Server

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

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

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

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

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.

The server.addContext() method adds a secure context that will be used if the client request's SNI name matches the supplied hostname (or wildcard).

When there are multiple matching contexts, the most recently added one is used.

@since v0.5.3

events.EventEmitter

1. tlsClientError
2. newSession
3. OCSPRequest
4. resumeSession
5. secureConnection
6. keylog

Stops the server from accepting new connections and keeps existing connections. This function is asynchronous, the server is finally closed when all connections are ended and the server emits a 'close' event. The optional callback will be called once the 'close' event occurs. Unlike that event, it will be called with an Error as its only argument if the server was not open when it was closed.

@since v0.1.90

Asynchronously get the number of concurrent connections on the server. Works when sockets were sent to forks.

Callback should take two arguments err and count.

@since v0.9.7

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 the session ticket keys.

See Session Resumption for more information.

@since v3.0.0

Returns. A 48-byte buffer containing the session ticket keys.

Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.

Possible signatures:

• server.listen(handle[, backlog][, callback])
• server.listen(options[, callback])
• server.listen(path[, backlog][, callback]) for IPC servers
• server.listen([port[, host[, backlog]]][, callback]) for TCP servers

This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.

All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).

All Socket are set to SO_REUSEADDR (see socket(7) for details).

The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.

One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:

server.on('error', (e) => {
if (e.code === 'EADDRINUSE') {
console.error('Address in use, retrying...');
setTimeout(() => {
server.close();
server.listen(PORT, HOST);
}, 1000);
}
});

Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.

Possible signatures:

• server.listen(handle[, backlog][, callback])
• server.listen(options[, callback])
• server.listen(path[, backlog][, callback]) for IPC servers
• server.listen([port[, host[, backlog]]][, callback]) for TCP servers

This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.

All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).

All Socket are set to SO_REUSEADDR (see socket(7) for details).

The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.

One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:

server.on('error', (e) => {
if (e.code === 'EADDRINUSE') {
console.error('Address in use, retrying...');
setTimeout(() => {
server.close();
server.listen(PORT, HOST);
}, 1000);
}
});

Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.

Possible signatures:

• server.listen(handle[, backlog][, callback])
• server.listen(options[, callback])
• server.listen(path[, backlog][, callback]) for IPC servers
• server.listen([port[, host[, backlog]]][, callback]) for TCP servers

This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.

All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).

All Socket are set to SO_REUSEADDR (see socket(7) for details).

The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.

One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:

server.on('error', (e) => {
if (e.code === 'EADDRINUSE') {
console.error('Address in use, retrying...');
setTimeout(() => {
server.close();
server.listen(PORT, HOST);
}, 1000);
}
});

Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.

Possible signatures:

• server.listen(handle[, backlog][, callback])
• server.listen(options[, callback])
• server.listen(path[, backlog][, callback]) for IPC servers
• server.listen([port[, host[, backlog]]][, callback]) for TCP servers

This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.

All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).

All Socket are set to SO_REUSEADDR (see socket(7) for details).

The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.

One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:

server.on('error', (e) => {
if (e.code === 'EADDRINUSE') {
console.error('Address in use, retrying...');
setTimeout(() => {
server.close();
server.listen(PORT, HOST);
}, 1000);
}
});

Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.

Possible signatures:

• server.listen(handle[, backlog][, callback])
• server.listen(options[, callback])
• server.listen(path[, backlog][, callback]) for IPC servers
• server.listen([port[, host[, backlog]]][, callback]) for TCP servers

This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.

All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).

All Socket are set to SO_REUSEADDR (see socket(7) for details).

The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.

One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:

server.on('error', (e) => {
if (e.code === 'EADDRINUSE') {
console.error('Address in use, retrying...');
setTimeout(() => {
server.close();
server.listen(PORT, HOST);
}, 1000);
}
});

Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.

Possible signatures:

• server.listen(handle[, backlog][, callback])
• server.listen(options[, callback])
• server.listen(path[, backlog][, callback]) for IPC servers
• server.listen([port[, host[, backlog]]][, callback]) for TCP servers

This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.

All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).

All Socket are set to SO_REUSEADDR (see socket(7) for details).

The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.

One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:

server.on('error', (e) => {
if (e.code === 'EADDRINUSE') {
console.error('Address in use, retrying...');
setTimeout(() => {
server.close();
server.listen(PORT, HOST);
}, 1000);
}
});

Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.

Possible signatures:

• server.listen(handle[, backlog][, callback])
• server.listen(options[, callback])
• server.listen(path[, backlog][, callback]) for IPC servers
• server.listen([port[, host[, backlog]]][, callback]) for TCP servers

This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.

All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).

All Socket are set to SO_REUSEADDR (see socket(7) for details).

The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.

One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:

server.on('error', (e) => {
if (e.code === 'EADDRINUSE') {
console.error('Address in use, retrying...');
setTimeout(() => {
server.close();
server.listen(PORT, HOST);
}, 1000);
}
});

Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.

Possible signatures:

• server.listen(handle[, backlog][, callback])
• server.listen(options[, callback])
• server.listen(path[, backlog][, callback]) for IPC servers
• server.listen([port[, host[, backlog]]][, callback]) for TCP servers

This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.

All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).

All Socket are set to SO_REUSEADDR (see socket(7) for details).

The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.

One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:

server.on('error', (e) => {
if (e.code === 'EADDRINUSE') {
console.error('Address in use, retrying...');
setTimeout(() => {
server.close();
server.listen(PORT, HOST);
}, 1000);
}
});

Start a server listening for connections. A net.Server can be a TCP or an IPC server depending on what it listens to.

Possible signatures:

• server.listen(handle[, backlog][, callback])
• server.listen(options[, callback])
• server.listen(path[, backlog][, callback]) for IPC servers
• server.listen([port[, host[, backlog]]][, callback]) for TCP servers

This function is asynchronous. When the server starts listening, the 'listening' event will be emitted. The last parameter callbackwill be added as a listener for the 'listening' event.

All listen() methods can take a backlog parameter to specify the maximum length of the queue of pending connections. The actual length will be determined by the OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on Linux. The default value of this parameter is 511 (not 512).

All Socket are set to SO_REUSEADDR (see socket(7) for details).

The server.listen() method can be called again if and only if there was an error during the first server.listen() call or server.close() has been called. Otherwise, an ERR_SERVER_ALREADY_LISTEN error will be thrown.

One of the most common errors raised when listening is EADDRINUSE. This happens when another server is already listening on the requestedport/path/handle. One way to handle this would be to retry after a certain amount of time:

server.on('error', (e) => {
if (e.code === 'EADDRINUSE') {
console.error('Address in use, retrying...');
setTimeout(() => {
server.close();
server.listen(PORT, HOST);
}, 1000);
}
});

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);
}

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

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.

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.

Opposite of unref(), calling ref() on a previously unrefed server will not let the program exit if it's the only server left (the default behavior). If the server is refed calling ref() again will have no effect.

@since v0.9.1

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.

@since v0.1.26

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.

@since v0.1.26

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

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

setMaxListeners(5, target, emitter);

@since v15.4.0

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.

@since v0.3.5

The server.setSecureContext() method replaces the secure context of an existing server. Existing connections to the server are not interrupted.

@since v11.0.0

Sets the session ticket keys.

Changes to the ticket keys are effective only for future server connections. Existing or currently pending server connections will use the previous keys.

See Session Resumption for more information.

@since v3.0.0

Calling unref() on a server will allow the program to exit if this is the only active server in the event system. If the server is already unrefed callingunref() again will have no effect.

@since v0.9.1