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Bun

Node.js module

worker_threads

The 'node:worker_threads' module enables multithreaded execution in Node.js by spawning Worker threads that run JavaScript in isolated V8 instances.

Features include message passing, transferable objects, SharedArrayBuffer support, and performance measurement, allowing compute-intensive tasks to be offloaded from the main event loop.

Works in Bun

Core worker creation and communication works. Several worker options (stdio, resourceLimits, etc.) and some advanced methods related to transferable objects and heap snapshots are not supported.

  • class BroadcastChannel

    Instances of BroadcastChannel allow asynchronous one-to-many communication with all other BroadcastChannel instances bound to the same channel name.

    'use strict';

import {
  isMainThread,
  BroadcastChannel,
  Worker,
} from 'node:worker_threads';

const bc = new BroadcastChannel('hello');

if (isMainThread) {
  let c = 0;
  bc.onmessage = (event) => {
    console.log(event.data);
    if (++c === 10) bc.close();
  };
  for (let n = 0; n < 10; n++)
    new Worker(__filename);
} else {
  bc.postMessage('hello from every worker');
  bc.close();
}
    • readonly name: string
    • onmessage: (message: unknown) => void

      Invoked with a single MessageEvent argument when a message is received.

    • onmessageerror: (message: unknown) => void

      Invoked with a received message cannot be deserialized.

    • close(): void;

      Closes the BroadcastChannel connection.

    • message: unknown
      ): void;
      @param message

      Any cloneable JavaScript value.

    • ref(): this;
    • unref(): this;

  • class MessageChannel

    Instances of the worker.MessageChannel class represent an asynchronous, two-way communications channel. The MessageChannel has no methods of its own. new MessageChannel() yields an object with port1 and port2 properties, which refer to linked MessagePort instances.

    import { MessageChannel } from 'node:worker_threads';

const { port1, port2 } = new MessageChannel();
port1.on('message', (message) => console.log('received', message));
port2.postMessage({ foo: 'bar' });
// Prints: received { foo: 'bar' } from the `port1.on('message')` listener

  • class MessagePort

    Instances of the worker.MessagePort class represent one end of an asynchronous, two-way communications channel. It can be used to transfer structured data, memory regions and other MessagePorts between different Workers.

    This implementation matches browser MessagePort s.

    • addEventListener: {(type: string, callback: null | EventListenerOrEventListenerObject, options?: boolean | AddEventListenerOptions) => void; (type: string, listener: EventListener | EventListenerObject, options?: boolean | AddEventListenerOptions) => void}
    • dispatchEvent: (event: Event) => boolean
    • removeEventListener: {(type: string, callback: null | EventListenerOrEventListenerObject, options?: boolean | EventListenerOptions) => void; (type: string, listener: EventListener | EventListenerObject, options?: boolean | EventListenerOptions) => void}
    • static captureRejections: boolean

      Value: boolean

      Change the default captureRejections option on all new EventEmitter objects.

    • readonly static captureRejectionSymbol: typeof captureRejectionSymbol

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

      See how to write a custom rejection handler.

    • 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'.

    • 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.

    • error: Error,
      event: string | symbol,
      ...args: AnyRest
      ): void;
    • event: 'close',
      listener: () => void
      ): this;

      Alias for emitter.on(eventName, listener).

      event: 'message',
      listener: (value: any) => void
      ): this;
      event: 'messageerror',
      listener: (error: Error) => void
      ): this;
      event: string | symbol,
      listener: (...args: any[]) => void
      ): this;
    • close(): void;

      Disables further sending of messages on either side of the connection. This method can be called when no further communication will happen over this MessagePort.

      The 'close' event is emitted on both MessagePort instances that are part of the channel.

    • event: 'close'
      ): 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
      event: 'message',
      value: any
      ): boolean;
      event: 'messageerror',
      error: Error
      ): boolean;
      event: string | symbol,
      ...args: any[]
      ): boolean;
    • 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) ]
    • getMaxListeners(): number;

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

    • eventName: string | symbol,
      listener?: Function
      ): 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.

      @param eventName

      The name of the event being listened for

      @param listener

      The event handler function

    • eventName: string | symbol
      ): 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] ]
    • event: 'close',
      listener: () => void
      ): this;

      Alias for emitter.removeListener().

      event: 'message',
      listener: (value: any) => void
      ): this;
      event: 'messageerror',
      listener: (error: Error) => void
      ): this;
      event: string | symbol,
      listener: (...args: any[]) => void
      ): this;
    • event: 'close',
      listener: () => void
      ): 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
      @param listener

      The callback function

      event: 'message',
      listener: (value: any) => void
      ): this;
      event: 'messageerror',
      listener: (error: Error) => void
      ): this;
      event: string | symbol,
      listener: (...args: any[]) => void
      ): this;
    • event: 'close',
      listener: () => void
      ): 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
      @param listener

      The callback function

      event: 'message',
      listener: (value: any) => void
      ): this;
      event: 'messageerror',
      listener: (error: Error) => void
      ): this;
      event: string | symbol,
      listener: (...args: any[]) => void
      ): this;
    • value: any,
      transferList?: readonly TransferListItem[]
      ): void;

      Sends a JavaScript value to the receiving side of this channel. value is transferred in a way which is compatible with the HTML structured clone algorithm.

      In particular, the significant differences to JSON are:

      • value may contain circular references.
      • value may contain instances of builtin JS types such as RegExps, BigInts, Maps, Sets, etc.
      • value may contain typed arrays, both using ArrayBuffers and SharedArrayBuffers.
      • value may contain WebAssembly.Module instances.
      • value may not contain native (C++-backed) objects other than:
      import { MessageChannel } from 'node:worker_threads';
const { port1, port2 } = new MessageChannel();

port1.on('message', (message) => console.log(message));

const circularData = {};
circularData.foo = circularData;
// Prints: { foo: [Circular] }
port2.postMessage(circularData);

      transferList may be a list of ArrayBuffer, MessagePort, and FileHandle objects. After transferring, they are not usable on the sending side of the channel anymore (even if they are not contained in value). Unlike with child processes, transferring handles such as network sockets is currently not supported.

      If value contains SharedArrayBuffer instances, those are accessible from either thread. They cannot be listed in transferList.

      value may still contain ArrayBuffer instances that are not in transferList; in that case, the underlying memory is copied rather than moved.

      import { MessageChannel } from 'node:worker_threads';
const { port1, port2 } = new MessageChannel();

port1.on('message', (message) => console.log(message));

const uint8Array = new Uint8Array([ 1, 2, 3, 4 ]);
// This posts a copy of `uint8Array`:
port2.postMessage(uint8Array);
// This does not copy data, but renders `uint8Array` unusable:
port2.postMessage(uint8Array, [ uint8Array.buffer ]);

// The memory for the `sharedUint8Array` is accessible from both the
// original and the copy received by `.on('message')`:
const sharedUint8Array = new Uint8Array(new SharedArrayBuffer(4));
port2.postMessage(sharedUint8Array);

// This transfers a freshly created message port to the receiver.
// This can be used, for example, to create communication channels between
// multiple `Worker` threads that are children of the same parent thread.
const otherChannel = new MessageChannel();
port2.postMessage({ port: otherChannel.port1 }, [ otherChannel.port1 ]);

      The message object is cloned immediately, and can be modified after posting without having side effects.

      For more information on the serialization and deserialization mechanisms behind this API, see the serialization API of the node:v8 module.

    • event: 'close',
      listener: () => void
      ): 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.

      @param listener

      The callback function

      event: 'message',
      listener: (value: any) => void
      ): this;
      event: 'messageerror',
      listener: (error: Error) => void
      ): this;
      event: string | symbol,
      listener: (...args: any[]) => void
      ): this;
    • event: 'close',
      listener: () => void
      ): 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.

      @param listener

      The callback function

      event: 'message',
      listener: (value: any) => void
      ): this;
      event: 'messageerror',
      listener: (error: Error) => void
      ): this;
      event: string | symbol,
      listener: (...args: any[]) => void
      ): this;
    • eventName: string | symbol
      ): 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');
    • ref(): void;

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

      If listeners are attached or removed using .on('message'), the port is ref()ed and unref()ed automatically depending on whether listeners for the event exist.

    • eventName?: string | symbol
      ): 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.

    • event: 'close',
      listener: () => void
      ): 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.

      event: 'message',
      listener: (value: any) => void
      ): this;
      event: 'messageerror',
      listener: (error: Error) => void
      ): this;
      event: string | symbol,
      listener: (...args: any[]) => void
      ): this;
    • n: number
      ): 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.

    • start(): void;

      Starts receiving messages on this MessagePort. When using this port as an event emitter, this is called automatically once 'message' listeners are attached.

      This method exists for parity with the Web MessagePort API. In Node.js, it is only useful for ignoring messages when no event listener is present. Node.js also diverges in its handling of .onmessage. Setting it automatically calls .start(), but unsetting it lets messages queue up until a new handler is set or the port is discarded.

    • unref(): void;

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

      If listeners are attached or removed using .on('message'), the port is ref()ed and unref()ed automatically depending on whether listeners for the event exist.

    • signal: AbortSignal,
      resource: (event: Event) => void
      ): Disposable;

      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]();
  }
}
      @returns

      Disposable that removes the abort listener.

    • emitter: EventEmitter<DefaultEventMap> | EventTarget,
      name: string | symbol
      ): 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] ]
}
    • emitter: EventEmitter<DefaultEventMap> | EventTarget
      ): 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
}
    • static on(
      emitter: EventEmitter,
      eventName: string | symbol,
      options?: StaticEventEmitterIteratorOptions
      ): AsyncIterator<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'
      @returns

      An AsyncIterator that iterates eventName events emitted by the emitter

      static on(
      emitter: EventTarget,
      eventName: string,
      options?: StaticEventEmitterIteratorOptions
      ): AsyncIterator<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'
      @returns

      An AsyncIterator that iterates eventName events emitted by the emitter

    • static once(
      emitter: EventEmitter,
      eventName: string | symbol,
      options?: StaticEventEmitterOptions
      ): 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!
      static once(
      emitter: EventTarget,
      eventName: string,
      options?: StaticEventEmitterOptions
      ): 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!
    • n?: number,
      ...eventTargets: EventEmitter<DefaultEventMap> | EventTarget[]
      ): void;
      import { setMaxListeners, EventEmitter } from 'node:events';

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

setMaxListeners(5, target, emitter);
      @param n

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

      @param eventTargets

      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.

  • 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 streams may be redirected by the parent thread.
    • The import { isMainThread } from 'node:worker_threads' variable is set to false.
    • The import { parentPort } from 'node:worker_threads' 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). On Windows, unlike the main thread, a copy of the environment variables operates in a case-sensitive manner.
    • 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 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();
  });
}
    • readonly performance: WorkerPerformance

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

    • readonly 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.

    • 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.

    • 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.

    • 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.

    • 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.

    • static captureRejections: boolean

      Value: boolean

      Change the default captureRejections option on all new EventEmitter objects.

    • readonly static captureRejectionSymbol: typeof captureRejectionSymbol

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

      See how to write a custom rejection handler.

    • 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'.

    • 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.

    • error: Error,
      event: string | symbol,
      ...args: AnyRest
      ): void;
    • event: 'error',
      listener: (err: Error) => void
      ): this;

      Alias for emitter.on(eventName, listener).

      event: 'exit',
      listener: (exitCode: number) => void
      ): this;
      event: 'message',
      listener: (value: any) => void
      ): this;
      event: 'messageerror',
      listener: (error: Error) => void
      ): this;
      event: 'online',
      listener: () => void
      ): this;
      event: string | symbol,
      listener: (...args: any[]) => void
      ): this;
    • event: 'error',
      err: Error
      ): 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
      event: 'exit',
      exitCode: number
      ): boolean;
      event: 'message',
      value: any
      ): boolean;
      event: 'messageerror',
      error: Error
      ): boolean;
      event: 'online'
      ): boolean;
      event: string | symbol,
      ...args: any[]
      ): boolean;
    • 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 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

      A promise for a Readable Stream containing a V8 heap snapshot

    • getMaxListeners(): number;

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

    • eventName: string | symbol,
      listener?: Function
      ): 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.

      @param eventName

      The name of the event being listened for

      @param listener

      The event handler function

    • eventName: string | symbol
      ): 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] ]
    • event: 'error',
      listener: (err: Error) => void
      ): this;

      Alias for emitter.removeListener().

      event: 'exit',
      listener: (exitCode: number) => void
      ): this;
      event: 'message',
      listener: (value: any) => void
      ): this;
      event: 'messageerror',
      listener: (error: Error) => void
      ): this;
      event: 'online',
      listener: () => void
      ): this;
      event: string | symbol,
      listener: (...args: any[]) => void
      ): this;
    • event: 'error',
      listener: (err: Error) => void
      ): 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
      @param listener

      The callback function

      event: 'exit',
      listener: (exitCode: number) => void
      ): this;
      event: 'message',
      listener: (value: any) => void
      ): this;
      event: 'messageerror',
      listener: (error: Error) => void
      ): this;
      event: 'online',
      listener: () => void
      ): this;
      event: string | symbol,
      listener: (...args: any[]) => void
      ): this;
    • event: 'error',
      listener: (err: Error) => void
      ): 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
      @param listener

      The callback function

      event: 'exit',
      listener: (exitCode: number) => void
      ): this;
      event: 'message',
      listener: (value: any) => void
      ): this;
      event: 'messageerror',
      listener: (error: Error) => void
      ): this;
      event: 'online',
      listener: () => void
      ): this;
      event: string | symbol,
      listener: (...args: any[]) => void
      ): this;
    • value: any,
      transferList?: readonly TransferListItem[]
      ): void;

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

    • threadId: number,
      value: any,
      timeout?: number
      ): Promise<void>;

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

      @param threadId

      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.

      @param value

      The value to send.

      @param timeout

      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.

      threadId: number,
      value: any,
      transferList: readonly TransferListItem[],
      timeout?: number
      ): Promise<void>;

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

      @param threadId

      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.

      @param value

      The value to send.

      @param transferList

      If one or more MessagePort-like objects are passed in value, a transferList is required for those items or ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST is thrown. See port.postMessage() for more information.

      @param timeout

      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.

    • event: 'error',
      listener: (err: Error) => void
      ): 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.

      @param listener

      The callback function

      event: 'exit',
      listener: (exitCode: number) => void
      ): this;
      event: 'message',
      listener: (value: any) => void
      ): this;
      event: 'messageerror',
      listener: (error: Error) => void
      ): this;
      event: 'online',
      listener: () => void
      ): this;
      event: string | symbol,
      listener: (...args: any[]) => void
      ): this;
    • event: 'error',
      listener: (err: Error) => void
      ): 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.

      @param listener

      The callback function

      event: 'exit',
      listener: (exitCode: number) => void
      ): this;
      event: 'message',
      listener: (value: any) => void
      ): this;
      event: 'messageerror',
      listener: (error: Error) => void
      ): this;
      event: 'online',
      listener: () => void
      ): this;
      event: string | symbol,
      listener: (...args: any[]) => void
      ): this;
    • eventName: string | symbol
      ): 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');
    • 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.

    • eventName?: string | symbol
      ): 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.

    • event: 'error',
      listener: (err: Error) => void
      ): 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.

      event: 'exit',
      listener: (exitCode: number) => void
      ): this;
      event: 'message',
      listener: (value: any) => void
      ): this;
      event: 'messageerror',
      listener: (error: Error) => void
      ): this;
      event: 'online',
      listener: () => void
      ): this;
      event: string | symbol,
      listener: (...args: any[]) => void
      ): this;
    • n: number
      ): 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.

    • 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.

    • 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.

    • signal: AbortSignal,
      resource: (event: Event) => void
      ): Disposable;

      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]();
  }
}
      @returns

      Disposable that removes the abort listener.

    • emitter: EventEmitter<DefaultEventMap> | EventTarget,
      name: string | symbol
      ): 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] ]
}
    • emitter: EventEmitter<DefaultEventMap> | EventTarget
      ): 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
}
    • static on(
      emitter: EventEmitter,
      eventName: string | symbol,
      options?: StaticEventEmitterIteratorOptions
      ): AsyncIterator<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'
      @returns

      An AsyncIterator that iterates eventName events emitted by the emitter

      static on(
      emitter: EventTarget,
      eventName: string,
      options?: StaticEventEmitterIteratorOptions
      ): AsyncIterator<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'
      @returns

      An AsyncIterator that iterates eventName events emitted by the emitter

    • static once(
      emitter: EventEmitter,
      eventName: string | symbol,
      options?: StaticEventEmitterOptions
      ): 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!
      static once(
      emitter: EventTarget,
      eventName: string,
      options?: StaticEventEmitterOptions
      ): 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!
    • n?: number,
      ...eventTargets: EventEmitter<DefaultEventMap> | EventTarget[]
      ): void;
      import { setMaxListeners, EventEmitter } from 'node:events';

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

setMaxListeners(5, target, emitter);
      @param n

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

      @param eventTargets

      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.

  • const isInternalThread: boolean
  • const isMainThread: boolean
  • const parentPort: null | MessagePort
  • const SHARE_ENV: unique symbol
  • const threadId: number
  • const workerData: any
  • key: Serializable

    Within a worker thread, worker.getEnvironmentData() returns a clone of data passed to the spawning thread's worker.setEnvironmentData(). Every new Worker receives its own copy of the environment data automatically.

    import {
  Worker,
  isMainThread,
  setEnvironmentData,
  getEnvironmentData,
} from 'node:worker_threads';

if (isMainThread) {
  setEnvironmentData('Hello', 'World!');
  const worker = new Worker(__filename);
} else {
  console.log(getEnvironmentData('Hello'));  // Prints 'World!'.
}
    @param key

    Any arbitrary, cloneable JavaScript value that can be used as a {Map} key.

  • object: object
    ): boolean;

    Check if an object is marked as not transferable with markAsUntransferable.

  • object: object
    ): void;

    Mark an object as not cloneable. If object is used as message in a port.postMessage() call, an error is thrown. This is a no-op if object is a primitive value.

    This has no effect on ArrayBuffer, or any Buffer like objects.

    This operation cannot be undone.

    const { markAsUncloneable } = require('node:worker_threads');

const anyObject = { foo: 'bar' };
markAsUncloneable(anyObject);
const { port1 } = new MessageChannel();
try {
  // This will throw an error, because anyObject is not cloneable.
  port1.postMessage(anyObject)
} catch (error) {
  // error.name === 'DataCloneError'
}

    There is no equivalent to this API in browsers.

  • object: object
    ): void;

    Mark an object as not transferable. If object occurs in the transfer list of a port.postMessage() call, it is ignored.

    In particular, this makes sense for objects that can be cloned, rather than transferred, and which are used by other objects on the sending side. For example, Node.js marks the ArrayBuffers it uses for its Buffer pool with this.

    This operation cannot be undone.

    import { MessageChannel, markAsUntransferable } from 'node:worker_threads';

const pooledBuffer = new ArrayBuffer(8);
const typedArray1 = new Uint8Array(pooledBuffer);
const typedArray2 = new Float64Array(pooledBuffer);

markAsUntransferable(pooledBuffer);

const { port1 } = new MessageChannel();
port1.postMessage(typedArray1, [ typedArray1.buffer ]);

// The following line prints the contents of typedArray1 -- it still owns
// its memory and has been cloned, not transferred. Without
// `markAsUntransferable()`, this would print an empty Uint8Array.
// typedArray2 is intact as well.
console.log(typedArray1);
console.log(typedArray2);

    There is no equivalent to this API in browsers.

  • port: MessagePort,
    contextifiedSandbox: Context

    Transfer a MessagePort to a different vm Context. The original port object is rendered unusable, and the returned MessagePort instance takes its place.

    The returned MessagePort is an object in the target context and inherits from its global Object class. Objects passed to the port.onmessage() listener are also created in the target context and inherit from its global Object class.

    However, the created MessagePort no longer inherits from EventTarget, and only port.onmessage() can be used to receive events using it.

    @param port

    The message port to transfer.

    @param contextifiedSandbox

    A contextified object as returned by the vm.createContext() method.

  • port: MessagePort
    ): undefined | { message: any };

    Receive a single message from a given MessagePort. If no message is available,undefined is returned, otherwise an object with a single message property that contains the message payload, corresponding to the oldest message in the MessagePort's queue.

    import { MessageChannel, receiveMessageOnPort } from 'node:worker_threads';
const { port1, port2 } = new MessageChannel();
port1.postMessage({ hello: 'world' });

console.log(receiveMessageOnPort(port2));
// Prints: { message: { hello: 'world' } }
console.log(receiveMessageOnPort(port2));
// Prints: undefined

    When this function is used, no 'message' event is emitted and the onmessage listener is not invoked.

  • key: Serializable,
    value: Serializable
    ): void;

    The worker.setEnvironmentData() API sets the content of worker.getEnvironmentData() in the current thread and all new Worker instances spawned from the current context.

    @param key

    Any arbitrary, cloneable JavaScript value that can be used as a {Map} key.

    @param value

    Any arbitrary, cloneable JavaScript value that will be cloned and passed automatically to all new Worker instances. If value is passed as undefined, any previously set value for the key will be deleted.

Type definitions