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8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 1/1014 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 View another version ▼ Table of Contents 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 2/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 3/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 4/1014 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() 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 5/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 6/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 7/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 8/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 9/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 10/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 11/1014 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]) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 12/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 13/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 14/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 15/1014 --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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 16/1014 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() 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 17/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 18/1014 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) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 19/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 20/1014 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) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 21/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 22/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 23/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 24/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 25/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 26/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 27/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 28/1014 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) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 29/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 30/1014 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) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 31/1014 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) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 32/1014 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() 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 33/1014 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' 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 34/1014 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]) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 35/1014 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' 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 36/1014 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' 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 37/1014 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' 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 38/1014 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() 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 39/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 40/1014 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]]]) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 41/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 42/1014 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]) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 43/1014 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]) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 44/1014 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() 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 45/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 46/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 47/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 48/1014 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) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 49/1014 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' 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 50/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 51/1014 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]) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 52/1014 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) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 53/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 54/1014 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' 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 55/1014 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' 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 56/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 57/1014 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() 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 58/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 59/1014 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]) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 60/1014 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() 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 61/1014 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) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 62/1014 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]) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 63/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 64/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 65/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 66/1014 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 $env:USERPROFILE\projects Next, create a new source file in the projects folder and call it hello-world.js . In Node.js it is considered good style to use hyphens ( - ) or underscores ( _ ) to separate multiple words in filenames. Open hello-world.js in any preferred text editor and paste in the following content: const http = require('http'); const hostname = '127.0.0.1'; const port = 3000; const server = http.createServer((req, res) => { res.statusCode = 200; res.setHeader('Content-Type', 'text/plain'); res.end('Hello World!\n'); }); server.listen(port, hostname, () => { 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 67/1014 console.log(`Server running at http://${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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 68/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 69/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 70/1014 } }; 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 71/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 72/1014 // 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 73/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 74/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 75/1014 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]]]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 76/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 77/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 78/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 79/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 80/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 81/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 82/1014 // 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 83/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 84/1014 /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: 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 85/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 86/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 87/1014 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) { } 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 88/1014 // 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 89/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 90/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 91/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 92/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 93/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 94/1014 }, 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() . 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 95/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 96/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 97/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 98/1014 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. 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 99/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 100/1014 // 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 101/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 102/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 103/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 104/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 105/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 106/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 107/1014 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: 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 108/1014 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. Buffers and iteration # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 109/1014 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]]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 110/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 111/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 112/1014 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]]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 113/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 114/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 115/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 116/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 117/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 118/1014 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]]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 119/1014 // 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 120/1014 // 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]]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 121/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 122/1014 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] # buf.buffer # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 123/1014 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]]]]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 124/1014 // 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]]]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 125/1014 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); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 126/1014 // 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() # buf.equals(otherBuffer) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 127/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 128/1014 // 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 129/1014 // 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 130/1014 // 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)); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 131/1014 // 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() # buf.lastIndexOf(value[, byteOffset][, encoding]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 132/1014 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')); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 133/1014 // 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 134/1014 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 # buf.readDoubleBE(offset[, noAssert]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 135/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 136/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 137/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 138/1014 // 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 139/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 140/1014 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]) # buf.readUInt16LE(offset[, noAssert]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 141/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 142/1014 // 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]]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 143/1014 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: 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 144/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 145/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 146/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 147/1014 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]]]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 148/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 149/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 150/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 151/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 152/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 153/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 154/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 155/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 156/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 157/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 158/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 159/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 160/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 161/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 162/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 163/1014 } // 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 164/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 165/1014 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() # Native Abstractions for Node.js # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 166/1014 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; N-API # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 167/1014 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 Addon examples # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 168/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 169/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 170/1014 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' }); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 171/1014 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: Object factory # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 172/1014 // 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(); Function factory # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 173/1014 // 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 174/1014 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: 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 175/1014 // 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()); } 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 176/1014 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": [ 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 177/1014 "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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 178/1014 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: 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 179/1014 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() { } 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 180/1014 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); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 181/1014 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()); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 182/1014 // 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 183/1014 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_; } 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 184/1014 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() { } 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 185/1014 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(); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 186/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 187/1014 // 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()); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 188/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 189/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 190/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 191/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 192/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 193/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 194/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 195/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 196/1014 [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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 197/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 198/1014 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: 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 199/1014 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 # napi_throw_error # napi_throw_type_error # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 200/1014 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 # napi_is_error # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 201/1014 [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 # napi_create_type_error # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 202/1014 [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); napi_create_range_error # napi_get_and_clear_last_exception # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 203/1014 [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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 204/1014 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 # Object Lifetime management # Making handle lifespan shorter than that of the native method # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 205/1014 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) { 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 206/1014 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); napi_open_handle_scope # napi_close_handle_scope # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 207/1014 [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 # napi_close_escapable_handle_scope # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 208/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 209/1014 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 # napi_delete_reference # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 210/1014 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 napi_reference_ref # napi_reference_unref # napi_get_reference_value # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 211/1014 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 # napi_add_env_cleanup_hook # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 212/1014 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 # Module registration # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 213/1014 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; } 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 214/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 215/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 216/1014 [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. napi_create_array_with_length # napi_create_arraybuffer # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 217/1014 [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, napi_create_buffer # napi_create_buffer_copy # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 218/1014 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. napi_create_external # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 219/1014 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, napi_create_external_arraybuffer # napi_create_external_buffer # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 220/1014 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. napi_create_function # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 221/1014 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 # napi_create_symbol # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 222/1014 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, napi_create_typedarray # napi_create_dataview # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 223/1014 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 # napi_create_int32 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 224/1014 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. napi_create_uint32 # napi_create_int64 # napi_create_double # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 225/1014 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 napi_create_string_latin1 # napi_create_string_utf16 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 226/1014 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. napi_create_string_utf8 # Functions to convert from N-API to C types # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 227/1014 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 # napi_get_arraybuffer_info # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 228/1014 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. napi_get_buffer_info # napi_get_prototype # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 229/1014 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 # napi_get_dataview_info # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 230/1014 [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 . napi_get_value_bool # napi_get_value_double # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 231/1014 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 # napi_get_value_int32 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 232/1014 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 # napi_get_value_string_latin1 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 233/1014 [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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 234/1014 [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 # napi_get_boolean # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 235/1014 [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 # napi_get_null # napi_get_undefined # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 236/1014 [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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 237/1014 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 # napi_coerce_to_string # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 238/1014 [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 # napi_instanceof # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 239/1014 [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 # napi_is_arraybuffer # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 240/1014 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 # napi_is_typedarray # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 241/1014 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 # napi_strict_equals # Working with JavaScript Properties # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 242/1014 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: 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 243/1014 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] 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 244/1014 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; 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 245/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 246/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 247/1014 [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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 248/1014 [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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 249/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 250/1014 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 # napi_has_named_property # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 251/1014 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 # napi_get_element # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 252/1014 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 # napi_delete_element # napi_define_properties # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 253/1014 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 # napi_call_function # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 254/1014 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; 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 255/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 256/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 257/1014 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. napi_get_new_target # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 258/1014 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; napi_new_instance # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 259/1014 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); Object Wrap # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 260/1014 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: napi_define_class # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 261/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 262/1014 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. napi_unwrap # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 263/1014 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 . napi_remove_wrap # Simple Asynchronous Operations # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 264/1014 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); napi_create_async_work # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 265/1014 [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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 266/1014 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 # napi_cancel_async_work # Custom Asynchronous Operations # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 267/1014 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 # napi_async_destroy # napi_make_callback # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 268/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 269/1014 [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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 270/1014 [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 # Memory Management # napi_adjust_external_memory # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 271/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 272/1014 // 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 273/1014 [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 # napi_reject_deferred # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 274/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 275/1014 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 # napi_get_uv_event_loop # Asynchronous Thread-safe Function Calls # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 276/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 277/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 278/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 279/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 280/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 281/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 282/1014 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. 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 283/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 284/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 285/1014 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) => { 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 286/1014 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(); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 287/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 288/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 289/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 290/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 291/1014 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']); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 292/1014 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.'); }); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 293/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 294/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 295/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 296/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 297/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 298/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 299/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 300/1014 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 # Event: 'close' # Event: 'disconnect' # Event: 'error' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 301/1014 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' # Event: 'message' # subprocess.channel # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 302/1014 <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 # subprocess.disconnect() # subprocess.kill([signal]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 303/1014 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'] 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 304/1014 } ); 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: ${grep.pid}`); grep.stdin.end(); subprocess.killed # subprocess.pid # subprocess.send(message[, sendHandle[, options]][, callback]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 305/1014 History message <Object> sendHandle <Handle> options <Object> The options argument, if present, is an object used to parameterize the sending of certain types of handles. options supports the following properties: keepOpen - A Boolean value that can be used when passing instances of net.Socket . When true , the socket is kept open in the sending process. Default: false . callback <Function> Returns: <boolean> When an IPC channel has been established between the parent and child ( i.e. when using child_process.fork() ), the subprocess.send() method can be used to send messages to the child process. When the child process is a Node.js instance, these messages can be received via the process.on('message') event. Note: The message goes through serialization and parsing. The resulting message might not be the same as what is originally sent. For example, in the parent script: const cp = require('child_process'); const n = cp.fork(`${__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 }); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 306/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 307/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 308/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 309/1014 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 # subprocess.stdio # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 310/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 311/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 312/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 313/1014 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' # Event: 'error' 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 314/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 315/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 316/1014 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. History 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 317/1014 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) => { 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 318/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 319/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 320/1014 History 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); }); } History 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 321/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 322/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 323/1014 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) History 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 324/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 325/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 326/1014 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. History cluster.settings # cluster.setupMaster([settings]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 327/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 328/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 329/1014 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. History 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" # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 330/1014 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. History 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] # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 331/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 332/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 333/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 334/1014 │ ├── 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 335/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 336/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 337/1014 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... # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 338/1014 --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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 339/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 340/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 341/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 342/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 343/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 344/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 345/1014 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']) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 346/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 347/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 348/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 349/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 350/1014 // 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 351/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 352/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 353/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 354/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 355/1014 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'); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 356/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 357/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 358/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 359/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 360/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 361/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 362/1014 // 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 363/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 364/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 365/1014 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]]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 366/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 367/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 368/1014 ); // 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 369/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 370/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 371/1014 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]) # hmac.update(data[, inputEncoding]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 372/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 373/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 374/1014 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]) # Class: Verify # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 375/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 376/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 377/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 378/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 379/1014 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) # crypto.createDecipher(algorithm, password[, options]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 380/1014 History 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 381/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 382/1014 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')} ${filename}`); } }); Added in: v0.1.94 algorithm <string> key <string> | <Buffer> | <TypedArray> | <DataView> options <Object> stream.transform options Returns: <Hmac> Creates and returns an Hmac object that uses the given algorithm and key . Optional options argument controls stream behavior. The algorithm is dependent on the available algorithms supported by the version of OpenSSL on the platform. Examples are 'sha256' , 'sha512' , etc. On recent releases of OpenSSL, openssl list-message-digest-algorithms will display the available digest algorithms. The key is the HMAC key used to generate the cryptographic HMAC hash. Example: generating the sha256 HMAC of a file crypto.createHmac(algorithm, key[, options]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 383/1014 const filename = process.argv[2]; const crypto = require('crypto'); const fs = require('fs'); const hmac = crypto.createHmac('sha256', 'a secret'); const input = fs.createReadStream(filename); input.on('readable', () => { const data = input.read(); if (data) hmac.update(data); else { console.log(`${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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 384/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 385/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 386/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 387/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 388/1014 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) # crypto.publicDecrypt(key, buffer) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 389/1014 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) # crypto.randomBytes(size[, callback]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 390/1014 // 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 391/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 392/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 393/1014 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) # Notes # Legacy Streams API (pre Node.js v0.10) # Recent ECDH Changes # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 394/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 395/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 396/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 397/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 398/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 399/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 400/1014 < 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 401/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 402/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 403/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 404/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 405/1014 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 # DEP0007: cluster worker.suicide # DEP0008: require('constants') # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 406/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 407/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 408/1014 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 # DEP0022: os.tmpDir() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 409/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 410/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 411/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 412/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 413/1014 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() # DEP0046: util.isBuffer() # DEP0047: util.isDate() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 414/1014 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() # DEP0051: util.isNullOrUndefined() # DEP0052: util.isNumber() # DEP0053 util.isObject() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 415/1014 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() # DEP0056: util.isString() # DEP0057: util.isSymbol() # DEP0058: util.isUndefined() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 416/1014 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() # DEP0061: fs.SyncWriteStream # DEP0062: node --debug # DEP0063: ServerResponse.prototype.writeHeader() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 417/1014 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() # DEP0065: repl.REPL_MODE_MAGIC and NODE_REPL_MODE=magic # DEP0066: outgoingMessage._headers, outgoingMessage._headerNames # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 418/1014 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 # DEP0068: node debug # DEP0069: vm.runInDebugContext(string) # DEP0070: async_hooks.currentId() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 419/1014 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: DEP0071: async_hooks.triggerId() # DEP0072: async_hooks.AsyncResource.triggerId() # DEP0076: tls.parseCertString() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 420/1014 > 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() # DEP0085: AsyncHooks Sensitive API # DEP0086: Remove runInAsyncIdScope # DEP0089: require('assert') # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 421/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 422/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 423/1014 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() # dns.getServers() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 424/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 425/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 426/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 427/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 428/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 429/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 430/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 431/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 432/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 433/1014 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 } ] 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 434/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 435/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 436/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 437/1014 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! # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 438/1014 }).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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 439/1014 // 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. 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 440/1014 // 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 441/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 442/1014 // 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 443/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 444/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 445/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 446/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 447/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 448/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 449/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 450/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 451/1014 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' 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 452/1014 }; } 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 453/1014 <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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 454/1014 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). 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 455/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 456/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 457/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 458/1014 // 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 459/1014 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) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 460/1014 // 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 461/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 462/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 463/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 464/1014 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. 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 465/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 466/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 467/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 468/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 469/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 470/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 471/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 472/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 473/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 474/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 475/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 476/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 477/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 478/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 479/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 480/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 481/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 482/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 483/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 484/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 485/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 486/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 487/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 488/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 489/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 490/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 491/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 492/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 493/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 494/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 495/1014 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(); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 496/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 497/1014 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; 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 498/1014 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. 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 499/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 500/1014 // - 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: 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 501/1014 // 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 502/1014 '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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 503/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 504/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 505/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 506/1014 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); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 507/1014 }); }); write (RECOMMENDED) fs.open('myfile', 'wx', (err, fd) => { if (err) { if (err.code === 'EEXIST') { console.error('myfile already exists'); return; } throw err; } writeMyData(fd); }); read (NOT RECOMMENDED) fs.access('myfile', (err) => { if (err) { if (err.code === 'ENOENT') { console.error('myfile does not exist'); return; } throw err; } fs.open('myfile', 'r', (err, fd) => { if (err) throw err; readMyData(fd); }); }); read (RECOMMENDED) 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 508/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 509/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 510/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 511/1014 /* 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 512/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 513/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 514/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 515/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 516/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 517/1014 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: 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 518/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 519/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 520/1014 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'); } }); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 521/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 522/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 523/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 524/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 525/1014 }); // 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 526/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 527/1014 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. History fs.lchown(path, uid, gid, callback) # fs.lchownSync(path, uid, gid) # fs.link(existingPath, newPath, callback) # fs.linkSync(existingPath, newPath) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 528/1014 existingPath <string> | <Buffer> | <URL> newPath <string> | <Buffer> | <URL> Synchronous link(2) . Returns undefined . History 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. History path <string> | <Buffer> | <URL> Synchronous lstat(2) . Returns an instance of fs.Stats . History 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 529/1014 History 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) History 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 530/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 531/1014 The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use. History 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 532/1014 '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> }); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 533/1014 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. History 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 534/1014 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. History 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. History 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 535/1014 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. History 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 536/1014 // 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. History 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 537/1014 // FreeBSD fs.readFileSync('<directory>'); // => null, <data> History 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 538/1014 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. History path <string> | <Buffer> | <URL> fs.realpath(path[, options], callback) # fs.realpathSync(path[, options]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 539/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 540/1014 newPath <string> | <Buffer> | <URL> Synchronous rename(2) . Returns undefined . History 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. History 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 541/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 542/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 543/1014 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) History 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. History fs.unlinkSync(path) # fs.unwatchFile(filename[, listener]) # fs.utimes(path, atime, mtime, callback) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 544/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 545/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 546/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 547/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 548/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 549/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 550/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 551/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 552/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 553/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 554/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 555/1014 __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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 556/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 557/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 558/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 559/1014 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 ) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 560/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 561/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 562/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 563/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 564/1014 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); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 565/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 566/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 567/1014 // 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 568/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 569/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 570/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 571/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 572/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 573/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 574/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 575/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 576/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 577/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 578/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 579/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 580/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 581/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 582/1014 // 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 583/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 584/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 585/1014 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. 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 586/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 587/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 588/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 589/1014 // 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 590/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 591/1014 '/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' } 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 592/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 593/1014 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}`); }); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 594/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 595/1014 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', 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 596/1014 '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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 597/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 598/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 599/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 600/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 601/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 602/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 603/1014 /** 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 604/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 605/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 606/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 607/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 608/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 609/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 610/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 611/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 612/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 613/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 614/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 615/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 616/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 617/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 618/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 619/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 620/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 621/1014 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 # http2stream.session # http2stream.setTimeout(msecs, callback) # http2stream.state # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 622/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 623/1014 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' # Event: 'headers' # Event: 'push' # Event: 'response' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 624/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 625/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 626/1014 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]]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 627/1014 }); 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]]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 628/1014 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' }); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 629/1014 }); 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]]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 630/1014 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. 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 631/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 632/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 633/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 634/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 635/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 636/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 637/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 638/1014 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', 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 639/1014 ':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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 640/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 641/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 642/1014 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 **/ 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 643/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 644/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 645/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 646/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 647/1014 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 # Error Handling # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 648/1014 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 # Push streams on the client # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 649/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 650/1014 }); 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 651/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 652/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 653/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 654/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 655/1014 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) # request.socket # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 656/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 657/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 658/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 659/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 660/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 661/1014 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) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 662/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 663/1014 <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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 664/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 665/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 666/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 667/1014 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 . 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 668/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 669/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 670/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 671/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 672/1014 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() : 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 673/1014 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'); 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 674/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 675/1014 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> # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 676/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 677/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 678/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 679/1014 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 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 680/1014 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 ) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 681/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 682/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 683/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 684/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 685/1014 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... # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 686/1014 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, 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 687/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 688/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 689/1014 $ 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 690/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 691/1014 /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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 692/1014 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 # __filename # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 693/1014 <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 # module # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 694/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 695/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 696/1014 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 # module.exports # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 697/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 698/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 699/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 700/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 701/1014 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' # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 702/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 703/1014 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() # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 704/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 705/1014 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]) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 706/1014 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 # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 707/1014 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 ) # 8/28/2019 About this Documentation | Node.js v8.16.1 Documentation https://nodejs.org/docs/latest-v8.x/api/all.html 708/1014 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