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delphirtl / Worker

Class: Worker

The Worker class represents an independent JavaScript execution thread. Most Node.js APIs are available inside of it.

Notable differences inside a Worker environment are:

Creating Worker instances inside of other Workers is possible.

Like Web Workers and the node:cluster module, two-way communication can be achieved through inter-thread message passing. Internally, a Worker has a built-in pair of MessagePort s that are already associated with each other when the Worker is created. While the MessagePort object on the parent side is not directly exposed, its functionalities are exposed through worker.postMessage() and the worker.on('message') event on the Worker object for the parent thread.

To create custom messaging channels (which is encouraged over using the default global channel because it facilitates separation of concerns), users can create a MessageChannel object on either thread and pass one of theMessagePorts on that MessageChannel to the other thread through a pre-existing channel, such as the global one.

See port.postMessage() for more information on how messages are passed, and what kind of JavaScript values can be successfully transported through the thread barrier.

import assert from 'node:assert';
import {
  Worker, MessageChannel, MessagePort, isMainThread, parentPort,
} from 'node:worker_threads';
if (isMainThread) {
  const worker = new Worker(__filename);
  const subChannel = new MessageChannel();
  worker.postMessage({ hereIsYourPort: subChannel.port1 }, [subChannel.port1]);
  subChannel.port2.on('message', (value) => {
    console.log('received:', value);
  });
} else {
  parentPort.once('message', (value) => {
    assert(value.hereIsYourPort instanceof MessagePort);
    value.hereIsYourPort.postMessage('the worker is sending this');
    value.hereIsYourPort.close();
  });
}

Since

v10.5.0

Extends

Constructors

new Worker()

new Worker(filename, options?): Worker

Parameters

• filename: string | URL

The path to the Worker’s main script or module. Must be either an absolute path or a relative path (i.e. relative to the current working directory) starting with ./ or ../, or a WHATWG URL object using file: protocol. If options.eval is true, this is a string containing JavaScript code rather than a path.

• options?: WorkerOptions

Returns

Worker

Overrides

EventEmitter.constructor

Defined in

node_modules/@types/node/worker_threads.d.ts:405

Properties

performance

readonly performance: WorkerPerformance

An object that can be used to query performance information from a worker instance. Similar to perf_hooks.performance.

Since

v15.1.0, v14.17.0, v12.22.0

Defined in

node_modules/@types/node/worker_threads.d.ts:399

resourceLimits?

readonly optional resourceLimits: ResourceLimits

Provides the set of JS engine resource constraints for this Worker thread. If the resourceLimits option was passed to the Worker constructor, this matches its values.

If the worker has stopped, the return value is an empty object.

Since

v13.2.0, v12.16.0

Defined in

node_modules/@types/node/worker_threads.d.ts:393

stderr

readonly stderr: Readable

This is a readable stream which contains data written to process.stderr inside the worker thread. If stderr: true was not passed to the Worker constructor, then data is piped to the parent thread’s process.stderr stream.

Since

v10.5.0

Defined in

node_modules/@types/node/worker_threads.d.ts:377

stdin

readonly stdin: null | Writable

If stdin: true was passed to the Worker constructor, this is a writable stream. The data written to this stream will be made available in the worker thread as process.stdin.

Since

v10.5.0

Defined in

node_modules/@types/node/worker_threads.d.ts:365

stdout

readonly stdout: Readable

This is a readable stream which contains data written to process.stdout inside the worker thread. If stdout: true was not passed to the Worker constructor, then data is piped to the parent thread’s process.stdout stream.

Since

v10.5.0

Defined in

node_modules/@types/node/worker_threads.d.ts:371

threadId

readonly threadId: number

An integer identifier for the referenced thread. Inside the worker thread, it is available as import { threadId } from 'node:worker_threads'. This value is unique for each Worker instance inside a single process.

Since

v10.5.0

Defined in

node_modules/@types/node/worker_threads.d.ts:384

captureRejections

static captureRejections: boolean

Value: boolean

Change the default captureRejections option on all new EventEmitter objects.

Since

v13.4.0, v12.16.0

Inherited from

EventEmitter.captureRejections

Defined in

node_modules/@types/node/events.d.ts:459

captureRejectionSymbol

readonly static captureRejectionSymbol: typeof captureRejectionSymbol

Value: Symbol.for('nodejs.rejection')

See how to write a custom rejection handler.

Since

v13.4.0, v12.16.0

Inherited from

EventEmitter.captureRejectionSymbol

Defined in

node_modules/@types/node/events.d.ts:452

defaultMaxListeners

static defaultMaxListeners: number

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 allEventEmitter instances, the events.defaultMaxListeners property can be used. If this value is not a positive number, a RangeError is thrown.

Take caution when setting the events.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 events.defaultMaxListeners.

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:

import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
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 and the number of attached listeners, respectively. Its name property is set to 'MaxListenersExceededWarning'.

Since

v0.11.2

Inherited from

EventEmitter.defaultMaxListeners

Defined in

node_modules/@types/node/events.d.ts:498

errorMonitor

readonly static errorMonitor: typeof errorMonitor

This symbol shall be used to install a listener for only monitoring 'error' events. Listeners installed using this symbol are called before the regular 'error' listeners are called.

Installing a listener using this symbol does not change the behavior once an 'error' event is emitted. Therefore, the process will still crash if no regular 'error' listener is installed.

Since

v13.6.0, v12.17.0

Inherited from

EventEmitter.errorMonitor

Defined in

node_modules/@types/node/events.d.ts:445

Methods

[captureRejectionSymbol]()?

optional [captureRejectionSymbol]<K>(error, event, …args): void

Type Parameters

• K

Parameters

• error: Error

• event: string | symbol

• …args: AnyRest

Returns

void

Inherited from

EventEmitter.[captureRejectionSymbol]

Defined in

node_modules/@types/node/events.d.ts:136

addListener()

addListener(event, listener)

addListener(event, listener): this

Alias for emitter.on(eventName, listener).

Parameters

• event: "error"

• listener

Returns

this

Since

v0.1.26

Overrides

EventEmitter.addListener

Defined in

node_modules/@types/node/worker_threads.d.ts:459

addListener(event, listener)

addListener(event, listener): this

Parameters

• event: "exit"

• listener

Returns

this

Overrides

EventEmitter.addListener

Defined in

node_modules/@types/node/worker_threads.d.ts:460

addListener(event, listener)

addListener(event, listener): this

Parameters

• event: "message"

• listener

Returns

this

Overrides

EventEmitter.addListener

Defined in

node_modules/@types/node/worker_threads.d.ts:461

addListener(event, listener)

addListener(event, listener): this

Parameters

• event: "messageerror"

• listener

Returns

this

Overrides

EventEmitter.addListener

Defined in

node_modules/@types/node/worker_threads.d.ts:462

addListener(event, listener)

addListener(event, listener): this

Parameters

• event: "online"

• listener

Returns

this

Overrides

EventEmitter.addListener

Defined in

node_modules/@types/node/worker_threads.d.ts:463

addListener(event, listener)

addListener(event, listener): this

Parameters

• event: string | symbol

• listener

Returns

this

Overrides

EventEmitter.addListener

Defined in

node_modules/@types/node/worker_threads.d.ts:464

emit()

emit(event, err)

emit(event, err): boolean

Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.

Returns true if the event had listeners, false otherwise.

import { EventEmitter } from 'node:events';
const myEmitter = new EventEmitter();

// First listener
myEmitter.on('event', function firstListener() {
  console.log('Helloooo! first listener');
});
// Second listener
myEmitter.on('event', function secondListener(arg1, arg2) {
  console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
});
// Third listener
myEmitter.on('event', function thirdListener(...args) {
  const parameters = args.join(', ');
  console.log(`event with parameters ${parameters} in third listener`);
});

console.log(myEmitter.listeners('event'));

myEmitter.emit('event', 1, 2, 3, 4, 5);

// Prints:
// [
//   [Function: firstListener],
//   [Function: secondListener],
//   [Function: thirdListener]
// ]
// Helloooo! first listener
// event with parameters 1, 2 in second listener
// event with parameters 1, 2, 3, 4, 5 in third listener
Parameters

• event: "error"

• err: Error

Returns

boolean

Since

v0.1.26

Overrides

EventEmitter.emit

Defined in

node_modules/@types/node/worker_threads.d.ts:465

emit(event, exitCode)

emit(event, exitCode): boolean

Parameters

• event: "exit"

• exitCode: number

Returns

boolean

Overrides

EventEmitter.emit

Defined in

node_modules/@types/node/worker_threads.d.ts:466

emit(event, value)

emit(event, value): boolean

Parameters

• event: "message"

• value: any

Returns

boolean

Overrides

EventEmitter.emit

Defined in

node_modules/@types/node/worker_threads.d.ts:467

emit(event, error)

emit(event, error): boolean

Parameters

• event: "messageerror"

• error: Error

Returns

boolean

Overrides

EventEmitter.emit

Defined in

node_modules/@types/node/worker_threads.d.ts:468

emit(event)

emit(event): boolean

Parameters

• event: "online"

Returns

boolean

Overrides

EventEmitter.emit

Defined in

node_modules/@types/node/worker_threads.d.ts:469

emit(event, args)

emit(event, …args): boolean

Parameters

• event: string | symbol

• …args: any[]

Returns

boolean

Overrides

EventEmitter.emit

Defined in

node_modules/@types/node/worker_threads.d.ts:470

eventNames()

eventNames(): (string | symbol)[]

Returns an array listing the events for which the emitter has registered listeners. The values in the array are strings or Symbols.

import { EventEmitter } from 'node:events';

const myEE = new EventEmitter();
myEE.on('foo', () => {});
myEE.on('bar', () => {});

const sym = Symbol('symbol');
myEE.on(sym, () => {});

console.log(myEE.eventNames());
// Prints: [ 'foo', 'bar', Symbol(symbol) ]

Returns

(string | symbol)[]

Since

v6.0.0

Inherited from

EventEmitter.eventNames

Defined in

node_modules/@types/node/events.d.ts:922

getHeapSnapshot()

getHeapSnapshot(): Promise<Readable>

Returns a readable stream for a V8 snapshot of the current state of the Worker. See v8.getHeapSnapshot() for more details.

If the Worker thread is no longer running, which may occur before the 'exit' event is emitted, the returned Promise is rejected immediately with an ERR_WORKER_NOT_RUNNING error.

Returns

Promise<Readable>

A promise for a Readable Stream containing a V8 heap snapshot

Since

v13.9.0, v12.17.0

Defined in

node_modules/@types/node/worker_threads.d.ts:458

getMaxListeners()

getMaxListeners(): number

Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to defaultMaxListeners.

Returns

number

Since

v1.0.0

Inherited from

EventEmitter.getMaxListeners

Defined in

node_modules/@types/node/events.d.ts:774

listenerCount()

listenerCount<K>(eventName, listener?): number

Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.

Type Parameters

• K

Parameters

• eventName: string | symbol

The name of the event being listened for

• listener?: Function

The event handler function

Returns

number

Since

v3.2.0

Inherited from

EventEmitter.listenerCount

Defined in

node_modules/@types/node/events.d.ts:868

listeners()

listeners<K>(eventName): Function[]

Returns a copy of the array of listeners for the event named eventName.

server.on('connection', (stream) => {
  console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection')));
// Prints: [ [Function] ]

Type Parameters

• K

Parameters

• eventName: string | symbol

Returns

Function[]

Since

v0.1.26

Inherited from

EventEmitter.listeners

Defined in

node_modules/@types/node/events.d.ts:787

off()

off(event, listener)

off(event, listener): this

Alias for emitter.removeListener().

Parameters

• event: "error"

• listener

Returns

this

Since

v10.0.0

Overrides

EventEmitter.off

Defined in

node_modules/@types/node/worker_threads.d.ts:501

off(event, listener)

off(event, listener): this

Parameters

• event: "exit"

• listener

Returns

this

Overrides

EventEmitter.off

Defined in

node_modules/@types/node/worker_threads.d.ts:502

off(event, listener)

off(event, listener): this

Parameters

• event: "message"

• listener

Returns

this

Overrides

EventEmitter.off

Defined in

node_modules/@types/node/worker_threads.d.ts:503

off(event, listener)

off(event, listener): this

Parameters

• event: "messageerror"

• listener

Returns

this

Overrides

EventEmitter.off

Defined in

node_modules/@types/node/worker_threads.d.ts:504

off(event, listener)

off(event, listener): this

Parameters

• event: "online"

• listener

Returns

this

Overrides

EventEmitter.off

Defined in

node_modules/@types/node/worker_threads.d.ts:505

off(event, listener)

off(event, listener): this

Parameters

• event: string | symbol

• listener

Returns

this

Overrides

EventEmitter.off

Defined in

node_modules/@types/node/worker_threads.d.ts:506

on()

on(event, listener)

on(event, listener): this

Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

server.on('connection', (stream) => {
  console.log('someone connected!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a
Parameters

• event: "error"

• listener

The callback function

Returns

this

Since

v0.1.101

Overrides

EventEmitter.on

Defined in

node_modules/@types/node/worker_threads.d.ts:471

on(event, listener)

on(event, listener): this

Parameters

• event: "exit"

• listener

Returns

this

Overrides

EventEmitter.on

Defined in

node_modules/@types/node/worker_threads.d.ts:472

on(event, listener)

on(event, listener): this

Parameters

• event: "message"

• listener

Returns

this

Overrides

EventEmitter.on

Defined in

node_modules/@types/node/worker_threads.d.ts:473

on(event, listener)

on(event, listener): this

Parameters

• event: "messageerror"

• listener

Returns

this

Overrides

EventEmitter.on

Defined in

node_modules/@types/node/worker_threads.d.ts:474

on(event, listener)

on(event, listener): this

Parameters

• event: "online"

• listener

Returns

this

Overrides

EventEmitter.on

Defined in

node_modules/@types/node/worker_threads.d.ts:475

on(event, listener)

on(event, listener): this

Parameters

• event: string | symbol

• listener

Returns

this

Overrides

EventEmitter.on

Defined in

node_modules/@types/node/worker_threads.d.ts:476

once()

once(event, listener)

once(event, listener): this

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.

import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a
Parameters

• event: "error"

• listener

The callback function

Returns

this

Since

v0.3.0

Overrides

EventEmitter.once

Defined in

node_modules/@types/node/worker_threads.d.ts:477

once(event, listener)

once(event, listener): this

Parameters

• event: "exit"

• listener

Returns

this

Overrides

EventEmitter.once

Defined in

node_modules/@types/node/worker_threads.d.ts:478

once(event, listener)

once(event, listener): this

Parameters

• event: "message"

• listener

Returns

this

Overrides

EventEmitter.once

Defined in

node_modules/@types/node/worker_threads.d.ts:479

once(event, listener)

once(event, listener): this

Parameters

• event: "messageerror"

• listener

Returns

this

Overrides

EventEmitter.once

Defined in

node_modules/@types/node/worker_threads.d.ts:480

once(event, listener)

once(event, listener): this

Parameters

• event: "online"

• listener

Returns

this

Overrides

EventEmitter.once

Defined in

node_modules/@types/node/worker_threads.d.ts:481

once(event, listener)

once(event, listener): this

Parameters

• event: string | symbol

• listener

Returns

this

Overrides

EventEmitter.once

Defined in

node_modules/@types/node/worker_threads.d.ts:482

postMessage()

postMessage(value, transferList?): void

Send a message to the worker that is received via require('node:worker_threads').parentPort.on('message'). See port.postMessage() for more details.

Parameters

• value: any

• transferList?: readonly TransferListItem[]

Returns

void

Since

v10.5.0

Defined in

node_modules/@types/node/worker_threads.d.ts:411

postMessageToThread()

postMessageToThread(threadId, value, timeout)

postMessageToThread(threadId, value, timeout?): Promise<void>

Sends a value to another worker, identified by its thread ID.

Parameters

• threadId: number

The target thread ID. If the thread ID is invalid, a ERR_WORKER_MESSAGING_FAILED error will be thrown. If the target thread ID is the current thread ID, a ERR_WORKER_MESSAGING_SAME_THREAD error will be thrown.

• value: any

The value to send.

• timeout?: number

Time to wait for the message to be delivered in milliseconds. By default it’s undefined, which means wait forever. If the operation times out, a ERR_WORKER_MESSAGING_TIMEOUT error is thrown.

Returns

Promise<void>

Since

v22.5.0

Defined in

node_modules/@types/node/worker_threads.d.ts:423

postMessageToThread(threadId, value, transferList, timeout)

postMessageToThread(threadId, value, transferList, timeout?): Promise<void>

Parameters

• threadId: number

• value: any

• transferList: readonly TransferListItem[]

• timeout?: number

Returns

Promise<void>

Defined in

node_modules/@types/node/worker_threads.d.ts:424

prependListener()

prependListener(event, listener)

prependListener(event, listener): this

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.

Parameters

• event: "error"

• listener

The callback function

Returns

this

Since

v6.0.0

Overrides

EventEmitter.prependListener

Defined in

node_modules/@types/node/worker_threads.d.ts:483

prependListener(event, listener)

prependListener(event, listener): this

Parameters

• event: "exit"

• listener

Returns

this

Overrides

EventEmitter.prependListener

Defined in

node_modules/@types/node/worker_threads.d.ts:484

prependListener(event, listener)

prependListener(event, listener): this

Parameters

• event: "message"

• listener

Returns

this

Overrides

EventEmitter.prependListener

Defined in

node_modules/@types/node/worker_threads.d.ts:485

prependListener(event, listener)

prependListener(event, listener): this

Parameters

• event: "messageerror"

• listener

Returns

this

Overrides

EventEmitter.prependListener

Defined in

node_modules/@types/node/worker_threads.d.ts:486

prependListener(event, listener)

prependListener(event, listener): this

Parameters

• event: "online"

• listener

Returns

this

Overrides

EventEmitter.prependListener

Defined in

node_modules/@types/node/worker_threads.d.ts:487

prependListener(event, listener)

prependListener(event, listener): this

Parameters

• event: string | symbol

• listener

Returns

this

Overrides

EventEmitter.prependListener

Defined in

node_modules/@types/node/worker_threads.d.ts:488

prependOnceListener()

prependOnceListener(event, listener)

prependOnceListener(event, listener): this

Adds a one-timelistener 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!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

Parameters

• event: "error"

• listener

The callback function

Returns

this

Since

v6.0.0

Overrides

EventEmitter.prependOnceListener

Defined in

node_modules/@types/node/worker_threads.d.ts:489

prependOnceListener(event, listener)

prependOnceListener(event, listener): this

Parameters

• event: "exit"

• listener

Returns

this

Overrides

EventEmitter.prependOnceListener

Defined in

node_modules/@types/node/worker_threads.d.ts:490

prependOnceListener(event, listener)

prependOnceListener(event, listener): this

Parameters

• event: "message"

• listener

Returns

this

Overrides

EventEmitter.prependOnceListener

Defined in

node_modules/@types/node/worker_threads.d.ts:491

prependOnceListener(event, listener)

prependOnceListener(event, listener): this

Parameters

• event: "messageerror"

• listener

Returns

this

Overrides

EventEmitter.prependOnceListener

Defined in

node_modules/@types/node/worker_threads.d.ts:492

prependOnceListener(event, listener)

prependOnceListener(event, listener): this

Parameters

• event: "online"

• listener

Returns

this

Overrides

EventEmitter.prependOnceListener

Defined in

node_modules/@types/node/worker_threads.d.ts:493

prependOnceListener(event, listener)

prependOnceListener(event, listener): this

Parameters

• event: string | symbol

• listener

Returns

this

Overrides

EventEmitter.prependOnceListener

Defined in

node_modules/@types/node/worker_threads.d.ts:494

rawListeners()

rawListeners<K>(eventName): Function[]

Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).

import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.once('log', () => console.log('log once'));

// Returns a new Array with a function `onceWrapper` which has a property
// `listener` which contains the original listener bound above
const listeners = emitter.rawListeners('log');
const logFnWrapper = listeners[0];

// Logs "log once" to the console and does not unbind the `once` event
logFnWrapper.listener();

// Logs "log once" to the console and removes the listener
logFnWrapper();

emitter.on('log', () => console.log('log persistently'));
// Will return a new Array with a single function bound by `.on()` above
const newListeners = emitter.rawListeners('log');

// Logs "log persistently" twice
newListeners[0]();
emitter.emit('log');

Type Parameters

• K

Parameters

• eventName: string | symbol

Returns

Function[]

Since

v9.4.0

Inherited from

EventEmitter.rawListeners

Defined in

node_modules/@types/node/events.d.ts:818

ref()

ref(): void

Opposite of unref(), calling ref() on a previously unref()ed worker does not let the program exit if it’s the only active handle left (the default behavior). If the worker is ref()ed, calling ref() again has no effect.

Returns

void

Since

v10.5.0

Defined in

node_modules/@types/node/worker_threads.d.ts:436

removeAllListeners()

removeAllListeners(eventName?): this

Removes all listeners, or those of the specified eventName.

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.

Parameters

• eventName?: string | symbol

Returns

this

Since

v0.1.26

Inherited from

EventEmitter.removeAllListeners

Defined in

node_modules/@types/node/events.d.ts:758

removeListener()

removeListener(event, listener)

removeListener(event, listener): this

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.

Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them fromemit() in progress. Subsequent events behave as expected.

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
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:

import { EventEmitter } from 'node:events';
const ee = new EventEmitter();

function pong() {
  console.log('pong');
}

ee.on('ping', pong);
ee.once('ping', pong);
ee.removeListener('ping', pong);

ee.emit('ping');
ee.emit('ping');

Returns a reference to the EventEmitter, so that calls can be chained.

Parameters

• event: "error"

• listener

Returns

this

Since

v0.1.26

Overrides

EventEmitter.removeListener

Defined in

node_modules/@types/node/worker_threads.d.ts:495

removeListener(event, listener)

removeListener(event, listener): this

Parameters

• event: "exit"

• listener

Returns

this

Overrides

EventEmitter.removeListener

Defined in

node_modules/@types/node/worker_threads.d.ts:496

removeListener(event, listener)

removeListener(event, listener): this

Parameters

• event: "message"

• listener

Returns

this

Overrides

EventEmitter.removeListener

Defined in

node_modules/@types/node/worker_threads.d.ts:497

removeListener(event, listener)

removeListener(event, listener): this

Parameters

• event: "messageerror"

• listener

Returns

this

Overrides

EventEmitter.removeListener

Defined in

node_modules/@types/node/worker_threads.d.ts:498

removeListener(event, listener)

removeListener(event, listener): this

Parameters

• event: "online"

• listener

Returns

this

Overrides

EventEmitter.removeListener

Defined in

node_modules/@types/node/worker_threads.d.ts:499

removeListener(event, listener)

removeListener(event, listener): this

Parameters

• event: string | symbol

• listener

Returns

this

Overrides

EventEmitter.removeListener

Defined in

node_modules/@types/node/worker_threads.d.ts:500

setMaxListeners()

setMaxListeners(n): this

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. 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.

Parameters

• n: number

Returns

this

Since

v0.3.5

Inherited from

EventEmitter.setMaxListeners

Defined in

node_modules/@types/node/events.d.ts:768

terminate()

terminate(): Promise<number>

Stop all JavaScript execution in the worker thread as soon as possible. Returns a Promise for the exit code that is fulfilled when the 'exit' event is emitted.

Returns

Promise<number>

Since

v10.5.0

Defined in

node_modules/@types/node/worker_threads.d.ts:448

unref()

unref(): void

Calling unref() on a worker allows the thread to exit if this is the only active handle in the event system. If the worker is already unref()ed calling unref() again has no effect.

Returns

void

Since

v10.5.0

Defined in

node_modules/@types/node/worker_threads.d.ts:442

addAbortListener()

static addAbortListener(signal, resource): Disposable

Experimental

Listens once to the abort event on the provided signal.

Listening to the abort event on abort signals is unsafe and may lead to resource leaks since another third party with the signal can call e.stopImmediatePropagation(). Unfortunately Node.js cannot change this since it would violate the web standard. Additionally, the original API makes it easy to forget to remove listeners.

This API allows safely using AbortSignals in Node.js APIs by solving these two issues by listening to the event such that stopImmediatePropagation does not prevent the listener from running.

Returns a disposable so that it may be unsubscribed from more easily.

import { addAbortListener } from 'node:events';

function example(signal) {
  let disposable;
  try {
    signal.addEventListener('abort', (e) => e.stopImmediatePropagation());
    disposable = addAbortListener(signal, (e) => {
      // Do something when signal is aborted.
    });
  } finally {
    disposable?.[Symbol.dispose]();
  }
}

Parameters

• signal: AbortSignal

• resource

Returns

Disposable

Disposable that removes the abort listener.

Since

v20.5.0

Inherited from

EventEmitter.addAbortListener

Defined in

node_modules/@types/node/events.d.ts:437

getEventListeners()

static getEventListeners(emitter, name): Function[]

Returns a copy of the array of listeners for the event named eventName.

For EventEmitters this behaves exactly the same as calling .listeners on the emitter.

For EventTargets this is the only way to get the event listeners for the event target. This is useful for debugging and diagnostic purposes.

import { getEventListeners, EventEmitter } from 'node:events';

{
  const ee = new EventEmitter();
  const listener = () => console.log('Events are fun');
  ee.on('foo', listener);
  console.log(getEventListeners(ee, 'foo')); // [ [Function: listener] ]
}
{
  const et = new EventTarget();
  const listener = () => console.log('Events are fun');
  et.addEventListener('foo', listener);
  console.log(getEventListeners(et, 'foo')); // [ [Function: listener] ]
}

Parameters

• emitter: EventEmitter<DefaultEventMap> | EventTarget

• name: string | symbol

Returns

Function[]

Since

v15.2.0, v14.17.0

Inherited from

EventEmitter.getEventListeners

Defined in

node_modules/@types/node/events.d.ts:358

getMaxListeners()

static getMaxListeners(emitter): number

Returns the currently set max amount of listeners.

For EventEmitters this behaves exactly the same as calling .getMaxListeners on the emitter.

For EventTargets this is the only way to get the max event listeners for the event target. If the number of event handlers on a single EventTarget exceeds the max set, the EventTarget will print a warning.

import { getMaxListeners, setMaxListeners, EventEmitter } from 'node:events';

{
  const ee = new EventEmitter();
  console.log(getMaxListeners(ee)); // 10
  setMaxListeners(11, ee);
  console.log(getMaxListeners(ee)); // 11
}
{
  const et = new EventTarget();
  console.log(getMaxListeners(et)); // 10
  setMaxListeners(11, et);
  console.log(getMaxListeners(et)); // 11
}

Parameters

• emitter: EventEmitter<DefaultEventMap> | EventTarget

Returns

number

Since

v19.9.0

Inherited from

EventEmitter.getMaxListeners

Defined in

node_modules/@types/node/events.d.ts:387

listenerCount()

static listenerCount(emitter, eventName): number

A class method that returns the number of listeners for the given eventName registered on the given emitter.

import { EventEmitter, listenerCount } from 'node:events';

const myEmitter = new EventEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(listenerCount(myEmitter, 'event'));
// Prints: 2

Parameters

• emitter: EventEmitter<DefaultEventMap>

The emitter to query

• eventName: string | symbol

The event name

Returns

number

Since

v0.9.12

Deprecated

Since v3.2.0 - Use listenerCount instead.

Inherited from

EventEmitter.listenerCount

Defined in

node_modules/@types/node/events.d.ts:330

on()

on(emitter, eventName, options)

static on(emitter, eventName, options?): AsyncIterator<any[], any, any>

import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

// Emit later on
process.nextTick(() => {
  ee.emit('foo', 'bar');
  ee.emit('foo', 42);
});

for await (const event of on(ee, 'foo')) {
  // The execution of this inner block is synchronous and it
  // processes one event at a time (even with await). Do not use
  // if concurrent execution is required.
  console.log(event); // prints ['bar'] [42]
}
// Unreachable here

Returns an AsyncIterator that iterates eventName events. It will throw if the EventEmitter emits 'error'. It removes all listeners when exiting the loop. The value returned by each iteration is an array composed of the emitted event arguments.

An AbortSignal can be used to cancel waiting on events:

import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ac = new AbortController();

(async () => {
  const ee = new EventEmitter();

  // Emit later on
  process.nextTick(() => {
    ee.emit('foo', 'bar');
    ee.emit('foo', 42);
  });

  for await (const event of on(ee, 'foo', { signal: ac.signal })) {
    // The execution of this inner block is synchronous and it
    // processes one event at a time (even with await). Do not use
    // if concurrent execution is required.
    console.log(event); // prints ['bar'] [42]
  }
  // Unreachable here
})();

process.nextTick(() => ac.abort());

Use the close option to specify an array of event names that will end the iteration:

import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

// Emit later on
process.nextTick(() => {
  ee.emit('foo', 'bar');
  ee.emit('foo', 42);
  ee.emit('close');
});

for await (const event of on(ee, 'foo', { close: ['close'] })) {
  console.log(event); // prints ['bar'] [42]
}
// the loop will exit after 'close' is emitted
console.log('done'); // prints 'done'
Parameters

• emitter: EventEmitter<DefaultEventMap>

• eventName: string | symbol

• options?: StaticEventEmitterIteratorOptions

Returns

AsyncIterator<any[], any, any>

An AsyncIterator that iterates eventName events emitted by the emitter

Since

v13.6.0, v12.16.0

Inherited from

EventEmitter.on

Defined in

node_modules/@types/node/events.d.ts:303

on(emitter, eventName, options)

static on(emitter, eventName, options?): AsyncIterator<any[], any, any>

Parameters

• emitter: EventTarget

• eventName: string

• options?: StaticEventEmitterIteratorOptions

Returns

AsyncIterator<any[], any, any>

Inherited from

EventEmitter.on

Defined in

node_modules/@types/node/events.d.ts:308

once()

once(emitter, eventName, options)

static once(emitter, eventName, options?): Promise<any[]>

Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.

This method is intentionally generic and works with the web platform EventTarget interface, which has no special'error' event semantics and does not listen to the 'error' event.

import { once, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

process.nextTick(() => {
  ee.emit('myevent', 42);
});

const [value] = await once(ee, 'myevent');
console.log(value);

const err = new Error('kaboom');
process.nextTick(() => {
  ee.emit('error', err);
});

try {
  await once(ee, 'myevent');
} catch (err) {
  console.error('error happened', err);
}

The special handling of the 'error' event is only used when events.once() is used to wait for another event. If events.once() is used to wait for the ‘error' event itself, then it is treated as any other kind of event without special handling:

import { EventEmitter, once } from 'node:events';

const ee = new EventEmitter();

once(ee, 'error')
  .then(([err]) => console.log('ok', err.message))
  .catch((err) => console.error('error', err.message));

ee.emit('error', new Error('boom'));

// Prints: ok boom

An AbortSignal can be used to cancel waiting for the event:

import { EventEmitter, once } from 'node:events';

const ee = new EventEmitter();
const ac = new AbortController();

async function foo(emitter, event, signal) {
  try {
    await once(emitter, event, { signal });
    console.log('event emitted!');
  } catch (error) {
    if (error.name === 'AbortError') {
      console.error('Waiting for the event was canceled!');
    } else {
      console.error('There was an error', error.message);
    }
  }
}

foo(ee, 'foo', ac.signal);
ac.abort(); // Abort waiting for the event
ee.emit('foo'); // Prints: Waiting for the event was canceled!
Parameters

• emitter: EventEmitter<DefaultEventMap>

• eventName: string | symbol

• options?: StaticEventEmitterOptions

Returns

Promise<any[]>

Since

v11.13.0, v10.16.0

Inherited from

EventEmitter.once

Defined in

node_modules/@types/node/events.d.ts:217

once(emitter, eventName, options)

static once(emitter, eventName, options?): Promise<any[]>

Parameters

• emitter: EventTarget

• eventName: string

• options?: StaticEventEmitterOptions

Returns

Promise<any[]>

Inherited from

EventEmitter.once

Defined in

node_modules/@types/node/events.d.ts:222

setMaxListeners()

static setMaxListeners(n?, …eventTargets?): void

import { setMaxListeners, EventEmitter } from 'node:events';

const target = new EventTarget();
const emitter = new EventEmitter();

setMaxListeners(5, target, emitter);

Parameters

• n?: number

A non-negative number. The maximum number of listeners per EventTarget event.

• …eventTargets?: (EventEmitter<DefaultEventMap> | EventTarget)[]

Zero or more {EventTarget} or {EventEmitter} instances. If none are specified, n is set as the default max for all newly created {EventTarget} and {EventEmitter} objects.

Returns

void

Since

v15.4.0

Inherited from

EventEmitter.setMaxListeners

Defined in

node_modules/@types/node/events.d.ts:402