Node.JS v8 Documentation

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Technology
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Index  View on single page  View as JSON  
About this Documentation
Contributing
Stability Index
JSON Output
Syscalls and man pages
Usage
Example
Assert
Strict mode
Legacy mode deprecated
assert(value[, message])
assert.deepEqual(actual, expected[, message])
assert.deepStrictEqual(actual, expected[, message])
Comparison details
assert.doesNotReject(block[, error][, message])
assert.doesNotThrow(block[, error][, message])
assert.equal(actual, expected[, message])
assert.fail(message)
assert.fail(actual, expected[, message[, operator[, stackStartFunction]]])
assert.ifError(value)
assert.notDeepEqual(actual, expected[, message])
assert.notDeepStrictEqual(actual, expected[, message])
assert.notEqual(actual, expected[, message])
assert.notStrictEqual(actual, expected[, message])
assert.ok(value[, message])
assert.strictEqual(actual, expected[, message])
assert.rejects(block[, error][, message])
Node.js v8.16.1 Documentation
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Table of Contents
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assert.throws(block[, error][, message])
Caveats
Async Hooks
Terminology
Public API
Overview
async_hooks.createHook(callbacks)
Error Handling
Printing in AsyncHooks callbacks
asyncHook.enable()
asyncHook.disable()
Hook Callbacks
init(asyncId, type, triggerAsyncId, resource)
type
triggerId
resource
Asynchronous context example
before(asyncId)
after(asyncId)
destroy(asyncId)
promiseResolve(asyncId)
async_hooks.executionAsyncId()
async_hooks.triggerAsyncId()
Promise execution tracking
JavaScript Embedder API
class AsyncResource()
AsyncResource(type[, options])
asyncResource.runInAsyncScope(fn[, thisArg, ...args])
asyncResource.emitBefore() deprecated
asyncResource.emitAfter() deprecated
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asyncResource.emitDestroy()
asyncResource.asyncId()
asyncResource.triggerAsyncId()
Buffer
Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe()
The --zero-fill-buffers command line option
What makes Buffer.allocUnsafe() and Buffer.allocUnsafeSlow() "unsafe"?
Buffers and Character Encodings
Buffers and TypedArray
Buffers and iteration
Class: Buffer
new Buffer(array) deprecated
new Buffer(arrayBuffer[, byteOffset[, length]]) deprecated
new Buffer(buffer) deprecated
new Buffer(size) deprecated
new Buffer(string[, encoding]) deprecated
Class Method: Buffer.alloc(size[, fill[, encoding]])
Class Method: Buffer.allocUnsafe(size)
Class Method: Buffer.allocUnsafeSlow(size)
Class Method: Buffer.byteLength(string[, encoding])
Class Method: Buffer.compare(buf1, buf2)
Class Method: Buffer.concat(list[, totalLength])
Class Method: Buffer.from(array)
Class Method: Buffer.from(arrayBuffer[, byteOffset[, length]])
Class Method: Buffer.from(buffer)
Class Method: Buffer.from(string[, encoding])
Class Method: Buffer.from(object[, offsetOrEncoding[, length]])
Class Method: Buffer.isBuffer(obj)
Class Method: Buffer.isEncoding(encoding)
Class Property: Buffer.poolSize
buf[index]
buf.buffer
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buf.compare(target[, targetStart[, targetEnd[, sourceStart[, sourceEnd]]]])
buf.copy(target[, targetStart[, sourceStart[, sourceEnd]]])
buf.entries()
buf.equals(otherBuffer)
buf.fill(value[, offset[, end]][, encoding])
buf.includes(value[, byteOffset][, encoding])
buf.indexOf(value[, byteOffset][, encoding])
buf.keys()
buf.lastIndexOf(value[, byteOffset][, encoding])
buf.length
buf.parent deprecated
buf.readDoubleBE(offset[, noAssert])
buf.readDoubleLE(offset[, noAssert])
buf.readFloatBE(offset[, noAssert])
buf.readFloatLE(offset[, noAssert])
buf.readInt8(offset[, noAssert])
buf.readInt16BE(offset[, noAssert])
buf.readInt16LE(offset[, noAssert])
buf.readInt32BE(offset[, noAssert])
buf.readInt32LE(offset[, noAssert])
buf.readIntBE(offset, byteLength[, noAssert])
buf.readIntLE(offset, byteLength[, noAssert])
buf.readUInt8(offset[, noAssert])
buf.readUInt16BE(offset[, noAssert])
buf.readUInt16LE(offset[, noAssert])
buf.readUInt32BE(offset[, noAssert])
buf.readUInt32LE(offset[, noAssert])
buf.readUIntBE(offset, byteLength[, noAssert])
buf.readUIntLE(offset, byteLength[, noAssert])
buf.slice([start[, end]])
buf.swap16()
buf.swap32()
buf.swap64()
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buf.toJSON()
buf.toString([encoding[, start[, end]]])
buf.values()
buf.write(string[, offset[, length]][, encoding])
buf.writeDoubleBE(value, offset[, noAssert])
buf.writeDoubleLE(value, offset[, noAssert])
buf.writeFloatBE(value, offset[, noAssert])
buf.writeFloatLE(value, offset[, noAssert])
buf.writeInt8(value, offset[, noAssert])
buf.writeInt16BE(value, offset[, noAssert])
buf.writeInt16LE(value, offset[, noAssert])
buf.writeInt32BE(value, offset[, noAssert])
buf.writeInt32LE(value, offset[, noAssert])
buf.writeIntBE(value, offset, byteLength[, noAssert])
buf.writeIntLE(value, offset, byteLength[, noAssert])
buf.writeUInt8(value, offset[, noAssert])
buf.writeUInt16BE(value, offset[, noAssert])
buf.writeUInt16LE(value, offset[, noAssert])
buf.writeUInt32BE(value, offset[, noAssert])
buf.writeUInt32LE(value, offset[, noAssert])
buf.writeUIntBE(value, offset, byteLength[, noAssert])
buf.writeUIntLE(value, offset, byteLength[, noAssert])
buffer.INSPECT_MAX_BYTES
buffer.kMaxLength
buffer.transcode(source, fromEnc, toEnc)
Class: SlowBuffer deprecated
new SlowBuffer(size) deprecated
Buffer Constants
buffer.constants.MAX_LENGTH
buffer.constants.MAX_STRING_LENGTH
C++ Addons
Hello world
Building
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Linking to Node.js' own dependencies
Loading Addons using require()
Native Abstractions for Node.js
N-API
Addon examples
Function arguments
Callbacks
Object factory
Function factory
Wrapping C++ objects
Factory of wrapped objects
Passing wrapped objects around
AtExit hooks
void AtExit(callback, args)
N-API
Usage
N-API Version Matrix
Basic N-API Data Types
napi_status
napi_extended_error_info
napi_env
napi_value
napi_threadsafe_function
napi_threadsafe_function_release_mode
napi_threadsafe_function_call_mode
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
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napi_async_execute_callback
napi_async_complete_callback
napi_threadsafe_function_call_js
Error Handling
Return values
napi_get_last_error_info
Exceptions
napi_throw
napi_throw_error
napi_throw_type_error
napi_throw_range_error
napi_is_error
napi_create_error
napi_create_type_error
napi_create_range_error
napi_get_and_clear_last_exception
napi_is_exception_pending
napi_fatal_exception
Fatal Errors
napi_fatal_error
Object Lifetime management
Making handle lifespan shorter than that of the native method
napi_open_handle_scope
napi_close_handle_scope
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
napi_create_reference
napi_delete_reference
napi_reference_ref
napi_reference_unref
napi_get_reference_value
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Cleanup on exit of the current Node.js instance
napi_add_env_cleanup_hook
napi_remove_env_cleanup_hook
Module registration
Working with JavaScript Values
Enum types
napi_valuetype
napi_typedarray_type
Object Creation Functions
napi_create_array
napi_create_array_with_length
napi_create_arraybuffer
napi_create_buffer
napi_create_buffer_copy
napi_create_external
napi_create_external_arraybuffer
napi_create_external_buffer
napi_create_function
napi_create_object
napi_create_symbol
napi_create_typedarray
napi_create_dataview
Functions to convert from C types to N-API
napi_create_int32
napi_create_uint32
napi_create_int64
napi_create_double
napi_create_string_latin1
napi_create_string_utf16
napi_create_string_utf8
Functions to convert from N-API to C types
napi_get_array_length
napi_get_arraybuffer_info
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napi_get_buffer_info
napi_get_prototype
napi_get_typedarray_info
napi_get_dataview_info
napi_get_value_bool
napi_get_value_double
napi_get_value_external
napi_get_value_int32
napi_get_value_int64
napi_get_value_string_latin1
napi_get_value_string_utf8
napi_get_value_string_utf16
napi_get_value_uint32
Functions to get global instances
napi_get_boolean
napi_get_global
napi_get_null
napi_get_undefined
Working with JavaScript Values - Abstract Operations
napi_coerce_to_bool
napi_coerce_to_number
napi_coerce_to_object
napi_coerce_to_string
napi_typeof
napi_instanceof
napi_is_array
napi_is_arraybuffer
napi_is_buffer
napi_is_error
napi_is_typedarray
napi_is_dataview
napi_strict_equals
Working with JavaScript Properties
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Structures
napi_property_attributes
napi_property_descriptor
Functions
napi_get_property_names
napi_set_property
napi_get_property
napi_has_property
napi_delete_property
napi_has_own_property
napi_set_named_property
napi_get_named_property
napi_has_named_property
napi_set_element
napi_get_element
napi_has_element
napi_delete_element
napi_define_properties
Working with JavaScript Functions
napi_call_function
napi_create_function
napi_get_cb_info
napi_get_new_target
napi_new_instance
Object Wrap
napi_define_class
napi_wrap
napi_unwrap
napi_remove_wrap
Simple Asynchronous Operations
napi_create_async_work
napi_delete_async_work
napi_queue_async_work
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napi_cancel_async_work
Custom Asynchronous Operations
napi_async_init
napi_async_destroy
napi_make_callback
napi_open_callback_scope
napi_close_callback_scope
Version Management
napi_get_node_version
napi_get_version
Memory Management
napi_adjust_external_memory
Promises
napi_create_promise
napi_resolve_deferred
napi_reject_deferred
napi_is_promise
Script execution
napi_run_script
libuv event loop
napi_get_uv_event_loop
Asynchronous Thread-safe Function Calls
napi_create_threadsafe_function
napi_get_threadsafe_function_context
napi_call_threadsafe_function
napi_acquire_threadsafe_function
napi_release_threadsafe_function
napi_ref_threadsafe_function
napi_unref_threadsafe_function
Child Process
Asynchronous Process Creation
Spawning .bat and .cmd files on Windows
child_process.exec(command[, options][, callback])
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child_process.execFile(file[, args][, options][, callback])
child_process.fork(modulePath[, args][, options])
child_process.spawn(command[, args][, options])
options.detached
options.stdio
Synchronous Process Creation
child_process.execFileSync(file[, args][, options])
child_process.execSync(command[, options])
child_process.spawnSync(command[, args][, options])
Class: ChildProcess
Event: 'close'
Event: 'disconnect'
Event: 'error'
Event: 'exit'
Event: 'message'
subprocess.channel
subprocess.connected
subprocess.disconnect()
subprocess.kill([signal])
subprocess.killed
subprocess.pid
subprocess.send(message[, sendHandle[, options]][, callback])
Example: sending a server object
Example: sending a socket object
subprocess.stderr
subprocess.stdin
subprocess.stdio
subprocess.stdout
maxBuffer and Unicode
Shell Requirements
Default Windows Shell
Cluster
How It Works
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Class: Worker
Event: 'disconnect'
Event: 'error'
Event: 'exit'
Event: 'listening'
Event: 'message'
Event: 'online'
worker.disconnect()
worker.exitedAfterDisconnect
worker.id
worker.isConnected()
worker.isDead()
worker.kill([signal='SIGTERM'])
worker.process
worker.send(message[, sendHandle][, callback])
worker.suicide deprecated
Event: 'disconnect'
Event: 'exit'
Event: 'fork'
Event: 'listening'
Event: 'message'
Event: 'online'
Event: 'setup'
cluster.disconnect([callback])
cluster.fork([env])
cluster.isMaster
cluster.isWorker
cluster.schedulingPolicy
cluster.settings
cluster.setupMaster([settings])
cluster.worker
cluster.workers
Command Line Options
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Synopsis
Options
-v, --version
-h, --help
-e, --eval "script"
-p, --print "script"
-c, --check
-i, --interactive
-r, --require module
--inspect[=[host:]port]
--inspect-brk[=[host:]port]
--inspect-port=[host:]port
--no-deprecation
--trace-deprecation
--throw-deprecation
--pending-deprecation
--no-warnings
--expose-http2
--abort-on-uncaught-exception
--trace-warnings
--redirect-warnings=file
--trace-sync-io
--force-async-hooks-checks
--trace-events-enabled
--trace-event-categories
--trace-event-file-pattern
--zero-fill-buffers
--preserve-symlinks
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--track-heap-objects
--prof-process
--v8-options
--tls-cipher-list=list
--enable-fips
--force-fips
--openssl-config=file
--use-openssl-ca, --use-bundled-ca
--icu-data-dir=file
-
--
--max-http-header-size=size
Environment Variables
NODE_DEBUG=module[,…]
NODE_PATH=path[:…]
NODE_DISABLE_COLORS=1
NODE_ICU_DATA=file
NODE_NO_WARNINGS=1
NODE_NO_HTTP2=1
NODE_OPTIONS=options...
NODE_PENDING_DEPRECATION=1
NODE_PRESERVE_SYMLINKS=1
NODE_REPL_HISTORY=file
NODE_EXTRA_CA_CERTS=file
OPENSSL_CONF=file
SSL_CERT_DIR=dir
SSL_CERT_FILE=file
NODE_REDIRECT_WARNINGS=file
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UV_THREADPOOL_SIZE=size
Console
Class: Console
new Console(stdout[, stderr])
console.assert(value[, message][, ...args])
console.clear()
console.count([label])
console.countReset([label='default'])
console.debug(data[, ...args])
console.dir(obj[, options])
console.error([data][, ...args])
console.group([...label])
console.groupCollapsed()
console.groupEnd()
console.info([data][, ...args])
console.log([data][, ...args])
console.time(label)
console.timeEnd(label)
console.trace([message][, ...args])
console.warn([data][, ...args])
Inspector only methods
console.dirxml(object)
console.markTimeline(label)
console.profile([label])
console.profileEnd()
console.table(array[, columns])
console.timeStamp([label])
console.timeline([label])
console.timelineEnd([label])
Crypto
Determining if crypto support is unavailable
Class: Certificate
new crypto.Certificate()
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certificate.exportChallenge(spkac)
certificate.exportPublicKey(spkac)
certificate.verifySpkac(spkac)
Class: Cipher
cipher.final([outputEncoding])
cipher.setAAD(buffer)
cipher.getAuthTag()
cipher.setAutoPadding([autoPadding])
cipher.update(data[, inputEncoding][, outputEncoding])
Class: Decipher
decipher.final([outputEncoding])
decipher.setAAD(buffer)
decipher.setAuthTag(buffer)
decipher.setAutoPadding([autoPadding])
decipher.update(data[, inputEncoding][, outputEncoding])
Class: DiffieHellman
diffieHellman.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])
diffieHellman.generateKeys([encoding])
diffieHellman.getGenerator([encoding])
diffieHellman.getPrime([encoding])
diffieHellman.getPrivateKey([encoding])
diffieHellman.getPublicKey([encoding])
diffieHellman.setPrivateKey(privateKey[, encoding])
diffieHellman.setPublicKey(publicKey[, encoding])
diffieHellman.verifyError
Class: ECDH
ecdh.computeSecret(otherPublicKey[, inputEncoding][, outputEncoding])
ecdh.generateKeys([encoding[, format]])
ecdh.getPrivateKey([encoding])
ecdh.getPublicKey([encoding][, format])
ecdh.setPrivateKey(privateKey[, encoding])
ecdh.setPublicKey(publicKey[, encoding]) deprecated
Class: Hash
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hash.digest([encoding])
hash.update(data[, inputEncoding])
Class: Hmac
hmac.digest([encoding])
hmac.update(data[, inputEncoding])
Class: Sign
sign.sign(privateKey[, outputFormat])
sign.update(data[, inputEncoding])
Class: Verify
verify.update(data[, inputEncoding])
verify.verify(object, signature[, signatureFormat])
crypto module methods and properties
crypto.constants
crypto.DEFAULT_ENCODING
crypto.fips
crypto.createCipher(algorithm, password[, options])
crypto.createCipheriv(algorithm, key, iv[, options])
crypto.createCredentials(details) deprecated
crypto.createDecipher(algorithm, password[, options])
crypto.createDecipheriv(algorithm, key, iv[, options])
crypto.createDiffieHellman(prime[, primeEncoding][, generator][,
generatorEncoding])
crypto.createDiffieHellman(primeLength[, generator])
crypto.createECDH(curveName)
crypto.createHash(algorithm[, options])
crypto.createHmac(algorithm, key[, options])
crypto.createSign(algorithm[, options])
crypto.createVerify(algorithm[, options])
crypto.getCiphers()
crypto.getCurves()
crypto.getDiffieHellman(groupName)
crypto.getHashes()
crypto.pbkdf2(password, salt, iterations, keylen, digest, callback)
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crypto.pbkdf2Sync(password, salt, iterations, keylen, digest)
crypto.privateDecrypt(privateKey, buffer)
crypto.privateEncrypt(privateKey, buffer)
crypto.publicDecrypt(key, buffer)
crypto.publicEncrypt(key, buffer)
crypto.randomBytes(size[, callback])
crypto.randomFillSync(buffer[, offset][, size])
crypto.randomFill(buffer[, offset][, size], callback)
crypto.setEngine(engine[, flags])
crypto.timingSafeEqual(a, b)
Notes
Legacy Streams API (pre Node.js v0.10)
Recent ECDH Changes
Support for weak or compromised algorithms
Crypto Constants
OpenSSL Options
OpenSSL Engine Constants
Other OpenSSL Constants
Node.js Crypto Constants
Debugger
Watchers
Command reference
Stepping
Breakpoints
Information
Execution control
Various
Advanced Usage
V8 Inspector Integration for Node.js
Deprecated APIs
Un-deprecation
List of Deprecated APIs
DEP0001: http.OutgoingMessage.prototype.flush
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DEP0002: require('_linklist')
DEP0003: _writableState.buffer
DEP0004: CryptoStream.prototype.readyState
DEP0005: Buffer() constructor
DEP0006: child_process options.customFds
DEP0007: cluster worker.suicide
DEP0008: require('constants')
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
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
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()
DEP0030: SlowBuffer
DEP0031: ecdh.setPublicKey()
DEP0032: domain module
DEP0033: EventEmitter.listenerCount()
DEP0034: fs.exists(path, callback)
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DEP0035: fs.lchmod(path, mode, callback)
DEP0036: fs.lchmodSync(path, mode)
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
DEP0044: util.isArray()
DEP0045: util.isBoolean()
DEP0046: util.isBuffer()
DEP0047: util.isDate()
DEP0048: util.isError()
DEP0049: util.isFunction()
DEP0050: util.isNull()
DEP0051: util.isNullOrUndefined()
DEP0052: util.isNumber()
DEP0053 util.isObject()
DEP0054: util.isPrimitive()
DEP0055: util.isRegExp()
DEP0056: util.isString()
DEP0057: util.isSymbol()
DEP0058: util.isUndefined()
DEP0059: util.log()
DEP0060: util._extend()
DEP0061: fs.SyncWriteStream
DEP0062: node --debug
DEP0063: ServerResponse.prototype.writeHeader()
DEP0064: tls.createSecurePair()
DEP0065: repl.REPL_MODE_MAGIC and NODE_REPL_MODE=magic
DEP0066: outgoingMessage._headers, outgoingMessage._headerNames
DEP0067: OutgoingMessage.prototype._renderHeaders
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DEP0068: node debug
DEP0069: vm.runInDebugContext(string)
DEP0070: async_hooks.currentId()
DEP0071: async_hooks.triggerId()
DEP0072: async_hooks.AsyncResource.triggerId()
DEP0076: tls.parseCertString()
DEP0079: Custom inspection function on Objects via .inspect()
DEP0085: AsyncHooks Sensitive API
DEP0086: Remove runInAsyncIdScope
DEP0089: require('assert')
DEP0098: AsyncHooks Embedder AsyncResource.emit<Before,After> APIs
DNS
Class dns.Resolver
resolver.cancel()
dns.getServers()
dns.lookup(hostname[, options], callback)
Supported getaddrinfo flags
dns.lookupService(address, port, callback)
dns.resolve(hostname[, rrtype], callback)
dns.resolve4(hostname[, options], callback)
dns.resolve6(hostname[, options], callback)
dns.resolveCname(hostname, callback)
dns.resolveMx(hostname, callback)
dns.resolveNaptr(hostname, callback)
dns.resolveNs(hostname, callback)
dns.resolvePtr(hostname, callback)
dns.resolveSoa(hostname, callback)
dns.resolveSrv(hostname, callback)
dns.resolveTxt(hostname, callback)
dns.resolveAny(hostname, callback)
dns.reverse(ip, callback)
dns.setServers(servers)
Error codes
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Implementation considerations
dns.lookup()
dns.resolve(), dns.resolve*() and dns.reverse()
Domain
Warning: Don't Ignore Errors!
Additions to Error objects
Implicit Binding
Explicit Binding
domain.create()
Class: Domain
domain.members
domain.add(emitter)
domain.bind(callback)
Example
domain.dispose() deprecated
domain.enter()
domain.exit()
domain.intercept(callback)
Example
domain.remove(emitter)
domain.run(fn[, ...args])
Domains and Promises
ECMAScript Modules
Enabling
Features
Supported
Unsupported
Notable differences between import and require
No NODE_PATH
No require.extensions
No require.cache
URL based paths
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Interop with existing modules
Loader hooks
Resolve hook
Dynamic instantiate hook
Errors
Error Propagation and Interception
Error-first callbacks
Class: Error
new Error(message)
Error.captureStackTrace(targetObject[, constructorOpt])
Error.stackTraceLimit
error.code
error.message
error.stack
Class: AssertionError
Class: RangeError
Class: ReferenceError
Class: SyntaxError
Class: TypeError
Exceptions vs. Errors
System Errors
Class: System Error
error.code
error.errno
error.syscall
error.path
error.address
error.port
Common System Errors
Node.js Error Codes
ERR_ARG_NOT_ITERABLE
ERR_ASYNC_CALLBACK
ERR_ASYNC_TYPE
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ERR_ENCODING_INVALID_ENCODED_DATA
ERR_ENCODING_NOT_SUPPORTED
ERR_FALSY_VALUE_REJECTION
ERR_HTTP_HEADERS_SENT
ERR_HTTP_INVALID_CHAR
ERR_HTTP_INVALID_STATUS_CODE
ERR_HTTP_TRAILER_INVALID
ERR_HTTP2_ALREADY_SHUTDOWN
ERR_HTTP2_ALTSVC_INVALID_ORIGIN
ERR_HTTP2_ALTSVC_LENGTH
ERR_HTTP2_CONNECT_AUTHORITY
ERR_HTTP2_CONNECT_PATH
ERR_HTTP2_CONNECT_SCHEME
ERR_HTTP2_FRAME_ERROR
ERR_HTTP2_GOAWAY_SESSION
ERR_HTTP2_HEADER_REQUIRED
ERR_HTTP2_HEADER_SINGLE_VALUE
ERR_HTTP2_HEADERS_AFTER_RESPOND
ERR_HTTP2_HEADERS_OBJECT
ERR_HTTP2_HEADERS_SENT
ERR_HTTP2_INFO_HEADERS_AFTER_RESPOND
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
ERR_HTTP2_INVALID_SETTING_VALUE
ERR_HTTP2_INVALID_STREAM
ERR_HTTP2_MAX_PENDING_SETTINGS_ACK
ERR_HTTP2_NESTED_PUSH
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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
ERR_HTTP2_PSEUDOHEADER_NOT_ALLOWED
ERR_HTTP2_PUSH_DISABLED
ERR_HTTP2_SEND_FILE
ERR_HTTP2_SESSION_ERROR
ERR_HTTP2_SETTINGS_CANCEL
ERR_HTTP2_SOCKET_BOUND
ERR_HTTP2_SOCKET_UNBOUND
ERR_HTTP2_STATUS_101
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_INVALID_ARG_TYPE
ERR_INVALID_ASYNC_ID
ERR_INVALID_CALLBACK
ERR_INVALID_FILE_URL_HOST
ERR_INVALID_FILE_URL_PATH
ERR_INVALID_HANDLE_TYPE
ERR_INVALID_OPT_VALUE
ERR_INVALID_PERFORMANCE_MARK
ERR_INVALID_PROTOCOL
ERR_INVALID_SYNC_FORK_INPUT
ERR_INVALID_THIS
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ERR_INVALID_TUPLE
ERR_INVALID_URL
ERR_INVALID_URL_SCHEME
ERR_IPC_CHANNEL_CLOSED
ERR_IPC_DISCONNECTED
ERR_IPC_ONE_PIPE
ERR_IPC_SYNC_FORK
ERR_MISSING_ARGS
ERR_MISSING_DYNAMIC_INSTANTIATE_HOOK
ERR_MISSING_MODULE
ERR_MODULE_RESOLUTION_LEGACY
ERR_MULTIPLE_CALLBACK
ERR_NAPI_CONS_FUNCTION
ERR_NAPI_CONS_PROTOTYPE_OBJECT
ERR_NAPI_INVALID_DATAVIEW_ARGS
ERR_NAPI_INVALID_TYPEDARRAY_ALIGNMENT
ERR_NAPI_INVALID_TYPEDARRAY_LENGTH
ERR_NAPI_TSFN_CALL_JS
ERR_NAPI_TSFN_GET_UNDEFINED
ERR_NAPI_TSFN_START_IDLE_LOOP
ERR_NAPI_TSFN_STOP_IDLE_LOOP
ERR_NO_ICU
ERR_SOCKET_ALREADY_BOUND
ERR_SOCKET_BAD_PORT
ERR_SOCKET_BAD_TYPE
ERR_SOCKET_CANNOT_SEND
ERR_SOCKET_CLOSED
ERR_SOCKET_DGRAM_NOT_RUNNING
ERR_STDERR_CLOSE
ERR_STDOUT_CLOSE
ERR_TLS_CERT_ALTNAME_INVALID
ERR_TLS_DH_PARAM_SIZE
ERR_TLS_HANDSHAKE_TIMEOUT
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ERR_TLS_RENEGOTIATION_FAILED
ERR_TLS_REQUIRED_SERVER_NAME
ERR_TLS_SESSION_ATTACK
ERR_TRANSFORM_ALREADY_TRANSFORMING
ERR_TRANSFORM_WITH_LENGTH_0
ERR_UNKNOWN_SIGNAL
ERR_UNKNOWN_STDIN_TYPE
ERR_UNKNOWN_STREAM_TYPE
ERR_V8BREAKITERATOR
ERR_VALID_PERFORMANCE_ENTRY_TYPE
ERR_VALUE_OUT_OF_RANGE
Events
Passing arguments and this to listeners
Asynchronous vs. Synchronous
Handling events only once
Error events
Class: EventEmitter
Event: 'newListener'
Event: 'removeListener'
EventEmitter.listenerCount(emitter, eventName) deprecated
EventEmitter.defaultMaxListeners
emitter.addListener(eventName, listener)
emitter.emit(eventName[, ...args])
emitter.eventNames()
emitter.getMaxListeners()
emitter.listenerCount(eventName)
emitter.listeners(eventName)
emitter.on(eventName, listener)
emitter.once(eventName, listener)
emitter.prependListener(eventName, listener)
emitter.prependOnceListener(eventName, listener)
emitter.removeAllListeners([eventName])
emitter.removeListener(eventName, listener)
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emitter.setMaxListeners(n)
File System
Threadpool Usage
WHATWG URL object support
Buffer API
Class: fs.FSWatcher
Event: 'change'
Event: 'error'
watcher.close()
Class: fs.ReadStream
Event: 'close'
Event: 'open'
readStream.bytesRead
readStream.path
Class: fs.Stats
Stat Time Values
Class: fs.WriteStream
Event: 'close'
Event: 'open'
writeStream.bytesWritten
writeStream.path
fs.access(path[, mode], callback)
fs.accessSync(path[, mode])
fs.appendFile(file, data[, options], callback)
fs.appendFileSync(file, data[, options])
fs.chmod(path, mode, callback)
File modes
fs.chmodSync(path, mode)
fs.chown(path, uid, gid, callback)
fs.chownSync(path, uid, gid)
fs.close(fd, callback)
fs.closeSync(fd)
fs.constants
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fs.copyFile(src, dest[, flags], callback)
fs.copyFileSync(src, dest[, flags])
fs.createReadStream(path[, options])
fs.createWriteStream(path[, options])
fs.exists(path, callback) deprecated
fs.existsSync(path)
fs.fchmod(fd, mode, callback)
fs.fchmodSync(fd, mode)
fs.fchown(fd, uid, gid, callback)
fs.fchownSync(fd, uid, gid)
fs.fdatasync(fd, callback)
fs.fdatasyncSync(fd)
fs.fstat(fd, callback)
fs.fstatSync(fd)
fs.fsync(fd, callback)
fs.fsyncSync(fd)
fs.ftruncate(fd[, len], callback)
fs.ftruncateSync(fd[, len])
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)
fs.lchownSync(path, uid, gid)
fs.link(existingPath, newPath, callback)
fs.linkSync(existingPath, newPath)
fs.lstat(path, callback)
fs.lstatSync(path)
fs.mkdir(path[, mode], callback)
fs.mkdirSync(path[, mode])
fs.mkdtemp(prefix[, options], callback)
fs.mkdtempSync(prefix[, options])
fs.open(path, flags[, mode], callback)
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fs.openSync(path, flags[, mode])
fs.read(fd, buffer, offset, length, position, callback)
fs.readdir(path[, options], callback)
fs.readdirSync(path[, options])
fs.readFile(path[, options], callback)
fs.readFileSync(path[, options])
fs.readlink(path[, options], callback)
fs.readlinkSync(path[, options])
fs.readSync(fd, buffer, offset, length, position)
fs.realpath(path[, options], callback)
fs.realpathSync(path[, options])
fs.rename(oldPath, newPath, callback)
fs.renameSync(oldPath, newPath)
fs.rmdir(path, callback)
fs.rmdirSync(path)
fs.stat(path, callback)
fs.statSync(path)
fs.symlink(target, path[, type], callback)
fs.symlinkSync(target, path[, type])
fs.truncate(path[, len], callback)
fs.truncateSync(path[, len])
fs.unlink(path, callback)
fs.unlinkSync(path)
fs.unwatchFile(filename[, listener])
fs.utimes(path, atime, mtime, callback)
fs.utimesSync(path, atime, mtime)
fs.watch(filename[, options][, listener])
Caveats
Availability
Inodes
Filename Argument
fs.watchFile(filename[, options], listener)
fs.write(fd, buffer[, offset[, length[, position]]], callback)
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fs.write(fd, string[, position[, encoding]], callback)
fs.writeFile(file, data[, options], callback)
fs.writeFileSync(file, data[, options])
fs.writeSync(fd, buffer[, offset[, length[, position]]])
fs.writeSync(fd, string[, position[, encoding]])
FS Constants
File Access Constants
File Open Constants
File Type Constants
File Mode Constants
Global Objects
Class: Buffer
__dirname
__filename
clearImmediate(immediateObject)
clearInterval(intervalObject)
clearTimeout(timeoutObject)
console
exports
global
module
process
require()
setImmediate(callback[, ...args])
setInterval(callback, delay[, ...args])
setTimeout(callback, delay[, ...args])
HTTP
Class: http.Agent
new Agent([options])
agent.createConnection(options[, callback])
agent.keepSocketAlive(socket)
agent.reuseSocket(socket, request)
agent.destroy()
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agent.freeSockets
agent.getName(options)
agent.maxFreeSockets
agent.maxSockets
agent.requests
agent.sockets
Class: http.ClientRequest
Event: 'abort'
Event: 'connect'
Event: 'continue'
Event: 'response'
Event: 'socket'
Event: 'timeout'
Event: 'upgrade'
request.abort()
request.aborted
request.connection
request.end([data[, encoding]][, callback])
request.flushHeaders()
request.getHeader(name)
request.removeHeader(name)
request.setHeader(name, value)
request.setNoDelay([noDelay])
request.setSocketKeepAlive([enable][, initialDelay])
request.setTimeout(timeout[, callback])
request.socket
request.write(chunk[, encoding][, callback])
Class: http.Server
Event: 'checkContinue'
Event: 'checkExpectation'
Event: 'clientError'
Event: 'close'
Event: 'connect'
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Event: 'connection'
Event: 'request'
Event: 'upgrade'
server.close([callback])
server.listen()
server.listening
server.maxHeadersCount
server.headersTimeout
server.setTimeout([msecs][, callback])
server.timeout
server.keepAliveTimeout
Class: http.ServerResponse
Event: 'close'
Event: 'finish'
response.addTrailers(headers)
response.connection
response.end([data][, encoding][, callback])
response.finished
response.getHeader(name)
response.getHeaderNames()
response.getHeaders()
response.hasHeader(name)
response.headersSent
response.removeHeader(name)
response.sendDate
response.setHeader(name, value)
response.setTimeout(msecs[, callback])
response.socket
response.statusCode
response.statusMessage
response.write(chunk[, encoding][, callback])
response.writeContinue()
response.writeHead(statusCode[, statusMessage][, headers])
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Class: http.IncomingMessage
Event: 'aborted'
Event: 'close'
message.aborted
message.complete
message.destroy([error])
message.headers
message.httpVersion
message.method
message.rawHeaders
message.rawTrailers
message.setTimeout(msecs, callback)
message.socket
message.statusCode
message.statusMessage
message.trailers
message.url
http.METHODS
http.STATUS_CODES
http.createServer([requestListener])
http.get(options[, callback])
http.globalAgent
http.maxHeaderSize
http.request(options[, callback])
HTTP/2
Core API
Server-side example
Client-side example
Class: Http2Session
Http2Session and Sockets
Event: 'close'
Event: 'connect'
Event: 'error'
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Event: 'frameError'
Event: 'goaway'
Event: 'localSettings'
Event: 'ping'
Event: 'remoteSettings'
Event: 'stream'
Event: 'timeout'
http2session.alpnProtocol
http2session.close([callback])
http2session.closed
http2session.connecting
http2session.destroy([error,][code])
http2session.destroyed
http2session.encrypted
http2session.goaway([code, [lastStreamID, [opaqueData]]])
http2session.localSettings
http2session.originSet
http2session.pendingSettingsAck
http2session.ping([payload, ]callback)
http2session.ref()
http2session.remoteSettings
http2session.setTimeout(msecs, callback)
http2session.socket
http2session.state
http2session.settings(settings)
http2session.type
http2session.unref()
Class: ServerHttp2Session
serverhttp2session.altsvc(alt, originOrStream)
Specifying alternative services
serverhttp2session.origin(...origins)
Class: ClientHttp2Session
Event: 'altsvc'
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Event: 'origin'
clienthttp2session.request(headers[, options])
Class: Http2Stream
Http2Stream Lifecycle
Creation
Destruction
Event: 'aborted'
Event: 'close'
Event: 'error'
Event: 'frameError'
Event: 'timeout'
Event: 'trailers'
Event: 'wantTrailers'
http2stream.aborted
http2stream.close(code[, callback])
http2stream.closed
http2stream.destroyed
http2stream.endAfterHeaders
http2stream.pending
http2stream.priority(options)
http2stream.rstCode
http2stream.sentHeaders
http2stream.sentInfoHeaders
http2stream.sentTrailers
http2stream.session
http2stream.setTimeout(msecs, callback)
http2stream.state
http2stream.sendTrailers(headers)
Class: ClientHttp2Stream
Event: 'continue'
Event: 'headers'
Event: 'push'
Event: 'response'
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Class: ServerHttp2Stream
http2stream.additionalHeaders(headers)
http2stream.headersSent
http2stream.pushAllowed
http2stream.pushStream(headers[, options], callback)
http2stream.respond([headers[, options]])
http2stream.respondWithFD(fd[, headers[, options]])
http2stream.respondWithFile(path[, headers[, options]])
Class: Http2Server
Event: 'checkContinue'
Event: 'request'
Event: 'session'
Event: 'sessionError'
Event: 'stream'
Event: 'timeout'
server.close([callback])
server.setTimeout([msecs][, callback])
Class: Http2SecureServer
Event: 'checkContinue'
Event: 'request'
Event: 'session'
Event: 'sessionError'
Event: 'stream'
Event: 'timeout'
Event: 'unknownProtocol'
server.close([callback])
server.setTimeout([msecs][, callback])
http2.createServer(options[, onRequestHandler])
http2.createSecureServer(options[, onRequestHandler])
http2.connect(authority[, options][, listener])
http2.constants
Error Codes for RST_STREAM and GOAWAY
http2.getDefaultSettings()
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http2.getPackedSettings(settings)
http2.getUnpackedSettings(buf)
Headers Object
Settings Object
Using options.selectPadding
Error Handling
Invalid character handling in header names and values
Push streams on the client
Supporting the CONNECT method
Compatibility API
ALPN negotiation
Class: http2.Http2ServerRequest
Event: 'aborted'
Event: 'close'
request.aborted
request.destroy([error])
request.headers
request.httpVersion
request.method
request.rawHeaders
request.rawTrailers
request.setTimeout(msecs, callback)
request.socket
request.stream
request.trailers
request.url
Class: http2.Http2ServerResponse
Event: 'close'
Event: 'finish'
response.addTrailers(headers)
response.connection
response.end([data][, encoding][, callback])
response.finished
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response.getHeader(name)
response.getHeaderNames()
response.getHeaders()
response.hasHeader(name)
response.headersSent
response.removeHeader(name)
response.sendDate
response.setHeader(name, value)
response.setTimeout(msecs[, callback])
response.socket
response.statusCode
response.statusMessage
response.stream
response.write(chunk[, encoding][, callback])
response.writeContinue()
response.writeHead(statusCode[, statusMessage][, headers])
response.createPushResponse(headers, callback)
Collecting HTTP/2 Performance Metrics
HTTPS
Class: https.Agent
Class: https.Server
server.close([callback])
server.listen()
server.headersTimeout
server.setTimeout([msecs][, callback])
server.timeout
server.keepAliveTimeout
https.createServer([options][, requestListener])
https.get(options[, callback])
https.globalAgent
https.request(options[, callback])
Inspector
inspector.open([port[, host[, wait]]])
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inspector.close()
inspector.url()
Class: inspector.Session
Constructor: new inspector.Session()
Event: 'inspectorNotification'
Event: <inspector-protocol-method>
session.connect()
session.post(method[, params][, callback])
session.disconnect()
Example usage
CPU Profiler
Internationalization Support
Options for building Node.js
Disable all internationalization features (none)
Build with a pre-installed ICU (system-icu)
Embed a limited set of ICU data (small-icu)
Providing ICU data at runtime
Embed the entire ICU (full-icu)
Detecting internationalization support
Modules
Accessing the main module
Addenda: Package Manager Tips
All Together...
Caching
Module Caching Caveats
Core Modules
Cycles
File Modules
Folders as Modules
Loading from node_modules Folders
Loading from the global folders
The module wrapper
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The module scope
__dirname
__filename
exports
module
require()
require.cache
require.extensions deprecated
require.resolve(request[, options])
require.resolve.paths(request)
The module Object
module.children
module.exports
exports shortcut
module.filename
module.id
module.loaded
module.parent
module.paths
module.require(id)
The Module Object
module.builtinModules
Net
IPC Support
Identifying paths for IPC connections
Class: net.Server
new net.Server([options][, connectionListener])
Event: 'close'
Event: 'connection'
Event: 'error'
Event: 'listening'
server.address()
server.close([callback])
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server.connections deprecated
server.getConnections(callback)
server.listen()
server.listen(handle[, backlog][, callback])
server.listen(options[, callback])
server.listen(path[, backlog][, callback])
server.listen([port][, host][, backlog][, callback])
server.listening
server.maxConnections
server.ref()
server.unref()
Class: net.Socket
new net.Socket([options])
Event: 'close'
Event: 'connect'
Event: 'data'
Event: 'drain'
Event: 'end'
Event: 'error'
Event: 'lookup'
Event: 'timeout'
socket.address()
socket.bufferSize
socket.bytesRead
socket.bytesWritten
socket.connect()
socket.connect(options[, connectListener])
socket.connect(path[, connectListener])
socket.connect(port[, host][, connectListener])
socket.connecting
socket.destroy([exception])
socket.destroyed
socket.end([data][, encoding])
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socket.localAddress
socket.localPort
socket.pause()
socket.ref()
socket.remoteAddress
socket.remoteFamily
socket.remotePort
socket.resume()
socket.setEncoding([encoding])
socket.setKeepAlive([enable][, initialDelay])
socket.setNoDelay([noDelay])
socket.setTimeout(timeout[, callback])
socket.unref()
socket.write(data[, encoding][, callback])
net.connect()
net.connect(options[, connectListener])
net.connect(path[, connectListener])
net.connect(port[, host][, connectListener])
net.createConnection()
net.createConnection(options[, connectListener])
net.createConnection(path[, connectListener])
net.createConnection(port[, host][, connectListener])
net.createServer([options][, connectionListener])
net.isIP(input)
net.isIPv4(input)
net.isIPv6(input)
OS
os.EOL
os.arch()
os.constants
os.cpus()
os.endianness()
os.freemem()
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os.homedir()
os.hostname()
os.loadavg()
os.networkInterfaces()
os.platform()
os.release()
os.tmpdir()
os.totalmem()
os.type()
os.uptime()
os.userInfo([options])
OS Constants
Signal Constants
Error Constants
POSIX Error Constants
Windows Specific Error Constants
libuv Constants
Path
Windows vs. POSIX
path.basename(path[, ext])
path.delimiter
path.dirname(path)
path.extname(path)
path.format(pathObject)
path.isAbsolute(path)
path.join([...paths])
path.normalize(path)
path.parse(path)
path.posix
path.relative(from, to)
path.resolve([...paths])
path.sep
path.win32
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Performance Timing API
Class: Performance
performance.clearEntries(name)
performance.clearFunctions([name])
performance.clearGC()
performance.clearMarks([name])
performance.clearMeasures([name])
performance.getEntries()
performance.getEntriesByName(name[, type])
performance.getEntriesByType(type)
performance.mark([name])
performance.maxEntries
performance.measure(name, startMark, endMark)
performance.nodeTiming
performance.now()
performance.timeOrigin
performance.timerify(fn)
Class: PerformanceEntry
performanceEntry.duration
performanceEntry.name
performanceEntry.startTime
performanceEntry.entryType
performanceEntry.kind
Class: PerformanceNodeTiming extends PerformanceEntry
performanceNodeTiming.bootstrapComplete
performanceNodeTiming.clusterSetupEnd
performanceNodeTiming.clusterSetupStart
performanceNodeTiming.loopExit
performanceNodeTiming.loopStart
performanceNodeTiming.moduleLoadEnd
performanceNodeTiming.moduleLoadStart
performanceNodeTiming.nodeStart
performanceNodeTiming.preloadModuleLoadEnd
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performanceNodeTiming.preloadModuleLoadStart
performanceNodeTiming.thirdPartyMainEnd
performanceNodeTiming.thirdPartyMainStart
performanceNodeTiming.v8Start
Class: PerformanceObserver(callback)
Callback: PerformanceObserverCallback(list, observer)
Class: PerformanceObserverEntryList
performanceObserverEntryList.getEntries()
performanceObserverEntryList.getEntriesByName(name[, type])
performanceObserverEntryList.getEntriesByType(type)
performanceObserver.disconnect()
performanceObserver.observe(options)
Examples
Measuring the duration of async operations
Measuring how long it takes to load dependencies
Process
Process Events
Event: 'beforeExit'
Event: 'disconnect'
Event: 'exit'
Event: 'message'
Event: 'rejectionHandled'
Event: 'uncaughtException'
Warning: Using 'uncaughtException' correctly
Event: 'unhandledRejection'
Event: 'warning'
Emitting custom warnings
Signal Events
process.abort()
process.arch
process.argv
process.argv0
process.channel
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process.chdir(directory)
process.config
process.connected
process.cpuUsage([previousValue])
process.cwd()
process.debugPort
process.disconnect()
process.emitWarning(warning[, options])
process.emitWarning(warning[, type[, code]][, ctor])
Avoiding duplicate warnings
process.env
process.execArgv
process.execPath
process.exit([code])
process.exitCode
process.getegid()
process.geteuid()
process.getgid()
process.getgroups()
process.getuid()
process.hrtime([time])
process.initgroups(user, extra_group)
process.kill(pid[, signal])
process.mainModule
process.memoryUsage()
process.nextTick(callback[, ...args])
process.noDeprecation
process.pid
process.platform
process.ppid
process.release
process.send(message[, sendHandle[, options]][, callback])
process.setegid(id)
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process.seteuid(id)
process.setgid(id)
process.setgroups(groups)
process.setuid(id)
process.stderr
process.stdin
process.stdout
A note on process I/O
process.throwDeprecation
process.title
process.traceDeprecation
process.umask([mask])
process.uptime()
process.version
process.versions
Exit Codes
Punycode
punycode.decode(string)
punycode.encode(string)
punycode.toASCII(domain)
punycode.toUnicode(domain)
punycode.ucs2
punycode.ucs2.decode(string)
punycode.ucs2.encode(codePoints)
punycode.version
Query String
querystring.escape(str)
querystring.parse(str[, sep[, eq[, options]]])
querystring.stringify(obj[, sep[, eq[, options]]])
querystring.unescape(str)
Readline
Class: Interface
Event: 'close'
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Event: 'line'
Event: 'pause'
Event: 'resume'
Event: 'SIGCONT'
Event: 'SIGINT'
Event: 'SIGTSTP'
rl.close()
rl.pause()
rl.prompt([preserveCursor])
rl.question(query, callback)
rl.resume()
rl.setPrompt(prompt)
rl.write(data[, key])
readline.clearLine(stream, dir)
readline.clearScreenDown(stream)
readline.createInterface(options)
Use of the completer Function
readline.cursorTo(stream, x, y)
readline.emitKeypressEvents(stream[, interface])
readline.moveCursor(stream, dx, dy)
Example: Tiny CLI
Example: Read File Stream Line-by-Line
REPL
Design and Features
Commands and Special Keys
Default Evaluation
JavaScript Expressions
Global and Local Scope
Accessing Core Node.js Modules
Assignment of the _ (underscore) variable
Custom Evaluation Functions
Recoverable Errors
Customizing REPL Output
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Class: REPLServer
Event: 'exit'
Event: 'reset'
replServer.defineCommand(keyword, cmd)
replServer.displayPrompt([preserveCursor])
repl.start([options])
The Node.js REPL
Environment Variable Options
Persistent History
NODE_REPL_HISTORY_FILE deprecated
Using the Node.js REPL with advanced line-editors
Starting multiple REPL instances against a single running instance
Stream
Organization of this Document
Types of Streams
Object Mode
Buffering
API for Stream Consumers
Writable Streams
Class: stream.Writable
Event: 'close'
Event: 'drain'
Event: 'error'
Event: 'finish'
Event: 'pipe'
Event: 'unpipe'
writable.cork()
writable.end([chunk][, encoding][, callback])
writable.setDefaultEncoding(encoding)
writable.uncork()
writable.writableHighWaterMark
writable.write(chunk[, encoding][, callback])
writable.destroy([error])
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Readable Streams
Two Modes
Three States
Choose One
Class: stream.Readable
Event: 'close'
Event: 'data'
Event: 'end'
Event: 'error'
Event: 'readable'
readable.isPaused()
readable.pause()
readable.pipe(destination[, options])
readable.readableHighWaterMark
readable.read([size])
readable.resume()
readable.setEncoding(encoding)
readable.unpipe([destination])
readable.unshift(chunk)
readable.wrap(stream)
readable.destroy([error])
Duplex and Transform Streams
Class: stream.Duplex
Class: stream.Transform
transform.destroy([error])
API for Stream Implementers
Simplified Construction
Implementing a Writable Stream
Constructor: new stream.Writable([options])
writable._write(chunk, encoding, callback)
writable._writev(chunks, callback)
writable._destroy(err, callback)
writable._final(callback)
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Errors While Writing
An Example Writable Stream
Decoding buffers in a Writable Stream
Implementing a Readable Stream
new stream.Readable([options])
readable._read(size)
readable._destroy(err, callback)
readable.push(chunk[, encoding])
Errors While Reading
An Example Counting Stream
Implementing a Duplex Stream
new stream.Duplex(options)
An Example Duplex Stream
Object Mode Duplex Streams
Implementing a Transform Stream
new stream.Transform([options])
Events: 'finish' and 'end'
transform._flush(callback)
transform._transform(chunk, encoding, callback)
Class: stream.PassThrough
Additional Notes
Compatibility with Older Node.js Versions
readable.read(0)
readable.push('')
highWaterMark discrepancy after calling readable.setEncoding()
String Decoder
Class: new StringDecoder([encoding])
stringDecoder.end([buffer])
stringDecoder.write(buffer)
Timers
Class: Immediate
Class: Timeout
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timeout.ref()
timeout.unref()
Scheduling Timers
setImmediate(callback[, ...args])
setInterval(callback, delay[, ...args])
setTimeout(callback, delay[, ...args])
Cancelling Timers
clearImmediate(immediate)
clearInterval(timeout)
clearTimeout(timeout)
TLS (SSL)
TLS/SSL Concepts
Perfect Forward Secrecy
ALPN, NPN, and SNI
Client-initiated renegotiation attack mitigation
Modifying the Default TLS Cipher suite
Class: tls.Server
Event: 'newSession'
Event: 'OCSPRequest'
Event: 'resumeSession'
Event: 'secureConnection'
Event: 'tlsClientError'
server.addContext(hostname, context)
server.address()
server.close([callback])
server.connections deprecated
server.getTicketKeys()
server.listen()
server.setTicketKeys(keys)
Class: tls.TLSSocket
new tls.TLSSocket(socket[, options])
Event: 'OCSPResponse'
Event: 'secureConnect'
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tlsSocket.address()
tlsSocket.authorizationError
tlsSocket.authorized
tlsSocket.disableRenegotiation()
tlsSocket.encrypted
tlsSocket.getCipher()
tlsSocket.getEphemeralKeyInfo()
tlsSocket.getFinished()
tlsSocket.getPeerCertificate([detailed])
tlsSocket.getPeerFinished()
tlsSocket.getProtocol()
tlsSocket.getSession()
tlsSocket.getTLSTicket()
tlsSocket.localAddress
tlsSocket.localPort
tlsSocket.remoteAddress
tlsSocket.remoteFamily
tlsSocket.remotePort
tlsSocket.renegotiate(options, callback)
tlsSocket.setMaxSendFragment(size)
tls.checkServerIdentity(host, cert)
tls.connect(options[, callback])
tls.connect(path[, options][, callback])
tls.connect(port[, host][, options][, callback])
tls.createSecureContext(options)
tls.createServer([options][, secureConnectionListener])
tls.getCiphers()
tls.DEFAULT_ECDH_CURVE
Deprecated APIs
Class: CryptoStream deprecated
cryptoStream.bytesWritten
Class: SecurePair deprecated
Event: 'secure'
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tls.createSecurePair([context][, isServer][, requestCert][, rejectUnauthorized][,
options]) deprecated
Tracing
TTY
Class: tty.ReadStream
readStream.isRaw
readStream.isTTY
readStream.setRawMode(mode)
Class: tty.WriteStream
Event: 'resize'
writeStream.columns
writeStream.isTTY
writeStream.rows
tty.isatty(fd)
UDP / Datagram Sockets
Class: dgram.Socket
Event: 'close'
Event: 'error'
Event: 'listening'
Event: 'message'
socket.addMembership(multicastAddress[, multicastInterface])
socket.address()
socket.bind([port][, address][, callback])
socket.bind(options[, callback])
socket.close([callback])
socket.dropMembership(multicastAddress[, multicastInterface])
socket.getRecvBufferSize()
socket.getSendBufferSize()
socket.ref()
socket.send(msg, [offset, length,] port [, address] [, callback])
socket.setBroadcast(flag)
socket.setMulticastInterface(multicastInterface)
Examples: IPv6 Outgoing Multicast Interface
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Example: IPv4 Outgoing Multicast Interface
Call Results
socket.setMulticastLoopback(flag)
socket.setMulticastTTL(ttl)
socket.setRecvBufferSize(size)
socket.setSendBufferSize(size)
socket.setTTL(ttl)
socket.unref()
Change to asynchronous socket.bind() behavior
dgram module functions
dgram.createSocket(options[, callback])
dgram.createSocket(type[, callback])
URL
URL Strings and URL Objects
The WHATWG URL API
Class: URL
Constructor: new URL(input[, base])
url.hash
url.host
url.hostname
url.href
url.origin
url.password
url.pathname
url.port
url.protocol
url.search
url.searchParams
url.username
url.toString()
url.toJSON()
Class: URLSearchParams
Constructor: new URLSearchParams()
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Constructor: new URLSearchParams(string)
Constructor: new URLSearchParams(obj)
Constructor: new URLSearchParams(iterable)
urlSearchParams.append(name, value)
urlSearchParams.delete(name)
urlSearchParams.entries()
urlSearchParams.forEach(fn[, thisArg])
urlSearchParams.get(name)
urlSearchParams.getAll(name)
urlSearchParams.has(name)
urlSearchParams.keys()
urlSearchParams.set(name, value)
urlSearchParams.sort()
urlSearchParams.toString()
urlSearchParams.values()
urlSearchParams[@@iterator]()
url.domainToASCII(domain)
url.domainToUnicode(domain)
url.format(URL[, options])
Legacy URL API
Legacy urlObject
urlObject.auth
urlObject.hash
urlObject.host
urlObject.hostname
urlObject.href
urlObject.path
urlObject.pathname
urlObject.port
urlObject.protocol
urlObject.query
urlObject.search
urlObject.slashes
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url.format(urlObject)
url.parse(urlString[, parseQueryString[, slashesDenoteHost]])
url.resolve(from, to)
Percent-Encoding in URLs
Legacy API
WHATWG API
Util
util.callbackify(original)
util.debuglog(section)
util.deprecate(function, string)
util.format(format[, ...args])
util.getSystemErrorName(err)
util.inherits(constructor, superConstructor)
util.inspect(object[, options])
Customizing util.inspect colors
Custom inspection functions on Objects
util.inspect.custom
util.inspect.defaultOptions
util.promisify(original)
Custom promisified functions
util.promisify.custom
Class: util.TextDecoder
WHATWG Supported Encodings
Encodings Supported Without ICU
Encodings Supported by Default (With ICU)
Encodings Requiring Full ICU Data
new TextDecoder([encoding[, options]])
textDecoder.decode([input[, options]])
textDecoder.encoding
textDecoder.fatal
textDecoder.ignoreBOM
Class: util.TextEncoder
textEncoder.encode([input])
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textEncoder.encoding
Deprecated APIs
util._extend(target, source) deprecated
util.debug(string) deprecated
util.error([...strings]) deprecated
util.isArray(object) deprecated
util.isBoolean(object) deprecated
util.isBuffer(object) deprecated
util.isDate(object) deprecated
util.isError(object) deprecated
util.isFunction(object) deprecated
util.isNull(object) deprecated
util.isNullOrUndefined(object) deprecated
util.isNumber(object) deprecated
util.isObject(object) deprecated
util.isPrimitive(object) deprecated
util.isRegExp(object) deprecated
util.isString(object) deprecated
util.isSymbol(object) deprecated
util.isUndefined(object) deprecated
util.log(string) deprecated
util.print([...strings]) deprecated
util.puts([...strings]) deprecated
V8
v8.cachedDataVersionTag()
v8.getHeapSpaceStatistics()
v8.getHeapStatistics()
v8.setFlagsFromString(string)
Serialization API
v8.serialize(value)
v8.deserialize(buffer)
class: v8.Serializer
new Serializer()
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serializer.writeHeader()
serializer.writeValue(value)
serializer.releaseBuffer()
serializer.transferArrayBuffer(id, arrayBuffer)
serializer.writeUint32(value)
serializer.writeUint64(hi, lo)
serializer.writeDouble(value)
serializer.writeRawBytes(buffer)
serializer._writeHostObject(object)
serializer._getDataCloneError(message)
serializer._getSharedArrayBufferId(sharedArrayBuffer)
serializer._setTreatArrayBufferViewsAsHostObjects(flag)
class: v8.Deserializer
new Deserializer(buffer)
deserializer.readHeader()
deserializer.readValue()
deserializer.transferArrayBuffer(id, arrayBuffer)
deserializer.getWireFormatVersion()
deserializer.readUint32()
deserializer.readUint64()
deserializer.readDouble()
deserializer.readRawBytes(length)
deserializer._readHostObject()
class: v8.DefaultSerializer
class: v8.DefaultDeserializer
VM (Executing JavaScript)
Class: vm.Script
new vm.Script(code, options)
script.runInContext(contextifiedSandbox[, options])
script.runInNewContext([sandbox[, options]])
script.runInThisContext([options])
vm.createContext([sandbox])
vm.isContext(sandbox)
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vm.runInContext(code, contextifiedSandbox[, options])
vm.runInDebugContext(code) deprecated
vm.runInNewContext(code[, sandbox][, options])
vm.runInThisContext(code[, options])
Example: Running an HTTP Server within a VM
What does it mean to "contextify" an object?
Zlib
Threadpool Usage
Compressing HTTP requests and responses
Memory Usage Tuning
Flushing
Constants
Class Options
Class: zlib.Deflate
Class: zlib.DeflateRaw
Class: zlib.Gunzip
Class: zlib.Gzip
Class: zlib.Inflate
Class: zlib.InflateRaw
Class: zlib.Unzip
Class: zlib.Zlib
zlib.bytesRead
zlib.close([callback])
zlib.flush([kind], callback)
zlib.params(level, strategy, callback)
zlib.reset()
zlib.constants
zlib.createDeflate([options])
zlib.createDeflateRaw([options])
zlib.createGunzip([options])
zlib.createGzip([options])
zlib.createInflate([options])
zlib.createInflateRaw([options])
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zlib.createUnzip([options])
Convenience Methods
zlib.deflate(buffer[, options], callback)
zlib.deflateSync(buffer[, options])
zlib.deflateRaw(buffer[, options], callback)
zlib.deflateRawSync(buffer[, options])
zlib.gunzip(buffer[, options], callback)
zlib.gunzipSync(buffer[, options])
zlib.gzip(buffer[, options], callback)
zlib.gzipSync(buffer[, options])
zlib.inflate(buffer[, options], callback)
zlib.inflateSync(buffer[, options])
zlib.inflateRaw(buffer[, options], callback)
zlib.inflateRawSync(buffer[, options])
zlib.unzip(buffer[, options], callback)
zlib.unzipSync(buffer[, options])
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 .
About this Documentation
#
Contributing
#
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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 This feature is known to be problematic, and changes may be
planned. Do not rely on it. Use of the feature may cause warnings to be emitted. Backwards
compatibility across major versions should not be expected.
Stability: 1  - Experimental This feature is still under active development and subject to non-
backwards compatible changes, or even 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 The API has proven satisfactory. Compatibility with the npm ecosystem is
a high priority, and will not be broken unless absolutely necessary.
Note: 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 process.on('warning')  event.
Added in: v0.6.12
Stability: 1  - Experimental
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
Stability Index
#
JSON Output
#
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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 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.
Some syscalls, like lchown(2) , are BSD-specific. That means, for example, that fs.lchown()  only
works on macOS and other BSD-derived systems, and is not available on Linux.
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.
Syscalls and man pages
#
Usage
#
Example
#
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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.
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, () => { 
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  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 .
History
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() .
It can be accessed using:
Assert
#
Strict mode
#
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const assert = require('assert').strict;
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());
Added in: v0.5.9
value  <any>
message  <any>
An alias of assert.ok() .
History
Legacy mode
#
assert(value[, message])
#
assert.deepEqual(actual, expected[,
message])
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actual  <any>
expected  <any>
message  <any>
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, attached symbols, or non-enumerable properties —
for such checks, consider using assert.deepStrictEqual()  instead. This can lead to some
potentially surprising results. For example, 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 . Maps and Sets 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 
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  }
};
const obj3 = { 
  a: { 
    b: 1 
  }
};
const obj4 = Object.create(obj1); 
 
assert.deepEqual(obj1, obj1);
// OK, object is equal to itself 
 
assert.deepEqual(obj1, obj2);
// AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }
// values of b are different 
 
assert.deepEqual(obj1, obj3);
// OK, objects are equal 
 
assert.deepEqual(obj1, obj4);
// AssertionError: { a: { b: 1 } } deepEqual {}
// Prototypes are ignored
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.
History
actual  <any>
expected  <any>
message  <any>
Generally identical to assert.deepEqual()  with a few exceptions:
assert.deepStrictEqual(actual, expected[,
message])
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Primitive values are compared using the Strict Equality Comparison  ( ===  ).
Set values and Map keys are compared using the SameValueZero  comparison. (Which means
they are free of the caveats ).
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  messages are always compared, even though this property is non-enumerable.
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.
const assert = require('assert').strict; 
 
assert.deepEqual({ a: 1 }, { a: '1' });
// OK, because 1 == '1' 
 
assert.deepStrictEqual({ a: 1 }, { a: '1' });
// AssertionError: { a: 1 } deepStrictEqual { a: '1' }
// because 1 !== '1' using strict equality 
 
// The following objects don't have own properties
const date = new Date();
const object = {};
const fakeDate = {}; 
 
Object.setPrototypeOf(fakeDate, Date.prototype); 
 
assert.deepEqual(object, fakeDate);
// OK, doesn't check [[Prototype]] 
assert.deepStrictEqual(object, fakeDate);
// AssertionError: {} deepStrictEqual Date {}
// Different [[Prototype]] 
 
assert.deepEqual(date, fakeDate);
// OK, doesn't check type tags 
assert.deepStrictEqual(date, fakeDate);
Comparison details
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// AssertionError: 2017-03-11T14:25:31.849Z deepStrictEqual Date {}
// Different type tags 
 
assert.deepStrictEqual(new Number(1), new Number(2));
// Fails because the wrapped number is unwrapped and compared as well. 
assert.deepStrictEqual(new String('foo'), Object('foo'));
// OK because the object and the string are identical when unwrapped.
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.
Added in: V8.13.0
block  <Function>
error  <RegExp>  | <Function>
message  <any>
Awaits for the promise returned by function block  to complete and not be rejected. See
assert.rejects()  for more details.
When assert.doesNotReject()  is called, it will immediately call the block  function, and awaits
for completion.
Besides the async nature to await the completion behaves identical to [ assert.doesNotThrow() ][].
(async () => { 
  await assert.doesNotReject( 
    async () => { 
      throw new TypeError('Wrong value'); 
    }, 
    SyntaxError 
  );
})();
assert.doesNotReject(block[, error][,
message])
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assert.doesNotReject( 
  () => Promise.reject(new TypeError('Wrong value')), 
  SyntaxError
).then(() => { 
  // ...
});
History
block  <Function>
error  <RegExp>  | <Function>
message  <any>
Asserts that the function block  does not throw an error. See assert.throws()  for more details.
Please note: Using assert.doesNotThrow()  is actually not useful because there is no benefit by
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 block  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.
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
);
assert.doesNotThrow(block[, error][,
message])
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However, the following will result in an AssertionError  with the message 'Got unwanted
exception (TypeError)..':
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'); 
  }, 
  TypeError, 
  'Whoops'
);
// Throws: AssertionError: Got unwanted exception (TypeError). Whoops
Added in: v0.1.21
actual  <any>
expected  <any>
message  <any>
Strict mode
An alias of assert.strictEqual() .
Legacy mode
Stability: 0  - Deprecated: Use assert.strictEqual()  instead.
assert.equal(actual, expected[, message])
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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.
Added in: v0.1.21
actual  <any>
expected  <any>
message  <any>
operator  <string>  Default: '!='
stackStartFunction  <Function>  Default: assert.fail
Throws an AssertionError . 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 only it will be used
as the error message, the other arguments will be stored as properties on the thrown object. If
stackStartFunction  is provided, all stack frames above that function will be removed from
stacktrace (see Error.captureStackTrace ).
assert.fail(message)
#
assert.fail(actual, expected[, message[,
operator[, stackStartFunction]]])
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const assert = require('assert').strict; 
 
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
Note: In the last two cases actual , expected , and operator  have no influence on the error
message.
assert.fail();
// AssertionError [ERR_ASSERTION]: Failed 
 
assert.fail('boom');
// AssertionError [ERR_ASSERTION]: boom 
 
assert.fail('a', 'b');
// AssertionError [ERR_ASSERTION]: 'a' != 'b'
Example use of stackStartFunction  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)
//     ...
Added in: v0.1.97
assert.ifError(value)
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value  <any>
Throws value  if value  is truthy. This is useful when testing the error  argument in callbacks.
const assert = require('assert').strict; 
 
assert.ifError(null);
// OK 
assert.ifError(0);
// OK 
assert.ifError(1);
// Throws 1 
assert.ifError('error');
// Throws 'error' 
assert.ifError(new Error());
// Throws Error
Added in: v0.1.21
actual  <any>
expected  <any>
message  <any>
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 = { 
assert.notDeepEqual(actual, expected[,
message])
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  a: { 
    b: 1 
  }
};
const obj2 = { 
  a: { 
    b: 2 
  }
};
const obj3 = { 
  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: obj1 and obj2 are not deeply equal 
 
assert.notDeepEqual(obj1, obj3);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } } 
 
assert.notDeepEqual(obj1, obj4);
// OK: obj1 and obj4 are not deeply equal
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.
Added in: v1.2.0
actual  <any>
expected  <any>
assert.notDeepStrictEqual(actual, expected[,
message])
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message  <any>
Tests for deep strict inequality. Opposite of assert.deepStrictEqual() .
const assert = require('assert').strict; 
 
assert.notDeepEqual({ a: 1 }, { a: '1' });
// AssertionError: { a: 1 } notDeepEqual { a: '1' } 
 
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.
Added in: v0.1.21
actual  <any>
expected  <any>
message  <any>
Strict mode
An alias of assert.notStrictEqual() .
Legacy mode
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(actual, expected[, message]) #
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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.
Added in: v0.1.21
actual  <any>
expected  <any>
message  <any>
Tests strict inequality as determined by the Strict Equality Comparison  ( !==  ).
const assert = require('assert').strict; 
 
assert.notStrictEqual(1, 2);
// OK 
 
assert.notStrictEqual(1, 1);
// AssertionError: 1 !== 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.
Added in: v0.1.21
assert.notStrictEqual(actual, expected[,
message])
#
assert.ok(value[, message])
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value  <any>
message  <any>
Tests if value  is truthy. It is equivalent to assert.equal(!!value, true, message) .
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.
const assert = require('assert').strict; 
 
assert.ok(true);
// OK 
assert.ok(1);
// OK 
assert.ok(false);
// throws "AssertionError: false == true" 
assert.ok(0);
// throws "AssertionError: 0 == true" 
assert.ok(false, 'it\'s false');
// throws "AssertionError: it's false"
Added in: v0.1.21
actual  <any>
expected  <any>
message  <any>
Tests strict equality as determined by the Strict Equality Comparison  ( ===  ).
const assert = require('assert').strict; 
 
assert.strictEqual(1, 2);
// AssertionError: 1 === 2 
 
assert.strictEqual(1, 1);
assert.strictEqual(actual, expected[,
message])
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// OK 
 
assert.strictEqual(1, '1');
// AssertionError: 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.
Added in: V8.13.0
block  <Function>
error  <RegExp>  | <Function>  | <Object>
message  <any>
Awaits for promise returned by function block  to be rejected.
When assert.rejects()  is called, it will immediately call the block  function, and awaits for
completion.
Besides the async nature to await the completion behaves identical to assert.throws() .
If specified, error  can be a constructor, RegExp , a validation function, or an object where each
property will be tested for.
If specified, message  will be the message provided by the AssertionError  if the block fails to
reject.
(async () => { 
  await assert.rejects( 
    async () => { 
      throw new Error('Wrong value'); 
    }, 
    Error 
  );
})();
assert.rejects(block[, error][, message])
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assert.rejects( 
  () => Promise.reject(new Error('Wrong value')), 
  Error
).then(() => { 
  // ...
});
History
block  <Function>
error  <RegExp>  | <Function>  | <object>
message  <any>
Expects the function block  to throw an error.
If specified, error  can be a constructor, RegExp , a validation function, or an object where each
property will be tested for.
If specified, message  will be the message provided by the AssertionError  if the block fails to
throw.
Validate instanceof using constructor:
assert.throws( 
  () => { 
    throw new Error('Wrong value'); 
  }, 
  Error
);
Validate error message using RegExp :
assert.throws( 
  () => { 
    throw new Error('Wrong value'); 
  }, 
assert.throws(block[, error][, message])
#
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  /value/
);
Custom error validation:
assert.throws( 
  () => { 
    throw new Error('Wrong value'); 
  }, 
  function(err) { 
    if ((err instanceof Error) && /value/.test(err)) { 
      return true; 
    } 
  }, 
  'unexpected error'
);
Custom error object / error instance:
assert.throws( 
  () => { 
    const err = new TypeError('Wrong value'); 
    err.code = 404; 
    throw err; 
  }, 
  { 
    name: 'TypeError', 
    message: 'Wrong value' 
    // Note that only properties on the error object will be tested! 
  }
);
Note that error  can not 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 below carefully if using a string as the second argument
gets considered:
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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.
// In that case both cases do not throw as neither is going to try to
// match for the error message thrown by the input function! 
assert.throws(throwingFirst, 'Second'); 
assert.throws(throwingSecond, 'Second'); 
 
// 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$/);
// Does not throw because the error messages match. 
assert.throws(throwingFirst, /Second$/);
// Throws a 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.
For the following cases, consider using ES2015 Object.is() , which uses the SameValueZero
comparison.
const a = 0;
const b = -a; 
assert.notStrictEqual(a, b);
// AssertionError: 0 !== -0
// Strict Equality Comparison doesn't distinguish between -0 and +0...
Caveats
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assert(!Object.is(a, b));
// but Object.is() does! 
 
const str1 = 'foo';
const str2 = 'foo'; 
assert.strictEqual(str1 / 1, str2 / 1);
// AssertionError: NaN === NaN
// Strict Equality Comparison can't be used to check NaN...
assert(Object.is(str1 / 1, str2 / 1));
// but Object.is() can!
For more information, see MDN's guide on equality comparisons and sameness .
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.
Following is a simple overview of the public API.
Async Hooks
#
Terminology
#
Public API
#
Overview
#
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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(); 
 
//
// 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) { } 
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// 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) { }
Added in: v8.1.0
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:
class MyAsyncCallbacks { 
  init(asyncId, type, triggerAsyncId, resource) { } 
  destroy(asyncId) {}
} 
async_hooks.createHook(callbacks)
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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.writeSync(1, msg) .
This will print to stdout because 1  is the file descriptor for stdout 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.writeSync(1, `${util.format(...args)}\n`);
}
Error Handling
#
Printing in AsyncHooks callbacks
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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.
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.
asyncHook.enable()
#
asyncHook.disable()
#
Hook Callbacks
#
init(asyncId, type, triggerAsyncId, resource)
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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 process.
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.
Note: 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 :
type
#
triggerId
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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(2): trigger: 1 execution: 1
TCPWRAP(4): trigger: 2 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 (side note: a
executionAsyncId()  of 0  means it's 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 hostname 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 a 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.
resource
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Note: 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({ 
  init(asyncId, type, triggerAsyncId) { 
    const eid = async_hooks.executionAsyncId(); 
    const indentStr = ' '.repeat(indent); 
    fs.writeSync( 
      1, 
      `${indentStr}${type}(${asyncId}):` + 
      ` trigger: ${triggerAsyncId} execution: ${eid}\n`); 
  }, 
  before(asyncId) { 
    const indentStr = ' '.repeat(indent); 
    fs.writeSync(1, `${indentStr}before:  ${asyncId}\n`); 
    indent += 2; 
  }, 
  after(asyncId) { 
    indent -= 2; 
    const indentStr = ' '.repeat(indent); 
    fs.writeSync(1, `${indentStr}after:   ${asyncId}\n`); 
  }, 
  destroy(asyncId) { 
    const indentStr = ' '.repeat(indent); 
    fs.writeSync(1, `${indentStr}destroy: ${asyncId}\n`); 
  },
}).enable(); 
 
require('net').createServer(() => {}).listen(8080, () => { 
  // Let's wait 10ms before logging the server started. 
  setTimeout(() => { 
    console.log('>>>', async_hooks.executionAsyncId()); 
Asynchronous context example
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  }, 10);
});
Output from only starting the server:
TCPSERVERWRAP(2): trigger: 1 execution: 1
TickObject(3): trigger: 2 execution: 1 
before:  3 
  Timeout(4): trigger: 3 execution: 3 
  TIMERWRAP(5): trigger: 3 execution: 3 
after:   3 
destroy: 3 
before:  5 
  before:  4 
    TTYWRAP(6): trigger: 4 execution: 4 
    SIGNALWRAP(7): trigger: 4 execution: 4 
    TTYWRAP(8): trigger: 4 execution: 4
>>> 4 
    TickObject(9): trigger: 4 execution: 4 
  after:   4 
after:   5 
before:  9 
after:   9 
destroy: 4 
destroy: 9 
destroy: 5
Note: 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:
TTYWRAP(6) -> Timeout(4) -> TIMERWRAP(5) -> TickObject(3) -> 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() .
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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.
Note: 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() .
Note: 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>
before(asyncId)
#
after(asyncId)
#
destroy(asyncId)
#
promiseResolve(asyncId)
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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.
Note: This does not necessarily mean that the Promise  is fulfilled or rejected at this point, if the
Promise  was resolved by assuming the state of another Promise .
For example:
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 6               # the then() callback is entered 
  promise resolve 6      # the then() callback resolves the promise by returning 
  after 6
History
Returns: <number>  The asyncId  of the current execution context. Useful to track when
something calls.
For example:
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() ). For example:
async_hooks.executionAsyncId()
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const server = net.createServer(function onConnection(conn) { 
  // Returns the ID of the server, not of the new connection, because the 
  // onConnection callback runs in the execution scope of the server's 
  // MakeCallback(). 
  async_hooks.executionAsyncId(); 
 
}).listen(port, function onListening() { 
  // 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.
For example:
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 triggerAsyncIds by default. See the section on
promise execution tracking .
async_hooks.triggerAsyncId()
#
Promise execution tracking
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By default, promise executions are not assigned asyncIds 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.
Here's an example:
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. Example:
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.
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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( 
  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(); 
JavaScript Embedder API
#
class AsyncResource()
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// Call AsyncHooks before callbacks.
// Deprecated: Use asyncResource.runInAsyncScope instead. 
asyncResource.emitBefore(); 
 
// Call AsyncHooks after callbacks.
// Deprecated: Use asyncResource.runInAsyncScope instead. 
asyncResource.emitAfter();
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(); 
  }
}
AsyncResource(type[, options])
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Added in: v8.12.0
fn  <Function>  The function to call in the execution context of this async resource.
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.
Deprecated since: v8.12.0
Stability: 0  - Deprecated: Use asyncResource.runInAsyncScope()  instead.
Returns: <undefined>
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.
Deprecated since: v8.12.0
Stability: 0  - Deprecated: Use asyncResource.runInAsyncScope()  instead.
Returns: <undefined>
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.
asyncResource.runInAsyncScope(fn[, thisArg, ...args])
#
asyncResource.emitBefore()
#
asyncResource.emitAfter()
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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: <undefined>
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
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 .
asyncResource.emitDestroy()
#
asyncResource.asyncId()
#
asyncResource.triggerAsyncId()
#
Buffer
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Examples:
// 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 v6, 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 Buffer
completely. 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 .
Buffer.from() , Buffer.alloc() , and
Buffer.allocUnsafe()
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Passing an ArrayBuffer  or a SharedArrayBuffer  returns a Buffer  that shares allocated
memory with the given array buffer.
Because the behavior of new Buffer()  changes significantly based on the type of value passed as
the first argument, applications that do not properly validate the input arguments passed to new
Buffer() , or that fail to appropriately initialize newly allocated Buffer  content, can inadvertently
introduce security and reliability issues into their code.
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(array)  returns a new Buffer  containing 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  containing a copy of the contents of the given
Buffer .
Buffer.from(string[, encoding])  returns a new Buffer  containing a copy of the provided
string.
Buffer.alloc(size[, fill[, encoding]])  returns a "filled" Buffer  instance of the specified
size. This method can be significantly slower than Buffer.allocUnsafe(size)  but ensures
that newly created Buffer  instances never contain old and potentially sensitive data.
Buffer.allocUnsafe(size)  and Buffer.allocUnsafeSlow(size)  each return a new Buffer
of the specified size  whose content must be initialized using either buf.fill(0)  or written
to completely.
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.
Added in: v5.10.0
Node.js can be started using the --zero-fill-buffers  command line option to force all newly
allocated Buffer  instances created using either new Buffer(size) , Buffer.allocUnsafe() ,
Buffer.allocUnsafeSlow()  or new SlowBuffer(size)  to be automatically zero-filled upon
creation. Use of this flag changes the default behavior of these methods and can have a significant
impact on performance. Use of the --zero-fill-buffers  option is recommended only when
The --zero-fill-buffers  command line option
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necessary to enforce that newly allocated Buffer  instances cannot contain potentially sensitive
data.
Example:
$ 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.
History
Buffer  instances are commonly used to represent sequences of encoded characters such as UTF-
8, UCS2, Base64, or even Hex-encoded data. It is possible to convert back and forth between
Buffer  instances and ordinary JavaScript strings by using an explicit character encoding.
Example:
const buf = Buffer.from('hello world', 'ascii'); 
 
// Prints: 68656c6c6f20776f726c64 
console.log(buf.toString('hex')); 
 
// Prints: aGVsbG8gd29ybGQ= 
console.log(buf.toString('base64'));
What makes Buffer.allocUnsafe()  and
Buffer.allocUnsafeSlow()  "unsafe"?
#
Buffers and Character Encodings
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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.
Note: Today's 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.
History
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.
Buffers and TypedArray
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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 TypeArray object's .buffer  property.
Example:
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); 
 
// Prints: <Buffer 88 a0> 
console.log(buf1); 
 
// Prints: <Buffer 88 13 a0 0f> 
console.log(buf2); 
 
arr[1] = 6000; 
 
// Prints: <Buffer 88 a0> 
console.log(buf1); 
 
// Prints: <Buffer 88 13 70 17> 
console.log(buf2);
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.
Example:
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const arr = new Uint16Array(20);
const buf = Buffer.from(arr.buffer, 0, 16); 
 
// Prints: 16 
console.log(buf.length);
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:
Example:
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.
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The Buffer  class is a global type for dealing with binary data directly. It can be constructed in a
variety of ways.
History
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.
Example:
// Creates a new Buffer containing the UTF-8 bytes of the string 'buffer'
const buf = new Buffer([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);
History
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 .
Class: Buffer
#
new Buffer(array)
#
new Buffer(arrayBuffer[, byteOffset[, length]])
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The optional byteOffset  and length  arguments specify a memory range within the arrayBuffer
that will be shared by the Buffer .
Example:
const arr = new Uint16Array(2); 
 
arr[0] = 5000; 
arr[1] = 4000; 
 
// Shares memory with `arr`
const buf = new Buffer(arr.buffer); 
 
// Prints: <Buffer 88 13 a0 0f> 
console.log(buf); 
 
// Changing the original Uint16Array changes the Buffer also 
arr[1] = 6000; 
 
// Prints: <Buffer 88 13 70 17> 
console.log(buf);
History
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.
Example:
const buf1 = new Buffer('buffer');
const buf2 = new Buffer(buf1); 
 
new Buffer(buffer)
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buf1[0] = 0x61; 
 
// Prints: auffer 
console.log(buf1.toString()); 
 
// Prints: buffer 
console.log(buf2.toString());
History
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 the size  is larger than buffer.constants.MAX_LENGTH  or
smaller than 0, a RangeError  will be thrown. A zero-length Buffer  will be 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  to zeroes.
Example:
const buf = new Buffer(10); 
 
// Prints: <Buffer 00 00 00 00 00 00 00 00 00 00> 
console.log(buf);
History
Stability: 0  - Deprecated: Use Buffer.from(string[, encoding])  instead.
new Buffer(size)
#
new Buffer(string[, encoding])
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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 .
Examples:
const buf1 = new Buffer('this is a tést'); 
 
// Prints: this is a tést 
console.log(buf1.toString()); 
 
// Prints: this is a tC)st 
console.log(buf1.toString('ascii')); 
 
const buf2 = new Buffer('7468697320697320612074c3a97374', 'hex'); 
 
// Prints: this is a tést 
console.log(buf2.toString());
History
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.
Example:
const buf = Buffer.alloc(5); 
 
// Prints: <Buffer 00 00 00 00 00> 
console.log(buf);
Class Method: Buffer.alloc(size[, fill[, encoding]])
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Allocates a new Buffer  of size  bytes. If the size  is larger than buffer.constants.MAX_LENGTH  or
smaller than 0, a RangeError  will be thrown. A zero-length Buffer  will be created if size  is 0.
If fill  is specified, the allocated Buffer  will be initialized by calling buf.fill(fill) .
Example:
const buf = Buffer.alloc(5, 'a'); 
 
// Prints: <Buffer 61 61 61 61 61> 
console.log(buf);
If both fill  and encoding  are specified, the allocated Buffer  will be initialized by calling
buf.fill(fill, encoding) .
Example:
const buf = Buffer.alloc(11, 'aGVsbG8gd29ybGQ=', 'base64'); 
 
// Prints: <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64> 
console.log(buf);
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.
History
size  <integer>  The desired length of the new Buffer .
Allocates a new Buffer  of size  bytes. If the size  is larger than buffer.constants.MAX_LENGTH  or
smaller than 0, a RangeError  will be thrown. A zero-length Buffer  will be 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 to zeroes.
Class Method: Buffer.allocUnsafe(size)
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Example:
const buf = Buffer.allocUnsafe(10); 
 
// Prints: (contents may vary): <Buffer a0 8b 28 3f 01 00 00 00 50 32> 
console.log(buf); 
 
buf.fill(0); 
 
// Prints: <Buffer 00 00 00 00 00 00 00 00 00 00> 
console.log(buf);
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.
Added in: v5.12.0
size  <integer>  The desired length of the new Buffer .
Allocates a new Buffer  of size  bytes. If the size  is larger than buffer.constants.MAX_LENGTH  or
smaller than 0, a RangeError  will be thrown. A zero-length Buffer  will be 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 to zeroes.
Class Method: Buffer.allocUnsafeSlow(size)
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When using Buffer.allocUnsafe()  to allocate new Buffer  instances, allocations under 4KB are,
by default, 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 cleanup 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()  then copy out the relevant bits.
Example:
// Need to keep around a few small chunks of memory
const store = []; 
 
socket.on('readable', () => { 
  const data = socket.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);
});
Use of 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.
History
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.byteLength(string[, encoding]) #
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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.
Note: 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.
Example:
const str = '\u00bd + \u00bc = \u00be'; 
 
// Prints: ½ + ¼ = ¾: 9 characters, 12 bytes 
console.log(`${str}: ${str.length} characters, ` + 
            `${Buffer.byteLength(str, 'utf8')} bytes`);
When string  is a Buffer / DataView / TypedArray / ArrayBuffer / SharedArrayBuffer , the actual
byte length is returned.
History
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) .
Example:
const buf1 = Buffer.from('1234');
const buf2 = Buffer.from('0123');
const arr = [buf1, buf2]; 
 
// Prints: [ <Buffer 30 31 32 33>, <Buffer 31 32 33 34> ]
// (This result is equal to: [buf2, buf1]) 
console.log(arr.sort(Buffer.compare));
Class Method: Buffer.compare(buf1, buf2)
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History
list  <Array>  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 .
Example: 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);
const totalLength = buf1.length + buf2.length + buf3.length; 
 
// Prints: 42 
console.log(totalLength); 
 
const bufA = Buffer.concat([buf1, buf2, buf3], totalLength); 
 
// Prints: <Buffer 00 00 00 00 ...> 
console.log(bufA); 
 
// Prints: 42 
console.log(bufA.length);
Class Method: Buffer.concat(list[, totalLength])
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Added in: v5.10.0
array  <Array>
Allocates a new Buffer  using an array  of octets.
Example:
// 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 .
Added in: v5.10.0
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 .
Example:
const arr = new Uint16Array(2); 
 
arr[0] = 5000; 
arr[1] = 4000; 
 
// Shares memory with `arr`
const buf = Buffer.from(arr.buffer); 
 
// Prints: <Buffer 88 13 a0 0f> 
console.log(buf); 
Class Method: Buffer.from(array)
#
Class Method: Buffer.from(arrayBuffer[,
byteOffset[, length]])
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// Changing the original Uint16Array changes the Buffer also 
arr[1] = 6000; 
 
// Prints: <Buffer 88 13 70 17> 
console.log(buf);
The optional byteOffset  and length  arguments specify a memory range within the arrayBuffer
that will be shared by the Buffer .
Example:
const ab = new ArrayBuffer(10);
const buf = Buffer.from(ab, 0, 2); 
 
// Prints: 2 
console.log(buf.length);
A TypeError  will be thrown if arrayBuffer  is not an ArrayBuffer  or a SharedArrayBuffer .
Added in: v5.10.0
buffer  <Buffer>  | <Uint8Array>  An existing Buffer  or Uint8Array  from which to copy
data.
Copies the passed buffer  data onto a new Buffer  instance.
Example:
const buf1 = Buffer.from('buffer');
const buf2 = Buffer.from(buf1); 
 
buf1[0] = 0x61; 
 
// Prints: auffer 
console.log(buf1.toString()); 
 
Class Method: Buffer.from(buffer)
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// Prints: buffer 
console.log(buf2.toString());
A TypeError  will be thrown if buffer  is not a Buffer .
Added in: v5.10.0
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 .
Examples:
const buf1 = Buffer.from('this is a tést'); 
 
// Prints: this is a tést 
console.log(buf1.toString()); 
 
// Prints: this is a tC)st 
console.log(buf1.toString('ascii')); 
 
const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex'); 
 
// Prints: this is a tést 
console.log(buf2.toString());
A TypeError  will be thrown if string  is not a string.
Added in: v8.2.0
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]() .
Class Method: Buffer.from(string[, encoding])
#
Class Method: Buffer.from(object[,
offsetOrEncoding[, length]])
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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) .
For example:
const buf = Buffer.from(new String('this is a test'));
// <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) .
For example:
class Foo { 
  [Symbol.toPrimitive]() { 
    return 'this is a test'; 
  }
} 
 
const buf = Buffer.from(new Foo(), 'utf8');
// <Buffer 74 68 69 73 20 69 73 20 61 20 74 65 73 74>
Added in: v0.1.101
obj  <Object>
Returns: <boolean>
Returns true  if obj  is a Buffer , false  otherwise.
Added in: v0.9.1
encoding  <string>  A character encoding name to check.
Returns: <boolean>
Class Method: Buffer.isBuffer(obj)
#
Class Method: Buffer.isEncoding(encoding)
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Returns true  if encoding  contains a supported character encoding, or false  otherwise.
Added in: v0.11.3
<integer>  Default: 8192
This is the number of bytes used to determine the size 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.
Example: 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);
} 
 
// Prints: Node.js 
console.log(buf.toString('ascii'));
The buffer  property references the underlying ArrayBuffer  object based on which this Buffer
object is created.
const arrayBuffer = new ArrayBuffer(16);
const buffer = Buffer.from(arrayBuffer); 
Class Property: Buffer.poolSize
#
buf[index]
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buf.buffer
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console.log(buffer.buffer === arrayBuffer);
// Prints: true
History
target  <Buffer>  | <Uint8Array>  A Buffer  or Uint8Array  to compare to.
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.
-1  is returned if target  should come after buf  when sorted.
Examples:
const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('BCD');
const buf3 = Buffer.from('ABCD'); 
 
// Prints: 0 
console.log(buf1.compare(buf1)); 
 
// Prints: -1 
console.log(buf1.compare(buf2)); 
buf.compare(target[, targetStart[, targetEnd[,
sourceStart[, sourceEnd]]]])
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// Prints: -1 
console.log(buf1.compare(buf3)); 
 
// Prints: 1 
console.log(buf2.compare(buf1)); 
 
// Prints: 1 
console.log(buf2.compare(buf3)); 
 
// Prints: [ <Buffer 41 42 43>, <Buffer 41 42 43 44>, <Buffer 42 43 44> ]
// (This result is equal to: [buf1, buf3, buf2]) 
console.log([buf1, buf2, buf3].sort(Buffer.compare));
The optional targetStart , targetEnd , sourceStart , and sourceEnd  arguments can be used to
limit the comparison to specific ranges within target  and buf  respectively.
Examples:
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]); 
 
// Prints: 0 
console.log(buf1.compare(buf2, 5, 9, 0, 4)); 
 
// Prints: -1 
console.log(buf1.compare(buf2, 0, 6, 4)); 
 
// Prints: 1 
console.log(buf1.compare(buf2, 5, 6, 5));
A RangeError  will be thrown if: targetStart < 0 , sourceStart < 0 , targetEnd >
target.byteLength  or sourceEnd > source.byteLength .
Added in: v0.1.90
buf.copy(target[, targetStart[, sourceStart[,
sourceEnd]]])
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target  <Buffer>  | <Uint8Array>  A Buffer  or Uint8Array  to copy into.
targetStart  <integer>  The offset within target  at which to begin copying to. Default: 0 .
sourceStart  <integer>  The offset within buf  at which to begin copying from. 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 .
Example: Create two Buffer  instances, buf1  and buf2 , and copy buf1  from byte 16 through byte
19 into buf2 , starting at the 8th byte in buf2
const buf1 = Buffer.allocUnsafe(26);
const buf2 = Buffer.allocUnsafe(26).fill('!'); 
 
for (let i = 0; i < 26; i++) { 
  // 97 is the decimal ASCII value for 'a' 
  buf1[i] = i + 97;
} 
 
buf1.copy(buf2, 8, 16, 20); 
 
// Prints: !!!!!!!!qrst!!!!!!!!!!!!! 
console.log(buf2.toString('ascii', 0, 25));
Example: Create a single 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); 
 
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// Prints: efghijghijklmnopqrstuvwxyz 
console.log(buf.toString());
Added in: v1.1.0
Returns: <Iterator>
Creates and returns an iterator  of [index, byte]  pairs from the contents of buf .
Example: Log the entire contents of a Buffer
const buf = Buffer.from('buffer'); 
 
// Prints:
//   [0, 98]
//   [1, 117]
//   [2, 102]
//   [3, 102]
//   [4, 101]
//   [5, 114]
for (const pair of buf.entries()) { 
  console.log(pair);
}
History
otherBuffer  <Buffer>  A Buffer  or Uint8Array  to compare to.
Returns: <boolean>
Returns true  if both buf  and otherBuffer  have exactly the same bytes, false  otherwise.
Examples:
const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('414243', 'hex');
buf.entries()
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buf.equals(otherBuffer)
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const buf3 = Buffer.from('ABCD'); 
 
// Prints: true 
console.log(buf1.equals(buf2)); 
 
// Prints: false 
console.log(buf1.equals(buf3));
History
value  <string>  | <Buffer>  | <integer>  The value to fill buf  with.
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>  If value  is a string, this is its encoding. 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. This is meant to be a small simplification to allow the creation and filling of a Buffer  to be
done on a single line.
Example: Fill a Buffer  with the ASCII character 'h'
const b = Buffer.allocUnsafe(50).fill('h'); 
 
// Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh 
console.log(b.toString());
value  is coerced to a uint32  value if it is not a String or Integer.
If the final write of a fill()  operation falls on a multi-byte character, then only the first bytes of
that character that fit into buf  are written.
Example: Fill a Buffer  with a two-byte character
buf.fill(value[, offset[, end]][, encoding])
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// Prints: <Buffer c8 a2 c8> 
console.log(Buffer.allocUnsafe(3).fill('\u0222'));
If value  contains invalid characters, it is truncated.
If no valid fill data remains, then the buffer is either zero-filled or no filling is performed, depending
on the input type. That behavior is dictated by compatibility reasons and was changed to throwing
an exception in Node.js v10, so it's not recommended to rely on that.
const buf = Buffer.allocUnsafe(5);
// Prints: <Buffer 61 61 61 61 61> 
console.log(buf.fill('a'));
// Prints: <Buffer aa aa aa aa aa> 
console.log(buf.fill('aazz', 'hex'));
// Prints: <Buffer aa aa aa aa aa> 
console.log(buf.fill('zz', 'hex'));
Added in: v5.3.0
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 .
Examples:
const buf = Buffer.from('this is a buffer'); 
 
// Prints: true 
console.log(buf.includes('this')); 
 
// Prints: true 
console.log(buf.includes('is')); 
 
buf.includes(value[, byteOffset][, encoding])
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// Prints: true 
console.log(buf.includes(Buffer.from('a buffer'))); 
 
// Prints: true
// (97 is the decimal ASCII value for 'a') 
console.log(buf.includes(97)); 
 
// Prints: false 
console.log(buf.includes(Buffer.from('a buffer example'))); 
 
// Prints: true 
console.log(buf.includes(Buffer.from('a buffer example').slice(0, 8))); 
 
// Prints: false 
console.log(buf.includes('this', 4));
History
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 .
Examples:
const buf = Buffer.from('this is a buffer'); 
 
buf.indexOf(value[, byteOffset][, encoding])
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// Prints: 0 
console.log(buf.indexOf('this')); 
 
// Prints: 2 
console.log(buf.indexOf('is')); 
 
// Prints: 8 
console.log(buf.indexOf(Buffer.from('a buffer'))); 
 
// Prints: 8
// (97 is the decimal ASCII value for 'a') 
console.log(buf.indexOf(97)); 
 
// Prints: -1 
console.log(buf.indexOf(Buffer.from('a buffer example'))); 
 
// Prints: 8 
console.log(buf.indexOf(Buffer.from('a buffer example').slice(0, 8))); 
 
const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'ucs2'); 
 
// Prints: 4 
console.log(utf16Buffer.indexOf('\u03a3', 0, 'ucs2')); 
 
// Prints: 6 
console.log(utf16Buffer.indexOf('\u03a3', -4, 'ucs2'));
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  or
0, like {} , [] , null  or undefined , will search the whole buffer. 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)); 
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// 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.
Added in: v1.1.0
Returns: <Iterator>
Creates and returns an iterator  of buf  keys (indices).
Example:
const buf = Buffer.from('buffer'); 
 
// Prints:
//   0
//   1
//   2
//   3
//   4
//   5
for (const key of buf.keys()) { 
  console.log(key);
}
History
value  <string>  | <Buffer>  | <Uint8Array>  | <integer>  What to search for.
buf.keys()
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buf.lastIndexOf(value[, byteOffset][, encoding])
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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.
Examples:
const buf = Buffer.from('this buffer is a buffer'); 
 
// Prints: 0 
console.log(buf.lastIndexOf('this')); 
 
// Prints: 17 
console.log(buf.lastIndexOf('buffer')); 
 
// Prints: 17 
console.log(buf.lastIndexOf(Buffer.from('buffer'))); 
 
// Prints: 15
// (97 is the decimal ASCII value for 'a') 
console.log(buf.lastIndexOf(97)); 
 
// Prints: -1 
console.log(buf.lastIndexOf(Buffer.from('yolo'))); 
 
// Prints: 5 
console.log(buf.lastIndexOf('buffer', 5)); 
 
// Prints: -1 
console.log(buf.lastIndexOf('buffer', 4)); 
 
const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'ucs2'); 
 
// Prints: 6 
console.log(utf16Buffer.lastIndexOf('\u03a3', undefined, 'ucs2')); 
 
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// Prints: 4 
console.log(utf16Buffer.lastIndexOf('\u03a3', -5, 'ucs2'));
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
// 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.
Added in: v0.1.90
<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 .
Example: Create a Buffer  and write a shorter ASCII string to it
buf.length
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const buf = Buffer.alloc(1234); 
 
// Prints: 1234 
console.log(buf.length); 
 
buf.write('some string', 0, 'ascii'); 
 
// Prints: 1234 
console.log(buf.length);
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 .
Examples:
let buf = Buffer.allocUnsafe(10); 
 
buf.write('abcdefghj', 0, 'ascii'); 
 
// Prints: 10 
console.log(buf.length); 
 
buf = buf.slice(0, 5); 
 
// Prints: 5 
console.log(buf.length);
Deprecated since: v8.0.0
Stability: 0  - Deprecated: Use buf.buffer  instead.
The buf.parent  property is a deprecated alias for buf.buffer .
buf.parent
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buf.readDoubleBE(offset[, noAssert])
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Added in: v0.11.15
offset  <integer>  Number of bytes to skip before starting to read. Must satisfy: 0 <= offset
<= buf.length - 8 .
noAssert  <boolean>  Skip offset  validation? Default: false
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).
Setting noAssert  to true  allows offset  to be beyond the end of buf , but the resulting behavior
is undefined.
Examples:
const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]); 
 
// Prints: 8.20788039913184e-304 
console.log(buf.readDoubleBE()); 
 
// Prints: 5.447603722011605e-270 
console.log(buf.readDoubleLE()); 
 
// Throws an exception: RangeError: Index out of range 
console.log(buf.readDoubleLE(1)); 
 
// Warning: reads passed end of buffer!
// This will result in a segmentation fault! Don't do this! 
console.log(buf.readDoubleLE(1, true));
Added in: v0.11.15
offset  <integer>  Number of bytes to skip before starting to read. Must satisfy: 0 <= offset
<= buf.length - 4 .
buf.readDoubleLE(offset[, noAssert])
#
buf.readFloatBE(offset[, noAssert])
#
buf.readFloatLE(offset[, noAssert])
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noAssert  <boolean>  Skip offset  validation? Default: false
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).
Setting noAssert  to true  allows offset  to be beyond the end of buf , but the resulting behavior
is undefined.
Examples:
const buf = Buffer.from([1, 2, 3, 4]); 
 
// Prints: 2.387939260590663e-38 
console.log(buf.readFloatBE()); 
 
// Prints: 1.539989614439558e-36 
console.log(buf.readFloatLE()); 
 
// Throws an exception: RangeError: Index out of range 
console.log(buf.readFloatLE(1)); 
 
// Warning: reads passed end of buffer!
// This will result in a segmentation fault! Don't do this! 
console.log(buf.readFloatLE(1, true));
Added in: v0.5.0
offset  <integer>  Number of bytes to skip before starting to read. Must satisfy: 0 <= offset
<= buf.length - 1 .
noAssert  <boolean>  Skip offset  validation? Default: false
Returns: <integer>
Reads a signed 8-bit integer from buf  at the specified offset .
Setting noAssert  to true  allows offset  to be beyond the end of buf , but the resulting behavior
is undefined.
buf.readInt8(offset[, noAssert])
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Integers read from a Buffer  are interpreted as two's complement signed values.
Examples:
const buf = Buffer.from([-1, 5]); 
 
// Prints: -1 
console.log(buf.readInt8(0)); 
 
// Prints: 5 
console.log(buf.readInt8(1)); 
 
// Throws an exception: RangeError: Index out of range 
console.log(buf.readInt8(2));
Added in: v0.5.5
offset  <integer>  Number of bytes to skip before starting to read. Must satisfy: 0 <= offset
<= buf.length - 2 .
noAssert  <boolean>  Skip offset  validation? Default: false
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).
Setting noAssert  to true  allows offset  to be beyond the end of buf , but the resulting behavior
is undefined.
Integers read from a Buffer  are interpreted as two's complement signed values.
Examples:
const buf = Buffer.from([0, 5]); 
 
// Prints: 5 
console.log(buf.readInt16BE()); 
buf.readInt16BE(offset[, noAssert])
#
buf.readInt16LE(offset[, noAssert])
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// Prints: 1280 
console.log(buf.readInt16LE()); 
 
// Throws an exception: RangeError: Index out of range 
console.log(buf.readInt16LE(1));
Added in: v0.5.5
offset  <integer>  Number of bytes to skip before starting to read. Must satisfy: 0 <= offset
<= buf.length - 4 .
noAssert  <boolean>  Skip offset  validation? Default: false
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).
Setting noAssert  to true  allows offset  to be beyond the end of buf , but the resulting behavior
is undefined.
Integers read from a Buffer  are interpreted as two's complement signed values.
Examples:
const buf = Buffer.from([0, 0, 0, 5]); 
 
// Prints: 5 
console.log(buf.readInt32BE()); 
 
// Prints: 83886080 
console.log(buf.readInt32LE()); 
 
// Throws an exception: RangeError: Index out of range 
console.log(buf.readInt32LE(1));
buf.readInt32BE(offset[, noAssert])
#
buf.readInt32LE(offset[, noAssert])
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Added in: v0.11.15
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 .
noAssert  <boolean>  Skip offset  and byteLength  validation? Default: false .
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.
Setting noAssert  to true  allows offset  to be beyond the end of buf , but the resulting behavior
is undefined.
Examples:
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]); 
 
// Prints: -546f87a9cbee 
console.log(buf.readIntLE(0, 6).toString(16)); 
 
// Prints: 1234567890ab 
console.log(buf.readIntBE(0, 6).toString(16)); 
 
// Throws an exception: RangeError: Index out of range 
console.log(buf.readIntBE(1, 6).toString(16));
Added in: v0.5.0
offset  <integer>  Number of bytes to skip before starting to read. Must satisfy: 0 <= offset
<= buf.length - 1 .
noAssert  <boolean>  Skip offset  validation? Default: false
Returns: <integer>
buf.readIntBE(offset, byteLength[, noAssert])
#
buf.readIntLE(offset, byteLength[, noAssert])
#
buf.readUInt8(offset[, noAssert])
#
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Reads an unsigned 8-bit integer from buf  at the specified offset .
Setting noAssert  to true  allows offset  to be beyond the end of buf , but the resulting behavior
is undefined.
Examples:
const buf = Buffer.from([1, -2]); 
 
// Prints: 1 
console.log(buf.readUInt8(0)); 
 
// Prints: 254 
console.log(buf.readUInt8(1)); 
 
// Throws an exception: RangeError: Index out of range 
console.log(buf.readUInt8(2));
Added in: v0.5.5
offset  <integer>  Number of bytes to skip before starting to read. Must satisfy: 0 <= offset
<= buf.length - 2 .
noAssert  <boolean>  Skip offset  validation? Default: false
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).
Setting noAssert  to true  allows offset  to be beyond the end of buf , but the resulting behavior
is undefined.
Examples:
const buf = Buffer.from([0x12, 0x34, 0x56]); 
 
// Prints: 1234 
buf.readUInt16BE(offset[, noAssert])
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buf.readUInt16LE(offset[, noAssert])
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console.log(buf.readUInt16BE(0).toString(16)); 
 
// Prints: 3412 
console.log(buf.readUInt16LE(0).toString(16)); 
 
// Prints: 3456 
console.log(buf.readUInt16BE(1).toString(16)); 
 
// Prints: 5634 
console.log(buf.readUInt16LE(1).toString(16)); 
 
// Throws an exception: RangeError: Index out of range 
console.log(buf.readUInt16LE(2).toString(16));
Added in: v0.5.5
offset  <integer>  Number of bytes to skip before starting to read. Must satisfy: 0 <= offset
<= buf.length - 4 .
noAssert  <boolean>  Skip offset  validation? Default: false
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).
Setting noAssert  to true  allows offset  to be beyond the end of buf , but the resulting behavior
is undefined.
Examples:
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]); 
 
// Prints: 12345678 
console.log(buf.readUInt32BE(0).toString(16)); 
 
// Prints: 78563412 
console.log(buf.readUInt32LE(0).toString(16)); 
buf.readUInt32BE(offset[, noAssert])
#
buf.readUInt32LE(offset[, noAssert])
#
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// Throws an exception: RangeError: Index out of range 
console.log(buf.readUInt32LE(1).toString(16));
Added in: v0.11.15
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 .
noAssert  <boolean>  Skip offset  and byteLength  validation? Default: false
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.
Setting noAssert  to true  allows offset  to be beyond the end of buf , but the resulting behavior
is undefined.
Examples:
const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]); 
 
// Prints: 1234567890ab 
console.log(buf.readUIntBE(0, 6).toString(16)); 
 
// Prints: ab9078563412 
console.log(buf.readUIntLE(0, 6).toString(16)); 
 
// Throws an exception: RangeError: Index out of range 
console.log(buf.readUIntBE(1, 6).toString(16));
History
buf.readUIntBE(offset, byteLength[, noAssert])
#
buf.readUIntLE(offset, byteLength[, noAssert])
#
buf.slice([start[, end]])
#
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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 .
Note: Modifying the new Buffer  slice will modify the memory in the original Buffer  because the
allocated memory of the two objects overlap.
Example: Create a Buffer  with the ASCII alphabet, take a slice, and then modify one byte 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); 
 
// Prints: abc 
console.log(buf2.toString('ascii', 0, buf2.length)); 
 
buf1[0] = 33; 
 
// Prints: !bc 
console.log(buf2.toString('ascii', 0, buf2.length));
Specifying negative indexes causes the slice to be generated relative to the end of buf  rather than
the beginning.
Examples:
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const buf = Buffer.from('buffer'); 
 
// Prints: buffe
// (Equivalent to buf.slice(0, 5)) 
console.log(buf.slice(-6, -1).toString()); 
 
// Prints: buff
// (Equivalent to buf.slice(0, 4)) 
console.log(buf.slice(-6, -2).toString()); 
 
// Prints: uff
// (Equivalent to buf.slice(1, 4)) 
console.log(buf.slice(-5, -2).toString());
Added in: v5.10.0
Returns: <Buffer>  A reference to buf .
Interprets buf  as an array of unsigned 16-bit integers and swaps the byte order in-place. Throws a
RangeError  if buf.length  is not a multiple of 2.
Examples:
const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]); 
 
// Prints: <Buffer 01 02 03 04 05 06 07 08> 
console.log(buf1); 
 
buf1.swap16(); 
 
// Prints: <Buffer 02 01 04 03 06 05 08 07> 
console.log(buf1); 
 
const buf2 = Buffer.from([0x1, 0x2, 0x3]); 
 
// Throws an exception: RangeError: Buffer size must be a multiple of 16-bits 
buf2.swap16();
buf.swap16()
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Added in: v5.10.0
Returns: <Buffer>  A reference to buf .
Interprets buf  as an array of unsigned 32-bit integers and swaps the byte order in-place. Throws a
RangeError  if buf.length  is not a multiple of 4.
Examples:
const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]); 
 
// Prints: <Buffer 01 02 03 04 05 06 07 08> 
console.log(buf1); 
 
buf1.swap32(); 
 
// Prints: <Buffer 04 03 02 01 08 07 06 05> 
console.log(buf1); 
 
const buf2 = Buffer.from([0x1, 0x2, 0x3]); 
 
// Throws an exception: RangeError: Buffer size must be a multiple of 32-bits 
buf2.swap32();
Added in: v6.3.0
Returns: <Buffer>  A reference to buf .
Interprets buf  as an array of 64-bit numbers and swaps the byte order in-place. Throws a
RangeError  if buf.length  is not a multiple of 8.
Examples:
const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]); 
 
// Prints: <Buffer 01 02 03 04 05 06 07 08> 
console.log(buf1); 
buf.swap32()
#
buf.swap64()
#
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buf1.swap64(); 
 
// Prints: <Buffer 08 07 06 05 04 03 02 01> 
console.log(buf1); 
 
const buf2 = Buffer.from([0x1, 0x2, 0x3]); 
 
// Throws an exception: RangeError: Buffer size must be a multiple of 64-bits 
buf2.swap64();
Note that JavaScript cannot encode 64-bit integers. This method is intended for working with 64-
bit floats.
Added in: v0.9.2
Returns: <Object>
Returns a JSON representation of buf . JSON.stringify()  implicitly calls this function when
stringifying a Buffer  instance.
Example:
const buf = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5]);
const json = JSON.stringify(buf); 
 
// Prints: {"type":"Buffer","data":[1,2,3,4,5]} 
console.log(json); 
 
const copy = JSON.parse(json, (key, value) => { 
  return value && value.type === 'Buffer' ? 
    Buffer.from(value.data) : 
    value;
}); 
 
// Prints: <Buffer 01 02 03 04 05> 
console.log(copy);
buf.toJSON()
#
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Added in: v0.1.90
encoding  <string>  The character encoding to decode to. 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 .
Examples:
const buf1 = Buffer.allocUnsafe(26); 
 
for (let i = 0; i < 26; i++) { 
  // 97 is the decimal ASCII value for 'a' 
  buf1[i] = i + 97;
} 
 
// Prints: abcdefghijklmnopqrstuvwxyz 
console.log(buf1.toString('ascii')); 
 
// Prints: abcde 
console.log(buf1.toString('ascii', 0, 5)); 
 
const buf2 = Buffer.from('tést'); 
 
// Prints: 74c3a97374 
console.log(buf2.toString('hex')); 
 
// Prints: té 
console.log(buf2.toString('utf8', 0, 3)); 
 
// Prints: té 
console.log(buf2.toString(undefined, 0, 3));
buf.toString([encoding[, start[, end]]])
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Added in: v1.1.0
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.
Examples:
const buf = Buffer.from('buffer'); 
 
// Prints:
//   98
//   117
//   102
//   102
//   101
//   114
for (const value of buf.values()) { 
  console.log(value);
} 
 
// Prints:
//   98
//   117
//   102
//   102
//   101
//   114
for (const value of buf) { 
  console.log(value);
}
Added in: v0.1.90
string  <string>  String to be written to buf .
buf.values()
#
buf.write(string[, offset[, length]][, encoding])
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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 a partial amount of string  will be written. However, partially encoded characters will
not be written.
Example:
const buf = Buffer.allocUnsafe(256); 
 
const len = buf.write('\u00bd + \u00bc = \u00be', 0); 
 
// Prints: 12 bytes: ½ + ¼ = ¾ 
console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`);
Added in: v0.11.15
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 .
noAssert  <boolean>  Skip value  and offset  validation? Default: false
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.
Setting noAssert  to true  allows the encoded form of value  to extend beyond the end of buf , but
the resulting behavior is undefined.
Examples:
buf.writeDoubleBE(value, offset[, noAssert])
#
buf.writeDoubleLE(value, offset[, noAssert])
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const buf = Buffer.allocUnsafe(8); 
 
buf.writeDoubleBE(0xdeadbeefcafebabe, 0); 
 
// Prints: <Buffer 43 eb d5 b7 dd f9 5f d7> 
console.log(buf); 
 
buf.writeDoubleLE(0xdeadbeefcafebabe, 0); 
 
// Prints: <Buffer d7 5f f9 dd b7 d5 eb 43> 
console.log(buf);
Added in: v0.11.15
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 .
noAssert  <boolean>  Skip value  and offset  validation? Default: false
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.
Setting noAssert  to true  allows the encoded form of value  to extend beyond the end of buf , but
the resulting behavior is undefined.
Examples:
const buf = Buffer.allocUnsafe(4); 
 
buf.writeFloatBE(0xcafebabe, 0); 
 
// Prints: <Buffer 4f 4a fe bb> 
console.log(buf); 
buf.writeFloatBE(value, offset[, noAssert])
#
buf.writeFloatLE(value, offset[, noAssert])
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buf.writeFloatLE(0xcafebabe, 0); 
 
// Prints: <Buffer bb fe 4a 4f> 
console.log(buf);
Added in: v0.5.0
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 .
noAssert  <boolean>  Skip value  and offset  validation? Default: false
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.
Setting noAssert  to true  allows the encoded form of value  to extend beyond the end of buf , but
the resulting behavior is undefined.
value  is interpreted and written as a two's complement signed integer.
Examples:
const buf = Buffer.allocUnsafe(2); 
 
buf.writeInt8(2, 0); 
buf.writeInt8(-2, 1); 
 
// Prints: <Buffer 02 fe> 
console.log(buf);
Added in: v0.5.5
buf.writeInt8(value, offset[, noAssert])
#
buf.writeInt16BE(value, offset[, noAssert])
#
buf.writeInt16LE(value, offset[, noAssert])
#
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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 .
noAssert  <boolean>  Skip value  and offset  validation? Default: false
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.
Setting noAssert  to true  allows the encoded form of value  to extend beyond the end of buf , but
the resulting behavior is undefined.
value  is interpreted and written as a two's complement signed integer.
Examples:
const buf = Buffer.allocUnsafe(4); 
 
buf.writeInt16BE(0x0102, 0); 
buf.writeInt16LE(0x0304, 2); 
 
// Prints: <Buffer 01 02 04 03> 
console.log(buf);
Added in: v0.5.5
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 .
noAssert  <boolean>  Skip value  and offset  validation? Default: false
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
buf.writeInt32BE(value, offset[, noAssert])
#
buf.writeInt32LE(value, offset[, noAssert])
#
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integer. Behavior is undefined when value  is anything other than a signed 32-bit integer.
Setting noAssert  to true  allows the encoded form of value  to extend beyond the end of buf , but
the resulting behavior is undefined.
value  is interpreted and written as a two's complement signed integer.
Examples:
const buf = Buffer.allocUnsafe(8); 
 
buf.writeInt32BE(0x01020304, 0); 
buf.writeInt32LE(0x05060708, 4); 
 
// Prints: <Buffer 01 02 03 04 08 07 06 05> 
console.log(buf);
Added in: v0.11.15
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 .
noAssert  <boolean>  Skip value , offset , and byteLength  validation? Default: false
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.
Setting noAssert  to true  allows the encoded form of value  to extend beyond the end of buf , but
the resulting behavior is undefined.
Examples:
buf.writeIntBE(value, offset, byteLength[,
noAssert])
#
buf.writeIntLE(value, offset, byteLength[,
noAssert])
#
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const buf = Buffer.allocUnsafe(6); 
 
buf.writeIntBE(0x1234567890ab, 0, 6); 
 
// Prints: <Buffer 12 34 56 78 90 ab> 
console.log(buf); 
 
buf.writeIntLE(0x1234567890ab, 0, 6); 
 
// Prints: <Buffer ab 90 78 56 34 12> 
console.log(buf);
Added in: v0.5.0
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 .
noAssert  <boolean>  Skip value  and offset  validation? Default: false
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.
Setting noAssert  to true  allows the encoded form of value  to extend beyond the end of buf , but
the resulting behavior is undefined.
Examples:
const buf = Buffer.allocUnsafe(4); 
 
buf.writeUInt8(0x3, 0); 
buf.writeUInt8(0x4, 1); 
buf.writeUInt8(0x23, 2); 
buf.writeUInt8(0x42, 3); 
 
// Prints: <Buffer 03 04 23 42> 
console.log(buf);
buf.writeUInt8(value, offset[, noAssert])
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Added in: v0.5.5
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 .
noAssert  <boolean>  Skip value  and offset  validation? Default: false
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.
Setting noAssert  to true  allows the encoded form of value  to extend beyond the end of buf , but
the resulting behavior is undefined.
Examples:
const buf = Buffer.allocUnsafe(4); 
 
buf.writeUInt16BE(0xdead, 0); 
buf.writeUInt16BE(0xbeef, 2); 
 
// Prints: <Buffer de ad be ef> 
console.log(buf); 
 
buf.writeUInt16LE(0xdead, 0); 
buf.writeUInt16LE(0xbeef, 2); 
 
// Prints: <Buffer ad de ef be> 
console.log(buf);
buf.writeUInt16BE(value, offset[, noAssert])
#
buf.writeUInt16LE(value, offset[, noAssert])
#
buf.writeUInt32BE(value, offset[, noAssert])
#
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Added in: v0.5.5
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 .
noAssert  <boolean>  Skip value  and offset  validation? Default: false
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.
Setting noAssert  to true  allows the encoded form of value  to extend beyond the end of buf , but
the resulting behavior is undefined.
Examples:
const buf = Buffer.allocUnsafe(4); 
 
buf.writeUInt32BE(0xfeedface, 0); 
 
// Prints: <Buffer fe ed fa ce> 
console.log(buf); 
 
buf.writeUInt32LE(0xfeedface, 0); 
 
// Prints: <Buffer ce fa ed fe> 
console.log(buf);
Added in: v0.5.5
buf.writeUInt32LE(value, offset[, noAssert])
#
buf.writeUIntBE(value, offset, byteLength[,
noAssert])
#
buf.writeUIntLE(value, offset, byteLength[,
noAssert])
#
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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 .
noAssert  <boolean>  Skip value , offset , and byteLength  validation? Default: false .
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.
Setting noAssert  to true  allows the encoded form of value  to extend beyond the end of buf , but
the resulting behavior is undefined.
Examples:
const buf = Buffer.allocUnsafe(6); 
 
buf.writeUIntBE(0x1234567890ab, 0, 6); 
 
// Prints: <Buffer 12 34 56 78 90 ab> 
console.log(buf); 
 
buf.writeUIntLE(0x1234567890ab, 0, 6); 
 
// Prints: <Buffer ab 90 78 56 34 12> 
console.log(buf);
Added in: v0.5.4
<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.
buffer.INSPECT_MAX_BYTES
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Added in: v3.0.0
<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.
History
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.
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'); 
 
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.
buffer.kMaxLength
#
buffer.transcode(source, fromEnc, toEnc)
#
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Deprecated since: v6.0.0
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.
Example:
// Need to keep around a few small chunks of memory
const store = []; 
 
socket.on('readable', () => { 
  const data = socket.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.
Deprecated since: v6.0.0
Class: SlowBuffer
#
new SlowBuffer(size)
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Stability: 0  - Deprecated: Use Buffer.allocUnsafeSlow()  instead.
size  <integer>  The desired length of the new SlowBuffer .
Allocates a new Buffer  of size  bytes. If the size  is larger than buffer.constants.MAX_LENGTH  or
smaller than 0, a RangeError  will be thrown. A zero-length Buffer  will be 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  to zeroes.
Example:
const { SlowBuffer } = require('buffer'); 
 
const buf = new SlowBuffer(5); 
 
// Prints: (contents may vary): <Buffer 78 e0 82 02 01> 
console.log(buf); 
 
buf.fill(0); 
 
// Prints: <Buffer 00 00 00 00 00> 
console.log(buf);
Added in: 8.2.0
Note that buffer.constants  is a property on the buffer  module returned by
require('buffer') , not on the Buffer  global or a Buffer  instance.
Added in: 8.2.0
<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).
Buffer Constants
#
buffer.constants.MAX_LENGTH
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This value is also available as buffer.kMaxLength .
Added in: 8.2.0
<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.
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,
buffer.constants.MAX_STRING_LENGTH
#
C++ Addons
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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::Object; 
using v8::String; 
using v8::Value; 
 
void Method(const FunctionCallbackInfo<Value>& args) { 
  Isolate* isolate = args.GetIsolate(); 
  args.GetReturnValue().Set(String::NewFromUtf8(isolate, "world"));
} 
 
void init(Local<Object> exports) { 
  NODE_SET_METHOD(exports, "hello", Method);
} 
 
NODE_MODULE(NODE_GYP_MODULE_NAME, init) 
Hello world
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}  // 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 init  and the Addon module name is
addon .
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" ] 
    } 
  ]
}
Note: 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.
Building
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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');
} catch (err) { 
  return require('./build/Debug/addon.node');
}
Linking to Node.js' own dependencies
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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.
Loading Addons using require()
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Stability: 1  - Experimental
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 version 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.
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.
// 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, "hello", 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; 
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  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. For example:
"sources": ["addon.cc", "myexample.cc"]
Once the binding.gyp  file is ready, the example Addons can be configured and built using node-
gyp :
$ node-gyp configure build 
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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::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"))); 
    return; 
  } 
 
  // Check the argument types 
  if (!args[0]->IsNumber() || !args[1]->IsNumber()) { 
    isolate->ThrowException(Exception::TypeError( 
Function arguments
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        String::NewFromUtf8(isolate, "Wrong arguments"))); 
    return; 
  } 
 
  // Perform the operation 
  double value = args[0]->NumberValue() + args[1]->NumberValue(); 
  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 { 
 
Callbacks
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using v8::Function; 
using v8::FunctionCallbackInfo; 
using v8::Isolate; 
using v8::Local; 
using v8::Null; 
using v8::Object; 
using v8::String; 
using v8::Value; 
 
void RunCallback(const FunctionCallbackInfo<Value>& args) { 
  Isolate* isolate = args.GetIsolate(); 
  Local<Function> cb = Local<Function>::Cast(args[0]); 
  const unsigned argc = 1; 
  Local<Value> argv[argc] = { String::NewFromUtf8(isolate, "hello world") }; 
  cb->Call(Null(isolate), argc, argv);
} 
 
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'
});
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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::FunctionCallbackInfo; 
using v8::Isolate; 
using v8::Local; 
using v8::Object; 
using v8::String; 
using v8::Value; 
 
void CreateObject(const FunctionCallbackInfo<Value>& args) { 
  Isolate* isolate = args.GetIsolate(); 
 
  Local<Object> obj = Object::New(isolate); 
  obj->Set(String::NewFromUtf8(isolate, "msg"), args[0]->ToString()); 
 
  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:
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// 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::Function; 
using v8::FunctionCallbackInfo; 
using v8::FunctionTemplate; 
using v8::Isolate; 
using v8::Local; 
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"));
} 
 
void CreateFunction(const FunctionCallbackInfo<Value>& args) { 
  Isolate* isolate = args.GetIsolate(); 
 
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, MyFunction); 
  Local<Function> fn = tpl->GetFunction(); 
 
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  // omit this to make it anonymous 
  fn->SetName(String::NewFromUtf8(isolate, "theFunction")); 
 
  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) { 
Wrapping C++ objects
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  MyObject::Init(exports);
} 
 
NODE_MODULE(NODE_GYP_MODULE_NAME, InitAll) 
 
}  // namespace demo
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:
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// 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::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(Local<Object> exports) { 
  Isolate* isolate = exports->GetIsolate(); 
 
  // Prepare constructor template 
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New); 
  tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject")); 
  tpl->InstanceTemplate()->SetInternalFieldCount(1); 
 
  // Prototype 
  NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne); 
 
  constructor.Reset(isolate, tpl->GetFunction()); 
  exports->Set(String::NewFromUtf8(isolate, "MyObject"), 
               tpl->GetFunction());
} 
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void MyObject::New(const FunctionCallbackInfo<Value>& args) { 
  Isolate* isolate = args.GetIsolate(); 
 
  if (args.IsConstructCall()) { 
    // Invoked as constructor: `new MyObject(...)` 
    double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue(); 
    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<Context> context = isolate->GetCurrentContext(); 
    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_));
} 
 
}  // namespace demo
To build this example, the myobject.cc  file must be added to the binding.gyp :
{ 
  "targets": [ 
    { 
      "target_name": "addon", 
      "sources": [ 
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        "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
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
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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: 
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  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 
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::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() {
} 
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void MyObject::Init(Isolate* isolate) { 
  // Prepare constructor template 
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New); 
  tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject")); 
  tpl->InstanceTemplate()->SetInternalFieldCount(1); 
 
  // Prototype 
  NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne); 
 
  constructor.Reset(isolate, tpl->GetFunction());
} 
 
void MyObject::New(const FunctionCallbackInfo<Value>& args) { 
  Isolate* isolate = args.GetIsolate(); 
 
  if (args.IsConstructCall()) { 
    // Invoked as constructor: `new MyObject(...)` 
    double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue(); 
    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<Context> context = isolate->GetCurrentContext(); 
    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); 
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  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:
// test.js
const createObject = require('./build/Release/addon'); 
 
const obj = createObject(10); 
console.log(obj.plusOne());
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// 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::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);
} 
 
Passing wrapped objects around
#
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void Add(const FunctionCallbackInfo<Value>& args) { 
  Isolate* isolate = args.GetIsolate(); 
 
  MyObject* obj1 = node::ObjectWrap::Unwrap<MyObject>( 
      args[0]->ToObject()); 
  MyObject* obj2 = node::ObjectWrap::Unwrap<MyObject>( 
      args[1]->ToObject()); 
 
  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_; } 
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 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" 
 
namespace demo { 
 
using v8::Context; 
using v8::Function; 
using v8::FunctionCallbackInfo; 
using v8::FunctionTemplate; 
using v8::Isolate; 
using v8::Local; 
using v8::Object; 
using v8::Persistent; 
using v8::String; 
using v8::Value; 
 
Persistent<Function> MyObject::constructor; 
 
MyObject::MyObject(double value) : value_(value) {
} 
 
MyObject::~MyObject() {
} 
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void MyObject::Init(Isolate* isolate) { 
  // Prepare constructor template 
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New); 
  tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject")); 
  tpl->InstanceTemplate()->SetInternalFieldCount(1); 
 
  constructor.Reset(isolate, tpl->GetFunction());
} 
 
void MyObject::New(const FunctionCallbackInfo<Value>& args) { 
  Isolate* isolate = args.GetIsolate(); 
 
  if (args.IsConstructCall()) { 
    // Invoked as constructor: `new MyObject(...)` 
    double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue(); 
    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<Context> context = isolate->GetCurrentContext(); 
    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(); 
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  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:
AtExit hooks
#
void AtExit(callback, args)
#
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// addon.cc 
#include <assert.h> 
#include <stdlib.h> 
#include <node.h> 
 
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()); 
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  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 (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 version to run on later versions of Node.js without recompilation.
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 .
N-API
#
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The documentation for N-API is structured as follows:
Basic N-API Data Types
Error Handling
Object Lifetime Management
Module Registration
Working with JavaScript Values
Working with JavaScript Values - Abstract Operations
Working with JavaScript Properties
Working with JavaScript Functions
Object Wrap
Simple Asynchronous Operations
Custom Asynchronous Operations
Promises
Script Execution
The N-API is a C API that ensures ABI stability across Node.js versions and different compiler
levels. However, we also understand that a C++ API can be easier to use in many cases. To support
these cases we expect there to be one or more C++ wrapper modules that provide an inlineable
C++ API. Binaries built with these wrapper modules will depend on the symbols for the N-API C
based functions exported by Node.js. These wrappers are not part of N-API, nor will they be
maintained as part of Node.js. One such example is: node-addon-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. For example:
#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>
Usage
#
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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
v4.x
v6.x
v6.14.2*
v8.x
v8.0.0*
v8.10.0*
v9.x
v9.0.0*
v9.3.0*
v9.11.0*
v10.x
v10.0.0
* 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.
N-API Version Matrix
#
Basic N-API Data Types
#
napi_status
#
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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, 
#ifdef NAPI_EXPERIMENTAL 
  napi_queue_full, 
  napi_closing, 
#endif  // NAPI_EXPERIMENTAL
} 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; 
  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.
napi_extended_error_info
#
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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;
napi_env
#
napi_value
#
napi_threadsafe_function
#
napi_threadsafe_function_release_mode
#
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Stability: 2  - Stable
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.
napi_threadsafe_function_call_mode
#
N-API Memory Management types
#
napi_handle_scope
#
napi_escapable_handle_scope
#
napi_ref
#
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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
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);
N-API Callback types
#
napi_callback_info
#
napi_callback
#
napi_finalize
#
napi_async_execute_callback
#
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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_complete_callback
#
napi_threadsafe_function_call_js
#
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[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 Handling
#
Return values
#
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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.
Note: 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.
Added in: v8.0.0
 N-API version: 1
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.
This API retrieves a napi_extended_error_info  structure with information about the last error
that occurred.
Note: The content of the napi_extended_error_info  returned is only valid up until an n-api
function is called on the same env .
Note: 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.
napi_get_last_error_info
#
Exceptions
#
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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.
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:
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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:
TypeError [ERR_ERROR_1]
Added in: v8.0.0
 N-API version: 1
NODE_EXTERN napi_status napi_throw(napi_env env, napi_value error);
[in] env : The environment that the API is invoked under.
[in] error : The napi_value  for the Error to be thrown.
Returns napi_ok  if the API succeeded.
This API throws the JavaScript Error provided.
Added in: v8.0.0
 N-API version: 1
NODE_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_throw
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napi_throw_error
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napi_throw_type_error
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Added in: v8.0.0
 N-API version: 1
NODE_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.
Added in: v8.0.0
 N-API version: 1
NODE_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.
Returns napi_ok  if the API succeeded.
This API throws a JavaScript RangeError with the text provided.
Added in: v8.0.0
 N-API version: 1
NODE_EXTERN napi_status napi_is_error(napi_env env, 
                                      napi_value value, 
                                      bool* result);
[in] env : The environment that the API is invoked under.
napi_throw_range_error
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napi_is_error
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[in] msg : 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.
Added in: v8.0.0
 N-API version: 1
NODE_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.
Added in: v8.0.0
 N-API version: 1
NODE_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.
napi_create_error
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napi_create_type_error
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[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.
Added in: v8.0.0
 N-API version: 1
NODE_EXTERN napi_status napi_create_range_error(napi_env env, 
                                                napi_value code, 
                                                const char* 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.
Added in: v8.0.0
 N-API version: 1
napi_status napi_get_and_clear_last_exception(napi_env env, 
                                              napi_value* result);
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[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.
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.11.2
 N-API version: 3
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 you want to pass 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_is_exception_pending
#
napi_fatal_exception
#
Fatal Errors
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Added in: v8.2.0
 N-API version: 1
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
than can be used to change the handle lifespan from the default.
napi_fatal_error
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Object Lifetime management
#
Making handle lifespan shorter than that of the
native method
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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.
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) { 
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    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.
Added in: v8.0.0
 N-API version: 1
NODE_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.
Added in: v8.0.0
 N-API version: 1
NODE_EXTERN napi_status napi_close_handle_scope(napi_env env, 
                                                napi_handle_scope scope);
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[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.
Added in: v8.0.0
 N-API version: 1
NODE_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.
Added in: v8.0.0
 N-API version: 1
NODE_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_open_escapable_handle_scope
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Added in: v8.0.0
 N-API version: 1
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 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_escape_handle
#
References to objects with a lifespan longer than
that of the native method
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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.
Added in: v8.0.0
 N-API version: 1
NODE_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.
Added in: v8.0.0
 N-API version: 1
NODE_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.
napi_create_reference
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napi_delete_reference
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This API deletes the reference passed in.
This API can be called even if there is a pending JavaScript exception.
Added in: v8.0.0
 N-API version: 1
NODE_EXTERN napi_status napi_reference_ref(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 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.
Added in: v8.0.0
 N-API version: 1
NODE_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.
Added in: v8.0.0
 N-API version: 1
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napi_reference_unref
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napi_get_reference_value
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NODE_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.
Added in: v8.12.0
 N-API version: 3
NODE_EXTERN napi_status napi_add_env_cleanup_hook(napi_env env, 
                                                  void (*fun)(void* arg), 
                                                  void* arg);
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.
Cleanup on exit of the current Node.js instance
#
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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.
Added in: v8.12.0
 N-API version: 3
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.
For example, to add the method hello  as a function so that it can be called as a method provided
by the addon:
napi_remove_env_cleanup_hook
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Module registration
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napi_value Init(napi_env env, napi_value exports) { 
  napi_status status; 
  napi_property_descriptor desc = 
    {"hello", Method, 0, 0, 0, napi_default, 0}; 
  if (status != napi_ok) return NULL; 
  status = napi_define_properties(env, exports, 1, &desc); 
  if (status != napi_ok) return NULL; 
  return exports;
}
For example, to set a function to be returned by the require()  for the addon:
For example, 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, GetValue, SetValue, 0, napi_default, 0 }, 
    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); 
napi_value Init(napi_env env, napi_value exports) { 
  napi_value method; 
  napi_status status; 
  status = napi_create_function(env, "exports", NAPI_AUTO_LENGTH, Method, NULL, &method); 
  if (status != napi_ok) return NULL; 
  return method;
}
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  if (status != napi_ok) return NULL; 
 
  status = napi_set_named_property(env, exports, "MyObject", cons); 
  if (status != napi_ok) return NULL; 
 
  return exports;
}
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, 
Working with JavaScript Values
#
Enum types
#
napi_valuetype
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  napi_number, 
  napi_string, 
  napi_symbol, 
  napi_object, 
  napi_function, 
  napi_external,
} 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 Functions and Objects with external data.
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,
} 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 .
Added in: v8.0.0
 N-API version: 1
napi_status napi_create_array(napi_env env, napi_value* result)
[in] env : The environment that the N-API call is invoked under.
napi_typedarray_type
#
Object Creation Functions
#
napi_create_array
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[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.
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
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.
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[out] data : Pointer to the underlying byte buffer of the ArrayBuffer.
[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. ArrayBuffers 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.
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
napi_status napi_create_buffer_copy(napi_env env, 
                                    size_t length, 
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                                    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.
Added in: v8.0.0
 N-API version: 1
napi_status napi_create_external(napi_env env, 
                                 void* data, 
                                 napi_finalize finalize_cb, 
                                 void* finalize_hint, 
                                 napi_value* result)
[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.
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Note: 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 .
Added in: v8.0.0
 N-API version: 1
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 ArrayBuffers are described in Section 24.1  of the ECMAScript Language Specification.
Added in: v8.0.0
 N-API version: 1
napi_status napi_create_external_buffer(napi_env env, 
                                        size_t length, 
                                        void* data, 
                                        napi_finalize finalize_cb, 
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                                        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).
[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.
Note: For Node.js >=4 Buffers  are Uint8Arrays.
Added in: v8.0.0
 N-API version: 1
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 : A string representing the name of the function encoded as UTF8.
[in] length : The length of the utf8name in bytes, or NAPI_AUTO_LENGTH  if it is null-
terminated.
[in] cb : A function pointer to the native function to be invoked when the created function is
invoked from JavaScript.
[in] data : Optional arbitrary context data to be passed into the native function when it is
invoked.
[out] result : A napi_value  representing a JavaScript function.
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Returns napi_ok  if the API succeeded.
This API returns an N-API value corresponding to a JavaScript Function object. It's used to wrap
native functions so that they can be invoked from JavaScript.
JavaScript Functions are described in Section 19.2  of the ECMAScript Language Specification.
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
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
napi_create_object
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The JavaScript Symbol type is described in Section 19.4  of the ECMAScript Language
Specification.
Added in: v8.0.0
 N-API version: 1
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.
[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.
Added in: v8.3.0
 N-API version: 1
napi_status napi_create_dataview(napi_env env, 
                                 size_t byte_length, 
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                                 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.
Added in: v8.4.0
 N-API version: 1
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.
Functions to convert from C types to N-API
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The JavaScript Number type is described in Section 6.1.6  of the ECMAScript Language
Specification.
Added in: v8.4.0
 N-API version: 1
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.
Added in: v8.4.0
 N-API version: 1
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.
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Added in: v8.4.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 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 a 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 a ISO-8859-1-encoded C string.
The JavaScript String type is described in Section 6.1.4  of the ECMAScript Language Specification.
Added in: v8.0.0
 N-API version: 1
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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 JavaScript String type is described in Section 6.1.4  of the ECMAScript Language Specification.
Added in: v8.0.0
 N-API version: 1
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.
This API creates a JavaScript String object from a UTF8-encoded C string
The JavaScript String type is described in Section 6.1.4  of the ECMAScript Language Specification.
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Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
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
napi_get_array_length
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napi_get_arraybuffer_info
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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.
Added in: v8.0.0
 N-API version: 1
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.
[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.
Added in: v8.0.0
 N-API version: 1
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.
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Added in: v8.0.0
 N-API version: 1
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 : Number of elements in the TypedArray.
[out] data : The data buffer underlying the typed array.
[out] byte_offset : The byte offset within the data buffer from which to start projecting the
TypedArray.
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
Added in: v8.3.0
 N-API version: 1
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.
napi_get_typedarray_info
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[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.
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
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 .
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This API returns the C double primitive equivalent of the given JavaScript Number.
Added in: v8.0.0
 N-API version: 1
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() .
Added in: v8.0.0
 N-API version: 1
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.
napi_get_value_external
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napi_get_value_int32
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Non-finite number values (NaN, positive infinity, or negative infinity) set the result to zero.
Added in: v8.0.0
 N-API version: 1
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 .
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, positive infinity, or negative infinity) set the result to zero.
Added in: v8.0.0
 N-API version: 1
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.
napi_get_value_int64
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[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.
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
napi_status napi_get_value_string_utf16(napi_env env, 
                                        napi_value value, 
                                        char16_t* buf, 
                                        size_t bufsize, 
                                        size_t* result)
napi_get_value_string_utf8
#
napi_get_value_string_utf16
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[in] env : The environment that the API is invoked under.
[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.
Added in: v8.0.0
 N-API version: 1
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 .
Added in: v8.0.0
 N-API version: 1
napi_status napi_get_boolean(napi_env env, bool value, napi_value* result)
napi_get_value_uint32
#
Functions to get global instances
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napi_get_boolean
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[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
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
napi_status napi_get_undefined(napi_env env, napi_value* result)
napi_get_global
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napi_get_null
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napi_get_undefined
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[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
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
Working with JavaScript Values - Abstract
Operations
#
napi_coerce_to_bool
#
napi_coerce_to_number
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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.
[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.
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
napi_status napi_coerce_to_string(napi_env env, 
                                  napi_value value, 
                                  napi_value* result)
napi_coerce_to_object
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napi_coerce_to_string
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[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.
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
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.
napi_typeof
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napi_instanceof
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[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.
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
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_is_array
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napi_is_arraybuffer
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Added in: v8.0.0
 N-API version: 1
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.
This API checks if the Object passed in is a buffer.
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
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.
napi_is_buffer
#
napi_is_error
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napi_is_typedarray
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This API checks if the Object passed in is a typed array.
Added in: v8.3.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
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.
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 .
napi_is_dataview
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napi_strict_equals
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Working with JavaScript Properties
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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:
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const arr = []; 
arr[123] = 'hello';
The equivalent can be done using N-API values with the following snippet:
napi_status status = napi_generic_failure; 
 
// 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] 
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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);
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, 0, 0, 0, fooValue, napi_default, 0 }, 
  { "bar", NULL, 0, 0, 0, barValue, napi_default, 0 }
} 
status = napi_define_properties(env, 
                                obj, 
                                sizeof(descriptors) / sizeof(descriptors[0]), 
                                descriptors);
if (status != napi_ok) return status;
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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; 
  napi_callback getter; 
Structures
#
napi_property_attributes
#
napi_property_descriptor
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  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.
Added in: v8.0.0
 N-API version: 1
napi_status napi_get_property_names(napi_env env, 
                                    napi_value object, 
                                    napi_value* result);
Functions
#
napi_get_property_names
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[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 array of properties for the Object passed in
Added in: v8.0.0
 N-API version: 1
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.
[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.
Added in: v8.0.0
 N-API version: 1
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.
napi_set_property
#
napi_get_property
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[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.
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.2.0
 N-API version: 1
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.
[out] result : Whether the property deletion succeeded or not. result  can optionally be
ignored by passing NULL .
napi_has_property
#
napi_delete_property
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Returns napi_ok  if the API succeeded.
This API attempts to delete the key  own property from object .
Added in: v8.2.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
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.
napi_has_own_property
#
napi_set_named_property
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This method is equivalent to calling napi_set_property  with a napi_value  created from the
string passed in as utf8Name
Added in: v8.0.0
 N-API version: 1
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.
This method is equivalent to calling napi_get_property  with a napi_value  created from the
string passed in as utf8Name
Added in: v8.0.0
 N-API version: 1
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.
napi_get_named_property
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napi_has_named_property
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This method is equivalent to calling napi_has_property  with a napi_value  created from the
string passed in as utf8Name
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
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_set_element
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napi_get_element
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Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.2.0
 N-API version: 1
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 .
Added in: v8.0.0
 N-API version: 1
napi_has_element
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napi_delete_element
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napi_define_properties
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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).
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.
Added in: v8.0.0
 N-API version: 1
napi_status napi_call_function(napi_env env, 
                               napi_value recv, 
Working with JavaScript Functions
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                               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 
status = napi_create_int32(env, 1337, &arg);
if (status != napi_ok) return; 
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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;
Added in: v8.0.0
 N-API version: 1
napi_status napi_create_function(napi_env env, 
                                 const char* utf8name, 
                                 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] 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.
napi_create_function
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Note: 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_value fn; 
  status = napi_create_function(env, NULL, 0, SayHello, nullptr, &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();
Note: The string passed to require is not necessarily the name passed into NAPI_MODULE  in the
earlier snippet but the name of the target in binding.gyp  responsible for creating the .node  file.
Added in: v8.0.0
 N-API version: 1
napi_get_cb_info
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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.
Added in: v8.6.0
 N-API version: 1
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.
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This API returns the new.target  of the constructor call. If the current callback is not a constructor
call, the result is NULL .
Added in: v8.0.0
 N-API version: 1
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; 
 
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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; 
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.
As an example:
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);
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if (is_instance) { 
  // napi_unwrap() ...
} else { 
  // otherwise...
}
The reference must be freed once it is no longer needed.
Added in: v8.0.0
 N-API version: 1
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.
[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:
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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.
Added in: v8.0.0
 N-API version: 1
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. This
object must have been created from the prototype  of a constructor that was created using
napi_define_class() .
[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.
napi_wrap
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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 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 proper of the reference.
Note: This API may modify the prototype chain of the wrapper object. Afterward, additional
manipulation of the wrapper's prototype chain may cause napi_unwrap()  to fail.
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.
Added in: v8.0.0
 N-API version: 1
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.
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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.
Added in: v8.5.0
 N-API version: 1
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, thereby restoring the JavaScript object's prototype
chain. If a finalize callback was associated with the wrapping, it will no longer be called when the
JavaScript object becomes garbage-collected.
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 .
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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. 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, 
                                             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.
History
N-API version: 1
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);
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[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.
Note: 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.
Added in: v8.0.0
 N-API version: 1
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.
napi_delete_async_work
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This API can be called even if there is a pending JavaScript exception.
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.0.0
 N-API version: 1
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_queue_async_work
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napi_cancel_async_work
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Custom Asynchronous Operations
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Added in: v8.6.0
 N-API version: 1
napi_status napi_async_init(napi_env env, 
                            napi_value async_resource, 
                            napi_value async_resource_name, 
                            napi_async_context* 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.
[out] result : The initialized async context.
Returns napi_ok  if the API succeeded.
Added in: v8.6.0
 N-API version: 1
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.
History
N-API version: 1
napi_status napi_make_callback(napi_env env, 
                               napi_async_context async_context, 
napi_async_init
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napi_async_destroy
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napi_make_callback
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                               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 .
Added in: v8.11.2
 N-API version: 3
NAPI_EXTERN napi_status napi_open_callback_scope(napi_env env, 
                                                 napi_value resource_object, 
                                                 napi_async_context context, 
                                                 napi_callback_scope* result)
napi_open_callback_scope
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[in] env : The environment that the API is invoked under.
[in] resource_object : An optional 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.
Added in: v8.11.2
 N-API version: 3
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.
Added in: v8.4.0
 N-API version: 1
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);
napi_close_callback_scope
#
Version Management
#
napi_get_node_version
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[in] env : The environment that the API is invoked under.
[out] version : A pointer to version information for Node 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.
Added in: v8.0.0
 N-API version: 1
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.
Added in: v8.5.0
 N-API version: 1
napi_get_version
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Memory Management
#
napi_adjust_external_memory
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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; 
 
// Pass the deferred to a function that performs an asynchronous action.
do_something_asynchronous(deferred); 
Promises
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// 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;
Added in: v8.5.0
 N-API version: 1
napi_status napi_create_promise(napi_env env, 
                                napi_deferred* deferred, 
                                napi_value* promise);
[in] env : The environment that the API is invoked under.
napi_create_promise
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[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.
Added in: v8.5.0
 N-API version: 1
napi_status napi_resolve_deferred(napi_env env, 
                                  napi_deferred deferred, 
                                  napi_value resolution);
[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.
Added in: v8.5.0
 N-API version: 1
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.
napi_resolve_deferred
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napi_reject_deferred
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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.
Added in: v8.5.0
 N-API version: 1
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.
Added in: v8.5.0
 N-API version: 1
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_is_promise
#
Script execution
#
napi_run_script
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N-API provides a function for getting the current event loop associated with a specific napi_env .
Added in: v8.10.0
 N-API version: 2
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
libuv event loop
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napi_get_uv_event_loop
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Asynchronous Thread-safe Function Calls
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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.
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
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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
Added in: v8.16.0
NAPI_EXTERN napi_status 
napi_create_threadsafe_function(napi_env env, 
napi_create_threadsafe_function
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                                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.
[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
Added in: v8.16.0
napi_get_threadsafe_function_context
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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] context : The location where to store the context.
This API may be called from any thread which makes use of func .
Stability: 2  - Stable
Added in: v8.16.0
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 .
Stability: 2  - Stable
napi_call_threadsafe_function
#
napi_acquire_threadsafe_function
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Added in: v8.16.0
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
Added in: v8.16.0
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 .
napi_release_threadsafe_function
#
napi_ref_threadsafe_function
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Stability: 2  - Stable
Added in: v8.16.0
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.
Stability: 2  - Stable
Added in: v8.16.0
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
napi_unref_threadsafe_function
#
Child Process
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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.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.
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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']); 
Asynchronous Process Creation
#
Spawning .bat  and .cmd  files on Windows
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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); 
    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) => { 
  // ...
});
History
command  <string>  The command to run, with space-separated arguments.
options  <Object>
cwd  <string>  Current working directory of the child process. Default: null .
child_process.exec(command[, options][, callback])
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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. 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
Note: Never pass unsanitized user input to this function. Any input containing shell metacharacters
may be used to trigger arbitrary command execution.
const { exec } = require('child_process');
exec('cat *.js bad_file | wc -l', (error, stdout, stderr) => { 
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  if (error) { 
    console.error(`exec error: ${error}`); 
    return; 
  } 
  console.log(`stdout: ${stdout}`); 
  console.log(`stderr: ${stderr}`);
});
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.
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.
Note: 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, a rejected promise is returned, with the same
error  object given in the callback, but with an additional two properties stdout  and stderr .
For example:
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();
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History
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
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 .
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
child_process.execFile(file[, args][, options][,
callback])
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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, 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();
Note: 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.
History
child_process.fork(modulePath[, args][, options])
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modulePath  <string>  The module to run in the child.
args  <Array>  List of string arguments.
options  <Object>
cwd  <string>  Current working directory of the child process.
env  <Object>  Environment key-value pairs.
execPath  <string>  Executable used to create the child process.
execArgv  <Array>  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
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child process.
Note: Unlike the fork(2)  POSIX system call, child_process.fork()  does not clone the current
process.
Note: The shell  option available in child_process.spawn()  is not supported by
child_process.fork()  and will be ignored if set.
History
command  <string>  The command to run.
args  <Array>  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.
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.
child_process.spawn(command[, args][, options])
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Note: 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 .
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
const { spawn } = require('child_process');
const ps = spawn('ps', ['ax']);
const grep = spawn('grep', ['ssh']); 
 
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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.');
});
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Note: Certain platforms (macOS, Linux) will use the value of argv[0]  for the process title while
others (Windows, SunOS) will use command .
Note: 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.
Added in: v0.7.10
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.
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 , 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 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();
options.detached
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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();
History
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 [process.stdin, process.stdout, process.stderr]  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:
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.
options.stdio
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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() , process.disconnect() ,
process.on('disconnect') , and process.on('message')  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. <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).
5. 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.
6. 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' .
Example:
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 process.on('disconnect')  event or the
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process.on('message')  event. This allows the child to exit normally without the process being held
open by the open IPC channel.
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.
Blocking calls like these are mostly useful for simplifying general-purpose scripting tasks and for
simplifying the loading/processing of application configuration at startup.
History
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>  | <Uint8Array>  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 .
Synchronous Process Creation
#
child_process.execFileSync(file[, args][, options])
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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.
Note: 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() .
Note: 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.
History
command  <string>  The command to run.
options  <Object>
cwd  <string>  Current working directory of the child process.
input  <string>  | <Buffer>  | <Uint8Array>  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.
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.
child_process.execSync(command[, options])
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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 .
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()
Note: Never pass unsanitized user input to this function. Any input containing shell metacharacters
may be used to trigger arbitrary command execution.
History
command  <string>  The command to run.
args  <Array>  List of string arguments.
options  <Object>
cwd  <string>  Current working directory of the child process.
child_process.spawnSync(command[, args][,
options])
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input  <string>  | <Buffer>  | <Uint8Array>  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.
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' .
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>  The exit code of the child process.
signal  <string>  The signal used to kill the child process.
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
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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.
Note: 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.
Added in: v2.2.0
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 .
Added in: v0.7.7
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.
Added in: v0.7.2
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
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Event: 'close'
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Event: 'disconnect'
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Event: 'error'
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3. Sending a message to the child process failed.
Note: 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() .
Added in: v0.1.90
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) .
Added in: v0.5.9
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.
Note: The message goes through serialization and parsing. The resulting message might not be the
same as what is originally sent.
Added in: v7.1.0
Event: 'exit'
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Event: 'message'
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subprocess.channel
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<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 .
Added in: v0.7.2
<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.
Added in: v0.7.2
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.
Added in: v0.1.90
signal  <string>
subprocess.connected
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subprocess.disconnect()
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subprocess.kill([signal])
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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.
Also note: 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 use of the
shell  option of ChildProcess , such as in this example:
'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'] 
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  }
); 
 
setTimeout(() => { 
  subprocess.kill(); // does not terminate the node process in the shell
}, 2000);
Added in: v0.5.10
<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.
Added in: v0.1.90
<number>  Integer
Returns the process identifier (PID) of the child process.
Example:
const { spawn } = require('child_process');
const grep = spawn('grep', ['ssh']); 
 
console.log(`Spawned child pid: ${grep.pid}`); 
grep.stdin.end();
subprocess.killed
#
subprocess.pid
#
subprocess.send(message[, sendHandle[, options]][,
callback])
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History
message  <Object>
sendHandle  <Handle>
options  <Object>  The options  argument, if present, is an object used to parameterize the
sending of certain types of handles. options  supports the following properties:
keepOpen  - A Boolean value that can be used when passing instances of net.Socket .
When true , the socket is kept open in the sending process. Default: false .
callback  <Function>
Returns: <boolean>
When an IPC channel has been established between the parent and child ( i.e. when using
child_process.fork() ), the subprocess.send()  method can be used to send messages to the
child process. When the child process is a Node.js instance, these messages can be received via the
process.on('message')  event.
Note: The message goes through serialization and parsing. The resulting message might not be the
same as what is originally sent.
For example, in the parent script:
const cp = require('child_process');
const n = cp.fork(`${__dirname}/sub.js`); 
 
n.on('message', (m) => { 
  console.log('PARENT got message:', m);
}); 
 
// 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 });
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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 process.on('message')  event. Rather, such messages are emitted using the
process.on('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 process.on('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');
}); 
Example: sending a server object
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server.listen(1337, () => { 
  subprocess.send('server', server);
});
The child would then receive the server object as:
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') { 
Example: sending a socket object
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    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`); 
    } 
  }
});
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.
Note: This function uses JSON.stringify()  internally to serialize the message .
Added in: v0.1.90
<stream.Readable>
A Readable Stream  that represents the child process's stderr .
subprocess.stderr
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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.
Added in: v0.1.90
<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.
Added in: v0.7.10
<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'); 
 
subprocess.stdin
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subprocess.stdio
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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); 
 
assert.strictEqual(subprocess.stdio[2], null); 
assert.strictEqual(subprocess.stdio[2], subprocess.stderr);
Added in: v0.1.90
<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.
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.
subprocess.stdout
#
maxBuffer  and Unicode
#
Shell Requirements
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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
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) => { 
Default Windows Shell
#
Cluster
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    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.
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:
How It Works
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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.
Note: 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.
Added in: v0.7.0
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 .
Added in: v0.7.7
Similar to the cluster.on('disconnect')  event, but specific to this worker.
cluster.fork().on('disconnect', () => { 
  // Worker has disconnected
});
Class: Worker
#
Event: 'disconnect'
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Event: 'error'
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Added in: v0.7.3
This event is the same as the one provided by child_process.fork() .
Within a worker, process.on('error')  may also be used.
Added in: v0.11.2
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.
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!'); 
  }
});
Added in: v0.7.0
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.
#
Event: 'exit'
#
Event: 'listening'
#
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Added in: v0.7.0
message  <Object>
handle  <undefined>  | <Object>
Similar to the cluster.on('message')  event, 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; 
    } 
  } 
 
  // 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) { 
Event: 'message'
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    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);
}
Added in: v0.7.0
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.
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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.
Event: 'online'
#
worker.disconnect()
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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 .
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) => { 
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    if (msg === 'shutdown') { 
      // initiate graceful close of any connections to server 
    } 
  });
}
Added in: v6.0.0
<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();
Added in: v0.8.0
<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
Added in: v0.11.14
worker.exitedAfterDisconnect
#
worker.id
#
worker.isConnected()
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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.
Added in: v0.11.14
This function returns true  if the worker's process has terminated (either because of exiting or
being signaled). Otherwise, it returns false .
Added in: v0.9.12
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 .
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 .
Added in: v0.7.0
<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.
worker.isDead()
#
worker.kill([signal='SIGTERM'])
#
worker.process
#
worker.send(message[, sendHandle][, callback])
#
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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() .
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); 
  });
}
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Stability: 0  - Deprecated: Use worker.exitedAfterDisconnect  instead.
An alias to worker.exitedAfterDisconnect .
Set by calling .kill()  or .disconnect() . Until then, it is undefined .
The boolean worker.suicide  is used to distinguish between voluntary and accidental exit, the
master may choose not to respawn a worker based on this value.
worker.suicide
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cluster.on('exit', (worker, code, signal) => { 
  if (worker.suicide === true) { 
    console.log('Oh, it was just voluntary – no need to worry'); 
  }
}); 
 
// kill worker 
worker.kill();
This API only exists for backwards compatibility and will be removed in the future.
Added in: v0.7.9
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`);
});
Added in: v0.7.9
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.
Event: 'disconnect'
#
Event: 'exit'
#
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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' .
Added in: v0.7.0
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.
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();
});
Added in: v0.7.0
worker  <cluster.Worker>
address  <Object>
Event: 'fork'
#
Event: 'listening'
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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)
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worker  <cluster.Worker>
message  <Object>
handle  <undefined>  | <Object>
Emitted when the cluster master receives a message from any worker.
See child_process event: 'message' .
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) { 
Event: 'message'
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    handle = message; 
    message = worker; 
    worker = undefined; 
  } 
  // ...
});
Added in: v0.7.0
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');
});
Added in: v0.7.1
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 .
Added in: v0.7.7
callback  <Function>  Called when all workers are disconnected and handles are closed.
Calls .disconnect()  on each worker in cluster.workers .
Event: 'online'
#
Event: 'setup'
#
cluster.disconnect([callback])
#
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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.
Added in: v0.6.0
env  <Object>  Key/value pairs to add to worker process environment.
Returns: <cluster.Worker>
Spawn a new worker process.
This can only be called from the master process.
Added in: v0.8.1
<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 .
Added in: v0.6.0
<boolean>
True if the process is not a master (it is the negation of cluster.isMaster ).
Added in: v0.11.2
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.
cluster.fork([env])
#
cluster.isMaster
#
cluster.isWorker
#
cluster.schedulingPolicy
#
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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' .
History
<Object>
execArgv  <Array>  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  <Array>  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.
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cluster.settings
#
cluster.setupMaster([settings])
#
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settings  <Object>  see cluster.settings
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.
Example:
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.
Added in: v0.7.0
<Object>
A reference to the current worker object. Not available in the master process.
const cluster = require('cluster'); 
 
cluster.worker
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if (cluster.isMaster) { 
  console.log('I am master'); 
  cluster.fork(); 
  cluster.fork();
} else if (cluster.isWorker) { 
  console.log(`I am worker #${cluster.worker.id}`);
}
Added in: v0.7.0
<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) { 
  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];
});
cluster.workers
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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 debug [script.js | -e "script" | <host>:<port>] …
node --v8-options
Execute without arguments to start the REPL .
For more info about node debug , please see the debugger  documentation.
Added in: v0.1.3
Print node's version.
Added in: v0.1.3
Print node command line options. The output of this option is less detailed than this document.
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Evaluate the following argument as JavaScript. The modules which are predefined in the REPL can
also be used in script .
Command Line Options
#
Synopsis
#
Options
#
-v , --version
#
-h , --help
#
-e , --eval "script"
#
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Note: 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.
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Identical to -e  but prints the result.
Added in: v5.0.0, v4.2.0
Syntax check the script without executing.
Added in: v0.7.7
Opens the REPL even if stdin does not appear to be a terminal.
Added in: v1.6.0
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.
Added in: v6.3.0
Activate inspector on host:port. Default is 127.0.0.1:9229.
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 Debugging Protocol .
Added in: v7.6.0
-p , --print "script"
#
-c , --check
#
-i , --interactive
#
-r , --require module
#
--inspect[=[host:]port]
#
--inspect-brk[=[host:]port]
#
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Activate inspector on host:port and break at start of user script. Default host:port is
127.0.0.1:9229.
Added in: v7.6.0
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.
Added in: v0.8.0
Silence deprecation warnings.
Added in: v0.8.0
Print stack traces for deprecations.
Added in: v0.11.14
Throw errors for deprecations.
Added in: v8.0.0
Emit pending deprecation warnings.
Note: 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.
--inspect-port=[host:]port
#
--no-deprecation
#
--trace-deprecation
#
--throw-deprecation
#
--pending-deprecation
#
--no-warnings
#
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Added in: v6.0.0
Silence all process warnings (including deprecations).
Added in: v8.4.0
Enable the experimental 'http2'  module.
Added in: v0.10
Aborting instead of exiting causes a core file to be generated for post-mortem analysis using a
debugger (such as lldb , gdb , and mdb ).
Added in: v6.0.0
Print stack traces for process warnings (including deprecations).
Added in: v8.0.0
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.
Added in: v2.1.0
Prints a stack trace whenever synchronous I/O is detected after the first turn of the event loop.
Added in: v8.8.0
Enables runtime checks for async_hooks . These can also be enabled dynamically by enabling one
of the async_hooks  hooks.
--expose-http2
#
--abort-on-uncaught-exception
#
--trace-warnings
#
--redirect-warnings=file
#
--trace-sync-io
#
--force-async-hooks-checks
#
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Added in: v7.7.0
Enables the collection of trace event tracing information.
Added in: v7.7.0
A comma separated list of categories that should be traced when trace event tracing is enabled
using --trace-events-enabled .
Added in: v8.12.0
Template string specifying the filepath for the trace event data, it supports ${rotation}  and
${pid} .
Added in: v6.0.0
Automatically zero-fills all newly allocated Buffer  and SlowBuffer  instances.
Added in: v6.3.0
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 
--trace-events-enabled
#
--trace-event-categories
#
--trace-event-file-pattern
#
--zero-fill-buffers
#
--preserve-symlinks
#
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 │       ├── 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).
Added in: v2.4.0
Track heap object allocations for heap snapshots.
Added in: v5.2.0
Process V8 profiler output generated using the V8 option --prof .
Added in: v0.1.3
Print V8 command line options.
Note: V8 options allow words to be separated by both dashes ( - ) or underscores ( _ ).
For example, --stack-trace-limit  is equivalent to --stack_trace_limit .
Added in: v4.0.0
--track-heap-objects
#
--prof-process
#
--v8-options
#
--tls-cipher-list=list
#
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Specify an alternative default TLS cipher list. (Requires Node.js to be built with crypto support.
(Default))
Added in: v6.0.0
Enable FIPS-compliant crypto at startup. (Requires Node.js to be built with ./configure --
openssl-fips )
Added in: v6.0.0
Force FIPS-compliant crypto on startup. (Cannot be disabled from script code.) (Same
requirements as --enable-fips )
Added in: v6.9.0
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 .
Added in: v7.5.0
Use OpenSSL's default CA store or use bundled Mozilla CA store as supplied by current Node.js
version. The default store is selectable at build-time.
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.
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.
See SSL_CERT_DIR  and SSL_CERT_FILE .
--enable-fips
#
--force-fips
#
--openssl-config=file
#
--use-openssl-ca , --use-bundled-ca
#
--icu-data-dir=file
#
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Added in: v0.11.15
Specify ICU data load path. (overrides NODE_ICU_DATA )
Added in: v8.0.0
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.
Added in: v7.5.0
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.
Added in: v8.15.0
Specify the maximum size, in bytes, of HTTP headers. Defaults to 8KB.
Added in: v0.1.32
',' -separated list of core modules that should print debug information.
Added in: v0.1.32
':' -separated list of directories prefixed to the module search path.
Note: On Windows, this is a ';' -separated list instead.
Added in: v0.3.0
-
#
--
#
--max-http-header-size=size
#
Environment Variables
#
NODE_DEBUG=module[,…]
#
NODE_PATH=path[:…]
#
NODE_DISABLE_COLORS=1
#
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When set to 1  colors will not be used in the REPL.
Added in: v0.11.15
Data path for ICU (Intl object) data. Will extend linked-in data when compiled with small-icu
support.
Added in: v7.5.0
When set to 1 , process warnings are silenced.
Added in: v8.8.0
When set to 1 , the http2  module is suppressed.
Added in: v8.0.0
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
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
--force-fips
--icu-data-dir
--inspect-brk
--inspect-port
--inspect
--max-http-header-size
--no-deprecation
NODE_ICU_DATA=file
#
NODE_NO_WARNINGS=1
#
NODE_NO_HTTP2=1
#
NODE_OPTIONS=options...
#
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--no-warnings
--openssl-config
--redirect-warnings
--require , -r
--throw-deprecation
--tls-cipher-list
--trace-deprecation
--trace-events-categories
--trace-events-enabled
--trace-event-file-pattern
--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
Added in: v8.0.0
When set to 1 , emit pending deprecation warnings.
Note: 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.
NODE_PENDING_DEPRECATION=1
#
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Pending deprecations are used to provide a kind of selective "early warning" mechanism that
developers may leverage to detect deprecated API usage.
Added in: v7.1.0
When set to 1 , instructs the module loader to preserve symbolic links when resolving and caching
modules.
Added in: v3.0.0
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.
Added in: v7.3.0
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.
Added in: v7.7.0
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.
Added in: v7.7.0
NODE_PRESERVE_SYMLINKS=1
#
NODE_REPL_HISTORY=file
#
NODE_EXTRA_CA_CERTS=file
#
OPENSSL_CONF=file
#
SSL_CERT_DIR=dir
#
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If --use-openssl-ca  is enabled, this overrides and sets OpenSSL's directory containing trusted
certificates.
Note: 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.
Added in: v7.7.0
If --use-openssl-ca  is enabled, this overrides and sets OpenSSL's file containing trusted
certificates.
Note: 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.
Added in: v8.0.0
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
equivalent to using the --redirect-warnings=file  command-line flag.
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()
SSL_CERT_FILE=file
#
NODE_REDIRECT_WARNINGS=file
#
UV_THREADPOOL_SIZE=size
#
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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
Console
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Example using the Console  class:
const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err); 
 
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
History
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;
stdout  <stream.Writable>
stderr  <stream.Writable>
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,
Class: Console
#
new Console(stdout[, stderr])
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stdout  is used for stderr .
const output = fs.createWriteStream('./stdout.log');
const errorOutput = fs.createWriteStream('./stderr.log');
// custom simple logger
const logger = new Console(output, 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(process.stdout, process.stderr);
Added in: v0.1.101
value  <any>
message  <any>
...args  <any>
A simple assertion test that verifies whether value  is truthy. If it is not, an AssertionError  is
thrown. If provided, the error message  is formatted using util.format()  and used as the error
message.
console.assert(true, 'does nothing');
// OK 
console.assert(false, 'Whoops %s', 'didn\'t work');
// AssertionError: Whoops didn't work
Note: The console.assert()  method is implemented differently in Node.js than the
console.assert()  method available in browsers .
console.assert(value[, message][, ...args])
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Specifically, in browsers, calling console.assert()  with a falsy assertion will cause the message  to
be printed to the console without interrupting execution of subsequent code. In Node.js, however,
a falsy assertion will cause an AssertionError  to be thrown.
Functionality approximating that implemented by browsers can be implemented by extending
Node.js' console  and overriding the console.assert()  method.
In the following example, a simple module is created that extends and overrides the default
behavior of console  in Node.js.
'use strict'; 
 
// Creates a simple extension of console with a
// new impl for assert without monkey-patching.
const myConsole = Object.create(console, { 
  assert: { 
    value: function assert(assertion, message, ...args) { 
      try { 
        console.assert(assertion, message, ...args); 
      } catch (err) { 
        console.error(err.stack); 
      } 
    }, 
    configurable: true, 
    enumerable: true, 
    writable: true, 
  },
}); 
 
module.exports = myConsole;
This can then be used as a direct replacement for the built in console:
const console = require('./myConsole'); 
console.assert(false, 'this message will print, but no error thrown'); 
console.log('this will also print');
console.clear()
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Added in: v8.3.0
When stdout  is a TTY, calling console.clear()  will attempt to clear the TTY. When stdout  is not
a TTY, this method does nothing.
Note: 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.
Added in: v8.3.0
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.count([label])
#
console.countReset([label='default'])
#
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Added in: v8.3.0
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
>
History
data  <any>
...args  <any>
The console.debug()  function is an alias for console.log() .
Added in: v0.1.101
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 .
console.debug(data[, ...args])
#
console.dir(obj[, options])
#
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Uses util.inspect()  on obj  and prints the resulting string to stdout . This function bypasses any
custom inspect()  function defined on obj .
Added in: v0.1.100
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
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.
Added in: v8.5.0
...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.
Added in: v8.5.0
An alias for console.group() .
console.error([data][, ...args])
#
console.group([...label])
#
console.groupCollapsed()
#
console.groupEnd()
#
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Added in: v8.5.0
Decreases indentation of subsequent lines by two spaces.
Added in: v0.1.100
data  <any>
...args  <any>
The console.info()  function is an alias for console.log() .
Added in: v0.1.100
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.
Added in: v0.1.104
label  <string>
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.
console.info([data][, ...args])
#
console.log([data][, ...args])
#
console.time(label)
#
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History
label  <string>
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
Note: As of Node.js v6.0.0, console.timeEnd()  deletes the timer to avoid leaking it. On older
versions, the timer persisted. This allowed console.timeEnd()  to be called multiple times for the
same label. This functionality was unintended and is no longer supported.
Added in: v0.1.104
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)
console.timeEnd(label)
#
console.trace([message][, ...args])
#
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//    at REPLServer.Interface._line (readline.js:549:8)
//    at REPLServer.Interface._ttyWrite (readline.js:826:14)
Added in: v0.1.100
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).
Added in: v8.0.0
object  <string>
This method does not display anything unless used in the inspector. The console.dirxml()
method displays in stdout  an XML interactive tree representation of the descendants of the
specified object  if possible, or the JavaScript representation if not. Calling console.dirxml()  on
an HTML or XML element is equivalent to calling console.log() .
Added in: v8.0.0
label  <string>  Defaults to 'default' .
This method does not display anything unless used in the inspector. The console.markTimeline()
method is the deprecated form of console.timeStamp() .
Added in: v8.0.0
label  <string>
console.warn([data][, ...args])
#
Inspector only methods
#
console.dirxml(object)
#
console.markTimeline(label)
#
console.profile([label])
#
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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();
// Adds the profile 'MyLabel' to the Profiles panel of the inspector.
Added in: v8.0.0
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.
Added in: v8.0.0
array  <Array>  | <Object>
columns  <Array>
This method does not display anything unless used in the inspector. Prints to stdout  the array
array  formatted as a table.
Added in: v8.0.0
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.
Added in: v8.0.0
label  <string>  Defaults to 'default' .
console.profileEnd()
#
console.table(array[, columns])
#
console.timeStamp([label])
#
console.timeline([label])
#
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This method does not display anything unless used in the inspector. The console.timeline()
method is the deprecated form of console.time() .
Added in: v8.0.0
label  <string>  Defaults to '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.
let crypto;
try { 
console.timelineEnd([label])
#
Crypto
#
Determining if crypto support is unavailable
#
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  crypto = require('crypto');
} catch (err) { 
  console.log('crypto support is disabled!');
}
Added in: v0.11.8
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.
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();
Added in: v0.11.8
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();
Class: Certificate
#
new crypto.Certificate()
#
certificate.exportChallenge(spkac)
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const challenge = cert.exportChallenge(spkac); 
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 string
Added in: v0.11.8
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 ...>
Added in: v0.11.8
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
Added in: v0.1.94
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
certificate.exportPublicKey(spkac)
#
certificate.verifySpkac(spkac)
#
Class: Cipher
#
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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 cipher = crypto.createCipher('aes192', 'a password'); 
 
let encrypted = ''; 
cipher.on('readable', () => { 
  const data = cipher.read(); 
  if (data) 
    encrypted += data.toString('hex');
}); 
cipher.on('end', () => { 
  console.log(encrypted); 
  // Prints: ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504
}); 
 
cipher.write('some clear text data'); 
cipher.end();
Example: Using Cipher  and piped streams:
const crypto = require('crypto');
const fs = require('fs');
const cipher = crypto.createCipher('aes192', 'a password'); 
 
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 cipher = crypto.createCipher('aes192', 'a password'); 
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let encrypted = cipher.update('some clear text data', 'utf8', 'hex'); 
encrypted += cipher.final('hex'); 
console.log(encrypted);
// Prints: ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504
Added in: v0.1.94
outputEncoding  <string>
Returns: <Buffer>  | <string>  Any remaining enciphered contents. If outputEncoding
parameter is one of 'latin1' , 'base64'  or 'hex' , 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.
Added in: v1.0.0
buffer  <Buffer>
Returns the <Cipher>  for method chaining.
When using an authenticated encryption mode (only GCM  is currently supported), the
cipher.setAAD()  method sets the value used for the additional authenticated data (AAD) input
parameter.
The cipher.setAAD()  method must be called before cipher.update() .
Added in: v1.0.0
Returns: <Buffer>  When using an authenticated encryption mode (only GCM  is 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.
cipher.final([outputEncoding])
#
cipher.setAAD(buffer)
#
cipher.getAuthTag()
#
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Added in: v0.7.1
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() .
History
data  <string>  | <Buffer>  | <TypedArray>  | <DataView>
inputEncoding  <string>
outputEncoding  <string>
Returns: <Buffer>  | <string>
Updates the cipher with data . If the inputEncoding  argument is given, its value must be one of
'utf8' , 'ascii' , or 'latin1'  and 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, and can be 'latin1' ,
'base64'  or 'hex' . 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.
cipher.setAutoPadding([autoPadding])
#
cipher.update(data[, inputEncoding][,
outputEncoding])
#
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Added in: v0.1.94
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 decipher = crypto.createDecipher('aes192', 'a password'); 
 
let decrypted = ''; 
decipher.on('readable', () => { 
  const data = decipher.read(); 
  if (data) 
    decrypted += data.toString('utf8');
}); 
decipher.on('end', () => { 
  console.log(decrypted); 
  // Prints: some clear text data
}); 
 
const encrypted = 
    'ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504'; 
decipher.write(encrypted, 'hex'); 
decipher.end();
Example: Using Decipher  and piped streams:
const crypto = require('crypto');
const fs = require('fs');
const decipher = crypto.createDecipher('aes192', 'a password'); 
 
Class: Decipher
#
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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 decipher = crypto.createDecipher('aes192', 'a password'); 
 
const encrypted = 
    'ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504'; 
let decrypted = decipher.update(encrypted, 'hex', 'utf8'); 
decrypted += decipher.final('utf8'); 
console.log(decrypted);
// Prints: some clear text data
Added in: v0.1.94
outputEncoding  <string>
Returns: <Buffer>  | <string>  Any remaining deciphered contents. If outputEncoding
parameter is one of 'latin1' , 'ascii'  or 'utf8' , 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.
History
buffer  <Buffer>  | <TypedArray>  | <DataView>
Returns: <Cipher>  for method chaining.
When using an authenticated encryption mode (only GCM  is currently supported), the
decipher.setAAD()  method sets the value used for the additional authenticated data (AAD) input
decipher.final([outputEncoding])
#
decipher.setAAD(buffer)
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parameter.
The decipher.setAAD()  method must be called before decipher.update() .
History
buffer  <Buffer>  | <TypedArray>  | <DataView>
Returns: <Cipher>  for method chaining.
When using an authenticated encryption mode (only GCM  is 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() .
Added in: v0.7.1
autoPadding  <boolean>  Default: true
Returns: <Cipher>  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() .
decipher.setAuthTag(buffer)
#
decipher.setAutoPadding([autoPadding])
#
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History
data  <string>  | <Buffer>  | <TypedArray>  | <DataView>
inputEncoding  <string>
outputEncoding  <string>
Returns: <Buffer>  | <string>
Updates the decipher with data . If the inputEncoding  argument is given, its value must be one of
'latin1' , 'base64' , or 'hex'  and 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, and can be 'latin1' ,
'ascii'  or 'utf8' . 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.
Added in: v0.5.0
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(); 
 
decipher.update(data[, inputEncoding][,
outputEncoding])
#
Class: DiffieHellman
#
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// 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'));
Added in: v0.5.0
otherPublicKey  <string>  | <Buffer>  | <TypedArray>  | <DataView>
inputEncoding  <string>
outputEncoding  <string>
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 . Encodings can be 'latin1' , 'hex' , or
'base64' . 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.
Added in: v0.5.0
encoding  <string>
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. Encoding can be 'latin1' , 'hex' , or
'base64' . If encoding  is provided a string is returned; otherwise a Buffer  is returned.
diffieHellman.computeSecret(otherPublicKey[,
inputEncoding][, outputEncoding])
#
diffieHellman.generateKeys([encoding])
#
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Added in: v0.5.0
encoding  <string>
Returns: <Buffer>  | <string>
Returns the Diffie-Hellman generator in the specified encoding , which can be 'latin1' , 'hex' ,
or 'base64' . If encoding  is provided a string is returned; otherwise a Buffer  is returned.
Added in: v0.5.0
encoding  <string>
Returns: <Buffer>  | <string>
Returns the Diffie-Hellman prime in the specified encoding , which can be 'latin1' , 'hex' , or
'base64' . If encoding  is provided a string is returned; otherwise a Buffer  is returned.
Added in: v0.5.0
encoding  <string>
Returns: <Buffer>  | <string>
Returns the Diffie-Hellman private key in the specified encoding , which can be 'latin1' , 'hex' ,
or 'base64' . If encoding  is provided a string is returned; otherwise a Buffer  is returned.
Added in: v0.5.0
encoding  <string>
Returns: <Buffer>  | <string>
Returns the Diffie-Hellman public key in the specified encoding , which can be 'latin1' , 'hex' ,
or 'base64' . If encoding  is provided a string is returned; otherwise a Buffer  is returned.
diffieHellman.getGenerator([encoding])
#
diffieHellman.getPrime([encoding])
#
diffieHellman.getPrivateKey([encoding])
#
diffieHellman.getPublicKey([encoding])
#
diffieHellman.setPrivateKey(privateKey[,
encoding])
#
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Added in: v0.5.0
privateKey  <string>  | <Buffer>  | <TypedArray>  | <DataView>
encoding  <string>
Sets the Diffie-Hellman private key. If the encoding  argument is provided and is either 'latin1' ,
'hex' , or 'base64' , privateKey  is expected to be a string. If no encoding  is provided,
privateKey  is expected to be a Buffer , TypedArray , or DataView .
Added in: v0.5.0
publicKey  <string>  | <Buffer>  | <TypedArray>  | <DataView>
encoding  <string>
Sets the Diffie-Hellman public key. If the encoding  argument is provided and is either 'latin1' ,
'hex'  or 'base64' , publicKey  is expected to be a string. If no encoding  is provided, publicKey  is
expected to be a Buffer , TypedArray , or DataView .
Added in: v0.11.12
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
Added in: v0.11.14
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.
diffieHellman.setPublicKey(publicKey[, encoding])
#
diffieHellman.verifyError
#
Class: ECDH
#
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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
History
otherPublicKey  <string>  | <Buffer>  | <TypedArray>  | <DataView>
inputEncoding  <string>
outputEncoding  <string>
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 . Encodings can be 'latin1' ,
'hex' , or 'base64' . 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(otherPublicKey[,
inputEncoding][, outputEncoding])
#
ecdh.generateKeys([encoding[, format]])
#
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Added in: v0.11.14
encoding  <string>
format  <string>  Default: uncompressed
Returns: <Buffer>  | <string>
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.
The encoding  argument can be 'latin1' , 'hex' , or 'base64' . If encoding  is provided a string is
returned; otherwise a Buffer  is returned.
Added in: v0.11.14
encoding  <string>
Returns: <Buffer>  | <string>  The EC Diffie-Hellman private key in the specified encoding ,
which can be 'latin1' , 'hex' , or 'base64' . If encoding  is provided a string is returned;
otherwise a Buffer  is returned.
Added in: v0.11.14
encoding  <string>
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.
The encoding  argument can be 'latin1' , 'hex' , or 'base64' . If encoding  is specified, a string is
returned; otherwise a Buffer  is returned.
Added in: v0.11.14
ecdh.getPrivateKey([encoding])
#
ecdh.getPublicKey([encoding][, format])
#
ecdh.setPrivateKey(privateKey[, encoding])
#
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privateKey  <string>  | <Buffer>  | <TypedArray>  | <DataView>
encoding  <string>
Sets the EC Diffie-Hellman private key. The encoding  can be 'latin1' , 'hex'  or 'base64' . 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.
Added in: v0.11.14 Deprecated since: v5.2.0
Stability: 0  - Deprecated
publicKey  <string>  | <Buffer>  | <TypedArray>  | <DataView>
encoding  <string>
Sets the EC Diffie-Hellman public key. Key encoding can be 'latin1' , 'hex'  or 'base64' . 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');
const alice = crypto.createECDH('secp256k1');
const bob = crypto.createECDH('secp256k1'); 
 
// Note: This is a shortcut way to specify 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()
ecdh.setPublicKey(publicKey[, encoding])
#
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); 
 
// 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);
Added in: v0.1.92
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', () => { 
  const data = hash.read(); 
  if (data) { 
    console.log(data.toString('hex')); 
    // Prints: 
    //   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50 
  }
}); 
 
Class: Hash
#
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hash.write('some data to hash'); 
hash.end();
Example: Using Hash  and piped streams:
const crypto = require('crypto');
const fs = require('fs');
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
Added in: v0.1.92
encoding  <string>
Returns: <Buffer>  | <string>
Calculates the digest of all of the data passed to be hashed (using the hash.update()  method). The
encoding  can be 'hex' , 'latin1'  or 'base64' . 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.
hash.digest([encoding])
#
hash.update(data[, inputEncoding])
#
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History
data  <string>  | <Buffer>  | <TypedArray>  | <DataView>
inputEncoding  <string>
Updates the hash content with the given data , the encoding of which is given in inputEncoding
and can be 'utf8' , 'ascii'  or 'latin1' . 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.
Added in: v0.1.94
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', () => { 
  const data = hmac.read(); 
  if (data) { 
    console.log(data.toString('hex')); 
    // Prints: 
    //   7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e 
  }
}); 
 
Class: Hmac
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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
Added in: v0.1.94
encoding  <string>
Returns: <Buffer>  | <string>
Calculates the HMAC digest of all of the data passed using hmac.update() . The encoding  can be
'hex' , 'latin1'  or 'base64' . 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.
hmac.digest([encoding])
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hmac.update(data[, inputEncoding])
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History
data  <string>  | <Buffer>  | <TypedArray>  | <DataView>
inputEncoding  <string>
Updates the Hmac  content with the given data , the encoding of which is given in inputEncoding
and can be 'utf8' , 'ascii'  or 'latin1' . 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.
Added in: v0.1.92
The Sign  Class is a utility for generating signatures. It can be used in one of two ways:
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:
Class: Sign
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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
History
privateKey  <string>  | <Object>
key  <string>
passphrase  <string>
outputFormat  <string>
Returns: <Buffer>  | <string>
Calculates the signature on all the data passed through using either sign.update()  or
sign.write() .
sign.sign(privateKey[, outputFormat])
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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.
The outputFormat  can specify one of 'latin1' , 'hex'  or 'base64' . If outputFormat  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.
History
data  <string>  | <Buffer>  | <TypedArray>  | <DataView>
inputEncoding  <string>
Updates the Sign  content with the given data , the encoding of which is given in inputEncoding
and can be 'utf8' , 'ascii'  or 'latin1' . 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.
Added in: v0.1.92
sign.update(data[, inputEncoding])
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Class: Verify
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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'); 
 
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
History
data  <string>  | <Buffer>  | <TypedArray>  | <DataView>
verify.update(data[, inputEncoding])
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inputEncoding  <string>
Updates the Verify  content with the given data , the encoding of which is given in inputEncoding
and can be 'utf8' , 'ascii'  or 'latin1' . 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.
History
object  <string>  | <Object>
signature  <string>  | <Buffer>  | <TypedArray>  | <DataView>
signatureFormat  <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.
The signature  argument is the previously calculated signature for the data, in the
signatureFormat  which can be 'latin1' , 'hex'  or 'base64' . If a signatureFormat  is specified,
the signature  is expected to be a string; otherwise signature  is expected to be a Buffer ,
TypedArray , or DataView .
verify.verify(object, signature[, signatureFormat])
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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.
Added in: v6.3.0
Returns: <Object>  An object containing commonly used constants for crypto and security
related operations. The specific constants currently defined are described in Crypto
Constants .
Added in: v0.9.3
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 may become
deprecated in a future Node.js release.
Added in: v6.0.0
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.
Added in: v0.1.94
algorithm  <string>
password  <string>  | <Buffer>  | <TypedArray>  | <DataView>
options  <Object>  stream.transform options
crypto  module methods and properties
#
crypto.constants
#
crypto.DEFAULT_ENCODING
#
crypto.fips
#
crypto.createCipher(algorithm, password[,
options])
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Returns: <Cipher>
Creates and returns a Cipher  object that uses the given algorithm  and password . Optional
options  argument controls stream behavior.
The algorithm  is dependent on OpenSSL, examples are 'aes192' , etc. On recent OpenSSL
releases, openssl list-cipher-algorithms  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 PBKDF2 instead of EVP_BytesToKey  it is
recommended that developers derive a key and IV on their own using crypto.pbkdf2()  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.
History
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 ).
Optional options  argument controls stream behavior.
The algorithm  is dependent on OpenSSL, examples are 'aes192' , etc. On recent OpenSSL
releases, openssl list-cipher-algorithms  will display the available cipher algorithms.
crypto.createCipheriv(algorithm, key, iv[, options])
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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 .
Added in: v0.1.92 Deprecated since: v0.11.13
Stability: 0  - Deprecated: Use tls.createSecureContext()  instead.
details  <Object>  Identical to tls.createSecureContext() .
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.
Returns a tls.SecureContext , as-if tls.createSecureContext()  had been called.
Added in: v0.1.94
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).
Optional options  argument controls stream behavior.
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 PBKDF2 instead of EVP_BytesToKey  it is
recommended that developers derive a key and IV on their own using crypto.pbkdf2()  and to use
crypto.createDecipheriv()  to create the Decipher  object.
crypto.createCredentials(details)
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crypto.createDecipher(algorithm, password[,
options])
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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 ). Optional options  argument controls stream behavior.
The algorithm  is dependent on OpenSSL, examples are 'aes192' , etc. On recent OpenSSL
releases, openssl list-cipher-algorithms  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 .
History
prime  <string>  | <Buffer>  | <TypedArray>  | <DataView>
primeEncoding  <string>
generator  <number>  | <string>  | <Buffer>  | <TypedArray>  | <DataView>  Default: 2
generatorEncoding  <string>
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.
The primeEncoding  and generatorEncoding  arguments can be 'latin1' , 'hex' , or 'base64' .
crypto.createDecipheriv(algorithm, key, iv[,
options])
#
crypto.createDiffieHellman(prime[, primeEncoding]
[, generator][, generatorEncoding])
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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.
Added in: v0.5.0
primeLength  <number>
generator  <number>  | <string>  | <Buffer>  | <TypedArray>  | <DataView>  Default: 2
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.
Added in: v0.11.14
curveName  <string>
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.
Added in: v0.1.92
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-message-digest-algorithms  will display the available digest algorithms.
crypto.createDiffieHellman(primeLength[,
generator])
#
crypto.createECDH(curveName)
#
crypto.createHash(algorithm[, options])
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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', () => { 
  const data = input.read(); 
  if (data) 
    hash.update(data); 
  else { 
    console.log(`${hash.digest('hex')} ${filename}`); 
  }
});
Added in: v0.1.94
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-message-digest-algorithms  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
crypto.createHmac(algorithm, key[, options])
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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', () => { 
  const data = input.read(); 
  if (data) 
    hmac.update(data); 
  else { 
    console.log(`${hmac.digest('hex')} ${filename}`); 
  }
});
Added in: v0.1.92
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.
Added in: v0.1.92
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.
crypto.createSign(algorithm[, options])
#
crypto.createVerify(algorithm[, options])
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Added in: v0.9.3
Returns: <string[]>  An array with the names of the supported cipher algorithms.
Example:
const ciphers = crypto.getCiphers(); 
console.log(ciphers); // ['aes-128-cbc', 'aes-128-ccm', ...]
Added in: v2.3.0
Returns: <string[]>  An array with the names of the supported elliptic curves.
Example:
const curves = crypto.getCurves(); 
console.log(curves); // ['Oakley-EC2N-3', 'Oakley-EC2N-4', ...]
Added in: v0.7.5
groupName  <string>
Returns: <Object>
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):
crypto.getCiphers()
#
crypto.getCurves()
#
crypto.getDiffieHellman(groupName)
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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);
Added in: v0.9.3
Returns: <string[]>  An array of the names of the supported hash algorithms, such as 'RSA-
SHA256' .
Example:
const hashes = crypto.getHashes(); 
console.log(hashes); // ['DSA', 'DSA-SHA', 'DSA-SHA1', ...]
History
password  <string>
salt  <string>
iterations  <number>
keylen  <number>
digest  <string>
callback  <Function>
crypto.getHashes()
#
crypto.pbkdf2(password, salt, iterations, keylen,
digest, callback)
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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, err  will be set; otherwise err  will be null. The successfully generated derivedKey  will be
passed as a Buffer .
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 also be as unique as possible. It is recommended that the salts are random and
their lengths are at least 16 bytes. See NIST SP 800-132  for details.
Example:
const crypto = require('crypto'); 
crypto.pbkdf2('secret', 'salt', 100000, 64, 'sha512', (err, derivedKey) => { 
  if (err) throw err; 
  console.log(derivedKey.toString('hex'));  // '3745e48...08d59ae'
});
An array of supported digest functions can be retrieved using crypto.getHashes() .
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.
History
password  <string>
salt  <string>
crypto.pbkdf2Sync(password, salt, iterations,
keylen, digest)
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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 .
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 also be as unique as possible. It is recommended that the salts are random and
their lengths are at least 16 bytes. See NIST SP 800-132  for details.
Example:
const crypto = require('crypto');
const key = crypto.pbkdf2Sync('secret', 'salt', 100000, 64, 'sha512'); 
console.log(key.toString('hex'));  // '3745e48...08d59ae'
An array of supported digest functions can be retrieved using crypto.getHashes() .
Added in: v0.11.14
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 , RSA_PKCS1_PADDING , or
crypto.constants.RSA_PKCS1_OAEP_PADDING .
buffer  <Buffer>  | <TypedArray>  | <DataView>
Returns: <Buffer>  A new Buffer  with the decrypted content.
crypto.privateDecrypt(privateKey, buffer)
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Decrypts buffer  with privateKey .
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 .
Added in: v1.1.0
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 RSA_PKCS1_PADDING .
buffer  <Buffer>  | <TypedArray>  | <DataView>
Returns: <Buffer>  A new Buffer  with the encrypted content.
Encrypts buffer  with privateKey .
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 .
Added in: v1.1.0
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 RSA_PKCS1_PADDING .
buffer  <Buffer>  | <TypedArray>  | <DataView>
Returns: <Buffer>  A new Buffer  with the decrypted content.
Decrypts buffer  with key .
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 .
crypto.privateEncrypt(privateKey, buffer)
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crypto.publicDecrypt(key, buffer)
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Because RSA public keys can be derived from private keys, a private key may be passed instead of a
public key.
Added in: v0.11.14
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 , 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.
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 .
Because RSA public keys can be derived from private keys, a private key may be passed instead of a
public key.
Added in: v0.5.8
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.
crypto.publicEncrypt(key, buffer)
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crypto.randomBytes(size[, callback])
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// 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.
Note: 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.
Added in: v7.10.0
buffer  <Buffer>  | <Uint8Array>  Must be supplied.
offset  <number>  Default: 0
size  <number>  Default: buffer.length - offset
Returns: <Buffer>
Synchronous version of crypto.randomFill() .
crypto.randomFillSync(buffer[, offset][, size])
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const buf = Buffer.alloc(10); 
console.log(crypto.randomFillSync(buf).toString('hex')); 
 
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'));
Added in: v7.10.0
buffer  <Buffer>  | <Uint8Array>  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; 
crypto.randomFill(buffer[, offset][, size], callback)
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  console.log(buf.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.
Note: 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.
Added in: v0.11.11
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_ECDH
crypto.constants.ENGINE_METHOD_ECDSA
crypto.constants.ENGINE_METHOD_CIPHERS
crypto.constants.ENGINE_METHOD_DIGESTS
crypto.constants.ENGINE_METHOD_STORE
crypto.constants.ENGINE_METHOD_PKEY_METHS
crypto.constants.ENGINE_METHOD_PKEY_ASN1_METHS
crypto.constants.ENGINE_METHOD_ALL
crypto.setEngine(engine[, flags])
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crypto.constants.ENGINE_METHOD_NONE
Added in: v6.6.0
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.
Note: 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 Buffers. 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.
crypto.timingSafeEqual(a, b)
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Notes
#
Legacy Streams API (pre Node.js v0.10)
#
Recent ECDH Changes
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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.
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 Op
https://www.openssl.org/docs/man1.0.2/ssl
for detail.
SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
Allows legacy insecure renegotiation betwe
clients or servers. See
https://www.openssl.org/docs/man1.0.2/ssl
Support for weak or compromised algorithms
#
Crypto Constants
#
OpenSSL Options
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SSL_OP_CIPHER_SERVER_PREFERENCE
Attempts to use the server's preferences ins
selecting a cipher. Behavior depends on prot
https://www.openssl.org/docs/man1.0.2/ssl
SSL_OP_CISCO_ANYCONNECT
Instructs OpenSSL to use Cisco's "speshul" v
SSL_OP_COOKIE_EXCHANGE
Instructs OpenSSL to turn on cookie exchan
SSL_OP_CRYPTOPRO_TLSEXT_BUG
Instructs OpenSSL to add server-hello exten
of the cryptopro draft.
SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
Instructs OpenSSL to disable a SSL 3.0/TLS 1
workaround added in OpenSSL 0.9.6d.
SSL_OP_EPHEMERAL_RSA
Instructs OpenSSL to always use the tmp_rs
operations.
SSL_OP_LEGACY_SERVER_CONNECT
Allows initial connection to servers that do n
SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
SSL_OP_MICROSOFT_SESS_ID_BUG
SSL_OP_MSIE_SSLV2_RSA_PADDING
Instructs OpenSSL to disable the workaroun
protocol-version vulnerability in the SSL 2.0
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 SS
SSL_OP_NO_QUERY_MTU
SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION
Instructs OpenSSL to always start a new ses
renegotiation.
SSL_OP_NO_SSLv2
Instructs OpenSSL to turn off SSL v2
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SSL_OP_NO_SSLv3
Instructs OpenSSL to turn off SSL v3
SSL_OP_NO_TICKET
Instructs OpenSSL to disable use of RFC450
SSL_OP_NO_TLSv1
Instructs OpenSSL to turn off TLS v1
SSL_OP_NO_TLSv1_1
Instructs OpenSSL to turn off TLS v1.1
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 ke
temporary/ephemeral DH parameters.
SSL_OP_SINGLE_ECDH_USE
Instructs OpenSSL to always create a new ke
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 rollbac
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
OpenSSL Engine Constants
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ENGINE_METHOD_RAND
Limit engine usage to RAND
ENGINE_METHOD_ECDH
Limit engine usage to ECDH
ENGINE_METHOD_ECDSA
Limit engine usage to ECDSA
ENGINE_METHOD_CIPHERS
Limit engine usage to CIPHERS
ENGINE_METHOD_DIGESTS
Limit engine usage to DIGESTS
ENGINE_METHOD_STORE
Limit engine usage to STORE
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
DH_CHECK_P_NOT_SAFE_PRIME
DH_CHECK_P_NOT_PRIME
DH_UNABLE_TO_CHECK_GENERATOR
DH_NOT_SUITABLE_GENERATOR
NPN_ENABLED
ALPN_ENABLED
RSA_PKCS1_PADDING
Other OpenSSL Constants
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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 Crypto Constants
#
Debugger
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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 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 
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< 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
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.
Watchers
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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('./m
  2 mod.hello(); 
  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
Command reference
#
Stepping
#
Breakpoints
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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 Debugging 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
>22 exports.hello = function() { 
 23   return 'hello from module'; 
 24 }; 
debug>
Information
#
Execution control
#
Various
#
Advanced Usage
#
V8 Inspector Integration for Node.js
#
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connections on port 9222.
To break on the first line of the application code, pass the --inspect-brk  flag instead of --
inspect .
(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.)
Node.js may deprecate APIs when either: (a) use of the API is considered to be unsafe, (b) an
improved alternative API has been made 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.
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 to identify code that either has been removed or will soon be
removed from Node.js.
From time-to-time the deprecation of an API may be reversed. Such action may happen in either a
semver-minor or semver-major release. In such situations, this document will be updated with
$ 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/inspector.html?experiments=true&v8only=true&ws=127
Deprecated APIs
#
Un-deprecation
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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.
Type: Documentation-only
The CryptoStream.prototype.readyState  property is deprecated and should not be used.
List of Deprecated APIs
#
DEP0001: http.OutgoingMessage.prototype.flush
#
DEP0002: require('_linklist')
#
DEP0003: _writableState.buffer
#
DEP0004: CryptoStream.prototype.readyState
#
DEP0005: Buffer() constructor
#
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Type: Documentation-only
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 .
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: Runtime
Within the cluster  module, the worker.suicide  property has been deprecated. Please use
worker.exitedAfterDisconnect  instead.
Type: Documentation-only
DEP0006: child_process options.customFds
#
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DEP0008: require('constants')
#
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The constants  module has been 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: Runtime
Domain.dispose()  is deprecated. Recover from failed I/O actions explicitly via error event
handlers set on the domain instead.
DEP0009: crypto.pbkdf2 without digest
#
DEP0010: crypto.createCredentials
#
DEP0011: crypto.Credentials
#
DEP0012: Domain.dispose
#
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Type: Runtime
Calling an asynchronous function without a callback is deprecated.
Type: End-of-Life
The fs.read()  legacy String interface is deprecated. Use the Buffer API as mentioned in the
documentation instead.
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 have been deprecated and should no longer
be used.
Type: Runtime
DEP0013: fs asynchronous function without
callback
#
DEP0014: fs.read legacy String interface
#
DEP0015: fs.readSync legacy String interface
#
DEP0016: GLOBAL/root
#
DEP0017: Intl.v8BreakIterator
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The Intl.v8BreakIterator  is deprecated and will be removed or replaced soon.
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.
DEP0018: Unhandled promise rejections
#
DEP0019: require('.') resolved outside directory
#
DEP0020: Server.connections
#
DEP0021: Server.listenFD
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DEP0022: os.tmpDir()
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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: Runtime
The REPLServer.prototype.convertToContext()  API is deprecated and should not be used.
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.
DEP0023: os.getNetworkInterfaces()
#
DEP0024:
REPLServer.prototype.convertToContext()
#
DEP0025: require('sys')
#
DEP0026: util.print()
#
DEP0027: util.puts()
#
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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.
Type: Runtime
The util.error()  API is deprecated. Please use console.error()  instead.
Type: Documentation-only
The SlowBuffer  class has been 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
DEP0028: util.debug()
#
DEP0029: util.error()
#
DEP0030: SlowBuffer
#
DEP0031: ecdh.setPublicKey()
#
DEP0032: domain module
#
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The domain  module is deprecated and should not be used.
Type: Documentation-only
The EventEmitter.listenerCount(emitter, eventName)  API has been deprecated. Please use
emitter.listenerCount(eventName)  instead.
Type: Documentation-only
The fs.exists(path, callback)  API has been deprecated. Please use fs.stat()  or
fs.access()  instead.
Type: Documentation-only
The fs.lchmod(path, mode, callback)  API has been deprecated.
Type: Documentation-only
The fs.lchmodSync(path, mode)  API has been deprecated.
Type: Documentation-only
DEP0033: EventEmitter.listenerCount()
#
DEP0034: fs.exists(path, callback)
#
DEP0035: fs.lchmod(path, mode, callback)
#
DEP0036: fs.lchmodSync(path, mode)
#
DEP0037: fs.lchown(path, uid, gid, callback)
#
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The fs.lchown(path, uid, gid, callback)  API has been deprecated.
Type: Documentation-only
The fs.lchownSync(path, uid, gid)  API has been deprecated.
Type: Documentation-only
The require.extensions  property has been deprecated.
Type: Documentation-only
The punycode  module has been deprecated. Please use a userland alternative instead.
Type: Documentation-only
The NODE_REPL_HISTORY_FILE  environment variable has been deprecated.
Type: Documentation-only
The tls.CryptoStream  class has been deprecated. Please use tls.TLSSocket  instead.
DEP0038: fs.lchownSync(path, uid, gid)
#
DEP0039: require.extensions
#
DEP0040: punycode module
#
DEP0041: NODE_REPL_HISTORY_FILE
environment variable
#
DEP0042: tls.CryptoStream
#
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Type: Documentation-only
The tls.SecurePair  class has been deprecated. Please use tls.TLSSocket  instead.
Type: Documentation-only
The util.isArray()  API has been deprecated. Please use Array.isArray()  instead.
Type: Documentation-only
The util.isBoolean()  API has been deprecated.
Type: Documentation-only
The util.isBuffer()  API has been deprecated. Please use Buffer.isBuffer()  instead.
Type: Documentation-only
The util.isDate()  API has been deprecated.
DEP0043: tls.SecurePair
#
DEP0044: util.isArray()
#
DEP0045: util.isBoolean()
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DEP0046: util.isBuffer()
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DEP0047: util.isDate()
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Type: Documentation-only
The util.isError()  API has been deprecated.
Type: Documentation-only
The util.isFunction()  API has been deprecated.
Type: Documentation-only
The util.isNull()  API has been deprecated.
Type: Documentation-only
The util.isNullOrUndefined()  API has been deprecated.
Type: Documentation-only
The util.isNumber()  API has been deprecated.
DEP0048: util.isError()
#
DEP0049: util.isFunction()
#
DEP0050: util.isNull()
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DEP0051: util.isNullOrUndefined()
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DEP0052: util.isNumber()
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Type: Documentation-only
The util.isObject()  API has been deprecated.
Type: Documentation-only
The util.isPrimitive()  API has been deprecated.
Type: Documentation-only
The util.isRegExp()  API has been deprecated.
Type: Documentation-only
The util.isString()  API has been deprecated.
Type: Documentation-only
The util.isSymbol()  API has been deprecated.
Type: Documentation-only
DEP0054: util.isPrimitive()
#
DEP0055: util.isRegExp()
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DEP0056: util.isString()
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DEP0057: util.isSymbol()
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DEP0058: util.isUndefined()
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The util.isUndefined()  API has been deprecated.
Type: Documentation-only
The util.log()  API has been deprecated.
Type: Documentation-only
The util._extend()  API has been 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 has been removed as of V8 5.8. It is
replaced by Inspector which is activated with --inspect  instead.
DEP0059: util.log()
#
DEP0060: util._extend()
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DEP0061: fs.SyncWriteStream
#
DEP0062: node --debug
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DEP0063:
ServerResponse.prototype.writeHeader()
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Type: Documentation-only
The http  module ServerResponse.prototype.writeHeader()  API has been deprecated. Please
use ServerResponse.prototype.writeHead()  instead.
Note: 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.
Type: Documentation-only
The repl  module's REPL_MODE_MAGIC  constant, used for replMode  option, has been deprecated.
Its behavior has been functionally identical to that of REPL_MODE_SLOPPY  since Node.js v6.0.0,
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 default value, magic , is similarly deprecated in favor of sloppy .
Type: Documentation-only
The http  module outgoingMessage._headers  and outgoingMessage._headerNames  properties
have been deprecated. Please instead use one of the public methods (e.g.
outgoingMessage.getHeader() , outgoingMessage.getHeaders() ,
DEP0064: tls.createSecurePair()
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NODE_REPL_MODE=magic
#
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outgoingMessage._headerNames
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outgoingMessage.getHeaderNames() , outgoingMessage.hasHeader() ,
outgoingMessage.removeHeader() , outgoingMessage.setHeader() ) for working with outgoing
headers.
Note: outgoingMessage._headers  and outgoingMessage._headerNames  were never documented
as officially supported properties.
Type: Documentation-only
The http  module OutgoingMessage.prototype._renderHeaders()  API has been deprecated.
Note: OutgoingMessage.prototype._renderHeaders  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: Documentation-only
The DebugContext will be removed in V8 soon and will not be available in Node 10+.
Note: DebugContext was an experimental API.
DEP0067:
OutgoingMessage.prototype._renderHeaders
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DEP0068: node debug
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DEP0069: vm.runInDebugContext(string)
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Type: Runtime
async_hooks.currentId()  was renamed to async_hooks.executionAsyncId()  for clarity.
Note: change was made while async_hooks  was an experimental API.
Type: Runtime
async_hooks.triggerId()  was renamed to async_hooks.triggerAsyncId()  for clarity.
Note: change was made while async_hooks  was an experimental API.
Type: Runtime
async_hooks.AsyncResource.triggerId()  was renamed to
async_hooks.AsyncResource.triggerAsyncId()  for clarity.
Note: change was made while async_hooks  was an experimental API.
Type: Documentation-only
tls.parseCertString()  is a trivial parsing helper that was made public by mistake. This function
can usually be replaced with:
const querystring = require('querystring'); 
querystring.parse(str, '\n', '=');
Note: This function is not completely equivalent to querystring.parse() . One difference is that
querystring.parse()  does url decoding:
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> querystring.parse('%E5%A5%BD=1', '\n', '=');
{ '好': '1' }
> tls.parseCertString('%E5%A5%BD=1');
{ '%E5%A5%BD': '1' }
Type: Documentation-only
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 backwards compatibility
with Node.js prior to version 6.4.0, both may be specified.
Type: Runtime
The AsyncHooks Sensitive API was never documented and had various of minor issues, see
https://github.com/nodejs/node/issues/15572 . Use the AsyncResource  API instead.
Type: Runtime
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
DEP0079: Custom inspection function on Objects
via .inspect()
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DEP0085: AsyncHooks Sensitive API
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DEP0086: Remove runInAsyncIdScope
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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
The embedded API provided by AsyncHooks exposes emit<Before,After> 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.
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);
});
// address: "192.0.43.8" family: IPv4
DEP0098: AsyncHooks Embedder
AsyncResource.emit APIs
<Before,After>#
DNS
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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.
Added in: v8.3.0
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 resolver:
Class dns.Resolver
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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.setServers()
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()
Added in: v8.3.0
Cancel all outstanding DNS queries made by this resolver. The corresponding callbacks will be
called with an error with code ECANCELLED .
resolver.cancel()
#
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Added in: v0.11.3
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.
For example:
[ 
  '4.4.4.4', 
  '2001:4860:4860::8888', 
  '4.4.4.4:1053', 
  '[2001:4860:4860::8888]:1053'
]
History
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 .
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.
dns.lookup(hostname[, options], callback)
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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.
Supported getaddrinfo flags
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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).
Added in: v0.11.14
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.
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.
Added in: v0.1.27
hostname  <string>  Hostname to resolve.
rrtype  <string>  Resource record type. Default: 'A' .
callback  <Function>
dns.lookupService(address, port, callback)
#
dns.resolve(hostname[, rrtype], callback)
#
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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()
'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()
'ANY'
any records
<Object>
dns.resolveAny()
On error, err  is an Error  object, where err.code  is one of the DNS error codes .
History
hostname  <string>  Hostname to resolve.
dns.resolve4(hostname[, options], callback)
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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'] ).
History
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.
Added in: v0.3.2
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'] ).
dns.resolve6(hostname[, options], callback)
#
dns.resolveCname(hostname, callback)
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Added in: v0.1.27
hostname  <string>
callback  <Function>
err  <Error>
addresses  <Object[]>
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'},
...] ).
Added in: v0.9.12
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
For example:
{ 
  flags: 's', 
  service: 'SIP+D2U', 
dns.resolveMx(hostname, callback)
#
dns.resolveNaptr(hostname, callback)
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  regexp: '', 
  replacement: '_sip._udp.example.com', 
  order: 30, 
  preference: 100
}
Added in: v0.1.90
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'] ).
Added in: v6.0.0
hostname  <string>
callback  <Function>
err  <Error>
addresses  <string[]>
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.
Added in: v0.11.10
hostname  <string>
callback  <Function>
err  <Error>
address  <Object>
dns.resolveNs(hostname, callback)
#
dns.resolvePtr(hostname, callback)
#
dns.resolveSoa(hostname, callback)
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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
}
Added in: v0.1.27
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
dns.resolveSrv(hostname, callback)
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port
name
{ 
  priority: 10, 
  weight: 5, 
  port: 21223, 
  name: 'service.example.com'
}
Added in: v0.1.27
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.
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
dns.resolveTxt(hostname, callback)
#
dns.resolveAny(hostname, callback)
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Type
Properties
'A'
address  / ttl
'AAAA'
address  / ttl
'CNAME'
value
'MX'
Refer to dns.resolveMx()
'NAPTR'
Refer to dns.resolveNaptr()
'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() , eg. { 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 } ]
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Added in: v0.1.16
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 .
Added in: v0.11.3
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.
For example:
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.
Each DNS query can return one of the following error codes:
dns.NODATA : DNS server returned answer with no data.
dns.reverse(ip, callback)
#
dns.setServers(servers)
#
Error codes
#
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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.
Implementation considerations
#
dns.lookup()
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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.
Note that various networking APIs will call dns.lookup()  internally to resolve host names. If that
is an issue, consider resolving the hostname to and 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 .
History
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.
dns.resolve() , dns.resolve*()  and dns.reverse() #
Domain
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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 you 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. 
  // 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); 
Warning: Don't Ignore Errors!
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  }).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 
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  // 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}`); 
 
      // Note: We're in dangerous territory! 
      // 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. 
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    // 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); 
      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.
Additions to Error objects
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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, so domain.dispose()  will not shut down the
EventEmitter. 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.
For example:
// 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 
Implicit Binding
#
Explicit Binding
#
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    // 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(); 
    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>
Returns a new Domain object.
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
domain.create()
#
Class: Domain
#
domain.members
#
domain.add(emitter)
#
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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 
    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.
});
domain.bind(callback)
#
Example
#
domain.dispose()
#
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Stability: 0  - Deprecated. Please recover from failed IO actions explicitly via error event
handlers set on the domain.
Once dispose  has been called, the domain will no longer be used by callbacks bound into the
domain via run , bind , or intercept , and a 'dispose'  event is emitted.
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.
If the domain on which enter  is called has been disposed, enter  will return without setting the
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.
If the domain on which exit  is called has been disposed, exit  will return without exiting the
domain.
callback  <Function>  The callback function
domain.enter()
#
domain.exit()
#
domain.intercept(callback)
#
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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. 
    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.
Example
#
domain.remove(emitter)
#
domain.run(fn[, ...args])
#
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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.
Example:
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 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; 
Domains and Promises
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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.
ECMAScript Modules
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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 
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.
Feature
Reason
require('./foo.mjs')
ES Modules have differing resolution and timing, use language
standard import()
import()
pending newer V8 release used in Node.js
import.meta
pending V8 implementation
NODE_PATH  is not part of resolving import  specifiers. Please use symlinks if this behavior is
desired.
Enabling
#
Features
#
Supported
#
Unsupported
#
Notable differences between import  and
require
#
No NODE_PATH
#
No require.extensions
#
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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 fs from 'fs'; 
fs.readFile('./foo.txt', (err, body) => { 
  if (err) { 
    console.error(err); 
  } else { 
    console.log(body); 
  }
});
No require.cache
#
URL based paths
#
Interop with existing modules
#
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To customize the default module resolution, loader hooks can optionally be provided via a --
loader ./loader-name.mjs  argument to Node.
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:
import url from 'url'; 
 
export async function resolve(specifier, parentModuleURL, defaultResolver) { 
  return { 
    url: new URL(specifier, parentModuleURL).href, 
    format: 'esm' 
  };
}
The default NodeJS 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
'commonjs'
Load a node-style CommonJS module
'builtin'
Load a node builtin CommonJS module
'json'
Load a JSON file
Loader hooks
#
Resolve hook
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format
Description
'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 builtin modules support could be written:
import url from 'url'; 
import path from 'path'; 
import process from 'process'; 
import Module from 'module'; 
 
const builtins = Module.builtinModules;
const JS_EXTENSIONS = new Set(['.js', '.mjs']); 
 
export function resolve(specifier, parentModuleURL/*, 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.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}.`); 
  } 
  return { 
    url: resolved.href, 
    format: 'esm' 
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  };
}
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>  : thrown when a call to eval()  fails.
<SyntaxError>  : thrown in response to improper JavaScript language syntax.
<RangeError>  : thrown when a value is not within an expected range
Dynamic instantiate hook
#
Errors
#
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<ReferenceError>  : thrown when using undefined variables
<TypeError>  : thrown when passing arguments of the wrong type
<URIError>  : thrown when a global URI handling function is misused.
System errors triggered by underlying operating system constraints such as attempting to
open a file that does not exist, attempting to send data over a closed socket, etc;
And User-specified errors triggered by application code.
Assertion Errors are a special class 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.
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.
// Throws with a ReferenceError because z is undefined
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:
Error Propagation and Interception
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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).
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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 unhandled exception and crash unless either: The domain
module is used appropriately or a handler has been registered for the
process.on('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 (sometimes referred to as a Node.js style 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);
} 
 
Error-first callbacks
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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);
}
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.
Class: Error
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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
new Error(message)
#
Error.captureStackTrace(targetObject[,
constructorOpt])
#
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// the constructor, we omit that frame, and retain all frames below it.
new MyError().stack;
<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. 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>
Error.stackTraceLimit
#
error.code
#
error.message
#
error.stack
#
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The error.stack  property is a string describing the point in the code at which the Error  was
instantiated.
For example:
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() { 
  // 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)
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//       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. Such errors commonly indicate
inequality of actual and expected value.
For example:
assert.strictEqual(1, 2);
// AssertionError [ERR_ASSERTION]: 1 === 2
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
Class: AssertionError
#
Class: RangeError
#
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given function parameter.
For example:
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.
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.
Class: ReferenceError
#
Class: SyntaxError
#
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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.
System errors are generated when exceptions occur within the program's runtime environment.
Typically, these are operational errors that occur when an application violates an operating system
constraint such as attempting to read a file that does not exist or when the user does not have
sufficient permissions.
System errors are typically generated at the syscall level: an exhaustive list of error codes and their
meanings is available by running man 2 intro  or man 3 errno  on most Unices; or online .
In Node.js, system errors are represented as augmented Error  objects with added properties.
<string>
Class: TypeError
#
Exceptions vs. Errors
#
System Errors
#
Class: System Error
#
error.code
#
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The error.code  property is a string representing the error code, which is typically E  followed by a
sequence of capital letters.
<string>  | <number>
The error.errno  property is a number or a string. The number is a negative value which
corresponds to the error code defined in libuv Error handling . See uv-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 .
<string>
The error.syscall  property is a string describing the syscall  that failed.
<string>
When present (e.g. in fs  or child_process ), the error.path  property is a string containing a
relevant invalid pathname.
<string>
When present (e.g. in net  or dgram ), the error.address  property is a string describing the
address to which the connection failed.
<number>
When present (e.g. in net  or dgram ), the error.port  property is a number representing the
connection's port that is not available.
This list is not exhaustive, but enumerates many of the common system errors encountered when
writing a Node.js program. An exhaustive list may be found here .
error.errno
#
error.syscall
#
error.path
#
error.address
#
error.port
#
Common System Errors
#
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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.
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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.
An iterable argument (i.e. a value that works with for...of  loops) was required, but not provided
to a Node.js API.
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 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 .
Node.js Error Codes
#
ERR_ARG_NOT_ITERABLE
#
ERR_ASYNC_CALLBACK
#
ERR_ASYNC_TYPE
#
ERR_ENCODING_INVALID_ENCODED_DATA
#
ERR_ENCODING_NOT_SUPPORTED
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A Promise  that was callbackified via util.callbackify()  was rejected with a falsy value.
An attempt was made to add more headers after the headers had already been sent.
An invalid character was found in an HTTP response status message (reason phrase).
Status code was outside the regular status code range (100-999).
The Trailer  header was set even though the transfer encoding does not support that.
Occurs with multiple attempts to shutdown an HTTP/2 session.
HTTP/2 ALTSVC frames require a valid origin.
ERR_FALSY_VALUE_REJECTION
#
ERR_HTTP_HEADERS_SENT
#
ERR_HTTP_INVALID_CHAR
#
ERR_HTTP_INVALID_STATUS_CODE
#
ERR_HTTP_TRAILER_INVALID
#
ERR_HTTP2_ALREADY_SHUTDOWN
#
ERR_HTTP2_ALTSVC_INVALID_ORIGIN
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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 failure occurred sending an individual frame on the HTTP/2 session.
New HTTP/2 Streams may not be opened after the Http2Session  has received a GOAWAY  frame
from the connected peer.
ERR_HTTP2_ALTSVC_LENGTH
#
ERR_HTTP2_CONNECT_AUTHORITY
#
ERR_HTTP2_CONNECT_PATH
#
ERR_HTTP2_CONNECT_SCHEME
#
ERR_HTTP2_FRAME_ERROR
#
ERR_HTTP2_GOAWAY_SESSION
#
ERR_HTTP2_HEADER_REQUIRED
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A required header was missing in an HTTP/2 message.
Multiple values were provided for an HTTP/2 header field that was required to have only a single
value.
An additional headers was specified after an HTTP/2 response was initiated.
An HTTP/2 Headers Object was expected.
An attempt was made to send multiple response headers.
HTTP/2 Informational headers must only be sent prior to calling the
Http2Stream.prototype.respond()  method.
Informational HTTP status codes ( 1xx ) may not be set as the response status code on HTTP/2
responses.
ERR_HTTP2_HEADER_SINGLE_VALUE
#
ERR_HTTP2_HEADERS_AFTER_RESPOND
#
ERR_HTTP2_HEADERS_OBJECT
#
ERR_HTTP2_HEADERS_SENT
#
ERR_HTTP2_INFO_HEADERS_AFTER_RESPOND
#
ERR_HTTP2_INFO_STATUS_NOT_ALLOWED
#
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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.
ERR_HTTP2_INVALID_CONNECTION_HEADERS
#
ERR_HTTP2_INVALID_HEADER_VALUE
#
ERR_HTTP2_INVALID_INFO_STATUS
#
ERR_HTTP2_INVALID_ORIGIN
#
ERR_HTTP2_INVALID_PACKED_SETTINGS_LENGT
H
#
ERR_HTTP2_INVALID_PSEUDOHEADER
#
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An action was performed on an Http2Session  object that had already been destroyed.
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 .
ERR_HTTP2_INVALID_SESSION
#
ERR_HTTP2_INVALID_SETTING_VALUE
#
ERR_HTTP2_INVALID_STREAM
#
ERR_HTTP2_MAX_PENDING_SETTINGS_ACK
#
ERR_HTTP2_NESTED_PUSH
#
ERR_HTTP2_NO_SOCKET_MANIPULATION
#
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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.
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.
ERR_HTTP2_ORIGIN_LENGTH
#
ERR_HTTP2_OUT_OF_STREAMS
#
ERR_HTTP2_PAYLOAD_FORBIDDEN
#
ERR_HTTP2_PING_CANCEL
#
ERR_HTTP2_PING_LENGTH
#
ERR_HTTP2_PSEUDOHEADER_NOT_ALLOWED
#
ERR_HTTP2_PUSH_DISABLED
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An attempt was made to use the Http2Stream.prototype.responseWithFile()  API to send
something other than a regular file.
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_SEND_FILE
#
ERR_HTTP2_SESSION_ERROR
#
ERR_HTTP2_SETTINGS_CANCEL
#
ERR_HTTP2_SOCKET_BOUND
#
ERR_HTTP2_SOCKET_UNBOUND
#
ERR_HTTP2_STATUS_101
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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 .
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
#
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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).
An argument of the wrong type was passed to a Node.js API.
An invalid asyncId  or triggerAsyncId  was passed using AsyncHooks . An id less than -1 should
never happen.
A callback function was required but was not been provided to a Node.js API.
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.
ERR_HTTP2_UNSUPPORTED_PROTOCOL
#
ERR_INDEX_OUT_OF_RANGE
#
ERR_INVALID_ARG_TYPE
#
ERR_INVALID_ASYNC_ID
#
ERR_INVALID_CALLBACK
#
ERR_INVALID_FILE_URL_HOST
#
ERR_INVALID_FILE_URL_PATH
#
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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 or unexpected value was passed in an options object.
While using the Performance Timing API ( perf_hooks ), a performance mark is invalid.
An invalid options.protocol  was passed.
A Buffer , Uint8Array  or string  was provided as stdio input to a synchronous fork. See the
documentation for the child_process  module for more information.
A Node.js API function was called with an incompatible this  value.
ERR_INVALID_HANDLE_TYPE
#
ERR_INVALID_OPT_VALUE
#
ERR_INVALID_PERFORMANCE_MARK
#
ERR_INVALID_PROTOCOL
#
ERR_INVALID_SYNC_FORK_INPUT
#
ERR_INVALID_THIS
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Example:
const { URLSearchParams } = require('url');
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 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 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_TUPLE
#
ERR_INVALID_URL
#
ERR_INVALID_URL_SCHEME
#
ERR_IPC_CHANNEL_CLOSED
#
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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.
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
Used when an [ES6 module][] loader hook specifies format: 'dynamic  but does not provide a
dynamicInstantiate  hook.
ERR_IPC_DISCONNECTED
#
ERR_IPC_ONE_PIPE
#
ERR_IPC_SYNC_FORK
#
ERR_MISSING_ARGS
#
ERR_MISSING_DYNAMIC_INSTANTIATE_HOOK
#
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Stability: 1  - Experimental
Used when an [ES6 module][] cannot be resolved.
Stability: 1  - Experimental
Used when a failure occurred resolving imports in an [ES6 module][].
A callback was called more than once.
Note: 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 using N-API , Constructor.prototype  was not an object.
ERR_MISSING_MODULE
#
ERR_MODULE_RESOLUTION_LEGACY
#
ERR_MULTIPLE_CALLBACK
#
ERR_NAPI_CONS_FUNCTION
#
ERR_NAPI_CONS_PROTOTYPE_OBJECT
#
ERR_NAPI_INVALID_DATAVIEW_ARGS
#
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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.
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 attemping to start the loop.
ERR_NAPI_INVALID_TYPEDARRAY_ALIGNMENT
#
ERR_NAPI_INVALID_TYPEDARRAY_LENGTH
#
ERR_NAPI_TSFN_CALL_JS
#
ERR_NAPI_TSFN_GET_UNDEFINED
#
ERR_NAPI_TSFN_START_IDLE_LOOP
#
ERR_NAPI_TSFN_STOP_IDLE_LOOP
#
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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 features that require ICU , but Node.js was not compiled with ICU
support.
An attempt was made to bind a socket that has already been bound.
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.
Data could be sent on a socket.
An attempt was made to operate on an already closed socket.
ERR_NO_ICU
#
ERR_SOCKET_ALREADY_BOUND
#
ERR_SOCKET_BAD_PORT
#
ERR_SOCKET_BAD_TYPE
#
ERR_SOCKET_CANNOT_SEND
#
ERR_SOCKET_CLOSED
#
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A call was made and the UDP subsystem was not running.
History
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.
History
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.
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.
ERR_SOCKET_DGRAM_NOT_RUNNING
#
ERR_STDERR_CLOSE
#
ERR_STDOUT_CLOSE
#
ERR_TLS_CERT_ALTNAME_INVALID
#
ERR_TLS_DH_PARAM_SIZE
#
ERR_TLS_HANDSHAKE_TIMEOUT
#
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A TLS/SSL handshake timed out. In this case, the server must also abort the connection.
A TLS renegotiation request has failed in a non-specific way.
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.
A Transform stream finished while it was still transforming.
A Transform stream finished with data still in the write buffer.
An invalid or unknown process signal was passed to an API expecting a valid signal (such as
subprocess.kill() ).
ERR_TLS_RENEGOTIATION_FAILED
#
ERR_TLS_REQUIRED_SERVER_NAME
#
ERR_TLS_SESSION_ATTACK
#
ERR_TRANSFORM_ALREADY_TRANSFORMING
#
ERR_TRANSFORM_WITH_LENGTH_0
#
ERR_UNKNOWN_SIGNAL
#
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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 given value is out of the accepted range.
Stability: 2  - Stable
ERR_UNKNOWN_STDIN_TYPE
#
ERR_UNKNOWN_STREAM_TYPE
#
ERR_V8BREAKITERATOR
#
ERR_VALID_PERFORMANCE_ENTRY_TYPE
#
ERR_VALUE_OUT_OF_RANGE
#
Events
#
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Much of the Node.js core API is built around an idiomatic asynchronous event-driven architecture
in which certain kinds of objects (called "emitters") periodically 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
by the EventEmitter , the standard this  keyword is intentionally set to reference the
EventEmitter  to which the listener is attached.
Passing arguments and this  to listeners
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const myEmitter = new MyEmitter(); 
myEmitter.on('event', function(a, b) { 
  console.log(a, b, this); 
  // Prints: 
  //   a b MyEmitter { 
  //     domain: null, 
  //     _events: { event: [Function] }, 
  //     _eventsCount: 1, 
  //     _maxListeners: undefined }
}); 
myEmitter.emit('event', 'a', 'b');
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');
Asynchronous vs. Synchronous
#
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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
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.
Handling events only once
#
Error events
#
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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 has been 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
Added in: v0.1.26
The EventEmitter  class is defined and exposed by the events  module:
const EventEmitter = require('events');
All EventEmitters emit the event 'newListener'  when new listeners are added and
'removeListener'  when existing listeners are removed.
Added in: v0.1.26
eventName  <any> 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.
Class: EventEmitter
#
Event: 'newListener'
#
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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'); 
    }); 
  }
}); 
myEmitter.on('event', () => { 
  console.log('A');
}); 
myEmitter.emit('event');
// Prints:
//   B
//   A
History
eventName  <any> The event name
listener  <Function>  The event handler function
The 'removeListener'  event is emitted after the listener  is removed.
Added in: v0.9.12 Deprecated since: v4.0.0
Stability: 0  - Deprecated: Use emitter.listenerCount()  instead.
Event: 'removeListener'
#
EventEmitter.listenerCount(emitter, eventName)
#
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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
Added in: v0.11.2
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));
});
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
EventEmitter.defaultMaxListeners
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and the number of attached listeners, respectively. Its name  property is set to
'MaxListenersExceededWarning' .
Added in: v0.1.26
eventName  <any>
listener  <Function>
Alias for emitter.on(eventName, listener) .
Added in: v0.1.26
eventName  <any>
...args  <any>
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.
Added in: v6.0.0
Returns an array listing the events for which the emitter has registered listeners. The values in the
array will be strings or Symbols.
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) ]
emitter.addListener(eventName, listener)
#
emitter.emit(eventName[, ...args])
#
emitter.eventNames()
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Added in: v1.0.0
Returns the current max listener value for the EventEmitter  which is either set by
emitter.setMaxListeners(n)  or defaults to EventEmitter.defaultMaxListeners .
Added in: v3.2.0
eventName  <any> The name of the event being listened for
Returns the number of listeners listening to the event named eventName .
History
eventName  <any>
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] ]
Added in: v0.1.101
eventName  <any> The name of the event.
listener  <Function>  The callback function
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.
emitter.getMaxListeners()
#
emitter.listenerCount(eventName)
#
emitter.listeners(eventName)
#
emitter.on(eventName, listener)
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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
Added in: v0.3.0
eventName  <any> The name of the event.
listener  <Function>  The callback function
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.
emitter.once(eventName, listener)
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const myEE = new EventEmitter(); 
myEE.once('foo', () => console.log('a')); 
myEE.prependOnceListener('foo', () => console.log('b')); 
myEE.emit('foo');
// Prints:
//   b
//   a
Added in: v6.0.0
eventName  <any> The name of the event.
listener  <Function>  The callback function
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.
Added in: v6.0.0
eventName  <any> The name of the event.
listener  <Function>  The callback function
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.
server.prependOnceListener('connection', (stream) => { 
  console.log('Ah, we have our first user!');
});
emitter.prependListener(eventName, listener)
#
emitter.prependOnceListener(eventName, listener) #
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Returns a reference to the EventEmitter , so that calls can be chained.
Added in: v0.1.26
eventName  <any>
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.
Added in: v0.1.26
eventName  <any>
listener  <Function>
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.
emitter.removeAllListeners([eventName])
#
emitter.removeListener(eventName, listener)
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const myEmitter = new MyEmitter(); 
 
const callbackA = () => { 
  console.log('A'); 
  myEmitter.removeListener('event', callbackB);
}; 
 
const callbackB = () => { 
  console.log('B');
}; 
 
myEmitter.on('event', callbackA); 
 
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(); 
 
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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.
Added in: v0.3.5
n  <integer>
By default EventEmitters 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.
Stability: 2  - Stable
File I/O is provided by simple wrappers around standard POSIX functions. To use this module do
require('fs') . All the methods have asynchronous and synchronous 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 .
emitter.setMaxListeners(n)
#
File System
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When using the synchronous form any exceptions are immediately thrown. Exceptions may be
handled using try / catch , or they may be allowed to bubble up.
Here is an example of the asynchronous version:
const fs = require('fs'); 
 
fs.unlink('/tmp/hello', (err) => { 
  if (err) throw err; 
  console.log('successfully deleted /tmp/hello');
});
Here is the synchronous version:
const fs = require('fs'); 
 
fs.unlinkSync('/tmp/hello'); 
console.log('successfully deleted /tmp/hello');
With the asynchronous methods there is no guaranteed ordering. So the following is prone to
error:
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; 
  console.log(`stats: ${JSON.stringify(stats)}`);
});
It could be that fs.stat  is executed before fs.rename . The correct way to do this is to chain the
callbacks.
fs.rename('/tmp/hello', '/tmp/world', (err) => { 
  if (err) throw err; 
  fs.stat('/tmp/world', (err, stats) => { 
    if (err) throw err; 
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    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.
The relative path to a filename can be used. Remember, however, that this path will be relative to
process.cwd() .
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:
Note: 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.>
Note: 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 .
Note: On Windows, opening an existing hidden file using the w  flag (either through fs.open  or
fs.writeFile ) will fail with EPERM . Existing hidden files can be opened for writing with the r+
flag. A call to fs.ftruncate  can be used to reset the file contents.
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Note that 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.
Added in: v7.6.0
Stability: 1  - Experimental
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 { URL } = require('url');
const fileUrl = new URL('file:///tmp/hello'); 
 
fs.readFileSync(fileUrl);
Note: 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')); 
Threadpool Usage
#
WHATWG URL object support
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// - 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
Note: 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:
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// 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 */
Added in: v6.0.0
fs  functions support passing and receiving paths as both strings and Buffers. The latter is
intended to make it possible to work with filesystems that allow for non-UTF-8 filenames. For most
typical uses, working with paths as Buffers will be unnecessary, as the string API converts to and
from UTF-8 automatically.
Note: On certain file systems (such as NTFS and HFS+) filenames will always be encoded as UTF-8.
On such file systems, passing non-UTF-8 encoded Buffers to fs  functions will not work as
expected.
Added in: v0.5.8
Objects returned from fs.watch()  are of this type.
The listener  callback provided to fs.watch()  receives the returned FSWatcher's change
events.
The object itself emits these events:
Added in: v0.5.8
eventType  <string>  The type of fs change
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 API
#
Class: fs.FSWatcher
#
Event: 'change'
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'buffer' , otherwise filename  will be a string.
// Example when handled through fs.watch listener 
fs.watch('./tmp', { encoding: 'buffer' }, (eventType, filename) => { 
  if (filename) { 
    console.log(filename); 
    // Prints: <Buffer ...> 
  }
});
Added in: v0.5.8
error  <Error>
Emitted when an error occurs.
Added in: v0.5.8
Stop watching for changes on the given fs.FSWatcher .
Added in: v0.1.93
ReadStream  is a Readable Stream .
Added in: v0.1.93
Emitted when the ReadStream 's underlying file descriptor has been closed.
Added in: v0.1.93
fd  <integer>  Integer file descriptor used by the ReadStream.
Event: 'error'
#
watcher.close()
#
Class: fs.ReadStream
#
Event: 'close'
#
Event: 'open'
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Emitted when the ReadStream's file is opened.
Added in: 6.4.0
The number of bytes read so far.
Added in: v0.1.93
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 .
History
Objects returned from fs.stat() , fs.lstat()  and fs.fstat()  and their synchronous
counterparts are of this type.
stats.isFile()
stats.isDirectory()
stats.isBlockDevice()
stats.isCharacterDevice()
stats.isSymbolicLink()  (only valid with fs.lstat() )
stats.isFIFO()
stats.isSocket()
For a regular file util.inspect(stats)  would return a string very similar to this:
Stats { 
  dev: 2114, 
  ino: 48064969, 
  mode: 33188, 
  nlink: 1, 
  uid: 85, 
readStream.bytesRead
#
readStream.path
#
Class: fs.Stats
#
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  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 }
Note: atimeMs , mtimeMs , ctimeMs , birthtimeMs  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 ). Note that 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 v0.12, the ctime  held the birthtime  on Windows systems. Note that as of v0.12,
ctime  is not "creation time", and on Unix systems, it never was.
Stat Time Values
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Added in: v0.1.93
WriteStream  is a Writable Stream .
Added in: v0.1.93
Emitted when the WriteStream 's underlying file descriptor has been closed.
Added in: v0.1.93
fd  <integer>  Integer file descriptor used by the WriteStream.
Emitted when the WriteStream's file is opened.
Added in: v0.4.7
The number of bytes written so far. Does not include data that is still queued for writing.
Added in: v0.1.93
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 .
History
path  <string>  | <Buffer>  | <URL>
mode  <integer>  Default: fs.constants.F_OK
callback  <Function>
err  <Error>
Class: fs.WriteStream
#
Event: 'close'
#
Event: 'open'
#
writeStream.bytesWritten
#
writeStream.path
#
fs.access(path[, mode], callback)
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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. The following constants
define the 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 ).
fs.constants.F_OK  - path  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.
fs.constants.R_OK  - path  can be read by the calling process.
fs.constants.W_OK  - path  can be written by the calling process.
fs.constants.X_OK  - path  can be executed by the calling process. This has no effect on
Windows (will behave like fs.constants.F_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
example checks if the file /etc/passwd  can be read and written by the current process.
fs.access('/etc/passwd', fs.constants.R_OK | fs.constants.W_OK, (err) => { 
  console.log(err ? 'no access!' : 'can read/write');
});
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
open/read/write the file directly and handle the error raised if the file is not accessible.
For example:
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); 
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  });
});
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)
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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.
History
path  <string>  | <Buffer>  | <URL>
mode  <integer>  Default: fs.constants.F_OK
Returns: <undefined>
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. The
following constants define the 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 ).
fs.constants.F_OK  - path  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.
fs.constants.R_OK  - path  can be read by the calling process.
fs.accessSync(path[, mode])
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fs.constants.W_OK  - path  can be written by the calling process.
fs.constants.X_OK  - path  can be executed by the calling process. This has no effect on
Windows (will behave like fs.constants.F_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!');
}
History
file  <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>  Default: 'a'
callback  <Function>
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 .
Example:
fs.appendFile('message.txt', 'data to append', (err) => { 
  if (err) throw err; 
  console.log('The "data to append" was appended to file!');
});
fs.appendFile(file, data[, options], callback)
#
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If options  is a string, then it specifies the encoding. Example:
fs.appendFile('message.txt', 'data to append', 'utf8', callback);
The file  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; 
  });
});
History
file  <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>  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 .
Example:
try { 
  fs.appendFileSync('message.txt', 'data to append'); 
  console.log('The "data to append" was appended to file!');
} catch (err) { 
fs.appendFileSync(file, data[, options])
#
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  /* Handle the error */
}
If options  is a string, then it specifies the encoding. Example:
fs.appendFileSync('message.txt', 'data to append', 'utf8');
The file  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);
}
History
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)
fs.chmod(path, mode, callback)
#
File modes
#
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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
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
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Number
Description
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.
History
path  <string>  | <Buffer>  | <URL>
mode  <integer>
Synchronously changes the permissions of a file. Returns undefined . This is the synchronous
version of fs.chmod() .
See also: chmod(2)
History
path  <string>  | <Buffer>  | <URL>
uid  <integer>
gid  <integer>
callback  <Function>
err  <Error>
fs.chmodSync(path, mode)
#
fs.chown(path, uid, gid, callback)
#
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Asynchronously changes owner and group of a file. No arguments other than a possible exception
are given to the completion callback.
See also: chown(2)
History
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)
History
fd  <integer>
callback  <Function>
err  <Error>
Asynchronous close(2) . No arguments other than a possible exception are given to the completion
callback.
Added in: v0.1.21
fd  <integer>
Synchronous close(2) . Returns undefined .
Returns an object containing commonly used constants for file system operations. The specific
constants currently defined are described in FS Constants .
fs.chownSync(path, uid, gid)
#
fs.close(fd, callback)
#
fs.closeSync(fd)
#
fs.constants
#
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Added in: v8.5.0
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. The only supported
flag is fs.constants.COPYFILE_EXCL , which causes the copy operation to fail if dest  already
exists.
Example:
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 , as shown in the following example.
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);
Added in: v8.5.0
fs.copyFile(src, dest[, flags], callback)
#
fs.copyFileSync(src, dest[, flags])
#
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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. The only supported
flag is fs.constants.COPYFILE_EXCL , which causes the copy operation to fail if dest  already
exists.
Example:
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 , as shown in the following example.
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);
History
path  <string>  | <Buffer>  | <URL>
options  <string>  | <Object>
flags  <string>
encoding  <string>
fs.createReadStream(path[, options])
#
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fd  <integer>
mode  <integer>
autoClose  <boolean>
start  <integer>
end  <integer>
highWaterMark  <integer>
Returns a new ReadStream  object. (See Readable Stream ).
Be aware that, unlike the default value set for highWaterMark  on a readable stream (16 kb), the
stream returned by this method has a default value of 64 kb for the same parameter.
options  is an object or string with the following defaults:
const defaults = { 
  flags: 'r', 
  encoding: null, 
  fd: null, 
  mode: 0o666, 
  autoClose: true, 
  highWaterMark: 64 * 1024
};
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. Note that fd  should be blocking;
non-blocking fd s should be passed to net.Socket .
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:
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fs.createReadStream('sample.txt', { start: 90, end: 99 });
If options  is a string, then it specifies the encoding.
History
path  <string>  | <Buffer>  | <URL>
options  <string>  | <Object>
flags  <string>
encoding  <string>
fd  <integer>
mode  <integer>
autoClose  <boolean>
start  <integer>
Returns a new WriteStream  object. (See Writable Stream ).
options  is an object or string with the following defaults:
const defaults = { 
  flags: 'w', 
  encoding: 'utf8', 
  fd: null, 
  mode: 0o666, 
  autoClose: true
};
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 end  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.
fs.createWriteStream(path[, options])
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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. Note that fd  should be
blocking; non-blocking fd s should be passed to net.Socket .
If options  is a string, then it specifies the encoding.
History
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. Example:
fs.exists('/etc/passwd', (exists) => { 
  console.log(exists ? 'it\'s there' : 'no passwd!');
});
Note that the parameter to this callback is 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.
For example:
write (NOT RECOMMENDED)
fs.exists(path, callback)
#
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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'); 
  }
});
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read (RECOMMENDED)
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 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.
History
path  <string>  | <Buffer>  | <URL>
Synchronous version of fs.exists() . Returns true  if the path exists, false  otherwise.
Note that 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.)
History
fd  <integer>
mode  <integer>
fs.existsSync(path)
#
fs.fchmod(fd, mode, callback)
#
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callback  <Function>
err  <Error>
Asynchronous fchmod(2) . No arguments other than a possible exception are given to the
completion callback.
Added in: v0.4.7
fd  <integer>
mode  <integer>
Synchronous fchmod(2) . Returns undefined .
History
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.
Added in: v0.4.7
fd  <integer>
uid  <integer>
gid  <integer>
Synchronous fchown(2) . Returns undefined .
History
fs.fchmodSync(fd, mode)
#
fs.fchown(fd, uid, gid, callback)
#
fs.fchownSync(fd, uid, gid)
#
fs.fdatasync(fd, callback)
#
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fd  <integer>
callback  <Function>
err  <Error>
Asynchronous fdatasync(2) . No arguments other than a possible exception are given to the
completion callback.
Added in: v0.1.96
fd  <integer>
Synchronous fdatasync(2) . Returns undefined .
History
fd  <integer>
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 .
Added in: v0.1.95
fd  <integer>
Synchronous fstat(2) . Returns an instance of fs.Stats .
History
fd  <integer>
fs.fdatasyncSync(fd)
#
fs.fstat(fd, callback)
#
fs.fstatSync(fd)
#
fs.fsync(fd, callback)
#
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callback  <Function>
err  <Error>
Asynchronous fsync(2) . No arguments other than a possible exception are given to the completion
callback.
Added in: v0.1.96
fd  <integer>
Synchronous fsync(2) . Returns undefined .
History
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'));
fs.fsyncSync(fd)
#
fs.ftruncate(fd[, len], callback)
#
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});
// Prints: Node
If the file previously was shorter than len  bytes, it is extended, and the extended part is filled with
null bytes ('\0'). For example,
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.
Added in: v0.8.6
fd  <integer>
len  <integer>  Default: 0
Synchronous ftruncate(2) . Returns undefined .
History
fd  <integer>
atime  <number>  | <string>  | <Date>
mtime  <number>  | <string>  | <Date>
fs.ftruncateSync(fd[, len])
#
fs.futimes(fd, atime, mtime, callback)
#
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callback  <Function>
err  <Error>
Change the file system timestamps of the object referenced by the supplied file descriptor. See
fs.utimes() .
Note: This function does not work on AIX versions before 7.1, it will return the error UV_ENOSYS .
History
fd  <integer>
atime  <integer>
mtime  <integer>
Synchronous version of fs.futimes() . Returns undefined .
History
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.
Deprecated since: v0.4.7
path  <string>  | <Buffer>  | <URL>
mode  <integer>
Synchronous lchmod(2) . Returns undefined .
fs.futimesSync(fd, atime, mtime)
#
fs.lchmod(path, mode, callback)
#
fs.lchmodSync(path, mode)
#
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History
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.
Deprecated since: v0.4.7
path  <string>  | <Buffer>  | <URL>
uid  <integer>
gid  <integer>
Synchronous lchown(2) . Returns undefined .
History
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.
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fs.lchown(path, uid, gid, callback)
#
fs.lchownSync(path, uid, gid)
#
fs.link(existingPath, newPath, callback)
#
fs.linkSync(existingPath, newPath)
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existingPath  <string>  | <Buffer>  | <URL>
newPath  <string>  | <Buffer>  | <URL>
Synchronous link(2) . Returns undefined .
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path  <string>  | <Buffer>  | <URL>
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.
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path  <string>  | <Buffer>  | <URL>
Synchronous lstat(2) . Returns an instance of fs.Stats .
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path  <string>  | <Buffer>  | <URL>
mode  <integer>  Default: 0o777
callback  <Function>
err  <Error>
Asynchronously creates a directory. No arguments other than a possible exception are given to the
completion callback.
See also: mkdir(2)
fs.lstat(path, callback)
#
fs.lstatSync(path)
#
fs.mkdir(path[, mode], callback)
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path  <string>  | <Buffer>  | <URL>
mode  <integer>  Default: 0o777
Synchronously creates a directory. Returns undefined . This is the synchronous version of
fs.mkdir() .
See also: mkdir(2)
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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.
The optional options  argument can be a string specifying an encoding, or an object with an
encoding  property specifying the character encoding to use.
Example:
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
});
fs.mkdirSync(path[, mode])
#
fs.mkdtemp(prefix[, options], callback)
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Note: 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`. 
  // Note that 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.
});
Added in: v5.10.0
prefix  <string>
options  <string>  | <Object>
encoding  <string>  Default: 'utf8'
The synchronous version of fs.mkdtemp() . Returns the created folder path.
fs.mkdtempSync(prefix[, options])
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The optional options  argument can be a string specifying an encoding, or an object with an
encoding  property specifying the character encoding to use.
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path  <string>  | <Buffer>  | <URL>
flags  <string>  | <number>
mode  <integer>  Default: 0o666  (readable and writable)
callback  <Function>
err  <Error>
fd  <integer>
Asynchronous file open. See open(2) . flags  can be:
'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.
Note that this doesn't turn fs.open()  into a synchronous blocking call. If synchronous
operation is desired 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 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 path  exists.
'a'  - Open file for appending. The file is created if it does not exist.
fs.open(path, flags[, mode], callback)
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'ax'  - Like 'a'  but fails if path  exists.
'as'  - Open file for appending in synchronous mode. The file is created if it does not exist.
'a+'  - Open file for reading and appending. The file is created if it does not exist.
'ax+'  - Like 'a+'  but fails if path  exists.
'as+'  - Open file for reading and appending in synchronous mode. The file is created if it does
not exist.
mode  sets the file mode (permission and sticky bits), but only if the file was created.
The callback gets two arguments (err, fd) .
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.
flags  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.
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.
Note: The behavior of fs.open()  is platform-specific for some flags. 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 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>
});
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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. eg. fs.writeFile() , fs.readFile() ,
etc.
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path  <string>  | <Buffer>  | <URL>
flags  <string>  | <number>
mode  <integer>  Default: 0o666
Synchronous version of fs.open() . Returns an integer representing the file descriptor.
History
fd  <integer>
buffer  <Buffer>  | <Uint8Array>
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.
fs.openSync(path, flags[, mode])
#
fs.read(fd, buffer, offset, length, position,
callback)
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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.
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.
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path  <string>  | <Buffer>  | <URL>
options  <string>  | <Object>
encoding  <string>  Default: 'utf8'
callback  <Function>
err  <Error>
files  <string[]>  | <Buffer[]>
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.
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path  <string>  | <Buffer>  | <URL>
options  <string>  | <Object>
encoding  <string>  Default: 'utf8'
Synchronous readdir(3) . Returns an array of filenames excluding '.'  and '..' .
fs.readdir(path[, options], callback)
#
fs.readdirSync(path[, options])
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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.
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path  <string>  | <Buffer>  | <URL>  | <integer>  filename or file descriptor
options  <Object>  | <string>
encoding  <string>  | <null>  Default: null
flag  <string>  Default: 'r'
callback  <Function>
err  <Error>
data  <string>  | <Buffer>
Asynchronously reads the entire contents of a file. Example:
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. Example:
fs.readFile('/etc/passwd', 'utf8', callback);
Note: 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.
fs.readFile(path[, options], callback)
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// 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>
});
Any specified file descriptor has to support reading.
Note: If a file descriptor is specified as the path , it will not be closed automatically.
Note: fs.readFile()  reads the entire file in a single threadpool request. To minimize threadpool
task length variation, prefer the partitioned APIs fs.read()  and fs.createReadStream()  when
reading files as part of fulfilling a client request.
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path  <string>  | <Buffer>  | <URL>  | <integer>  filename or file descriptor
options  <Object>  | <string>
encoding  <string>  | <null>  Default: null
flag  <string>  Default: 'r'
Synchronous version of fs.readFile() . Returns the contents of the path .
If the encoding  option is specified then this function returns a string. Otherwise it returns a buffer.
Note: 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>] 
 
fs.readFileSync(path[, options])
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//  FreeBSD 
fs.readFileSync('<directory>'); // => null, <data>
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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.
History
path  <string>  | <Buffer>  | <URL>
options  <string>  | <Object>
encoding  <string>  Default: 'utf8'
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 passed to the
callback. If the encoding  is set to 'buffer' , the link path returned will be passed as a Buffer
object.
fs.readlink(path[, options], callback)
#
fs.readlinkSync(path[, options])
#
fs.readSync(fd, buffer, offset, length,
position)
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History
fd  <integer>
buffer  <Buffer>  | <Uint8Array>
offset  <integer>
length  <integer>
position  <integer>
Synchronous version of fs.read() . Returns the number of bytesRead .
History
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) . 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.
Note: If path  resolves to a socket or a pipe, the function will return a system dependent name for
that object.
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path  <string>  | <Buffer>  | <URL>
fs.realpath(path[, options], callback)
#
fs.realpathSync(path[, options])
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options  <string>  | <Object>
encoding  <string>  Default: 'utf8'
Synchronous realpath(3) . Returns 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 returned value. If the
encoding  is set to 'buffer' , the path returned will be passed as a Buffer  object.
Note: If path  resolves to a socket or a pipe, the function will return a system dependent name for
that object.
History
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!');
});
History
oldPath  <string>  | <Buffer>  | <URL>
fs.rename(oldPath, newPath, callback)
#
fs.renameSync(oldPath, newPath)
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newPath  <string>  | <Buffer>  | <URL>
Synchronous rename(2) . Returns undefined .
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path  <string>  | <Buffer>  | <URL>
callback  <Function>
err  <Error>
Asynchronous rmdir(2) . No arguments other than a possible exception are given to the
completion callback.
Note: Using fs.rmdir()  on a file (not a directory) results in an ENOENT  error on Windows and an
ENOTDIR  error on POSIX.
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path  <string>  | <Buffer>  | <URL>
Synchronous rmdir(2) . Returns undefined .
Note: Using fs.rmdirSync()  on a file (not a directory) results in an ENOENT  error on Windows and
an ENOTDIR  error on POSIX.
History
path  <string>  | <Buffer>  | <URL>
callback  <Function>
err  <Error>
stats  <fs.Stats>
Asynchronous stat(2) . The callback gets two arguments (err, stats)  where stats  is an
fs.Stats  object.
fs.rmdir(path, callback)
#
fs.rmdirSync(path)
#
fs.stat(path, callback)
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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.
History
path  <string>  | <Buffer>  | <URL>
Synchronous stat(2) . Returns an instance of fs.Stats .
History
target  <string>  | <Buffer>  | <URL>
path  <string>  | <Buffer>  | <URL>
type  <string>  Default: 'file'
callback  <Function>
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). Note that 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".
fs.statSync(path)
#
fs.symlink(target, path[, type], callback)
#
fs.symlinkSync(target, path[, type])
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History
target  <string>  | <Buffer>  | <URL>
path  <string>  | <Buffer>  | <URL>
type  <string>  Default: 'file'
Synchronous symlink(2) . Returns undefined .
History
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.
Added in: v0.8.6
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.
History
path  <string>  | <Buffer>  | <URL>
callback  <Function>
err  <Error>
fs.truncate(path[, len], callback)
#
fs.truncateSync(path[, len])
#
fs.unlink(path, callback)
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Asynchronously removes a file or symbolic link. No arguments other than a possible exception are
given to the completion callback.
// 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)
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path  <string>  | <Buffer>  | <URL>
Synchronous unlink(2) . Returns undefined .
Added in: v0.1.31
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.
Note: 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.
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fs.unlinkSync(path)
#
fs.unwatchFile(filename[, listener])
#
fs.utimes(path, atime, mtime, callback)
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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 , a Error  will
be thrown.
History
path  <string>  | <Buffer>  | <URL>
atime  <integer>
mtime  <integer>
Synchronous version of fs.utimes() . Returns undefined .
History
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' .
fs.utimesSync(path, atime, mtime)
#
fs.watch(filename[, options][, listener])
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listener  <Function>  | <undefined>  Default: undefined
eventType  <string>
filename  <string>  | <Buffer>
Watch for changes on filename , where filename  is either a file or a directory. The returned
object is a fs.FSWatcher .
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.
Note that on most platforms, 'rename'  is emitted whenever a filename appears or disappears in
the directory.
Also note 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
On BSD systems, this uses kqueue
On macOS, this uses kqueue  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,
Caveats
#
Availability
#
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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.
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'); 
  }
});
History
filename  <string>  | <Buffer>  | <URL>
options  <Object>
persistent  <boolean>  Default: true
Inodes
#
Filename Argument
#
fs.watchFile(filename[, options], listener)
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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 .
Note: 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 again,
with the latest stat objects. This is a change in functionality since v0.10.
Note: 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.
Note: 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
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the file is renamed twice - the second time back to its original name
History
fd  <integer>
buffer  <Buffer>  | <Uint8Array>
offset  <integer>
length  <integer>
position  <integer>
callback  <Function>
err  <Error>
bytesWritten  <integer>
buffer  <Buffer>  | <Uint8Array>
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.
Note that it is unsafe to use fs.write  multiple times on the same file without waiting for the
callback. For this scenario, fs.createWriteStream  is strongly 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.
fs.write(fd, buffer[, offset[, length[,
position]]], callback)
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History
fd  <integer>
string  <string>
position  <integer>
encoding  <string>
callback  <Function>
err  <Error>
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. Note that bytes written is not the same as
string characters. See Buffer.byteLength .
Unlike when writing buffer , the entire string must be written. No substring may be specified. This
is because the byte offset of the resulting data may not be the same as the string offset.
Note that it is unsafe to use fs.write  multiple times on the same file without waiting for the
callback. For this scenario, fs.createWriteStream  is strongly 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.
History
fs.write(fd, string[, position[, encoding]],
callback)
#
fs.writeFile(file, data[, options], callback)
#
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file  <string>  | <Buffer>  | <URL>  | <integer>  filename or file descriptor
data  <string>  | <Buffer>  | <Uint8Array>
options  <Object>  | <string>
encoding  <string>  | <null>  Default: 'utf8'
mode  <integer>  Default: 0o666
flag  <string>  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.
Example:
fs.writeFile('message.txt', 'Hello Node.js', (err) => { 
  if (err) throw err; 
  console.log('The file has been saved!');
});
If options  is a string, then it specifies the encoding. Example:
fs.writeFile('message.txt', 'Hello Node.js', 'utf8', callback);
Any specified file descriptor has to support writing.
Note that it is unsafe to use fs.writeFile  multiple times on the same file without waiting for the
callback. For this scenario, fs.createWriteStream  is strongly recommended.
Note: If a file descriptor is specified as the file , it will not be closed automatically.
History
file  <string>  | <Buffer>  | <URL>  | <integer>  filename or file descriptor
data  <string>  | <Buffer>  | <Uint8Array>
fs.writeFileSync(file, data[, options])
#
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options  <Object>  | <string>
encoding  <string>  | <null>  Default: 'utf8'
mode  <integer>  Default: 0o666
flag  <string>  Default: 'w'
The synchronous version of fs.writeFile() . Returns undefined .
History
fd  <integer>
buffer  <Buffer>  | <Uint8Array>
offset  <integer>
length  <integer>
position  <integer>
History
fd  <integer>
string  <string>
position  <integer>
encoding  <string>
Synchronous versions of fs.write() . Returns the number of bytes written.
The following constants are exported by fs.constants .
Note: Not every constant will be available on every operating system.
fs.writeSync(fd, buffer[, offset[, length[,
position]]])
#
fs.writeSync(fd, string[, position[, encoding]]) #
FS Constants
#
File Access Constants
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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.
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.
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.
File Open Constants
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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.
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.
File Type Constants
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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.
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 :
__dirname
File Mode Constants
#
Global Objects
#
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__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.
Added in: v0.1.103
<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 .
Added in: v0.9.1
clearImmediate  is described in the timers  section.
Added in: v0.0.1
clearInterval  is described in the timers  section.
Added in: v0.0.1
clearTimeout  is described in the timers  section.
Class: Buffer
#
__dirname
#
__filename
#
clearImmediate(immediateObject)
#
clearInterval(intervalObject)
#
clearTimeout(timeoutObject)
#
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Added in: v0.1.100
<Object>
Used to print to stdout and stderr. See the console  section.
This variable may appear to be global but is not. See exports .
Added in: v0.1.27
<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.
This variable may appear to be global but is not. See module .
Added in: v0.1.7
<Object>
The process object. See the process object  section.
This variable may appear to be global but is not. See require() .
Added in: v0.9.1
console
#
exports
#
global
#
module
#
process
#
require()
#
setImmediate(callback[, ...args])
#
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setImmediate  is described in the timers  section.
Added in: v0.0.1
setInterval  is described in the timers  section.
Added in: v0.0.1
setTimeout  is described in the timers  section.
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:
{ '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.
setInterval(callback, delay[, ...args])
#
setTimeout(callback, delay[, ...args])
#
HTTP
#
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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', '*/*' ]
Added in: v0.3.4
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:
Class: http.Agent
#
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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.
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
});
Added in: v0.3.4
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. 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. 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 .
new Agent( options )
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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);
Added in: v0.11.4
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.
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) .
Added in: v8.1.0
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;
agent.createConnection(options[, callback])
#
agent.keepSocketAlive(socket)
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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.
Added in: v8.1.0
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.
Added in: v0.11.4
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.
Added in: v0.11.4
<Object>
An object which contains arrays of sockets currently awaiting use by the agent when keepAlive  is
enabled. Do not modify.
Added in: v0.11.4
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
agent.reuseSocket(socket, request)
#
agent.destroy()
#
agent.freeSockets
#
agent.getName(options)
#
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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 HTTPS/TLS-specific
options that determine socket reusability.
Added in: v0.11.7
<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.
Added in: v0.3.6
<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() .
Added in: v0.5.9
<Object>
An object which contains queues of requests that have not yet been assigned to sockets. Do not
modify.
Added in: v0.3.6
<Object>
An object which contains arrays of sockets currently in use by the agent. Do not modify.
agent.maxFreeSockets
#
agent.maxSockets
#
agent.requests
#
agent.sockets
#
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Added in: v0.1.17
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.
Note: Node.js does not check whether Content-Length and the length of the body which has been
transmitted are equal or not.
The request implements the Writable Stream  interface. This is an EventEmitter  with the
following events:
Added in: v1.4.1
Emitted when the request has been aborted by the client. This event is only emitted on the first call
to abort() .
Added in: v0.7.0
response  <http.IncomingMessage>
socket  <net.Socket>
Class: http.ClientRequest
#
Event: 'abort'
#
Event: 'connect'
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head  <Buffer>
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, 
    hostname: '127.0.0.1', 
    method: 'CONNECT', 
    path: 'www.google.com:80' 
  }; 
 
  const req = http.request(options); 
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  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(); 
    }); 
  });
});
Added in: v0.3.2
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.
Added in: v0.1.0
response  <http.IncomingMessage>
Emitted when a response is received to this request. This event is emitted only once.
Added in: v0.5.3
socket  <net.Socket>
Emitted after a socket is assigned to this request.
Event: 'continue'
#
Event: 'response'
#
Event: 'socket'
#
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Added in: v0.7.8
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()
Added in: v0.1.94
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, 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', () => { 
 
Event: 'timeout'
#
Event: 'upgrade'
#
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  // make a request 
  const options = { 
    port: 1337, 
    hostname: '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); 
  });
});
Added in: v0.3.8
Marks the request as aborting. Calling this will cause remaining data in the response to be dropped
and the socket to be destroyed.
Added in: v0.11.14
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.
Added in: v0.3.0
<net.Socket>
See request.socket
request.abort()
#
request.aborted
#
request.connection
#
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Added in: v0.1.90
data  <string>  | <Buffer>
encoding  <string>
callback  <Function>
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.
Added in: v1.6.0
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.
Added in: v1.6.0
name  <string>
Returns: <string>
Reads out a header on the request. Note that the name is case insensitive.
Example:
const contentType = request.getHeader('Content-Type');
request.end([data[, encoding]][, callback])
#
request.flushHeaders()
#
request.getHeader(name)
#
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Added in: v1.6.0
name  <string>
Removes a header that's already defined into headers object.
Example:
request.removeHeader('Content-Type');
Added in: v1.6.0
name  <string>
value  <string>
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.
Example:
request.setHeader('Content-Type', 'application/json');
or
request.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);
Added in: v0.5.9
noDelay  <boolean>
Once a socket is assigned to this request and is connected socket.setNoDelay()  will be called.
request.removeHeader(name)
#
request.setHeader(name, value)
#
request.setNoDelay([noDelay])
#
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Added in: v0.5.9
enable  <boolean>
initialDelay  <number>
Once a socket is assigned to this request and is connected socket.setKeepAlive()  will be called.
Added in: v0.5.9
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.
If no socket is assigned to this request then socket.setTimeout()  will be called immediately.
Otherwise socket.setTimeout()  will be called after the assigned socket is connected.
Returns request .
Added in: v0.3.0
<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
request.connection .
Example:
const http = require('http');
const options = { 
  host: 'www.google.com',
};
const req = http.get(options); 
req.end(); 
request.setSocketKeepAlive([enable][,
initialDelay])
#
request.setTimeout(timeout[, callback])
#
request.socket
#
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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
});
Added in: v0.1.29
chunk  <string>  | <Buffer>
encoding  <string>
callback  <Function>
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.
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.
Added in: v0.1.17
This class inherits from net.Server  and has the following additional events:
Added in: v0.3.0
request  <http.IncomingMessage>
response  <http.ServerResponse>
request.write(chunk[, encoding][, callback])
#
Class: http.Server
#
Event: 'checkContinue'
#
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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.
Note that when this event is emitted and handled, the 'request'  event will not be emitted.
Added in: v5.5.0
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.
History
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'); 
 
Event: 'checkExpectation'
#
Event: 'clientError'
#
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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.
Added in: v0.1.4
Emitted when the server closes.
Added in: v0.7.0
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)
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.
Event: 'close'
#
Event: 'connect'
#
Event: 'connection'
#
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Added in: v0.1.0
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 .
Note: 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.
Added in: v0.1.0
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).
Added in: v0.1.94
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. If this event is not listened for, then clients
requesting an upgrade 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.
Added in: v0.1.90
callback  <Function>
Stops the server from accepting new connections. See net.Server.close() .
Event: 'request'
#
Event: 'upgrade'
#
server.close([callback])
#
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Starts the HTTP server listening for connections. This method is identical to server.listen()
from net.Server .
Added in: v5.7.0
<boolean>
A Boolean indicating whether or not the server is listening for connections.
Added in: v0.7.0
<number>  Default: 2000
Limits maximum incoming headers count. If set to 0 - no limit will be applied.
Added in: v8.14.0
<number>  Default: 40000
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.
Added in: v0.9.12
msecs  <number>  Default: 120000  (2 minutes)
callback  <Function>
server.listen()
#
server.listening
#
server.maxHeadersCount
#
server.headersTimeout
#
server.setTimeout([msecs][, callback])
#
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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.
Returns server .
Added in: v0.9.12
<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.
Note: The socket timeout logic is set up on connection, so changing this value only affects new
connections to the server, not any existing connections.
Added in: v8.0.0
<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.
Note: The socket timeout logic is set up on connection, so changing this value only affects new
connections to the server, not any existing connections.
server.timeout
#
server.keepAliveTimeout
#
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Added in: v0.1.17
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 implements, but does not inherit from, the Writable Stream  interface. This is an
EventEmitter  with the following events:
Added in: v0.6.7
Indicates that the underlying connection was terminated before response.end()  was called or
able to flush.
Added in: v0.3.6
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.
After this event, no more events will be emitted on the response object.
Added in: v0.3.0
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' }); 
Class: http.ServerResponse
#
Event: 'close'
#
Event: 'finish'
#
response.addTrailers(headers)
#
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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.
Added in: v0.3.0
<net.Socket>
See response.socket .
Added in: v0.1.90
data  <string>  | <Buffer>
encoding  <string>
callback  <Function>
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.
Added in: v0.0.2
<boolean>
Boolean value that indicates whether the response has completed. Starts as false . After
response.end()  executes, the value will be true .
response.connection
#
response.end([data][, encoding][, callback])
#
response.finished
#
response.getHeader(name)
#
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Added in: v0.4.0
name  <string>
Returns: <string>
Reads out a header that's already been queued but not sent to the client. Note that the name is
case insensitive.
Example:
const contentType = response.getHeader('content-type');
Added in: v7.7.0
Returns: <Array>
Returns an array containing the unique names of the current outgoing headers. All header names
are lowercase.
Example:
response.setHeader('Foo', 'bar'); 
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']); 
 
const headerNames = response.getHeaderNames();
// headerNames === ['foo', 'set-cookie']
Added in: v7.7.0
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.
response.getHeaderNames()
#
response.getHeaders()
#
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Note: 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.
Example:
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'] }
Added in: v7.7.0
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.
Example:
const hasContentType = response.hasHeader('content-type');
Added in: v0.9.3
<boolean>
Boolean (read-only). True if headers were sent, false otherwise.
Added in: v0.4.0
name  <string>
Removes a header that's queued for implicit sending.
response.hasHeader(name)
#
response.headersSent
#
response.removeHeader(name)
#
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Example:
response.removeHeader('Content-Encoding');
Added in: v0.7.5
<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.
Added in: v0.4.0
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.
Example:
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.
response.sendDate
#
response.setHeader(name, value)
#
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// 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');
});
Added in: v0.9.12
msecs  <number>
callback  <Function>
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.
Returns response .
Added in: v0.3.0
<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 .
Example:
const http = require('http');
const server = http.createServer((req, res) => { 
  const ip = res.socket.remoteAddress; 
response.setTimeout(msecs[, callback])
#
response.socket
#
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  const port = res.socket.remotePort; 
  res.end(`Your IP address is ${ip} and your source port is ${port}.`);
}).listen(3000);
Added in: v0.4.0
<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.
Example:
response.statusCode = 404;
After response header was sent to the client, this property indicates the status code which was
sent out.
Added in: v0.11.8
<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.
Example:
response.statusMessage = 'Not found';
After response header was sent to the client, this property indicates the status message which was
sent out.
Added in: v0.1.29
response.statusCode
#
response.statusMessage
#
response.write(chunk[, encoding][, callback])
#
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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.
Note: 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.
Added in: v0.3.0
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 .
History
response.writeContinue()
#
response.writeHead(statusCode[, statusMessage][,
headers])
#
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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.
Example:
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.
// 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.
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Attempting to set a header field name or value that contains invalid characters will result in a
TypeError  being thrown.
Added in: v0.1.17
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.
Added in: v0.3.8
Emitted when the request has been aborted.
Added in: v0.4.2
Indicates that the underlying connection was closed. Just like 'end' , this event occurs only once
per response.
Added in: v8.13.0
<boolean>
The message.aborted  property will be true  if the request has been aborted.
Added in: v0.3.0
<boolean>
The message.complete  property will be true  if a complete HTTP message has been received and
successfully parsed.
Class: http.IncomingMessage
#
Event: 'aborted'
#
Event: 'close'
#
message.aborted
#
message.complete
#
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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', () => { 
    if (!res.complete) 
      console.error( 
        'The connection was terminated while the message was still being sent'); 
  });
});
Added in: v0.3.0
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.
Added in: v0.1.5
<Object>
The request/response headers object.
Key-value pairs of header names and values. Header names are lower-cased. Example:
// Prints something like:
//
// { 'user-agent': 'curl/7.22.0',
//   host: '127.0.0.1:8000',
//   accept: '*/*' } 
console.log(request.headers);
message.destroy([error])
#
message.headers
#
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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 all other headers, the values are joined together with ', '.
Added in: v0.1.1
<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.
Added in: v0.1.1
<string>
Only valid for request obtained from http.Server .
The request method as a string. Read only. Example: 'GET' , 'DELETE' .
Added in: v0.11.6
<Array>
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.
message.httpVersion
#
message.method
#
message.rawHeaders
#
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// 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);
Added in: v0.11.6
<Array>
The raw request/response trailer keys and values exactly as they were received. Only populated at
the 'end'  event.
Added in: v0.5.9
msecs  <number>
callback  <Function>
Calls message.connection.setTimeout(msecs, callback) .
Returns message .
Added in: v0.3.0
<net.Socket>
The net.Socket  object associated with the connection.
With HTTPS support, use request.socket.getPeerCertificate()  to obtain the client's
authentication details.
message.rawTrailers
#
message.setTimeout(msecs, callback)
#
message.socket
#
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Added in: v0.1.1
<number>
Only valid for response obtained from http.ClientRequest .
The 3-digit HTTP response status code. E.G. 404 .
Added in: v0.11.10
<string>
Only valid for response obtained from http.ClientRequest .
The HTTP response status message (reason phrase). E.G. OK  or Internal Server Error .
Added in: v0.3.0
<Object>
The request/response trailers object. Only populated at the 'end'  event.
Added in: v0.1.90
<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:
message.statusCode
#
message.statusMessage
#
message.trailers
#
message.url
#
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'/status?name=ryan'
To parse the url into its parts require('url').parse(request.url)  can be used. Example:
$ 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 . Example:
$ 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' }
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Added in: v0.11.8
<Array>
A list of the HTTP methods that are supported by the parser.
Added in: v0.1.22
<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' .
Added in: v0.1.13
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.
History
options  <Object>  | <string>  | <URL>  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
http.METHODS
#
http.STATUS_CODES
#
http.createServer([requestListener])
#
http.get(options[, callback])
#
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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.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}`);
});
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Added in: v0.5.9
<http.Agent>
Global instance of Agent  which is used as the default for all HTTP client requests.
Added in: v8.15.0
<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.
History
options  <Object>  | <string>  | <URL>
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  is preferred over
host .
family  <number>  IP address family to use when resolving host  and 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 (use one of host:port or socketPath).
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.
http.globalAgent
#
http.maxHeaderSize
#
http.request(options[, callback])
#
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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.
callback  <Function>
Returns: <http.ClientRequest>
Node.js maintains several connections per server to make HTTP requests. This function allows one
to transparently issue requests.
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.
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.
Example:
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', 
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    '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
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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 { URL } = require('url'); 
 
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
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
close
error  with an error with message Error: socket hang up  and code ECONNRESET
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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
close
aborted  on the res  object
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.
History
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.
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.
HTTP/2
#
Core API
#
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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');
Server-side example
#
Client-side example
#
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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]}`); 
  }
}); 
 
req.setEncoding('utf8'); 
let data = ''; 
req.on('data', (chunk) => { data += chunk; }); 
req.on('end', () => { 
  console.log(`\n${data}`); 
  client.close();
}); 
req.end();
Added in: v8.4.0
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
Class: Http2Session
#
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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 the 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 .
Added in: v8.4.0
The 'close'  event is emitted once the Http2Session  has been destroyed. Its listener does not
expect any arguments.
Added in: v8.4.0
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.
Note: User code will typically not listen for this event directly.
Added in: v8.4.0
error  <Error>
The 'error'  event is emitted when an error occurs during the processing of an Http2Session .
Http2Session and Sockets
#
Event: 'close'
#
Event: 'connect'
#
Event: 'error'
#
Event: 'frameError'
#
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Added in: v8.4.0
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.
Added in: v8.4.0
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.
Added in: v8.4.0
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.
Note: 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) => { 
Event: 'goaway'
#
Event: 'localSettings'
#
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  /** use the new settings **/
});
Added in: v8.13.0
payload  <Buffer>  The PING  frame 8-byte payload
The 'ping'  event is emitted whenever a PING  frame is received from the connected peer.
Added in: v8.4.0
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 **/
});
Added in: v8.4.0
stream  <Http2Stream>  A reference to the stream
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']; 
  // ... 
Event: 'ping'
#
Event: 'remoteSettings'
#
Event: 'stream'
#
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  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.end('<h1>Hello World</h1>');
}); 
 
server.listen(80);
Added in: v8.4.0
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'
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Added in: v8.11.2
Value: <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.
Added in: v8.11.2
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.
Added in: v8.11.2
Value: <boolean>
Will be true  if this Http2Session  instance has been closed, otherwise false .
Added in: v8.11.2
<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.
Added in: v8.4.0
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 .
http2session.alpnProtocol
#
http2session.close([callback])
#
http2session.closed
#
http2session.connecting
#
http2session.destroy([error,][code])
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Returns: <undefined>
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 after the 'close'  event.
If there are any remaining open Http2Streams  associated with the Http2Session , those will also
be destroyed.
Added in: v8.4.0
Value: <boolean>
Will be true  if this Http2Session  instance has been destroyed and must no longer be used,
otherwise false .
Added in: v8.11.2
Value: <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.
Added in: v8.11.2
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.
Transmits a GOAWAY  frame to the connected peer without shutting down the Http2Session .
Added in: v8.4.0
Value: <HTTP/2 Settings Object>
http2session.destroyed
#
http2session.encrypted
#
http2session.goaway([code, [lastStreamID, [opaqueData]]])
#
http2session.localSettings
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A prototype-less object describing the current local settings of this Http2Session . The local
settings are local to this Http2Session  instance.
Added in: v8.11.2
Value: <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.
Added in: v8.4.0
Value: <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.
Added in: v8.9.3
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
http2session.originSet
#
http2session.pendingSettingsAck
#
http2session.ping([payload, ]callback)
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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.
Added in: v8.11.2
Calls ref()  on this Http2Session  instance's underlying net.Socket .
Added in: v8.4.0
Value: <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.
Added in: v8.4.0
msecs  <number>
callback  <Function>
Returns: <undefined>
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.
Added in: v8.4.0
http2session.ref()
#
http2session.remoteSettings
#
http2session.setTimeout(msecs, callback)
#
http2session.socket
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Value: <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.
Added in: v8.4.0
Provides miscellaneous information about the current state of the Http2Session .
Value: <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 .
http2session.state
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Added in: v8.4.0
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.
Note: 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.
Added in: v8.4.0
Value: <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.
Added in: v8.11.2
Calls unref()  on this Http2Session  instance's underlying net.Socket .
Added in: v8.4.0
Added in: v8.11.2
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.
http2session.settings(settings)
#
http2session.type
#
http2session.unref()
#
Class: ServerHttp2Session
#
serverhttp2session.altsvc(alt, originOrStream)
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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.
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.
Specifying alternative services
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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.
Added in: v8.13.0
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:
serverhttp2session.origin(...origins)
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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');
});
Added in: v8.4.0
Added in: v8.11.2
alt  <string>
origin  <string>
streamId  <number>
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);
});
Added in: v8.13.0
origins  <string[]>
Class: ClientHttp2Session
#
Event: 'altsvc'
#
Event: 'origin'
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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.
Added in: v8.4.0
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.
clienthttp2session.request(headers[, options])
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This method is only available if http2session.type  is equal to
http2.constants.NGHTTP2_SESSION_CLIENT .
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  = /
Added in: v8.4.0
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.
Class: Http2Stream
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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.
Note: 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.
Note: 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.
Http2Stream Lifecycle
#
Creation
#
Destruction
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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.
Added in: v8.4.0
The 'aborted'  event is emitted whenever a Http2Stream  instance is abnormally aborted in mid-
communication.
Note: The 'aborted'  event will only be emitted if the Http2Stream  writable side has not been
ended.
Added in: v8.4.0
The 'close'  event is emitted when the Http2Stream  is destroyed. Once this event is emitted, the
Http2Stream  instance is no longer usable.
The listener callback is passed a single argument specifying the HTTP/2 error code specified when
closing the stream. If the code is any value other than NGHTTP2_NO_ERROR  ( 0 ), an 'error'  event
will also be emitted.
Added in: v8.4.0
error  <Error>
The 'error'  event is emitted when an error occurs during the processing of an Http2Stream .
Added in: v8.4.0
Event: 'aborted'
#
Event: 'close'
#
Event: 'error'
#
Event: 'frameError'
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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.
Added in: v8.4.0
The 'timeout'  event is emitted after no activity is received for this 'Http2Stream'  within the
number of milliseconds set using http2stream.setTimeout() .
Added in: v8.4.0
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.
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);
});
Added in: v8.13.0
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.
Added in: v8.4.0
Value: <boolean>
Event: 'timeout'
#
Event: 'trailers'
#
Event: 'wantTrailers'
#
http2stream.aborted
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Set to true  if the Http2Stream  instance was aborted abnormally. When set, the 'aborted'  event
will have been emitted.
Added in: v8.4.0
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.
Returns: <undefined>
Closes the Http2Stream  instance by sending an RST_STREAM  frame to the connected HTTP/2 peer.
Added in: v8.11.2
Value: <boolean>
Set to true  if the Http2Stream  instance has been closed.
Added in: v8.4.0
Value: <boolean>
Set to true  if the Http2Stream  instance has been destroyed and is no longer usable.
Added in: v8.13.0
<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.
Added in: v8.11.2
Value: <boolean>
http2stream.close(code[, callback])
#
http2stream.closed
#
http2stream.destroyed
#
http2stream.endAfterHeaders
#
http2stream.pending
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Set to true  if the Http2Stream  instance has not yet been assigned a numeric stream identifier.
Added in: v8.4.0
options  <Object>
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.
Returns: <undefined>
Updates the priority for this Http2Stream  instance.
Added in: v8.4.0
Value: <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.
Added in: v8.11.2
Value: <HTTP/2 Headers Object>
An object containing the outbound headers sent for this Http2Stream .
Added in: v8.11.2
Value: <HTTP/2 Headers Object[]>
http2stream.priority(options)
#
http2stream.rstCode
#
http2stream.sentHeaders
#
http2stream.sentInfoHeaders
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An array of objects containing the outbound informational (additional) headers sent for this
Http2Stream .
Added in: v8.11.2
Value: <HTTP/2 Headers Object>
An object containing the outbound trailers sent for this this HttpStream .
Added in: v8.4.0
Value: <Http2Session>
A reference to the Http2Session  instance that owns this Http2Stream . The value will be
undefined  after the Http2Stream  instance is destroyed.
Added in: v8.4.0
msecs  <number>
callback  <Function>
Returns: <undefined>
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));
Added in: v8.4.0
Provides miscellaneous information about the current state of the Http2Stream .
Value: <Object>
http2stream.sentTrailers
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http2stream.session
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http2stream.setTimeout(msecs, callback)
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http2stream.state
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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 .
Added in: v8.13.0
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).
Added in: v8.4.0
http2stream.sendTrailers(headers)
#
Class: ClientHttp2Stream
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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.
Added in: v8.5.0
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.
Added in: v8.4.0
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);
});
Added in: v8.4.0
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);
});
Added in: v8.4.0
Event: 'continue'
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Event: 'headers'
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Event: 'push'
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Event: 'response'
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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.
For example:
const http2 = require('http2');
const client = http2.connect('https://localhost');
const req = client.request({ ':path': '/' }); 
req.on('response', (headers, flags) => { 
  console.log(headers[':status']);
});
Added in: v8.4.0
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.
Added in: v8.4.0
headers  <HTTP/2 Headers Object>
Sends an additional informational HEADERS  frame to the connected HTTP/2 peer.
Added in: v8.4.0
Value: <boolean>
Boolean (read-only). True if headers were sent, false otherwise.
Added in: v8.4.0
Class: ServerHttp2Stream
#
http2stream.additionalHeaders(headers)
#
http2stream.headersSent
#
http2stream.pushAllowed
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Value: <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 .
Added in: v8.4.0
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.
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.
Returns: <undefined>
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');
});
http2stream.pushStream(headers[, options], callback)
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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.
Added in: v8.4.0
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 }); 
  stream.on('wantTrailers', () => { 
    stream.sendTrailers({ ABC: 'some value to send' }); 
http2stream.respond([headers[, options]])
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  }); 
  stream.end('some data');
});
Added in: v8.4.0
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); 
http2stream.respondWithFD(fd[, headers[, options]])
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  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.
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' }); 
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  }); 
 
  stream.on('close', () => fs.closeSync(fd));
});
Added in: v8.4.0
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:
const http2 = require('http2');
const server = http2.createServer(); 
server.on('stream', (stream) => { 
  function statCheck(stat, headers) { 
    headers['last-modified'] = stat.mtime.toUTCString(); 
  } 
http2stream.respondWithFile(path[, headers[, options]])
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  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.
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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.sendTrilers()  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.respondWithFile('/some/file', 
                         { 'content-type': 'text/plain' }, 
                         { waitForTrailers: true }); 
  stream.on('wantTrailers', () => { 
    stream.sendTrailers({ ABC: 'some value to send' }); 
  });
});
Added in: v8.4.0
Extends: <net.Server>
Instances of Http2Server  are created using the http2.createServer()  function. The
Http2Server  class is not exported directly by the http2  module.
Added in: v8.5.0
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.
Class: Http2Server
#
Event: 'checkContinue'
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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.
Added in: v8.4.0
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 .
Added in: v8.4.0
The 'session'  event is emitted when a new Http2Session  is created by the Http2Server .
Added in: v8.4.0
The 'sessionError'  event is emitted when an 'error'  event is emitted by an Http2Session
object associated with the Http2Server .
Added in: v8.4.0
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; 
Event: 'request'
#
Event: 'session'
#
Event: 'sessionError'
#
Event: 'stream'
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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');
});
Added in: v8.4.0
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.
Added in: v8.4.0
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.
Added in: v8.4.0
msecs  <number>  Default: 120000  (2 minutes)
callback  <Function>
Returns: <Http2Server>
Event: 'timeout'
#
server.close([callback])
#
server.setTimeout([msecs][, callback])
#
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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.
Added in: v8.4.0
Extends: <tls.Server>
Instances of Http2SecureServer  are created using the http2.createSecureServer()  function.
The Http2SecureServer  class is not exported directly by the http2  module.
Added in: v8.5.0
request  <http2.Http2ServerRequest>
response  <http2.Http2ServerResponse>
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.
Added in: v8.4.0
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 .
Class: Http2SecureServer
#
Event: 'checkContinue'
#
Event: 'request'
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Added in: v8.4.0
The 'session'  event is emitted when a new Http2Session  is created by the Http2SecureServer .
Added in: v8.4.0
The 'sessionError'  event is emitted when an 'error'  event is emitted by an Http2Session
object associated with the Http2SecureServer .
Added in: v8.4.0
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');
});
Event: 'session'
#
Event: 'sessionError'
#
Event: 'stream'
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Added in: v8.4.0
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.
Added in: v8.4.0
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 .
Added in: v8.4.0
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.
Added in: v8.4.0
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.
Event: 'timeout'
#
Event: 'unknownProtocol'
#
server.close([callback])
#
server.setTimeout([msecs][, callback])
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History
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.
http2.createServer(options[, onRequestHandler])
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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', 
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    ':status': 200 
  }); 
  stream.end('<h1>Hello World</h1>');
}); 
 
server.listen(80);
History
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 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.createSecureServer(options[,
onRequestHandler])
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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
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const server = http2.createSecureServer(options); 
 
server.on('stream', (stream, headers) => { 
  stream.respond({ 
    'content-type': 'text/html', 
    ':status': 200 
  }); 
  stream.end('<h1>Hello World</h1>');
}); 
 
server.listen(80);
History
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
http2.connect(authority[, options][, listener])
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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'); 
 
/** use the client **/ 
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client.close();
Added in: v8.4.0
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
http2.constants
#
Error Codes for RST_STREAM and GOAWAY
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Value
Name
Constant
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() .
Added in: v8.4.0
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.
Added in: v8.4.0
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
Added in: v8.4.0
buf  <Buffer>  | <Uint8Array>  The packed settings.
http2.getDefaultSettings()
#
http2.getPackedSettings(settings)
#
http2.getUnpackedSettings(buf)
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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).
For example:
const headers = { 
  ':status': '200', 
  'content-type': 'text-plain', 
  'ABC': ['has', 'more', 'than', 'one', 'value']
}; 
 
stream.respond(headers);
Note: 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 , 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.
cookie : the values are joined together with '; '.
For all other headers, the values are joined together with ', '.
Headers Object
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const http2 = require('http2');
const server = http2.createServer(); 
server.on('stream', (stream, headers) => { 
  console.log(headers[':path']); 
  console.log(headers.ABC);
});
History
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 .
All additional properties on the settings object are ignored.
Settings Object
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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; 
  }
});
Note: The options.selectPadding  function is invoked once for every HEADERS  and DATA  frame.
This has a definite noticeable impact on performance.
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.
Using options.selectPadding
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Error Handling
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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': '/' });
Invalid character handling in header names and
values
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Push streams on the client
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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:
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 { URL } = require('url'); 
 
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); 
Supporting the CONNECT method
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  }); 
  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}` 
}); 
 
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');
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.
Compatibility API
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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');
});
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' ? 
ALPN negotiation
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    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.
Added in: v8.4.0
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.
Added in: v8.4.0
The 'aborted'  event is emitted whenever a Http2ServerRequest  instance is abnormally aborted
in mid-communication.
Note: The 'aborted'  event will only be emitted if the Http2ServerRequest  writable side has not
been ended.
Added in: v8.4.0
Indicates that the underlying Http2Stream  was closed. Just like 'end' , this event occurs only once
per response.
Added in: v8.13.0
<boolean>
Class: http2.Http2ServerRequest
#
Event: 'aborted'
#
Event: 'close'
#
request.aborted
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The request.aborted  property will be true  if the request has been aborted.
Added in: v8.4.0
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.
Added in: v8.4.0
<Object>
The request/response headers object.
Key-value pairs of header names and values. Header names are lower-cased. Example:
// 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 .
Note: 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
request.destroy([error])
#
request.headers
#
request.httpVersion
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Added in: v8.4.0
<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.
Added in: v8.4.0
<string>
The request method as a string. Read-only. Example: 'GET' , 'DELETE' .
Added in: v8.4.0
<Array>
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);
request.method
#
request.rawHeaders
#
request.rawTrailers
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Added in: v8.4.0
<Array>
The raw request/response trailer keys and values exactly as they were received. Only populated at
the 'end'  event.
Added in: v8.4.0
msecs  <number>
callback  <Function>
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.
Returns request .
Added in: v8.4.0
<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 .
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.
request.setTimeout(msecs, callback)
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request.socket
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Added in: v8.4.0
<http2.Http2Stream>
The Http2Stream  object backing the request.
Added in: v8.4.0
<Object>
The request/response trailers object. Only populated at the 'end'  event.
Added in: v8.4.0
<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. Example:
$ node 
> require('url').parse('/status?name=ryan') 
Url { 
  protocol: null, 
  slashes: null, 
  auth: null, 
  host: null, 
request.stream
#
request.trailers
#
request.url
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  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 . Example:
$ 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' }
Added in: v8.4.0
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 implements, but does not inherit from, the Writable Stream  interface. This is an
EventEmitter  with the following events:
Class: http2.Http2ServerResponse
#
Event: 'close'
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Added in: v8.4.0
Indicates that the underlying Http2Stream  was terminated before response.end()  was called or
able to flush.
Added in: v8.4.0
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.
Added in: v8.4.0
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.
Added in: v8.4.0
<net.Socket>  | <tls.TLSSocket>
See response.socket .
Added in: v8.4.0
data  <string>  | <Buffer>
encoding  <string>
callback  <Function>
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.
Event: 'finish'
#
response.addTrailers(headers)
#
response.connection
#
response.end([data][, encoding][, callback])
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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.
Added in: v8.4.0
<boolean>
Boolean value that indicates whether the response has completed. Starts as false . After
response.end()  executes, the value will be true .
Added in: v8.4.0
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.
Example:
const contentType = response.getHeader('content-type');
Added in: v8.4.0
Returns: <Array>
Returns an array containing the unique names of the current outgoing headers. All header names
are lowercase.
Example:
response.setHeader('Foo', 'bar'); 
response.setHeader('Set-Cookie', ['foo=bar', 'bar=baz']); 
 
response.finished
#
response.getHeader(name)
#
response.getHeaderNames()
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const headerNames = response.getHeaderNames();
// headerNames === ['foo', 'set-cookie']
Added in: v8.4.0
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.
Note: 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.
Example:
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'] }
Added in: v8.4.0
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.
Example:
const hasContentType = response.hasHeader('content-type');
response.getHeaders()
#
response.hasHeader(name)
#
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Added in: v8.4.0
<boolean>
Boolean (read-only). True if headers were sent, false otherwise.
Added in: v8.4.0
name  <string>
Removes a header that has been queued for implicit sending.
Example:
response.removeHeader('Content-Encoding');
Added in: v8.4.0
<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.
Added in: v8.4.0
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.
Example:
response.setHeader('Content-Type', 'text/html');
response.headersSent
#
response.removeHeader(name)
#
response.sendDate
#
response.setHeader(name, value)
#
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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');
});
Added in: v8.4.0
msecs  <number>
callback  <Function>
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.
Returns response .
Added in: v8.4.0
response.setTimeout(msecs[, callback])
#
response.socket
#
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<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.
Example:
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);
Added in: v8.4.0
<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.
Example:
response.statusCode = 404;
After response header was sent to the client, this property indicates the status code which was
sent out.
response.statusCode
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Added in: v8.4.0
<string>
Status message is not supported by HTTP/2 (RFC7540 8.1.2.4). It returns an empty string.
Added in: v8.4.0
<http2.Http2Stream>
The Http2Stream  object backing the response.
Added in: v8.4.0
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.
Note: 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
response.statusMessage
#
response.stream
#
response.write(chunk[, encoding][, callback])
#
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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.
Added in: v8.4.0
Sends a status 100 Continue  to the client, indicating that the request body should be sent. See the
'checkContinue'  event on Http2Server  and Http2SecureServer .
Added in: v8.4.0
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.
Example:
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 not.
response.writeContinue()
#
response.writeHead(statusCode[, statusMessage][,
headers])
#
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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.
Added in: v8.4.0
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 .
response.createPushResponse(headers, callback)
#
Collecting HTTP/2 Performance Metrics
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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' .
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 .
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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.
Added in: v0.4.5
An Agent object for HTTPS similar to http.Agent . See https.request()  for more information.
Added in: v0.3.4
This class is a subclass of tls.Server  and emits events same as http.Server . See http.Server
for more information.
Added in: v0.1.90
callback  <Function>
See server.close()  from the HTTP module for details.
HTTPS
#
Class: https.Agent
#
Class: https.Server
#
server.close([callback])
#
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Starts the HTTPS server listening for encrypted connections. This method is identical to
server.listen()  from net.Server .
<number>  Default: 40000
See http.Server#headersTimeout .
Added in: v0.11.2
msecs  <number>  Default: 120000  (2 minutes)
callback  <Function>
See http.Server#setTimeout() .
Added in: v0.11.2
<number>  Default: 120000  (2 minutes)
See http.Server#timeout .
Added in: v8.0.0
<number>  Default: 5000  (5 seconds)
See http.Server#keepAliveTimeout .
Added in: v0.3.4
options  <Object>  Accepts options  from tls.createServer() ,
tls.createSecureContext()  and http.createServer() .
server.listen()
#
server.headersTimeout
#
server.setTimeout([msecs][, callback])
#
server.timeout
#
server.keepAliveTimeout
#
https.createServer([options][,
requestListener])
#
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requestListener  <Function>  A listener to be added to the request  event.
Example:
// 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
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);
History
options  <Object>  | <string>  | <URL>  Accepts the same options  as https.request() , with
the method  always set to GET .
https.get(options[, callback])
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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.
Example:
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);
});
Added in: v0.5.9
Global instance of https.Agent  for all HTTPS client requests.
History
options  <Object>  | <string>  | <URL>  Accepts all options  from http.request() , with some
differences in default values:
protocol  Default: https:
port  Default: 443
agent  Default: https.globalAgent
https.globalAgent
#
https.request(options[, callback])
#
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callback  <Function>
Makes a request to a secure web server.
The following additional options  from tls.connect()  are also accepted when using a custom
Agent : pfx , key , passphrase , cert , ca , ciphers , rejectUnauthorized , secureProtocol ,
servername
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.
Example:
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() :
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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) => { 
  // ...
});
Alternatively, opt out of connection pooling by not using an Agent .
Example:
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 { URL } = require('url'); 
 
const options = new URL('https://abc:xyz@example.com'); 
 
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const req = https.request(options, (res) => { 
  // ...
});
Stability: 1  - Experimental
The inspector  module provides an API for interacting with the V8 inspector.
It can be accessed using:
const inspector = require('inspector');
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.
Deactivate the inspector. Blocks until there are no active connections.
Return the URL of the active inspector, or undefined  if there is none.
Inspector
#
inspector.open([port[, host[, wait]]])
#
inspector.close()
#
inspector.url()
#
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The inspector.Session  is used for dispatching messages to the V8 inspector back-end and
receiving message responses and notifications.
Added in: v8.0.0
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:
Added in: v8.0.0
<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:
Added in: v8.0.0
<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):
Class: inspector.Session
#
Constructor: new inspector.Session()
#
Event: 'inspectorNotification'
#
Event: <inspector-protocol-method>
#
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session.on('Debugger.paused', ({ params }) => { 
  console.log(params.hitBreakpoints);
});
// [ '/the/file/that/has/the/breakpoint.js:11:0' ]
Added in: v8.0.0
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.
Added in: v8.0.0
method <string>
params <Object>
callback <Function>
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.
Added in: v8.0.0
session.connect()
#
session.post(method[, params][, callback])
#
session.disconnect()
#
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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.
Apart from the debugger, various V8 Profilers are available through the DevTools protocol. Here's
a simple example showing how to use the CPU profiler :
const inspector = require('inspector'); 
 
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', ({ profile }) => { 
      // write profile to disk, upload, etc. 
    }); 
  });
});
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):
Example usage
#
CPU Profiler
#
Internationalization Support
#
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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
String.prototype.to*Case()
full
full
full
full
Options for building Node.js
#
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none
system-
icu
small-
icu
full-
icu
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
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none
system-
icu
small-
icu
full-
icu
RegExp Unicode Property Escapes
none
(invalid
RegExp
error)
full
full
full
Note: 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 )
#
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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 icudt5X[bl].dat , where 5X  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
Providing ICU data at runtime
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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:
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.
Embed the entire ICU (full-icu )
#
Detecting internationalization support
#
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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:
const Square = require('./square.js');
const mySquare = new Square(2); 
console.log(`The area of mySquare is ${mySquare.area()}`);
Modules
#
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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
Accessing the main module
#
Addenda: Package Manager Tips
#
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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.
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, 
All Together...
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   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) 
 
NODE_MODULES_PATHS(START)
1. let PARTS = path split(START)
2. let I = count of PARTS - 1
3. let DIRS = []
4. while I >= 0, 
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   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.
Multiple calls to require('foo')  may 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.
Caching
#
Module Caching Caveats
#
Core Modules
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When there are circular require()  calls, a module might not have finished executing when it is
returned.
Consider this situation:
a.js :
console.log('a starting'); 
exports.done = false;
const b = require('./b.js'); 
console.log('in a, b.done = %j', b.done); 
exports.done = true; 
console.log('a done');
b.js :
console.log('b starting'); 
exports.done = false;
const a = require('./a.js'); 
console.log('in b, a.done = %j', a.done); 
exports.done = true; 
console.log('b done');
main.js :
console.log('main starting');
const a = require('./a.js');
const b = require('./b.js'); 
console.log('in main, a.done = %j, b.done = %j', a.done, b.done);
When main.js  loads a.js , then a.js  in turn loads b.js . At that point, b.js  tries to load a.js . In
order to prevent an infinite loop, an unfinished copy of the a.js  exports object is returned to the
b.js  module. b.js  then finishes loading, and its exports  object is provided to the a.js  module.
By the time main.js  has loaded both modules, they're both finished. The output of this program
would thus be:
Cycles
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$ node main.js 
main starting 
a starting 
b starting 
in b, a.done = false 
b done 
in a, b.done = true 
a done 
in main, a.done = true, b.done = true
Careful planning is required to allow cyclic module dependencies to work correctly within an
application.
If the exact filename is not found, then Node.js will attempt to load the required filename with the
added extensions: .js , .json , and finally .node .
.js  files are interpreted as JavaScript text files, and .json  files are parsed as JSON text files.
.node  files are interpreted as compiled addon modules loaded with dlopen .
A required module prefixed with '/'  is an absolute path to the file. For example,
require('/home/marco/foo.js')  will load the file at /home/marco/foo.js .
A required module prefixed with './'  is relative to the file calling require() . That is, circle.js
must be in the same directory as foo.js  for require('./circle')  to find it.
Without a leading '/', './', or '../' to indicate a file, the module must either be a core module or is
loaded from a node_modules  folder.
If the given path does not exist, require()  will throw an Error  with its code  property set to
'MODULE_NOT_FOUND' .
It is convenient to organize programs and libraries into self-contained directories, and then provide
a single entry point to that library. There are three ways in which a folder may be passed to
require()  as an argument.
File Modules
#
Folders as Modules
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The first is to create a package.json  file in the root of the folder, which specifies a main  module.
An example package.json file might look like this:
{ "name" : "some-library", 
  "main" : "./lib/some-library.js" }
If this was in a folder at ./some-library , then require('./some-library')  would attempt to load
./some-library/lib/some-library.js .
This is the extent of Node.js's awareness of package.json files.
Note: If the file specified by the 'main'  entry of package.json  is missing and can not be resolved,
Node.js will report the entire module as missing with the default error:
Error: Cannot find module 'some-library'
If there is no package.json file present in the directory, then Node.js will attempt to load an
index.js  or index.node  file out of that directory. For example, if there was no package.json file in
the above example, then require('./some-library')  would attempt to load:
./some-library/index.js
./some-library/index.node
If the module identifier passed to require()  is not a core  module, and does not begin with '/' ,
'../' , or './' , then Node.js starts at the parent directory of the current module, and adds
/node_modules , and attempts to load the module from that location. Node.js will not append
node_modules  to a path already ending in node_modules .
If it is not found there, then it moves to the parent directory, and so on, until the root of the file
system is reached.
For example, if the file at '/home/ry/projects/foo.js'  called require('bar.js') , then Node.js
would look in the following locations, in this order:
/home/ry/projects/node_modules/bar.js
/home/ry/node_modules/bar.js
Loading from node_modules  Folders
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/home/node_modules/bar.js
/node_modules/bar.js
This allows programs to localize their dependencies, so that they do not clash.
It is possible to require specific files or sub modules distributed with a module by including a path
suffix after the module name. For instance require('example-module/path/to/file')  would
resolve path/to/file  relative to where example-module  is located. The suffixed path follows the
same module resolution semantics.
If the NODE_PATH  environment variable is set to a colon-delimited list of absolute paths, then
Node.js will search those paths for modules if they are not found elsewhere.
Note: On Windows, NODE_PATH  is delimited by semicolons instead of colons.
NODE_PATH  was originally created to support loading modules from varying paths before the
current module resolution  algorithm was frozen.
NODE_PATH  is still supported, but is less necessary now that the Node.js ecosystem has settled on a
convention for locating dependent modules. Sometimes deployments that rely on NODE_PATH  show
surprising behavior when people are unaware that NODE_PATH  must be set. Sometimes a module's
dependencies change, causing a different version (or even a different module) to be loaded as the
NODE_PATH  is searched.
Additionally, Node.js will search in the following locations:
1: $HOME/.node_modules
2: $HOME/.node_libraries
3: $PREFIX/lib/node
Where $HOME  is the user's home directory, and $PREFIX  is Node.js's configured node_prefix .
These are mostly for historic reasons.
Note: It is strongly encouraged to place dependencies in the local node_modules  folder. These will
be loaded faster, and more reliably.
Loading from the global folders
#
The module wrapper
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Before a module's code is executed, Node.js will wrap it with a function wrapper that looks like the
following:
(function(exports, require, module, __filename, __dirname) {
// Module code actually lives in here
});
By doing this, Node.js achieves a few things:
It keeps top-level variables (defined with var , const  or let ) scoped to the module rather
than the global object.
It helps to provide some global-looking variables that are actually specific to the module, such
as:
The module  and exports  objects that the implementor can use to export values from the
module.
The convenience variables __filename  and __dirname , containing the module's absolute
filename and directory path.
Added in: v0.1.27
<string>
The directory name of the current module. This is the same as the path.dirname()  of the
__filename .
Example: running node example.js  from /Users/mjr
console.log(__dirname);
// Prints: /Users/mjr 
console.log(path.dirname(__filename));
// Prints: /Users/mjr
Added in: v0.0.1
The module scope
#
__dirname
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__filename
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<string>
The file name of the current module. This is the resolved absolute path of the current module file.
For a main program this is not necessarily the same as the file name used in the command line.
See __dirname  for the directory name of the current module.
Examples:
Running node example.js  from /Users/mjr
console.log(__filename);
// Prints: /Users/mjr/example.js 
console.log(__dirname);
// Prints: /Users/mjr
Given two modules: a  and b , where b  is a dependency of a  and there is a directory structure of:
/Users/mjr/app/a.js
/Users/mjr/app/node_modules/b/b.js
References to __filename  within b.js  will return /Users/mjr/app/node_modules/b/b.js  while
references to __filename  within a.js  will return /Users/mjr/app/a.js .
Added in: v0.1.12
A reference to the module.exports  that is shorter to type. See the section about the exports
shortcut  for details on when to use exports  and when to use module.exports .
Added in: v0.1.16
<Object>
A reference to the current module, see the section about the module object . In particular,
module.exports  is used for defining what a module exports and makes available through
require() .
exports
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module
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Added in: v0.1.13
<Function>
Used to import modules, JSON , and local files. Modules can be imported from node_modules . Local
modules and JSON files can be imported using a relative path (e.g. ./ , ./foo , ./bar/baz , ../foo )
that will be resolved against the directory named by __dirname  (if defined) or the current working
directory.
// Importing a local module:
const myLocalModule = require('./path/myLocalModule'); 
 
// Importing a JSON file:
const jsonData = require('./path/filename.json'); 
 
// Importing a module from node_modules or Node.js built-in module:
const crypto = require('crypto');
Added in: v0.3.0
<Object>
Modules are cached in this object when they are required. By deleting a key value from this object,
the next require  will reload the module. Note that this does not apply to native addons , for which
reloading will result in an Error.
Added in: v0.3.0 Deprecated since: v0.10.6
Stability: 0  - Deprecated
<Object>
Instruct require  on how to handle certain file extensions.
Process files with the extension .sjs  as .js :
require()
#
require.cache
#
require.extensions
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require.extensions['.sjs'] = require.extensions['.js'];
Deprecated In the past, this list has been used to load non-JavaScript modules into Node.js by
compiling them on-demand. However, in practice, there are much better ways to do this, such as
loading modules via some other Node.js program, or compiling them to JavaScript ahead of time.
Since the module system is locked, this feature will probably never go away. However, it may have
subtle bugs and complexities that are best left untouched.
Note that the number of file system operations that the module system has to perform in order to
resolve a require(...)  statement to a filename scales linearly with the number of registered
extensions.
In other words, adding extensions slows down the module loader and should be discouraged.
History
request  <string>  The module path to resolve.
options  <Object>
paths  <Array>  Paths to resolve module location from. If present, these paths are used
instead of the default resolution paths. Note that each of these paths is used as a starting
point for the module resolution algorithm, meaning that the node_modules  hierarchy is
checked from this location.
Returns: <string>
Use the internal require()  machinery to look up the location of a module, but rather than loading
the module, just return the resolved filename.
Added in: v8.9.0
request  <string>  The module path whose lookup paths are being retrieved.
Returns: <Array>  | <null>
Returns an array containing the paths searched during resolution of request  or null if the request
string references a core module, for example http  or fs .
require.resolve(request[, options])
#
require.resolve.paths(request)
#
The module  Object
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Added in: v0.1.16
<Object>
In each module, the module  free variable is a reference to the object representing the current
module. For convenience, module.exports  is also accessible via the exports  module-global.
module  is not actually a global but rather local to each module.
Added in: v0.1.16
<Array>
The module objects required by this one.
Added in: v0.1.16
<Object>
The module.exports  object is created by the Module system. Sometimes this is not acceptable;
many want their module to be an instance of some class. To do this, assign the desired export object
to module.exports . Note that assigning the desired object to exports  will simply rebind the local
exports  variable, which is probably not what is desired.
For example suppose we were making a module called a.js
const EventEmitter = require('events'); 
 
module.exports = new EventEmitter(); 
 
// Do some work, and after some time emit
// the 'ready' event from the module itself.
setTimeout(() => { 
  module.exports.emit('ready');
}, 1000);
Then in another file we could do
module.children
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module.exports
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const a = require('./a'); 
a.on('ready', () => { 
  console.log('module a is ready');
});
Note that assignment to module.exports  must be done immediately. It cannot be done in any
callbacks. This does not work:
x.js:
setTimeout(() => { 
  module.exports = { a: 'hello' };
}, 0);
y.js:
const x = require('./x'); 
console.log(x.a);
Added in: v0.1.16
The exports  variable is available within a module's file-level scope, and is assigned the value of
module.exports  before the module is evaluated.
It allows a shortcut, so that module.exports.f = ...  can be written more succinctly as exports.f
= ... . However, be aware that like any variable, if a new value is assigned to exports , it is no
longer bound to module.exports :
module.exports.hello = true; // Exported from require of module 
exports = { hello: false };  // Not exported, only available in the module
When the module.exports  property is being completely replaced by a new object, it is common to
also reassign exports , for example:
exports shortcut
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module.exports = exports = function Constructor() { 
  // ... etc.
};
To illustrate the behavior, imagine this hypothetical implementation of require() , which is quite
similar to what is actually done by require() :
function require(/* ... */) { 
  const module = { exports: {} }; 
  ((module, exports) => { 
    // Module code here. In this example, define a function. 
    function someFunc() {} 
    exports = someFunc; 
    // At this point, exports is no longer a shortcut to module.exports, and 
    // this module will still export an empty default object. 
    module.exports = someFunc; 
    // At this point, the module will now export someFunc, instead of the 
    // default object. 
  })(module, module.exports); 
  return module.exports;
}
Added in: v0.1.16
<string>
The fully resolved filename to the module.
Added in: v0.1.16
<string>
The identifier for the module. Typically this is the fully resolved filename.
module.filename
#
module.id
#
module.loaded
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Added in: v0.1.16
<boolean>
Whether or not the module is done loading, or is in the process of loading.
Added in: v0.1.16
<Object>  Module object
The module that first required this one.
Added in: v0.4.0
<string[]>
The search paths for the module.
Added in: v0.5.1
id  <string>
Returns: <Object>  module.exports  from the resolved module
The module.require  method provides a way to load a module as if require()  was called from the
original module.
Note: In order to do this, it is necessary to get a reference to the module  object. Since require()
returns the module.exports , and the module  is typically only available within a specific module's
code, it must be explicitly exported in order to be used.
Added in: v0.3.7
<Object>
Provides general utility methods when interacting with instances of Module  — the module  variable
often seen in file modules. Accessed via require('module') .
module.parent
#
module.paths
#
module.require(id)
#
The Module  Object
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Added in: v8.10.0
<string[]>
A list of the names of all modules provided by Node.js. Can be used to verify if a module is
maintained by a third-party module or not.
Stability: 2  - Stable
The net  module provides an asynchronous network API for creating stream-based TCP or IPC
servers ( net.createServer() ) and clients ( net.createConnection() ).
It can be accessed using:
const net = require('net');
The net  module supports IPC with named pipes on Windows, and UNIX domain sockets on other
operating systems.
net.connect() , net.createConnection() , server.listen()  and socket.connect()  take a path
parameter to identify IPC endpoints.
On UNIX, the local domain is also known as the UNIX domain. The path is a filesystem path name. It
gets truncated to sizeof(sockaddr_un.sun_path) - 1 , which varies on different operating
system between 91 and 107 bytes. The typical values are 107 on Linux and 103 on macOS. The
path is subject to the same naming conventions and permissions checks as would be done on file
creation. It will be visible in the filesystem, and will persist until unlinked.
On Windows, the local domain is implemented using a named pipe. The path must refer to an entry
in \\?\pipe\  or \\.\pipe\ . Any characters are permitted, but the latter may do some processing
of pipe names, such as resolving ..  sequences. Despite appearances, the pipe name space is flat.
module.builtinModules
#
Net
#
IPC Support
#
Identifying paths for IPC connections
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Pipes will not persist, they are removed when the last reference to them is closed. Do not forget
JavaScript string escaping requires paths to be specified with double-backslashes, such as:
net.createServer().listen( 
  path.join('\\\\?\\pipe', process.cwd(), 'myctl'));
Added in: v0.1.90
This class is used to create a TCP or IPC  server.
Returns: <net.Server>
See net.createServer([options][, connectionListener]) .
net.Server  is an EventEmitter  with the following events:
Added in: v0.5.0
Emitted when the server closes. Note that if connections exist, this event is not emitted until all
connections are ended.
Added in: v0.1.90
<net.Socket>  The connection object
Emitted when a new connection is made. socket  is an instance of net.Socket .
Added in: v0.1.90
<Error>
Emitted when an error occurs. Unlike net.Socket , the 'close'  event will not be emitted directly
following this event unless server.close()  is manually called. See the example in discussion of
Class: net.Server
#
new net.Server([options][, connectionListener])
#
Event: 'close'
#
Event: 'connection'
#
Event: 'error'
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server.listen() .
Added in: v0.1.90
Emitted when the server has been bound after calling server.listen() .
Added in: v0.1.90
Returns the bound address, the address family name, and port of the server as reported by the
operating system if listening on an IP socket. Useful to find which port was assigned when getting
an OS-assigned address. Returns an object with port , family , and address  properties: { port:
12346, family: 'IPv4', address: '127.0.0.1' }
For a server listening on a pipe or UNIX domain socket, the name is returned as a string.
Example:
const server = net.createServer((socket) => { 
  socket.end('goodbye\n');
}).on('error', (err) => { 
  // handle errors here 
  throw err;
}); 
 
// grab an arbitrary unused port. 
server.listen(() => { 
  console.log('opened server on', server.address());
});
Don't call server.address()  until the 'listening'  event has been emitted.
Added in: v0.1.90
Returns: <net.Server>
Event: 'listening'
#
server.address()
#
server.close([callback])
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Stops the server from accepting new connections and keeps existing connections. This function is
asynchronous, the server is finally closed when all connections are ended and the server emits a
'close'  event. The optional callback  will be called once the 'close'  event occurs. Unlike that
event, it will be called with an Error as its only argument if the server was not open when it was
closed.
Returns server .
Added in: v0.2.0 Deprecated since: v0.9.7
Stability: 0  - Deprecated: Use server.getConnections()  instead.
The number of concurrent connections on the server.
This becomes null  when sending a socket to a child with child_process.fork() . To poll forks and
get current number of active connections use asynchronous server.getConnections()  instead.
Added in: v0.9.7
Returns: <net.Server>
Asynchronously get the number of concurrent connections on the server. Works when sockets
were sent to forks.
Callback should take two arguments err  and count .
Start a server listening for connections. A net.Server  can be a TCP or a IPC  server depending on
what it listens to.
Possible signatures:
server.listen(handle[, backlog][, callback])
server.listen(options[, callback])
server.listen(path[, backlog][, callback])  for IPC  servers
server.listen([port][, host][, backlog][, callback])  for TCP servers
server.connections
#
server.getConnections(callback)
#
server.listen()
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This function is asynchronous. When the server starts listening, the 'listening'  event will be
emitted. The last parameter callback  will be added as a listener for the 'listening'  event.
All listen()  methods can take a backlog  parameter to specify the maximum length of the queue
of pending connections. The actual length will be determined by the OS through sysctl settings
such as tcp_max_syn_backlog  and somaxconn  on Linux. The default value of this parameter is 511
(not 512).
Note:
All net.Socket  are set to SO_REUSEADDR  (See socket(7)  for details).
The server.listen()  method may be called multiple times. Each subsequent call will re-open
the server using the provided options.
One of the most common errors raised when listening is EADDRINUSE . This happens when another
server is already listening on the requested port  / path  / handle . One way to handle this would
be to retry after a certain amount of time:
server.on('error', (e) => { 
  if (e.code === 'EADDRINUSE') { 
    console.log('Address in use, retrying...'); 
    setTimeout(() => { 
      server.close(); 
      server.listen(PORT, HOST); 
    }, 1000); 
  }
});
Added in: v0.5.10
handle  <Object>
backlog  <number>  Common parameter of server.listen()  functions
callback  <Function>  Common parameter of server.listen()  functions
Returns: <net.Server>
Start a server listening for connections on a given handle  that has already been bound to a port, a
UNIX domain socket, or a Windows named pipe.
server.listen(handle[, backlog][, callback])
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The handle  object can be either a server, a socket (anything with an underlying _handle  member),
or an object with an fd  member that is a valid file descriptor.
Note: Listening on a file descriptor is not supported on Windows.
Added in: v0.11.14
options  <Object>  Required. Supports the following properties:
port  <number>
host  <string>
path  <string>  Will be ignored if port  is specified. See Identifying paths for IPC
connections .
backlog  <number>  Common parameter of server.listen()  functions.
exclusive  <boolean>  Default: false
callback  <Function>  Common parameter of server.listen()  functions.
Returns: <net.Server>
If port  is specified, it behaves the same as server.listen([port][, hostname][, backlog][,
callback]) . Otherwise, if path  is specified, it behaves the same as server.listen(path[,
backlog][, callback]) . If none of them is specified, an error will be thrown.
If exclusive  is false  (default), then cluster workers will use the same underlying handle, allowing
connection handling duties to be shared. When exclusive  is true , the handle is not shared, and
attempted port sharing results in an error. An example which listens on an exclusive port is shown
below.
server.listen({ 
  host: 'localhost', 
  port: 80, 
  exclusive: true
});
Added in: v0.1.90
path  <string>  Path the server should listen to. See Identifying paths for IPC connections .
server.listen(options[, callback])
#
server.listen(path[, backlog][, callback])
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backlog  <number>  Common parameter of server.listen()  functions.
callback  <Function>  Common parameter of server.listen()  functions.
Returns: <net.Server>
Start a IPC  server listening for connections on the given path .
Added in: v0.1.90
port  <number>
host  <string>
backlog  <number>  Common parameter of server.listen()  functions.
callback  <Function>  Common parameter of server.listen()  functions.
Returns: <net.Server>
Start a TCP server listening for connections on the given port  and host .
If port  is omitted or is 0, the operating system will assign an arbitrary unused port, which can be
retrieved by using server.address().port  after the 'listening'  event has been emitted.
If host  is omitted, the server will accept connections on the unspecified IPv6 address  ( :: ) when
IPv6 is available, or the unspecified IPv4 address  ( 0.0.0.0 ) otherwise.
Note: In most operating systems, listening to the unspecified IPv6 address  ( :: ) may cause the
net.Server  to also listen on the unspecified IPv4 address  ( 0.0.0.0 ).
Added in: v5.7.0
A Boolean indicating whether or not the server is listening for connections.
Added in: v0.2.0
Set this property to reject connections when the server's connection count gets high.
It is not recommended to use this option once a socket has been sent to a child with
child_process.fork() .
server.listen([port][, host][, backlog][, callback])
#
server.listening
#
server.maxConnections
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Added in: v0.9.1
Returns: <net.Server>
Opposite of unref , calling ref  on a previously unref d server will not let the program exit if it's the
only server left (the default behavior). If the server is ref d calling ref  again will have no effect.
Added in: v0.9.1
Returns: <net.Server>
Calling unref  on a server will allow the program to exit if this is the only active server in the event
system. If the server is already unref d calling unref  again will have no effect.
Added in: v0.3.4
This class is an abstraction of a TCP socket or a streaming IPC  endpoint (uses named pipes on
Windows, and UNIX domain sockets otherwise). A net.Socket  is also a duplex stream , so it can be
both readable and writable, and it is also a EventEmitter .
A net.Socket  can be created by the user and used directly to interact with a server. For example, it
is returned by net.createConnection() , so the user can use it to talk to the server.
It can also be created by Node.js and passed to the user when a connection is received. For
example, it is passed to the listeners of a 'connection'  event emitted on a net.Server , so the user
can use it to interact with the client.
Added in: v0.3.4
Creates a new socket object.
options  <Object>  Available options are:
fd : <number>  If specified, wrap around an existing socket with the given file descriptor,
otherwise a new socket will be created.
allowHalfOpen  <boolean>  Indicates whether half-opened TCP connections are allowed.
See net.createServer()  and the 'end'  event for details. Default: false .
server.ref()
#
server.unref()
#
Class: net.Socket
#
new net.Socket( options )
#
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readable  <boolean>  Allow reads on the socket when an fd  is passed, otherwise ignored.
Default: false .
writable  <boolean>  Allow writes on the socket when an fd  is passed, otherwise
ignored. Default: false .
Returns: <net.Socket>
The newly created socket can be either a TCP socket or a streaming IPC  endpoint, depending on
what it connect()  to.
Added in: v0.1.90
had_error  <boolean>  true  if the so