Socket

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

This property is only present if the family autoselection algorithm is enabled in socket.connect(options) and it is an array of the addresses that have been attempted.

Each address is a string in the form of $IP:$PORT. If the connection was successful, then the last address is the one that the socket is currently connected to.

@since v19.4.0

This property shows the number of characters buffered for writing. The buffer may contain strings whose length after encoding is not yet known. So this number is only an approximation of the number of bytes in the buffer.

net.Socket has the property that socket.write() always works. This is to help users get up and running quickly. The computer cannot always keep up with the amount of data that is written to a socket. The network connection simply might be too slow. Node.js will internally queue up the data written to a socket and send it out over the wire when it is possible.

The consequence of this internal buffering is that memory may grow. Users who experience large or growing bufferSize should attempt to "throttle" the data flows in their program with socket.pause() and socket.resume().

@since v0.3.8

The amount of received bytes.

@since v0.5.3

The amount of bytes sent.

@since v0.5.3

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

If true, socket.connect(options[, connectListener]) was called and has not yet finished. It will stay true until the socket becomes connected, then it is set to false and the 'connect' event is emitted. Note that the socket.connect(options[, connectListener]) callback is a listener for the 'connect' event.

@since v6.1.0

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

See writable.destroyed for further details.

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

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.

The string representation of the local IP address the remote client is connecting on. For example, in a server listening on '0.0.0.0', if a client connects on '192.168.1.1', the value of socket.localAddress would be'192.168.1.1'.

@since v0.9.6

The string representation of the local IP family. 'IPv4' or 'IPv6'.

@since v18.8.0, v16.18.0

The numeric representation of the local port. For example, 80 or 21.

@since v0.9.6

This is true if the socket is not connected yet, either because .connect()has not yet been called or because it is still in the process of connecting (see socket.connecting).

@since v11.2.0, v10.16.0

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

This property represents the state of the connection as a string.

• If the stream is connecting socket.readyState is opening.
• If the stream is readable and writable, it is open.
• If the stream is readable and not writable, it is readOnly.
• If the stream is not readable and writable, it is writeOnly.

@since v0.5.0

The string representation of the remote IP address. For example,'74.125.127.100' or '2001:4860:a005::68'. Value may be undefined if the socket is destroyed (for example, if the client disconnected).

@since v0.5.10

The string representation of the remote IP family. 'IPv4' or 'IPv6'. Value may be undefined if the socket is destroyed (for example, if the client disconnected).

@since v0.11.14

The numeric representation of the remote port. For example, 80 or 21. Value may be undefined if the socket is destroyed (for example, if the client disconnected).

@since v0.5.10

The socket timeout in milliseconds as set by socket.setTimeout(). It is undefined if a timeout has not been set.

@since v10.7.0

Half-closes the socket. i.e., it sends a FIN packet. It is possible the server will still send some data.

See writable.end() for further details. Half-closes the socket, with one final chunk of data.

@since v0.1.90

Returns. The socket itself.

Half-closes the socket. i.e., it sends a FIN packet. It is possible the server will still send some data.

See writable.end() for further details. Half-closes the socket, with one final chunk of data.

@since v0.1.90

Returns. The socket itself.

Half-closes the socket. i.e., it sends a FIN packet. It is possible the server will still send some data.

See writable.end() for further details. Half-closes the socket, with one final chunk of data.

@since v0.1.90

Returns. The socket itself.

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

events.EventEmitter

1. close
2. connect
3. connectionAttempt
4. connectionAttemptFailed
5. connectionAttemptTimeout
6. data
7. drain
8. end
9. error
10. lookup
11. ready
12. timeout

Returns the bound address, the address family name and port of the socket as reported by the operating system:{ port: 12346, family: 'IPv4', address: '127.0.0.1' }

@since v0.1.90

Initiate a connection on a given socket.

Possible signatures:

• socket.connect(options[, connectListener])
• socket.connect(path[, connectListener]) for IPC connections.
• socket.connect(port[, host][, connectListener]) for TCP connections.
• Returns: net.Socket The socket itself.

This function is asynchronous. When the connection is established, the 'connect' event will be emitted. If there is a problem connecting, instead of a 'connect' event, an 'error' event will be emitted with the error passed to the 'error' listener. The last parameter connectListener, if supplied, will be added as a listener for the 'connect' event once.

This function should only be used for reconnecting a socket after'close' has been emitted or otherwise it may lead to undefined behavior.

Initiate a connection on a given socket.

Possible signatures:

• socket.connect(options[, connectListener])
• socket.connect(path[, connectListener]) for IPC connections.
• socket.connect(port[, host][, connectListener]) for TCP connections.
• Returns: net.Socket The socket itself.

This function is asynchronous. When the connection is established, the 'connect' event will be emitted. If there is a problem connecting, instead of a 'connect' event, an 'error' event will be emitted with the error passed to the 'error' listener. The last parameter connectListener, if supplied, will be added as a listener for the 'connect' event once.

This function should only be used for reconnecting a socket after'close' has been emitted or otherwise it may lead to undefined behavior.

Initiate a connection on a given socket.

Possible signatures:

• socket.connect(options[, connectListener])
• socket.connect(path[, connectListener]) for IPC connections.
• socket.connect(port[, host][, connectListener]) for TCP connections.
• Returns: net.Socket The socket itself.

This function is asynchronous. When the connection is established, the 'connect' event will be emitted. If there is a problem connecting, instead of a 'connect' event, an 'error' event will be emitted with the error passed to the 'error' listener. The last parameter connectListener, if supplied, will be added as a listener for the 'connect' event once.

This function should only be used for reconnecting a socket after'close' has been emitted or otherwise it may lead to undefined behavior.

Initiate a connection on a given socket.

Possible signatures:

• socket.connect(options[, connectListener])
• socket.connect(path[, connectListener]) for IPC connections.
• socket.connect(port[, host][, connectListener]) for TCP connections.
• Returns: net.Socket The socket itself.

This function is asynchronous. When the connection is established, the 'connect' event will be emitted. If there is a problem connecting, instead of a 'connect' event, an 'error' event will be emitted with the error passed to the 'error' listener. The last parameter connectListener, if supplied, will be added as a listener for the 'connect' event once.

This function should only be used for reconnecting a socket after'close' has been emitted or otherwise it may lead to undefined behavior.

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

Destroys the socket after all data is written. If the finish event was already emitted the socket is destroyed immediately. If the socket is still writable it implicitly calls socket.end().

@since v0.3.4

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

A utility method for creating duplex streams.

• Stream converts writable stream into writable Duplex and readable stream to Duplex.
• Blob converts into readable Duplex.
• string converts into readable Duplex.
• ArrayBuffer converts into readable Duplex.
• AsyncIterable converts into a readable Duplex. Cannot yield null.
• AsyncGeneratorFunction converts into a readable/writable transform Duplex. Must take a source AsyncIterable as first parameter. Cannot yield null.
• AsyncFunction converts into a writable Duplex. Must return either null or undefined
• Object ({ writable, readable }) converts readable and writable into Stream and then combines them into Duplex where the Duplex will write to the writable and read from the readable.
• Promise converts into readable Duplex. Value null is ignored.

@since v16.8.0

A utility method for creating a Duplex from a web ReadableStream and WritableStream.

@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 encountered an error.

@since v17.3.0, v16.14.0

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.

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

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);
}