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class

worker_threads.Worker

class Worker

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

Notable differences inside a Worker environment are:

  • 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.

  • readonly threadName: null | string

    A string identifier for the referenced thread or null if the thread is not running. Inside the worker thread, it is available as require('node:worker_threads').threadName.

  • 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.

  • [Symbol.asyncDispose](): Promise<void>;

    Calls worker.terminate() when the dispose scope is exited.

    async function example() {
  await using worker = new Worker('for (;;) {}', { eval: true });
  // Worker is automatically terminate when the scope is exited.
}
  • 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;
  • prev?: CpuUsage
    ): Promise<CpuUsage>;

    This method returns a Promise that will resolve to an object identical to process.threadCpuUsage(), or reject with an ERR_WORKER_NOT_RUNNING error if the worker is no longer running. This methods allows the statistics to be observed from outside the actual thread.

  • 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

  • This method returns a Promise that will resolve to an object identical to v8.getHeapStatistics(), or reject with an ERR_WORKER_NOT_RUNNING error if the worker is no longer running. This methods allows the statistics to be observed from outside the actual thread.

  • 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 Transferable[]
    ): void;

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

  • 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.

  • Starting a CPU profile then return a Promise that fulfills with an error or an CPUProfileHandle object. This API supports await using syntax.

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

const worker = new Worker(`
  const { parentPort } = require('worker_threads');
  parentPort.on('message', () => {});
  `, { eval: true });

worker.on('online', async () => {
  const handle = await worker.startCpuProfile();
  const profile = await handle.stop();
  console.log(profile);
  worker.terminate();
});

    await using example.

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

const w = new Worker(`
  const { parentPort } = require('node:worker_threads');
  parentPort.on('message', () => {});
  `, { eval: true });

w.on('online', async () => {
  // Stop profile automatically when return and profile will be discarded
  await using handle = await w.startCpuProfile();
});

  • Starting a Heap profile then return a Promise that fulfills with an error or an HeapProfileHandle object. This API supports await using syntax.

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

const worker = new Worker(`
  const { parentPort } = require('worker_threads');
  parentPort.on('message', () => {});
  `, { eval: true });

worker.on('online', async () => {
  const handle = await worker.startHeapProfile();
  const profile = await handle.stop();
  console.log(profile);
  worker.terminate();
});

    await using example.

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

const w = new Worker(`
  const { parentPort } = require('node:worker_threads');
  parentPort.on('message', () => {});
  `, { eval: true });

w.on('online', async () => {
  // Stop profile automatically when return and profile will be discarded
  await using handle = await w.startHeapProfile();
});
  • 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.