Readable

Duplex streams are streams that implement both the Readable and Writable interfaces.

Examples of Duplex streams include:

• TCP sockets
• zlib streams
• crypto streams

@since v0.9.4

Integrates EventEmitter with AsyncResource for EventEmitters that require manual async tracking. Specifically, all events emitted by instances of events.EventEmitterAsyncResource will run within its async context.

import { EventEmitterAsyncResource, EventEmitter } from 'node:events';
import { notStrictEqual, strictEqual } from 'node:assert';
import { executionAsyncId, triggerAsyncId } from 'node:async_hooks';

// Async tracking tooling will identify this as 'Q'.
const ee1 = new EventEmitterAsyncResource({ name: 'Q' });

// 'foo' listeners will run in the EventEmitters async context.
ee1.on('foo', () => {
strictEqual(executionAsyncId(), ee1.asyncId);
strictEqual(triggerAsyncId(), ee1.triggerAsyncId);
});

const ee2 = new EventEmitter();

// 'foo' listeners on ordinary EventEmitters that do not track async
// context, however, run in the same async context as the emit().
ee2.on('foo', () => {
notStrictEqual(executionAsyncId(), ee2.asyncId);
notStrictEqual(triggerAsyncId(), ee2.triggerAsyncId);
});

Promise.resolve().then(() => {
ee1.emit('foo');
ee2.emit('foo');
});

The EventEmitterAsyncResource class has the same methods and takes the same options as EventEmitter and AsyncResource themselves.

@since v17.4.0, v16.14.0

The stream.PassThrough class is a trivial implementation of a Transform stream that simply passes the input bytes across to the output. Its purpose is primarily for examples and testing, but there are some use cases where stream.PassThrough is useful as a building block for novel sorts of streams.

@since v0.9.4

Transform streams are Duplex streams where the output is in some way related to the input. Like all Duplex streams, Transform streams implement both the Readable and Writable interfaces.

Examples of Transform streams include:

• zlib streams
• crypto streams

@since v0.9.4

@since v0.9.4

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

See how to write a custom rejection handler.

@since v13.4.0, v12.16.0

Value: boolean

Change the default captureRejections option on all new EventEmitter objects.

@since v13.4.0, v12.16.0

Is true after 'close' has been emitted.

@since v18.0.0

By default, a maximum of 10 listeners can be registered for any single event. This limit can be changed for individual EventEmitter instances using the emitter.setMaxListeners(n) method. To change the default for allEventEmitter instances, the events.defaultMaxListeners property can be used. If this value is not a positive number, a RangeError is thrown.

Take caution when setting the events.defaultMaxListeners because the change affects all EventEmitter instances, including those created before the change is made. However, calling emitter.setMaxListeners(n) still has precedence over events.defaultMaxListeners.

This is not a hard limit. The EventEmitter instance will allow more listeners to be added but will output a trace warning to stderr indicating that a "possible EventEmitter memory leak" has been detected. For any single EventEmitter, the emitter.getMaxListeners() and emitter.setMaxListeners() methods can be used to temporarily avoid this warning:

import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
// do stuff
emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});

The --trace-warnings command-line flag can be used to display the stack trace for such warnings.

The emitted warning can be inspected with process.on('warning') and will have the additional emitter, type, and count properties, referring to the event emitter instance, the event's name and the number of attached listeners, respectively. Its name property is set to 'MaxListenersExceededWarning'.

@since v0.11.2

Is true after readable.destroy() has been called.

@since v8.0.0

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

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

@since v13.6.0, v12.17.0

Returns error if the stream has been destroyed with an error.

@since v18.0.0

A readable and/or writable stream/webstream.

A function to get notified when a stream is no longer readable, writable or has experienced an error or a premature close event.

import { finished } from 'node:stream';
import fs from 'node:fs';

const rs = fs.createReadStream('archive.tar');

finished(rs, (err) => {
if (err) {
console.error('Stream failed.', err);
} else {
console.log('Stream is done reading.');
}
});

rs.resume(); // Drain the stream.

Especially useful in error handling scenarios where a stream is destroyed prematurely (like an aborted HTTP request), and will not emit 'end' or 'finish'.

The finished API provides promise version.

stream.finished() leaves dangling event listeners (in particular 'error', 'end', 'finish' and 'close') after callback has been invoked. The reason for this is so that unexpected 'error' events (due to incorrect stream implementations) do not cause unexpected crashes. If this is unwanted behavior then the returned cleanup function needs to be invoked in the callback:

const cleanup = finished(rs, (err) => {
cleanup();
// ...
});

@since v10.0.0

Returns. A cleanup function which removes all registered listeners.

A module method to pipe between streams and generators forwarding errors and properly cleaning up and provide a callback when the pipeline is complete.

import { pipeline } from 'node:stream';
import fs from 'node:fs';
import zlib from 'node:zlib';

// Use the pipeline API to easily pipe a series of streams
// together and get notified when the pipeline is fully done.

// A pipeline to gzip a potentially huge tar file efficiently:

pipeline(
fs.createReadStream('archive.tar'),
zlib.createGzip(),
fs.createWriteStream('archive.tar.gz'),
(err) => {
if (err) {
console.error('Pipeline failed.', err);
} else {
console.log('Pipeline succeeded.');
}
},
);

The pipeline API provides a promise version.

stream.pipeline() will call stream.destroy(err) on all streams except:

• Readable streams which have emitted 'end' or 'close'.
• Writable streams which have emitted 'finish' or 'close'.

stream.pipeline() leaves dangling event listeners on the streams after the callback has been invoked. In the case of reuse of streams after failure, this can cause event listener leaks and swallowed errors. If the last stream is readable, dangling event listeners will be removed so that the last stream can be consumed later.

stream.pipeline() closes all the streams when an error is raised. The IncomingRequest usage with pipeline could lead to an unexpected behavior once it would destroy the socket without sending the expected response. See the example below:

import fs from 'node:fs';
import http from 'node:http';
import { pipeline } from 'node:stream';

const server = http.createServer((req, res) => {
const fileStream = fs.createReadStream('./fileNotExist.txt');
pipeline(fileStream, res, (err) => {
if (err) {
console.log(err); // No such file
// this message can't be sent once `pipeline` already destroyed the socket
return res.end('error!!!');
}
});
});

@since v10.0.0

Is true if it is safe to call read, which means the stream has not been destroyed or emitted 'error' or 'end'.

@since v11.4.0

Returns whether the stream was destroyed or errored before emitting 'end'.

@since v16.8.0

Returns whether 'data' has been emitted.

@since v16.7.0, v14.18.0

Getter for the property encoding of a given Readable stream. The encoding property can be set using the setEncoding method.

@since v12.7.0

Becomes true when 'end' event is emitted.

@since v12.9.0

This property reflects the current state of a Readable stream as described in the Three states section.

@since v9.4.0

Returns the value of highWaterMark passed when creating this Readable.

@since v9.3.0

This property contains the number of bytes (or objects) in the queue ready to be read. The value provides introspection data regarding the status of the highWaterMark.

@since v9.4.0

Getter for the property objectMode of a given Readable stream.

@since v12.3.0

Calls readable.destroy() with an AbortError and returns a promise that fulfills when the stream is finished.

@since v20.4.0

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]();
}
}

@since v20.5.0

Returns. Disposable that removes the abort listener.

A stream to attach a signal to.

Attaches an AbortSignal to a readable or writeable stream. This lets code control stream destruction using an AbortController.

Calling abort on the AbortController corresponding to the passed AbortSignal will behave the same way as calling .destroy(new AbortError()) on the stream, and controller.error(new AbortError()) for webstreams.

import fs from 'node:fs';

const controller = new AbortController();
const read = addAbortSignal(
controller.signal,
fs.createReadStream(('object.json')),
);
// Later, abort the operation closing the stream
controller.abort();

Or using an AbortSignal with a readable stream as an async iterable:

const controller = new AbortController();
setTimeout(() => controller.abort(), 10_000); // set a timeout
const stream = addAbortSignal(
controller.signal,
fs.createReadStream(('object.json')),
);
(async () => {
try {
for await (const chunk of stream) {
await process(chunk);
}
} catch (e) {
if (e.name === 'AbortError') {
// The operation was cancelled
} else {
throw e;
}
}
})();

Or using an AbortSignal with a ReadableStream:

const controller = new AbortController();
const rs = new ReadableStream({
start(controller) {
controller.enqueue('hello');
controller.enqueue('world');
controller.close();
},
});

addAbortSignal(controller.signal, rs);

finished(rs, (err) => {
if (err) {
if (err.name === 'AbortError') {
// The operation was cancelled
}
}
});

const reader = rs.getReader();

reader.read().then(({ value, done }) => {
console.log(value); // hello
console.log(done); // false
controller.abort();
});

@since v15.4.0

Event emitter The defined events on documents including:

1. close
2. data
3. end
4. error
5. pause
6. readable
7. resume

Event emitter The defined events on documents including:

1. close
2. data
3. end
4. error
5. pause
6. readable
7. resume

addListener(event: "close", listener: () => void): this

This method returns a new stream with chunks of the underlying stream paired with a counter in the form [index, chunk]. The first index value is 0 and it increases by 1 for each chunk produced.

@since v17.5.0

Returns. a stream of indexed pairs.

Destroy the stream. Optionally emit an 'error' event, and emit a 'close' event (unless emitClose is set to false). After this call, the readable stream will release any internal resources and subsequent calls to push() will be ignored.

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 readable._destroy().

@since v8.0.0

This method returns a new stream with the first limit chunks dropped from the start.

@since v17.5.0

Returns. a stream with limit chunks dropped from the start.

The utility function duplexPair returns an Array with two items, each being a Duplex stream connected to the other side:

const [ sideA, sideB ] = duplexPair();

Whatever is written to one stream is made readable on the other. It provides behavior analogous to a network connection, where the data written by the client becomes readable by the server, and vice-versa.

The Duplex streams are symmetrical; one or the other may be used without any difference in behavior.

@since v22.6.0

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

This method is similar to Array.prototype.every and calls fn on each chunk in the stream to check if all awaited return values are truthy value for fn. Once an fn call on a chunk awaited return value is falsy, the stream is destroyed and the promise is fulfilled with false. If all of the fn calls on the chunks return a truthy value, the promise is fulfilled with true.

@since v17.5.0

Returns. a promise evaluating to true if fn returned a truthy value for every one of the chunks.

This method allows filtering the stream. For each chunk in the stream the fn function will be called and if it returns a truthy value, the chunk will be passed to the result stream. If the fn function returns a promise - that promise will be awaited.

@since v17.4.0, v16.14.0

Returns. a stream filtered with the predicate fn.

This method is similar to Array.prototype.find and calls fn on each chunk in the stream to find a chunk with a truthy value for fn. Once an fn call's awaited return value is truthy, the stream is destroyed and the promise is fulfilled with value for which fn returned a truthy value. If all of the fn calls on the chunks return a falsy value, the promise is fulfilled with undefined.

@since v17.5.0

Returns. a promise evaluating to the first chunk for which fn evaluated with a truthy value, or undefined if no element was found.

This method is similar to Array.prototype.find and calls fn on each chunk in the stream to find a chunk with a truthy value for fn. Once an fn call's awaited return value is truthy, the stream is destroyed and the promise is fulfilled with value for which fn returned a truthy value. If all of the fn calls on the chunks return a falsy value, the promise is fulfilled with undefined.

@since v17.5.0

Returns. a promise evaluating to the first chunk for which fn evaluated with a truthy value, or undefined if no element was found.

This method returns a new stream by applying the given callback to each chunk of the stream and then flattening the result.

It is possible to return a stream or another iterable or async iterable from fn and the result streams will be merged (flattened) into the returned stream.

@since v17.5.0

Returns. a stream flat-mapped with the function fn.

This method allows iterating a stream. For each chunk in the stream the fn function will be called. If the fn function returns a promise - that promise will be awaited.

This method is different from for await...of loops in that it can optionally process chunks concurrently. In addition, a forEach iteration can only be stopped by having passed a signal option and aborting the related AbortController while for await...of can be stopped with break or return. In either case the stream will be destroyed.

This method is different from listening to the 'data' event in that it uses the readable event in the underlying machinary and can limit the number of concurrent fn calls.

@since v17.5.0

Returns. a promise for when the stream has finished.

A utility method for creating Readable Streams out of iterators.

@since v12.3.0, v10.17.0

A utility method for creating a Readable from a web ReadableStream.

@since v17.0.0

Returns the default highWaterMark used by streams. Defaults to 65536 (64 KiB), or 16 for objectMode.

@since v19.9.0

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] ]
}

@since v15.2.0, v14.17.0

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
}

@since v19.9.0

Returns whether the stream has been read from or cancelled.

@since v16.8.0

Returns whether the stream has encountered an error.

@since v17.3.0, v16.14.0

The readable.isPaused() method returns the current operating state of the Readable. This is used primarily by the mechanism that underlies the readable.pipe() method. In most typical cases, there will be no reason to use this method directly.

const readable = new stream.Readable();

readable.isPaused(); // === false
readable.pause();
readable.isPaused(); // === true
readable.resume();
readable.isPaused(); // === false

@since v0.11.14

Returns whether the stream is readable.

@since v17.4.0, v16.14.0

Returns. Only returns null if stream is not a valid Readable, Duplex or ReadableStream.

Returns whether the stream is writable.

@since v20.0.0

Returns. Only returns null if stream is not a valid Writable, Duplex or WritableStream.

The iterator created by this method gives users the option to cancel the destruction of the stream if the for await...of loop is exited by return, break, or throw, or if the iterator should destroy the stream if the stream emitted an error during iteration.

@since v16.3.0

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

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

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

@since v0.9.12

This method allows mapping over the stream. The fn function will be called for every chunk in the stream. If the fn function returns a promise - that promise will be awaited before being passed to the result stream.

@since v17.4.0, v16.14.0

Returns. a stream mapped with the function fn.

Alias for emitter.removeListener().

@since v10.0.0

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'

@since v13.6.0, v12.16.0

Returns. An AsyncIterator that iterates eventName events emitted by the emitter

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'

@since v13.6.0, v12.16.0

Returns. An AsyncIterator that iterates eventName events emitted by the emitter

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

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!

@since v11.13.0, v10.16.0

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!

@since v11.13.0, v10.16.0

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. 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