Writable

WritableWritableRequired

Inherited from

addListener(event, listener): Writable

Parameters

event"drain"Required

listener() => voidRequired

Returns

WritableWritableRequired

Inherited from

addListener(event, listener): Writable

Parameters

event"error"Required

listener(err: Error) => voidRequired

Returns

WritableWritableRequired

Inherited from

addListener(event, listener): Writable

Parameters

event"finish"Required

listener() => voidRequired

Returns

WritableWritableRequired

Inherited from

addListener(event, listener): Writable

Parameters

event"pipe"Required

listener(src: Readable) => voidRequired

Returns

WritableWritableRequired

Inherited from

addListener(event, listener): Writable

Parameters

event"unpipe"Required

listener(src: Readable) => voidRequired

Returns

WritableWritableRequired

Inherited from

addListener(event, listener): Writable

Parameters

eventstring | symbolRequired

listener(...args: any[]) => voidRequired

Returns

WritableWritableRequired

Inherited from

cork

cork(): void

The writable.cork() method forces all written data to be buffered in memory. The buffered data will be flushed when either the uncork or end methods are called.

The primary intent of writable.cork() is to accommodate a situation in which several small chunks are written to the stream in rapid succession. Instead of immediately forwarding them to the underlying destination, writable.cork()buffers all the chunks until writable.uncork() is called, which will pass them all to writable._writev(), if present. This prevents a head-of-line blocking situation where data is being buffered while waiting for the first small chunk to be processed. However, use of writable.cork() without implementingwritable._writev() may have an adverse effect on throughput.

See also: writable.uncork(), writable._writev().

Returns

void

voidvoid

Since

v0.11.2

Inherited from

WritableBase.cork

destroy

destroy(error?): Writable

Destroy the stream. Optionally emit an 'error' event, and emit a 'close'event (unless emitClose is set to false). After this call, the writable stream has ended and subsequent calls to write() or end() will result in an ERR_STREAM_DESTROYED error. This is a destructive and immediate way to destroy a stream. Previous calls towrite() may not have drained, and may trigger an ERR_STREAM_DESTROYED error. Use end() instead of destroy if data should flush before close, or wait for the 'drain' event before destroying the stream.

Once destroy() has been called any further calls will be a no-op and no further errors except from _destroy() may be emitted as 'error'.

Implementors should not override this method, but instead implement writable._destroy().

Parameters

errorError

Optional, an error to emit with 'error' event.

Returns

WritableWritableRequired

Since

v8.0.0

Inherited from

WritableBase.destroy

emit

emit(event): boolean

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

Returns true if the event had listeners, false otherwise.

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

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

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

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

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

Parameters

event"close"Required

Returns

boolean

booleanboolean

Since

v0.1.26

Inherited from

emit(event): boolean

Parameters

event"drain"Required

Returns

boolean

booleanboolean

Inherited from

emit(event, err): boolean

Parameters

event"error"Required

errErrorRequired

Returns

boolean

booleanboolean

Inherited from

emit(event): boolean

Parameters

event"finish"Required

Returns

boolean

booleanboolean

Inherited from

emit(event, src): boolean

Parameters

event"pipe"Required

srcReadableRequired

Returns

boolean

booleanboolean

Inherited from

emit(event, src): boolean

Parameters

event"unpipe"Required

srcReadableRequired

Returns

boolean

booleanboolean

Inherited from

emit(event, ...args): boolean

Parameters

eventstring | symbolRequired

argsany[]Required

Returns

boolean

booleanboolean

Inherited from

end

end(cb?): Writable

Calling the writable.end() method signals that no more data will be written to the Writable. The optional chunk and encoding arguments allow one final additional chunk of data to be written immediately before closing the stream.

Calling the write method after calling end will raise an error.

// Write 'hello, ' and then end with 'world!'.
const fs = require('node:fs');
const file = fs.createWriteStream('example.txt');
file.write('hello, ');
file.end('world!');
// Writing more now is not allowed!

Parameters

cb() => void

Returns

WritableWritableRequired

Since

v0.9.4

Inherited from

WritableBase.end

end(chunk, cb?): Writable

Parameters

chunkanyRequired

cb() => void

Returns

WritableWritableRequired

Inherited from

WritableBase.end

end(chunk, encoding, cb?): Writable

Parameters

chunkanyRequired

encodingBufferEncodingRequired

cb() => void

Returns

WritableBase.getMaxListeners

WritableWritableRequired

Inherited from

WritableBase.end

eventNames

eventNames(): (string | symbol)[]

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

import { EventEmitter } from 'node:events';

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

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

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

Returns

(string | symbol)[]

(string \| symbol)[](string | symbol)[]Required

Since

v6.0.0

Inherited from

WritableBase.eventNames

getMaxListeners

getMaxListeners(): number

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

Returns

number

numbernumber

Since

v1.0.0

Inherited from

listenerCount

listenerCount(eventName, listener?): number

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

Parameters

eventNamestring | symbolRequired

The name of the event being listened for

listenerFunction

The event handler function

Returns

number

numbernumber

Since

v3.2.0

Inherited from

WritableBase.listenerCount

listeners

listeners(eventName): Function[]

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

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

Parameters

eventNamestring | symbolRequired

Returns

Function[]

Function[]Function[]Required

Since

v0.1.26

Inherited from

WritableBase.listeners

off

off(eventName, listener): Writable

Alias for emitter.removeListener().

Parameters

eventNamestring | symbolRequired

listener(...args: any[]) => voidRequired

Returns

WritableWritableRequired

Since

v10.0.0

Inherited from

WritableBase.off

on

on(event, listener): Writable

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

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

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

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

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

Parameters

event"close"Required

The name of the event.

listener() => voidRequired

The callback function

Returns

WritableWritableRequired

Since

v0.1.101

Inherited from

on(event, listener): Writable

Parameters

event"drain"Required

listener() => voidRequired

Returns

WritableWritableRequired

Inherited from

on(event, listener): Writable

Parameters

event"error"Required

listener(err: Error) => voidRequired

Returns

WritableWritableRequired

Inherited from

on(event, listener): Writable

Parameters

event"finish"Required

listener() => voidRequired

Returns

WritableWritableRequired

Inherited from

on(event, listener): Writable

Parameters

event"pipe"Required

listener(src: Readable) => voidRequired

Returns

WritableWritableRequired

Inherited from

on(event, listener): Writable

Parameters

event"unpipe"Required

listener(src: Readable) => voidRequired

Returns

WritableWritableRequired

Inherited from

on(event, listener): Writable

Parameters

eventstring | symbolRequired

listener(...args: any[]) => voidRequired

Returns

WritableWritableRequired

Inherited from

once

once(event, listener): Writable

Adds a one-timelistener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

server.once('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

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

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

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

Parameters

event"close"Required

The name of the event.

listener() => voidRequired

The callback function

Returns

WritableWritableRequired

Since

v0.3.0

Inherited from

once(event, listener): Writable

Parameters

event"drain"Required

listener() => voidRequired

Returns

WritableWritableRequired

Inherited from

once(event, listener): Writable

Parameters

event"error"Required

listener(err: Error) => voidRequired

Returns

WritableWritableRequired

Inherited from

once(event, listener): Writable

Parameters

event"finish"Required

listener() => voidRequired

Returns

WritableWritableRequired

Inherited from

once(event, listener): Writable

Parameters

event"pipe"Required

listener(src: Readable) => voidRequired

Returns

WritableWritableRequired

Inherited from

once(event, listener): Writable

Parameters

event"unpipe"Required

listener(src: Readable) => voidRequired

Returns

WritableWritableRequired

Inherited from

once(event, listener): Writable

Parameters

eventstring | symbolRequired

listener(...args: any[]) => voidRequired

Returns

WritableWritableRequired

Inherited from

pipe

pipe<T>(destination, options?): T

TWritableStreamRequired

Parameters

destinationTRequired

optionsobject

options.endboolean

Returns

T

Inherited from

WritableBase.pipe

prependListener

prependListener(event, listener): Writable

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

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

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

Parameters

event"close"Required

The name of the event.

listener() => voidRequired

The callback function

Returns

WritableWritableRequired

Since

v6.0.0

Inherited from

prependListener(event, listener): Writable

Parameters

event"drain"Required

listener() => voidRequired

Returns

WritableWritableRequired

Inherited from

prependListener(event, listener): Writable

Parameters

event"error"Required

listener(err: Error) => voidRequired

Returns

WritableWritableRequired

Inherited from

prependListener(event, listener): Writable

Parameters

event"finish"Required

listener() => voidRequired

Returns

WritableWritableRequired

Inherited from

prependListener(event, listener): Writable

Parameters

event"pipe"Required

listener(src: Readable) => voidRequired

Returns

WritableWritableRequired

Inherited from

prependListener(event, listener): Writable

Parameters

event"unpipe"Required

listener(src: Readable) => voidRequired

Returns

WritableWritableRequired

Inherited from

prependListener(event, listener): Writable

Parameters

eventstring | symbolRequired

listener(...args: any[]) => voidRequired

Returns

WritableWritableRequired

Inherited from

prependOnceListener

prependOnceListener(event, listener): Writable

Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

server.prependOnceListener('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

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

Parameters

event"close"Required

The name of the event.

listener() => voidRequired

The callback function

Returns

WritableWritableRequired

Since

v6.0.0

Inherited from

prependOnceListener(event, listener): Writable

Parameters

event"drain"Required

listener() => voidRequired

Returns

WritableWritableRequired

Inherited from

prependOnceListener(event, listener): Writable

Parameters

event"error"Required

listener(err: Error) => voidRequired

Returns

WritableWritableRequired

Inherited from

prependOnceListener(event, listener): Writable

Parameters

event"finish"Required

listener() => voidRequired

Returns

WritableWritableRequired

Inherited from

prependOnceListener(event, listener): Writable

Parameters

event"pipe"Required

listener(src: Readable) => voidRequired

Returns

WritableWritableRequired

Inherited from

prependOnceListener(event, listener): Writable

Parameters

event"unpipe"Required

listener(src: Readable) => voidRequired

Returns

WritableWritableRequired

Inherited from

prependOnceListener(event, listener): Writable

Parameters

eventstring | symbolRequired

listener(...args: any[]) => voidRequired

Returns

WritableWritableRequired

Inherited from

WritableBase.rawListeners

rawListeners

rawListeners(eventName): Function[]

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

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');

Parameters

eventNamestring | symbolRequired

Returns

Function[]

Function[]Function[]Required

Since

v9.4.0

Inherited from

removeAllListeners

removeAllListeners(event?): Writable

Removes all listeners, or those of the specified eventName.

It is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).

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

Parameters

eventstring | symbol

Returns

WritableBase.removeAllListeners

WritableWritableRequired

Since

v0.1.26

Inherited from

removeListener

removeListener(event, listener): Writable

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

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

removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.

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

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

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

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

myEmitter.on('event', callbackA);

myEmitter.on('event', callbackB);

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

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

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

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

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

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

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

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

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

Parameters

event"close"Required

listener() => voidRequired

Returns

WritableWritableRequired

Since

v0.1.26

Inherited from

removeListener(event, listener): Writable

Parameters

event"drain"Required

listener() => voidRequired

Returns

WritableWritableRequired

Inherited from

removeListener(event, listener): Writable

Parameters

event"error"Required

listener(err: Error) => voidRequired

Returns

WritableWritableRequired

Inherited from

removeListener(event, listener): Writable

Parameters

event"finish"Required

listener() => voidRequired

Returns

WritableWritableRequired

Inherited from

removeListener(event, listener): Writable

Parameters

event"pipe"Required

listener(src: Readable) => voidRequired

Returns

WritableWritableRequired

Inherited from

removeListener(event, listener): Writable

Parameters

event"unpipe"Required

listener(src: Readable) => voidRequired

Returns

WritableWritableRequired

Inherited from

removeListener(event, listener): Writable

Parameters

eventstring | symbolRequired

listener(...args: any[]) => voidRequired

Returns

WritableWritableRequired

Inherited from

setDefaultEncoding

setDefaultEncoding(encoding): Writable

The writable.setDefaultEncoding() method sets the default encoding for a Writable stream.

Parameters

encodingBufferEncodingRequired

The new default encoding

Returns

WritableBase.setDefaultEncoding

WritableWritableRequired

Since

v0.11.15

Inherited from

setMaxListeners

setMaxListeners(n): Writable

By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set toInfinity (or 0) to indicate an unlimited number of listeners.

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

Parameters

nnumberRequired

Returns

WritableBase.setMaxListeners

WritableWritableRequired

Since

v0.3.5

Inherited from

uncork

uncork(): void

The writable.uncork() method flushes all data buffered since cork was called.

When using writable.cork() and writable.uncork() to manage the buffering of writes to a stream, defer calls to writable.uncork() usingprocess.nextTick(). Doing so allows batching of allwritable.write() calls that occur within a given Node.js event loop phase.

stream.cork();
stream.write('some ');
stream.write('data ');
process.nextTick(() => stream.uncork());

If the writable.cork() method is called multiple times on a stream, the same number of calls to writable.uncork() must be called to flush the buffered data.

stream.cork();
stream.write('some ');
stream.cork();
stream.write('data ');
process.nextTick(() => {
  stream.uncork();
  // The data will not be flushed until uncork() is called a second time.
  stream.uncork();
});

Returns

void

voidvoid

Since

v0.11.2

Inherited from

WritableBase.uncork

write

write(chunk, callback?): boolean

The writable.write() method writes some data to the stream, and calls the supplied callback once the data has been fully handled. If an error occurs, the callback will be called with the error as its first argument. The callback is called asynchronously and before 'error' is emitted.

The return value is true if the internal buffer is less than thehighWaterMark configured when the stream was created after admitting chunk. If false is returned, further attempts to write data to the stream should stop until the 'drain' event is emitted.

While a stream is not draining, calls to write() will buffer chunk, and return false. Once all currently buffered chunks are drained (accepted for delivery by the operating system), the 'drain' event will be emitted. Once write() returns false, do not write more chunks until the 'drain' event is emitted. While calling write() on a stream that is not draining is allowed, Node.js will buffer all written chunks until maximum memory usage occurs, at which point it will abort unconditionally. Even before it aborts, high memory usage will cause poor garbage collector performance and high RSS (which is not typically released back to the system, even after the memory is no longer required). Since TCP sockets may never drain if the remote peer does not read the data, writing a socket that is not draining may lead to a remotely exploitable vulnerability.

Writing data while the stream is not draining is particularly problematic for a Transform, because the Transform streams are paused by default until they are piped or a 'data' or 'readable' event handler is added.

If the data to be written can be generated or fetched on demand, it is recommended to encapsulate the logic into a Readable and use pipe. However, if calling write() is preferred, it is possible to respect backpressure and avoid memory issues using the 'drain' event:

function write(data, cb) {
  if (!stream.write(data)) {
    stream.once('drain', cb);
  } else {
    process.nextTick(cb);
  }
}

// Wait for cb to be called before doing any other write.
write('hello', () => {
  console.log('Write completed, do more writes now.');
});

A Writable stream in object mode will always ignore the encoding argument.

Parameters

chunkanyRequired

Optional data to write. For streams not operating in object mode, chunk must be a string, Buffer or Uint8Array. For object mode streams, chunk may be any JavaScript value other than null.

callback(error: undefined | null | Error) => void

Callback for when this chunk of data is flushed.

Returns

boolean

booleanboolean

false if the stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

Since

v0.9.4

Inherited from

WritableBase.write

write(chunk, encoding, callback?): boolean

Parameters

chunkanyRequired

encodingBufferEncodingRequired

callback(error: undefined | null | Error) => void

Returns

boolean

booleanboolean

Inherited from

WritableBase.write

fromWeb

Static fromWeb(writableStream, options?): Writable

A utility method for creating a Writable from a web WritableStream.

Parameters

writableStreamWritableStream<any>Required

optionsPick<WritableOptions, "signal" | "decodeStrings" | "highWaterMark" | "objectMode">

Returns

WritableBase.getEventListeners

WritableWritableRequired

Since

v17.0.0

getEventListeners

Static getEventListeners(emitter, name): Function[]

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

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

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

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

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

Parameters

emitterEventEmitter | _DOMEventTargetRequired

namestring | symbolRequired

Returns

Function[]

Function[]Function[]Required

Since

v15.2.0, v14.17.0

Inherited from

listenerCount

Static listenerCount(emitter, eventName): number

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

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

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

Parameters

emitterEventEmitterRequired

The emitter to query

eventNamestring | symbolRequired

The event name

Returns

number

numbernumber

Since

v0.9.12

Deprecated

Since v3.2.0 - Use listenerCount instead.

Inherited from

WritableBase.listenerCount

on

Static on(emitter, eventName, options?): AsyncIterableIterator<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());

Parameters

emitterEventEmitterRequired

eventNamestringRequired

The name of the event being listened for

optionsStaticEventEmitterOptions

Returns

AsyncIterableIterator<any>

AsyncIterableIteratorAsyncIterableIterator<any>Required

that iterates eventName events emitted by the emitter

Since

v13.6.0, v12.16.0

Inherited from

once

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

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

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

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

const ee = new EventEmitter();

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

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

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

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

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

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

const ee = new EventEmitter();

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

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

// Prints: ok boom

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

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

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

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

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

Parameters

emitter_NodeEventTargetRequired

eventNamestring | symbolRequired

optionsStaticEventEmitterOptions

Returns

Promise<any[]>

PromisePromise<any[]>Required

Since

v11.13.0, v10.16.0

Inherited from

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

Parameters

emitter_DOMEventTargetRequired

eventNamestringRequired

optionsStaticEventEmitterOptions

Returns

Promise<any[]>

PromisePromise<any[]>Required

Inherited from