/status?name=ryan' } This object is created internally by an HTTP server — not by the user. It is passed as the second parameter to the 'request' event. The response inherits from Stream , and additionally implements the following: Indicates that the underlying Http2Stream was terminated before response.end() was called or able to flush. Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the HTTP/2 multiplexing for transmission over the network. It does not imply that the client has received anything yet. After this event, no more events will be emitted on the response object. headers <Object> This method adds HTTP trailing headers (a header but at the end of the message) to the response. Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown. <net.Socket> | <tls.TLSSocket> See response.socket . data <string> | <Buffer> encoding <string> callback <Function> Returns: <this> Class: http2.Http2ServerResponse Event: 'close' Event: 'finish' response.addTrailers(headers) response.connection response.end([data][, encoding][, callback]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 461/739 This method signals to the server that all of the response headers and body have been sent; that server should consider this message complete. The method, response.end() , MUST be called on each response. If data is specified, it is equivalent to calling response.write(data, encoding) followed by response.end(callback) . If callback is specified, it will be called when the response stream is finished. <boolean> Boolean value that indicates whether the response has completed. Starts as false . After response.end() executes, the value will be true . name <string> Returns: <string> Reads out a header that has already been queued but not sent to the client. Note that the name is case insensitive. const contentType = response.getHeader('content-type'); Returns: <string[]> Returns an array containing the unique names of the current outgoing headers. All header names are lowercase. response.setHeader('Foo', 'bar'); response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']); const headerNames = response.getHeaderNames(); // headerNames === ['foo', 'set-cookie'] Returns: <Object> Returns a shallow copy of the current outgoing headers. Since a shallow copy is used, array values may be mutated without additional calls to various header-related http module methods. The keys of the returned object are the header names and the values are the respective header values. All header names are lowercase. The object returned by the response.getHeaders() method does not prototypically inherit from the JavaScript Object . This means that typical Object methods such as obj.toString() , obj.hasOwnProperty() , and others are not defined and will not work. response.setHeader('Foo', 'bar'); response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']); const headers = response.getHeaders(); // headers === { foo: 'bar', 'set-cookie': ['foo=bar', 'bar=baz'] } name <string> Returns: <boolean> response.finished response.getHeader(name) response.getHeaderNames() response.getHeaders() response.hasHeader(name) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 462/739 Returns true if the header identified by name is currently set in the outgoing headers. Note that the header name matching is case- insensitive. const hasContentType = response.hasHeader('content-type'); <boolean> True if headers were sent, false otherwise (read-only). name <string> Removes a header that has been queued for implicit sending. response.removeHeader('Content-Encoding'); <boolean> When true, the Date header will be automatically generated and sent in the response if it is not already present in the headers. Defaults to true. This should only be disabled for testing; HTTP requires the Date header in responses. name <string> value <string> | <string[]> Sets a single header value for implicit headers. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here to send multiple headers with the same name. response.setHeader('Content-Type', 'text/html'); or response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']); Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown. When headers have been set with response.setHeader() , they will be merged with any headers passed to response.writeHead() , with the headers passed to response.writeHead() given precedence. // returns content-type = text/plain const server = http2.createServer((req, res) => { res.setHeader('Content-Type', 'text/html'); res.setHeader('X-Foo', 'bar'); res.writeHead(200, { 'Content-Type': 'text/plain' }); res.end('ok'); }); response.headersSent response.removeHeader(name) response.sendDate response.setHeader(name, value) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 463/739 msecs <number> callback <Function> Returns: <http2.Http2ServerResponse> Sets the Http2Stream 's timeout value to msecs . If a callback is provided, then it is added as a listener on the 'timeout' event on the response object. If no 'timeout' listener is added to the request, the response, or the server, then Http2Stream s are destroyed when they time out. If a handler is assigned to the request, the response, or the server's 'timeout' events, timed out sockets must be handled explicitly. <net.Socket> | <tls.TLSSocket> Returns a Proxy object that acts as a net.Socket (or tls.TLSSocket ) but applies getters, setters, and methods based on HTTP/2 logic. destroyed , readable , and writable properties will be retrieved from and set on response.stream . destroy , emit , end , on and once methods will be called on response.stream . setTimeout method will be called on response.stream.session . pause , read , resume , and write will throw an error with code ERR_HTTP2_NO_SOCKET_MANIPULATION . See Http2Session and Sockets for more information. All other interactions will be routed directly to the socket. const http2 = require('http2'); const server = http2.createServer((req, res) => { const ip = req.socket.remoteAddress; const port = req.socket.remotePort; res.end(`Your IP address is ${ip} and your source port is ${port}.`); }).listen(3000); <number> When using implicit headers (not calling response.writeHead() explicitly), this property controls the status code that will be sent to the client when the headers get flushed. response.statusCode = 404; After response header was sent to the client, this property indicates the status code which was sent out. <string> Status message is not supported by HTTP/2 (RFC7540 8.1.2.4). It returns an empty string. response.setTimeout(msecs[, callback]) response.socket response.statusCode response.statusMessage 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 464/739 <Http2Stream> The Http2Stream object backing the response. chunk <string> | <Buffer> encoding <string> callback <Function> Returns: <boolean> If this method is called and response.writeHead() has not been called, it will switch to implicit header mode and flush the implicit headers. This sends a chunk of the response body. This method may be called multiple times to provide successive parts of the body. Note that in the http module, the response body is omitted when the request is a HEAD request. Similarly, the 204 and 304 responses must not include a message body. chunk can be a string or a buffer. If chunk is a string, the second parameter specifies how to encode it into a byte stream. By default the encoding is 'utf8' . callback will be called when this chunk of data is flushed. This is the raw HTTP body and has nothing to do with higher-level multi-part body encodings that may be used. The first time response.write() is called, it will send the buffered header information and the first chunk of the body to the client. The second time response.write() is called, Node.js assumes data will be streamed, and sends the new data separately. That is, the response is buffered up to the first chunk of the body. Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in user memory. 'drain' will be emitted when the buffer is free again. Sends a status 100 Continue to the client, indicating that the request body should be sent. See the 'checkContinue' event on Http2Server and Http2SecureServer . statusCode <number> statusMessage <string> headers <Object> Sends a response header to the request. The status code is a 3-digit HTTP status code, like 404 . The last argument, headers , are the response headers. For compatibility with HTTP/1 , a human-readable statusMessage may be passed as the second argument. However, because the statusMessage has no meaning within HTTP/2, the argument will have no effect and a process warning will be emitted. const body = 'hello world'; response.writeHead(200, { 'Content-Length': Buffer.byteLength(body), 'Content-Type': 'text/plain' }); Note that Content-Length is given in bytes not characters. The Buffer.byteLength() API may be used to determine the number of bytes in a given encoding. On outbound messages, Node.js does not check if Content-Length and the length of the body being transmitted are equal or response.stream response.write(chunk[, encoding][, callback]) response.writeContinue() response.writeHead(statusCode[, statusMessage][, headers]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 465/739 not. However, when receiving messages, Node.js will automatically reject messages when the Content-Length does not match the actual payload size. This method may be called at most one time on a message before response.end() is called. If response.write() or response.end() are called before calling this, the implicit/mutable headers will be calculated and call this function. When headers have been set with response.setHeader() , they will be merged with any headers passed to response.writeHead() , with the headers passed to response.writeHead() given precedence. // returns content-type = text/plain const server = http2.createServer((req, res) => { res.setHeader('Content-Type', 'text/html'); res.setHeader('X-Foo', 'bar'); res.writeHead(200, { 'Content-Type': 'text/plain' }); res.end('ok'); }); Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown. headers <HTTP/2 Headers Object> An object describing the headers callback <Function> Called once http2stream.pushStream() is finished, or either when the attempt to create the pushed Http2Stream has failed or has been rejected, or the state of Http2ServerRequest is closed prior to calling the http2stream.pushStream() method err <Error> stream <ServerHttp2Stream> The newly-created ServerHttp2Stream object Call http2stream.pushStream() with the given headers, and wrap the given Http2Stream on a newly created Http2ServerResponse as the callback parameter if successful. When Http2ServerRequest is closed, the callback is called with an error ERR_HTTP2_INVALID_STREAM . The Performance Observer API can be used to collect basic performance metrics for each Http2Session and Http2Stream instance. const { PerformanceObserver } = require('perf_hooks'); const obs = new PerformanceObserver((items) => { const entry = items.getEntries()[0]; console.log(entry.entryType); // prints 'http2' if (entry.name === 'Http2Session') { // entry contains statistics about the Http2Session } else if (entry.name === 'Http2Stream') { // entry contains statistics about the Http2Stream } }); obs.observe({ entryTypes: ['http2'] }); The entryType property of the PerformanceEntry will be equal to 'http2' . The name property of the PerformanceEntry will be equal to either 'Http2Stream' or 'Http2Session' . response.createPushResponse(headers, callback) Collecting HTTP/2 Performance Metrics 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 466/739 If name is equal to Http2Stream , the PerformanceEntry will contain the following additional properties: bytesRead <number> The number of DATA frame bytes received for this Http2Stream . bytesWritten <number> The number of DATA frame bytes sent for this Http2Stream . id <number> The identifier of the associated Http2Stream timeToFirstByte <number> The number of milliseconds elapsed between the PerformanceEntry startTime and the reception of the first DATA frame. timeToFirstByteSent <number> The number of milliseconds elapsed between the PerformanceEntry startTime and sending of the first DATA frame. timeToFirstHeader <number> The number of milliseconds elapsed between the PerformanceEntry startTime and the reception of the first header. If name is equal to Http2Session , the PerformanceEntry will contain the following additional properties: bytesRead <number> The number of bytes received for this Http2Session . bytesWritten <number> The number of bytes sent for this Http2Session . framesReceived <number> The number of HTTP/2 frames received by the Http2Session . framesSent <number> The number of HTTP/2 frames sent by the Http2Session . maxConcurrentStreams <number> The maximum number of streams concurrently open during the lifetime of the Http2Session . pingRTT <number> The number of milliseconds elapsed since the transmission of a PING frame and the reception of its acknowledgment. Only present if a PING frame has been sent on the Http2Session . streamAverageDuration <number> The average duration (in milliseconds) for all Http2Stream instances. streamCount <number> The number of Http2Stream instances processed by the Http2Session . type <string> Either 'server' or 'client' to identify the type of Http2Session . Stability: 2 - Stable HTTPS is the HTTP protocol over TLS/SSL. In Node.js this is implemented as a separate module. An Agent object for HTTPS similar to http.Agent . See https.request() for more information. This class is a subclass of tls.Server and emits events same as http.Server . See http.Server for more information. callback <Function> Returns: <https.Server> See server.close() from the HTTP module for details. Starts the HTTPS server listening for encrypted connections. This method is identical to server.listen() from net.Server . HTTPS Class: https.Agent Class: https.Server server.close([callback]) server.listen() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 467/739 <number> Default: 2000 See http.Server#maxHeadersCount . <number> Default: 40000 See http.Server#headersTimeout . msecs <number> Default: 120000 (2 minutes) callback <Function> Returns: <https.Server> See http.Server#setTimeout() . <number> Default: 120000 (2 minutes) See http.Server#timeout . <number> Default: 5000 (5 seconds) See http.Server#keepAliveTimeout . options <Object> Accepts options from tls.createServer() , tls.createSecureContext() and http.createServer() . requestListener <Function> A listener to be added to the 'request' event. Returns: <https.Server> // curl -k https://localhost:8000/ const https = require('https'); const fs = require('fs'); const options = { key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'), cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem') }; https.createServer(options, (req, res) => { res.writeHead(200); res.end('hello world\n'); }).listen(8000); Or server.maxHeadersCount server.headersTimeout server.setTimeout([msecs][, callback]) server.timeout server.keepAliveTimeout https.createServer([options][, requestlistener]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 468/739 const https = require('https'); const fs = require('fs'); const options = { pfx: fs.readFileSync('test/fixtures/test_cert.pfx'), passphrase: 'sample' }; https.createServer(options, (req, res) => { res.writeHead(200); res.end('hello world\n'); }).listen(8000); url <string> | <URL> options <Object> | <string> | <URL> Accepts the same options as https.request() , with the method always set to GET . callback <Function> Like http.get() but for HTTPS. options can be an object, a string, or a URL object. If options is a string, it is automatically parsed with url.parse() . If it is a URL object, it will be automatically converted to an ordinary options object. const https = require('https'); https.get('https://encrypted.google.com/', (res) => { console.log('statusCode:', res.statusCode); console.log('headers:', res.headers); res.on('data', (d) => { process.stdout.write(d); }); }).on('error', (e) => { console.error(e); }); Global instance of https.Agent for all HTTPS client requests. url <string> | <URL> options <Object> | <string> | <URL> Accepts all options from http.request() , with some differences in default values: protocol Default: 'https:' https.get(options[, callback]) https.get(url[, options][, callback]) https.globalAgent https.request(options[, callback]) https.request(url[, options][, callback]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 469/739 port Default: 443 agent Default: https.globalAgent callback <Function> Makes a request to a secure web server. The following additional options from tls.connect() are also accepted: ca , cert , ciphers , clientCertEngine , crl , dhparam , ecdhCurve , honorCipherOrder , key , passphrase , pfx , rejectUnauthorized , secureOptions , secureProtocol , servername , sessionIdContext . options can be an object, a string, or a URL object. If options is a string, it is automatically parsed with url.parse() . If it is a URL object, it will be automatically converted to an ordinary options object. const https = require('https'); const options = { hostname: 'encrypted.google.com', port: 443, path: '/', method: 'GET' }; const req = https.request(options, (res) => { console.log('statusCode:', res.statusCode); console.log('headers:', res.headers); res.on('data', (d) => { process.stdout.write(d); }); }); req.on('error', (e) => { console.error(e); }); req.end(); Example using options from tls.connect() : const options = { hostname: 'encrypted.google.com', port: 443, path: '/', method: 'GET', key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'), cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem') }; options.agent = new https.Agent(options); const req = https.request(options, (res) => { // ... }); 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 470/739 Alternatively, opt out of connection pooling by not using an Agent . const options = { hostname: 'encrypted.google.com', port: 443, path: '/', method: 'GET', key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'), cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem'), agent: false }; const req = https.request(options, (res) => { // ... }); Example using a URL as options : const options = new URL('https://abc:xyz@example.com'); const req = https.request(options, (res) => { // ... }); Example pinning on certificate fingerprint, or the public key (similar to pin-sha256 ): const tls = require('tls'); const https = require('https'); const crypto = require('crypto'); function sha256(s) { return crypto.createHash('sha256').update(s).digest('base64'); } const options = { hostname: 'github.com', port: 443, path: '/', method: 'GET', checkServerIdentity: function(host, cert) { // Make sure the certificate is issued to the host we are connected to const err = tls.checkServerIdentity(host, cert); if (err) { return err; } // Pin the public key, similar to HPKP pin-sha25 pinning const pubkey256 = 'pL1+qb9HTMRZJmuC/bB/ZI9d302BYrrqiVuRyW+DGrU='; if (sha256(cert.pubkey) !== pubkey256) { const msg = 'Certificate verification error: ' + `The public key of '${cert.subject.CN}' ` + 'does not match our pinned fingerprint'; t E ( ) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 471/739 return new Error(msg); } // Pin the exact certificate, rather then the pub key const cert256 = '25:FE:39:32:D9:63:8C:8A:FC:A1:9A:29:87:' + 'D8:3E:4C:1D:98:DB:71:E4:1A:48:03:98:EA:22:6A:BD:8B:93:16'; if (cert.fingerprint256 !== cert256) { const msg = 'Certificate verification error: ' + `The certificate of '${cert.subject.CN}' ` + 'does not match our pinned fingerprint'; return new Error(msg); } // This loop is informational only. // Print the certificate and public key fingerprints of all certs in the // chain. Its common to pin the public key of the issuer on the public // internet, while pinning the public key of the service in sensitive // environments. do { console.log('Subject Common Name:', cert.subject.CN); console.log(' Certificate SHA256 fingerprint:', cert.fingerprint256); hash = crypto.createHash('sha256'); console.log(' Public key ping-sha256:', sha256(cert.pubkey)); lastprint256 = cert.fingerprint256; cert = cert.issuerCertificate; } while (cert.fingerprint256 !== lastprint256); }, }; options.agent = new https.Agent(options); const req = https.request(options, (res) => { console.log('All OK. Server matched our pinned cert or public key'); console.log('statusCode:', res.statusCode); // Print the HPKP values console.log('headers:', res.headers['public-key-pins']); res.on('data', (d) => {}); }); req.on('error', (e) => { console.error(e.message); }); req.end(); Outputs for example: Subject Common Name: github.com Certificate SHA256 fingerprint: 25:FE:39:32:D9:63:8C:8A:FC:A1:9A:29:87:D8:3E:4C:1D:98:DB:71:E4:1A:48:03:98:EA:22:6A:BD:8B:93:16 Public key ping-sha256: pL1+qb9HTMRZJmuC/bB/ZI9d302BYrrqiVuRyW+DGrU= Subject Common Name: DigiCert SHA2 Extended Validation Server CA 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 472/739 Stability: 1 - Experimental The inspector module provides an API for interacting with the V8 inspector. It can be accessed using: const inspector = require('inspector'); Deactivate the inspector. Blocks until there are no active connections. <Object> An object to send messages to the remote inspector console. require('inspector').console.log('a message'); The inspector console does not have API parity with Node.js console. port <number> Port to listen on for inspector connections. Optional. Default: what was specified on the CLI. host <string> Host to listen on for inspector connections. Optional. Default: what was specified on the CLI. wait <boolean> Block until a client has connected. Optional. Default: false . Activate inspector on host and port. Equivalent to node --inspect=[[host:]port] , but can be done programmatically after node has started. If wait is true , will block until a client has connected to the inspect port and flow control has been passed to the debugger client. See the security warning regarding the host parameter usage. Returns: <string> | <undefined> Return the URL of the active inspector, or undefined if there is none. Certificate SHA256 fingerprint: 40:3E:06:2A:26:53:05:91:13:28:5B:AF:80:A0:D4:AE:42:2C:84:8C:9F:78:FA:D0:1F:C9:4B:C5:B8:7F:EF:1A Public key ping-sha256: RRM1dGqnDFsCJXBTHky16vi1obOlCgFFn/yOhI/y+ho= Subject Common Name: DigiCert High Assurance EV Root CA Certificate SHA256 fingerprint: 74:31:E5:F4:C3:C1:CE:46:90:77:4F:0B:61:E0:54:40:88:3B:A9:A0:1E:D0:0B:A6:AB:D7:80:6E:D3:B1:18:CF Public key ping-sha256: WoiWRyIOVNa9ihaBciRSC7XHjliYS9VwUGOIud4PB18= All OK. Server matched our pinned cert or public key statusCode: 200 headers: max-age=0; pin-sha256="WoiWRyIOVNa9ihaBciRSC7XHjliYS9VwUGOIud4PB18="; pin-sha256="RRM1dGqnDFsCJXBTHky16vi1obOlCgFFn/yOhI/y+ Inspector inspector.close() inspector.console inspector.open([port[, host[, wait]]]) inspector.url() 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 473/739 The inspector.Session is used for dispatching messages to the V8 inspector back-end and receiving message responses and notifications. Create a new instance of the inspector.Session class. The inspector session needs to be connected through session.connect() before the messages can be dispatched to the inspector backend. inspector.Session is an EventEmitter with the following events: <Object> The notification message object Emitted when any notification from the V8 Inspector is received. session.on('inspectorNotification', (message) => console.log(message.method)); // Debugger.paused // Debugger.resumed It is also possible to subscribe only to notifications with specific method: <Object> The notification message object Emitted when an inspector notification is received that has its method field set to the <inspector-protocol-method> value. The following snippet installs a listener on the 'Debugger.paused' event, and prints the reason for program suspension whenever program execution is suspended (through breakpoints, for example): session.on('Debugger.paused', ({ params }) => { console.log(params.hitBreakpoints); }); // [ '/the/file/that/has/the/breakpoint.js:11:0' ] Connects a session to the inspector back-end. An exception will be thrown if there is already a connected session established either through the API or by a front-end connected to the Inspector WebSocket port. Immediately close the session. All pending message callbacks will be called with an error. session.connect() will need to be called to be able to send messages again. Reconnected session will lose all inspector state, such as enabled agents or configured breakpoints. method <string> params <Object> callback <Function> Class: inspector.Session Constructor: new inspector.Session() Event: 'inspectorNotification' Event: <inspector-protocol-method> session.connect() session.disconnect() session.post(method[, params][, callback]) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 474/739 Posts a message to the inspector back-end. callback will be notified when a response is received. callback is a function that accepts two optional arguments - error and message-specific result. session.post('Runtime.evaluate', { expression: '2 + 2' }, (error, { result }) => console.log(result)); // Output: { type: 'number', value: 4, description: '4' } The latest version of the V8 inspector protocol is published on the Chrome DevTools Protocol Viewer . Node.js inspector supports all the Chrome DevTools Protocol domains declared by V8. Chrome DevTools Protocol domain provides an interface for interacting with one of the runtime agents used to inspect the application state and listen to the run-time events. Apart from the debugger, various V8 Profilers are available through the DevTools protocol. Here's an example showing how to use the CPU Profiler : const inspector = require('inspector'); const fs = require('fs'); const session = new inspector.Session(); session.connect(); session.post('Profiler.enable', () => { session.post('Profiler.start', () => { // invoke business logic under measurement here... // some time later... session.post('Profiler.stop', (err, { profile }) => { // write profile to disk, upload, etc. if (!err) { fs.writeFileSync('./profile.cpuprofile', JSON.stringify(profile)); } }); }); }); Here's an example showing how to use the Heap Profiler : const inspector = require('inspector'); const fs = require('fs'); const session = new inspector.Session(); const fd = fs.openSync('profile.heapsnapshot', 'w'); session.connect(); Example usage CPU Profiler Heap Profiler 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 475/739 session.on('HeapProfiler.addHeapSnapshotChunk', (m) => { fs.writeSync(fd, m.params.chunk); }); session.post('HeapProfiler.takeHeapSnapshot', null, (err, r) => { console.log('Runtime.takeHeapSnapshot done:', err, r); session.disconnect(); fs.closeSync(fd); }); Node.js has many features that make it easier to write internationalized programs. Some of them are: Locale-sensitive or Unicode-aware functions in the ECMAScript Language Specification : String.prototype.normalize() String.prototype.toLowerCase() String.prototype.toUpperCase() All functionality described in the ECMAScript Internationalization API Specification (aka ECMA-402): Intl object Locale-sensitive methods like String.prototype.localeCompare() and Date.prototype.toLocaleString() The WHATWG URL parser 's internationalized domain names (IDNs) support require('buffer').transcode() More accurate REPL line editing require('util').TextDecoder RegExp Unicode Property Escapes Node.js (and its underlying V8 engine) uses ICU to implement these features in native C/C++ code. However, some of them require a very large ICU data file in order to support all locales of the world. Because it is expected that most Node.js users will make use of only a small portion of ICU functionality, only a subset of the full ICU data set is provided by Node.js by default. Several options are provided for customizing and expanding the ICU data set either when building or running Node.js. To control how ICU is used in Node.js, four configure options are available during compilation. Additional details on how to compile Node.js are documented in BUILDING.md . --with-intl=none / --without-intl --with-intl=system-icu --with-intl=small-icu (default) --with-intl=full-icu An overview of available Node.js and JavaScript features for each configure option: none system-icu small-icu full- icu String.prototype.normalize() none (function is no-op) full full full Internationalization Support Options for building Node.js 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 476/739 none system-icu small-icu full- icu String.prototype.to*Case() full full full full Intl none (object does not exist) partial/full (depends on OS) partial (English- only) full String.prototype.localeCompare() partial (not locale-aware) full full full String.prototype.toLocale*Case() partial (not locale-aware) full full full Number.prototype.toLocaleString() partial (not locale-aware) partial/full (depends on OS) partial (English- only) full Date.prototype.toLocale*String() partial (not locale-aware) partial/full (depends on OS) partial (English- only) full WHATWG URL Parser partial (no IDN support) full full full require('buffer').transcode() none (function does not exist) full full full REPL partial (inaccurate line editing) full full full require('util').TextDecoder partial (basic encodings support) partial/full (depends on OS) partial (Unicode- only) full RegExp Unicode Property Escapes none (invalid RegExp error) full full full The "(not locale-aware)" designation denotes that the function carries out its operation just like the non- Locale version of the function, if one exists. For example, under none mode, Date.prototype.toLocaleString() 's operation is identical to that of Date.prototype.toString() . If this option is chosen, most internationalization features mentioned above will be unavailable in the resulting node binary. Node.js can link against an ICU build already installed on the system. In fact, most Linux distributions already come with ICU installed, and this option would make it possible to reuse the same set of data used by other components in the OS. Functionalities that only require the ICU library itself, such as String.prototype.normalize() and the WHATWG URL parser , are fully supported under system-icu . Features that require ICU locale data in addition, such as Intl.DateTimeFormat may be fully or partially supported, depending on the completeness of the ICU data installed on the system. This option makes the resulting binary link against the ICU library statically, and includes a subset of ICU data (typically only the English locale) within the node executable. Functionalities that only require the ICU library itself, such as String.prototype.normalize() and the WHATWG URL parser , are fully supported under small-icu . Features that require ICU locale data in addition, such as Intl.DateTimeFormat , generally only work with the English locale: Disable all internationalization features ( none ) Build with a pre-installed ICU ( system-icu ) Embed a limited set of ICU data ( small-icu ) 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 477/739 const january = new Date(9e8); const english = new Intl.DateTimeFormat('en', { month: 'long' }); const spanish = new Intl.DateTimeFormat('es', { month: 'long' }); console.log(english.format(january)); // Prints "January" console.log(spanish.format(january)); // Prints "M01" on small-icu // Should print "enero" This mode provides a good balance between features and binary size, and it is the default behavior if no --with-intl flag is passed. The official binaries are also built in this mode. If the small-icu option is used, one can still provide additional locale data at runtime so that the JS methods would work for all ICU locales. Assuming the data file is stored at /some/directory , it can be made available to ICU through either: The NODE_ICU_DATA environment variable: env NODE_ICU_DATA=/some/directory node The --icu-data-dir CLI parameter: node --icu-data-dir=/some/directory (If both are specified, the --icu-data-dir CLI parameter takes precedence.) ICU is able to automatically find and load a variety of data formats, but the data must be appropriate for the ICU version, and the file correctly named. The most common name for the data file is icudt6X[bl].dat , where 6X denotes the intended ICU version, and b or l indicates the system's endianness. Check "ICU Data" article in the ICU User Guide for other supported formats and more details on ICU data in general. The full-icu npm module can greatly simplify ICU data installation by detecting the ICU version of the running node executable and downloading the appropriate data file. After installing the module through npm i full-icu , the data file will be available at ./node_modules/full-icu . This path can be then passed either to NODE_ICU_DATA or --icu-data-dir as shown above to enable full Intl support. This option makes the resulting binary link against ICU statically and include a full set of ICU data. A binary created this way has no further external dependencies and supports all locales, but might be rather large. See BUILDING.md on how to compile a binary using this mode. To verify that ICU is enabled at all ( system-icu , small-icu , or full-icu ), simply checking the existence of Intl should suffice: const hasICU = typeof Intl === 'object'; Alternatively, checking for process.versions.icu , a property defined only when ICU is enabled, works too: Providing ICU data at runtime Embed the entire ICU ( full-icu ) Detecting internationalization support 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 478/739 const hasICU = typeof process.versions.icu === 'string'; To check for support for a non-English locale (i.e. full-icu or system-icu ), Intl.DateTimeFormat can be a good distinguishing factor: const hasFullICU = (() => { try { const january = new Date(9e8); const spanish = new Intl.DateTimeFormat('es', { month: 'long' }); return spanish.format(january) === 'enero'; } catch (err) { return false; } })(); For more verbose tests for Intl support, the following resources may be found to be helpful: btest402 : Generally used to check whether Node.js with Intl support is built correctly. Test262 : ECMAScript's official conformance test suite includes a section dedicated to ECMA-402. Stability: 2 - Stable In the Node.js module system, each file is treated as a separate module. For example, consider a file named foo.js : const circle = require('./circle.js'); console.log(`The area of a circle of radius 4 is ${circle.area(4)}`); On the first line, foo.js loads the module circle.js that is in the same directory as foo.js . Here are the contents of circle.js : const { PI } = Math; exports.area = (r) => PI * r ** 2; exports.circumference = (r) => 2 * PI * r; The module circle.js has exported the functions area() and circumference() . Functions and objects are added to the root of a module by specifying additional properties on the special exports object. Variables local to the module will be private, because the module is wrapped in a function by Node.js (see module wrapper ). In this example, the variable PI is private to circle.js . The module.exports property can be assigned a new value (such as a function or object). Below, bar.js makes use of the square module, which exports a Square class: Modules 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 479/739 const Square = require('./square.js'); const mySquare = new Square(2); console.log(`The area of mySquare is ${mySquare.area()}`); The square module is defined in square.js : // assigning to exports will not modify module, must use module.exports module.exports = class Square { constructor(width) { this.width = width; } area() { return this.width ** 2; } }; The module system is implemented in the require('module') module. When a file is run directly from Node.js, require.main is set to its module . That means that it is possible to determine whether a file has been run directly by testing require.main === module . For a file foo.js , this will be true if run via node foo.js , but false if run by require('./foo') . Because module provides a filename property (normally equivalent to __filename ), the entry point of the current application can be obtained by checking require.main.filename . The semantics of Node.js's require() function were designed to be general enough to support a number of reasonable directory structures. Package manager programs such as dpkg , rpm , and npm will hopefully find it possible to build native packages from Node.js modules without modification. Below we give a suggested directory structure that could work: Let's say that we wanted to have the folder at /usr/lib/node/<some-package>/<some-version> hold the contents of a specific version of a package. Packages can depend on one another. In order to install package foo , it may be necessary to install a specific version of package bar . The bar package may itself have dependencies, and in some cases, these may even collide or form cyclic dependencies. Since Node.js looks up the realpath of any modules it loads (that is, resolves symlinks), and then looks for their dependencies in the node_modules folders as described here , this situation is very simple to resolve with the following architecture: /usr/lib/node/foo/1.2.3/ - Contents of the foo package, version 1.2.3. /usr/lib/node/bar/4.3.2/ - Contents of the bar package that foo depends on. /usr/lib/node/foo/1.2.3/node_modules/bar - Symbolic link to /usr/lib/node/bar/4.3.2/ . /usr/lib/node/bar/4.3.2/node_modules/* - Symbolic links to the packages that bar depends on. Thus, even if a cycle is encountered, or if there are dependency conflicts, every module will be able to get a version of its dependency that it can use. Accessing the main module Addenda: Package Manager Tips 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 480/739 When the code in the foo package does require('bar') , it will get the version that is symlinked into /usr/lib/node/foo/1.2.3/node_modules/bar . Then, when the code in the bar package calls require('quux') , it'll get the version that is symlinked into /usr/lib/node/bar/4.3.2/node_modules/quux . Furthermore, to make the module lookup process even more optimal, rather than putting packages directly in /usr/lib/node , we could put them in /usr/lib/node_modules/<name>/<version> . Then Node.js will not bother looking for missing dependencies in /usr/node_modules or /node_modules . In order to make modules available to the Node.js REPL, it might be useful to also add the /usr/lib/node_modules folder to the $NODE_PATH environment variable. Since the module lookups using node_modules folders are all relative, and based on the real path of the files making the calls to require() , the packages themselves can be anywhere. To get the exact filename that will be loaded when require() is called, use the require.resolve() function. Putting together all of the above, here is the high-level algorithm in pseudocode of what require.resolve() does: require(X) from module at path Y 1. If X is a core module, a. return the core module b. STOP 2. If X begins with '/' a. set Y to be the filesystem root 3. If X begins with './' or '/' or '../' a. LOAD_AS_FILE(Y + X) b. LOAD_AS_DIRECTORY(Y + X) 4. LOAD_NODE_MODULES(X, dirname(Y)) 5. THROW "not found" LOAD_AS_FILE(X) 1. If X is a file, load X as JavaScript text. STOP 2. If X.js is a file, load X.js as JavaScript text. STOP 3. If X.json is a file, parse X.json to a JavaScript Object. STOP 4. If X.node is a file, load X.node as binary addon. STOP LOAD_INDEX(X) 1. If X/index.js is a file, load X/index.js as JavaScript text. STOP 2. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP 3. If X/index.node is a file, load X/index.node as binary addon. STOP LOAD_AS_DIRECTORY(X) 1. If X/package.json is a file, a. Parse X/package.json, and look for "main" field. b. let M = X + (json main field) c. LOAD_AS_FILE(M) d. LOAD_INDEX(M) 2. LOAD_INDEX(X) LOAD_NODE_MODULES(X, START) 1. let DIRS = NODE_MODULES_PATHS(START) 2. for each DIR in DIRS: a. LOAD_AS_FILE(DIR/X) b. LOAD AS DIRECTORY(DIR/X) All Together... 8/28/2019 Node.js v10.16.3 Documentation https://nodejs.org/dist/latest-v10.x/docs/api/all.html 481/739 b. O _ S_ C O ( / ) NODE_MODULES_PATHS(START) 1. let PARTS = path split(START) 2. let I = count of PARTS - 1 3. let DIRS = [GLOBAL_FOLDERS] 4. while I >= 0, a. if PARTS[I] = "node_modules" CONTINUE b. DIR = path join(PARTS[0 .. I] + "node_modules") c. DIRS = DIRS + DIR d. let I = I - 1 5. return DIRS Modules are cached after the first time they are loaded. This means (among other things) that every call to require('foo') will get exactly the same object returned, if it would resolve to the same file. Provided require.cache is not modified, multiple calls to require('foo') will not cause the module code to be executed multiple times. This is an important feature. With it, "partially done" objects can be returned, thus allowing transitive dependencies to be loaded even when they would cause cycles. To have a module execute code multiple times, export a function, and call that function. Modules are cached based on their resolved filename. Since modules may resolve to a different filename based on the location of the calling module (loading from node_modules folders), it is not a guarantee that require('foo') will always return the exact same object, if it would resolve to different files. Additionally, on case-insensitive file systems or operating systems, different resolved filenames can point to the same file, but the cache will still treat them as different modules and will reload the file multiple times. For example, require('./foo') and require('./FOO') return two different objects, irrespective of whether or not ./foo and ./FOO are the same file. Node.js has several modules compiled into the binary. These modules are described in greater detail elsewhere in this documentation. The core modules are defined within Node.js's source and are located in the lib/ folder. Core modules are always preferentially loaded if their identifier is passed to require() . For instance, require('http') will always return the built in HTTP module, even if there is a file by that name. When there are circular require() ca