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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:

The process.stdin, process.stdout and process.stderr may be redirected by the parent thread.
The require('worker_threads').isMainThread property is set to false.
The require('worker_threads').parentPort message port is available.
process.exit() does not stop the whole program, just the single thread, and process.abort() is not available.
process.chdir() and process methods that set group or user ids are not available.
process.env is a copy of the parent thread’s environment variables, unless otherwise specified. Changes to one copy are not visible in other threads, and are not visible to native add-ons (unless worker.SHARE_ENV is passed as the env option to the Worker constructor).
process.title cannot be modified.
Signals are not delivered through process.on('...').
Execution may stop at any point as a result of worker.terminate() being invoked.
IPC channels from parent processes are not accessible.
The trace_events module is not supported.
Native add-ons can only be loaded from multiple threads if they fulfill certain conditions.

Creating Worker instances inside of other Workers is possible.

Like Web Workers and the 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.

const assert = require('assert');
const {
  Worker, MessageChannel, MessagePort, isMainThread, parentPort
} = require('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

EventEmitter

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:395

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:389

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:383

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:367

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:355

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:361

threadId

readonly threadId: number

An integer identifier for the referenced thread. Inside the worker thread, it is available as require('worker_threads').threadId. 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:374

captureRejections

static captureRejections: boolean

Sets or gets the default captureRejection value for all emitters.

Inherited from

EventEmitter.captureRejections

Defined in

node_modules/@types/node/events.d.ts:306

captureRejectionSymbol

readonly static captureRejectionSymbol: typeof captureRejectionSymbol

Inherited from

EventEmitter.captureRejectionSymbol

Defined in

node_modules/@types/node/events.d.ts:301

defaultMaxListeners

static defaultMaxListeners: number

Inherited from

EventEmitter.defaultMaxListeners

Defined in

node_modules/@types/node/events.d.ts:307

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.

Inherited from

EventEmitter.errorMonitor

Defined in

node_modules/@types/node/events.d.ts:300

Methods

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:431

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:432

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:433

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:434

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:435

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:436

emit()

emit(event, err)

emit(event, err): boolean

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

Returns true if the event had listeners, false otherwise.

const EventEmitter = require('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:437

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:438

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:439

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:440

emit(event)

emit(event): boolean

Parameters

• event: "online"

Returns

boolean

Overrides

EventEmitter.emit

Defined in

node_modules/@types/node/worker_threads.d.ts:441

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:442

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.

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

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

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

Returns

(string | symbol)[]

Since

v6.0.0

Inherited from

EventEmitter.eventNames

Defined in

node_modules/@types/node/events.d.ts:642

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:430

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:499

listenerCount()

listenerCount(eventName): number

Returns the number of listeners listening to the event named eventName.

Parameters

• eventName: string | symbol

The name of the event being listened for

Returns

number

Since

v3.2.0

Inherited from

EventEmitter.listenerCount

Defined in

node_modules/@types/node/events.d.ts:589

listeners()

listeners(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] ]

Parameters

• eventName: string | symbol

Returns

Function[]

Since

v0.1.26

Inherited from

EventEmitter.listeners

Defined in

node_modules/@types/node/events.d.ts:512

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:473

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:474

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:475

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:476

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:477

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:478

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 eventNameand 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. Theemitter.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

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:443

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:444

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:445

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:446

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:447

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:448

once()

once(event, listener)

once(event, listener): this

Adds a one-timelistener 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. Theemitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a

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:449

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:450

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:451

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:452

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:453

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:454

postMessage()

postMessage(value, transferList?): void

Send a message to the worker that is received via require('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:401

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 eventNameand 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:455

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:456

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:457

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:458

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:459

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:460

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:461

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:462

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:463

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:464

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:465

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:466

rawListeners()

rawListeners(eventName): Function[]

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

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');

Parameters

• eventName: string | symbol

Returns

Function[]

Since

v9.4.0

Inherited from

EventEmitter.rawListeners

Defined in

node_modules/@types/node/events.d.ts:542

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:408

removeAllListeners()

removeAllListeners(event?): 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

• event?: string | symbol

Returns

this

Since

v0.1.26

Inherited from

EventEmitter.removeAllListeners

Defined in

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

removeListener()

removeListener(event, listener)

removeListener(event, listener): this

Removes the specified listener from the listener array for the event namedeventName.

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 anyremoveListener() or removeAllListeners() calls after emitting and_before_ the last listener finishes execution will not remove them fromemit() in progress. Subsequent events behave as expected.

const myEmitter = new MyEmitter();

const callbackA = () => {
  console.log('A');
  myEmitter.removeListener('event', callbackB);
};

const callbackB = () => {
  console.log('B');
};

myEmitter.on('event', callbackA);

myEmitter.on('event', callbackB);

// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit('event');
// Prints:
//   A
//   B

// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit('event');
// Prints:
//   A

Because listeners are managed using an internal array, calling this will change the position indices of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping')listener is removed:

const ee = new EventEmitter();

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

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

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

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

Parameters

• event: "error"

• listener

Returns

this

Since

v0.1.26

Overrides

EventEmitter.removeListener

Defined in

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

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:468

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:469

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:470

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:471

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:472

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 toInfinity (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:493

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:420

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 callingunref() again has no effect.

Returns

void

Since

v10.5.0

Defined in

node_modules/@types/node/worker_threads.d.ts:414

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.

const { getEventListeners, EventEmitter } = require('events');

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

Parameters

• emitter: EventEmitter | DOMEventTarget

• 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:270

listenerCount()

static listenerCount(emitter, eventName): number

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

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

Parameters

• emitter: EventEmitter

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:242

on()

static on(emitter, eventName, options?): AsyncIterableIterator<any, any, any>

const { on, EventEmitter } = require('events');

(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')) {
    // 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:

const { on, EventEmitter } = require('events');
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());

Parameters

• emitter: EventEmitter

• eventName: string

The name of the event being listened for

• options?: StaticEventEmitterOptions

Returns

AsyncIterableIterator<any, any, any>

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:221

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.

const { once, EventEmitter } = require('events');

async function run() {
  const ee = new EventEmitter();

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

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

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

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

run();

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:

const { EventEmitter, once } = require('events');

const ee = new EventEmitter();

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

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

// Prints: ok boom

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

const { EventEmitter, once } = require('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: NodeEventTarget

• 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:157

once(emitter, eventName, options)

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

Parameters

• emitter: DOMEventTarget

• eventName: string

• options?: StaticEventEmitterOptions

Returns

Promise<any[]>

Inherited from

EventEmitter.once

Defined in

node_modules/@types/node/events.d.ts:162

setMaxListeners()

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

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 EventEmitter.setMaxListeners() method allows the default limit to be modified (if eventTargets is empty) or modify the limit specified in every EventTarget | EventEmitter passed as arguments. The value can be set toInfinity (or 0) to indicate an unlimited number of listeners.

EventEmitter.setMaxListeners(20);
// Equivalent to
EventEmitter.defaultMaxListeners = 20;

const eventTarget = new EventTarget();
// Only way to increase limit for `EventTarget` instances
// as these doesn't expose its own `setMaxListeners` method
EventEmitter.setMaxListeners(20, eventTarget);

Parameters

• n?: number

• …eventTargets?: (EventEmitter | DOMEventTarget)[]

Returns

void

Since

v15.3.0, v14.17.0

Inherited from

EventEmitter.setMaxListeners

Defined in

node_modules/@types/node/events.d.ts:290