Module

Defined in: packages/core/src/module.ts:102

Module - Represents a worker process with configuration and communication.

Example

const worker = new Module('worker')
  .executable('python', ['worker.py'])
  .method('process', { codec: msgpackCodec })
  .event('progress');

// After manager.spawn():
const result = await worker.send('process', data);
worker.onEvent('progress', console.log);

Extends

EventEmitter

Type Parameters

S

S extends Schema = Schema

Constructors

Constructor

new Module<S>(name): Module<S>

Defined in: packages/core/src/module.ts:141

Parameters

name

string

Returns

Module<S>

Overrides

EventEmitter.constructor

Properties

__schema

readonly __schema: S

Defined in: packages/core/src/module.ts:103

name

readonly name: string

Defined in: packages/core/src/module.ts:105

captureRejections

static captureRejections: boolean

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:425

Value: boolean

Change the default captureRejections option on all new EventEmitter objects.

Since

v13.4.0, v12.16.0

Inherited from

EventEmitter.captureRejections

captureRejectionSymbol

readonly static captureRejectionSymbol: typeof captureRejectionSymbol

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:418

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

See how to write a custom rejection handler.

Since

v13.4.0, v12.16.0

Inherited from

EventEmitter.captureRejectionSymbol

defaultMaxListeners

static defaultMaxListeners: number

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:464

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

Inherited from

EventEmitter.defaultMaxListeners

errorMonitor

readonly static errorMonitor: typeof errorMonitor

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:411

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

Inherited from

EventEmitter.errorMonitor

Accessors

events

Get Signature

get events(): ReadonlyMap<string, EventConfig>

Defined in: packages/core/src/module.ts:287

Returns

ReadonlyMap<string, EventConfig>

executableConfig

Get Signature

get executableConfig(): ExecutableConfig | null

Defined in: packages/core/src/module.ts:279

Returns

ExecutableConfig | null

methods

Get Signature

get methods(): ReadonlyMap<string, MethodConfig>

Defined in: packages/core/src/module.ts:283

Returns

ReadonlyMap<string, MethodConfig>

process

Get Signature

get process(): ChildProcess | null

Defined in: packages/core/src/module.ts:295

Returns

ChildProcess | null

spawnPolicyConfig

Get Signature

get spawnPolicyConfig(): SpawnPolicy

Defined in: packages/core/src/module.ts:291

Returns

SpawnPolicy

state

Get Signature

get state(): ModuleState

Defined in: packages/core/src/module.ts:275

Returns

ModuleState

Methods

[captureRejectionSymbol]()?

optional [captureRejectionSymbol]<K>(error, event, …args): void

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:103

Type Parameters

K

K

Parameters

error

Error

event

string | symbol

args

…AnyRest

Returns

void

Inherited from

EventEmitter.[captureRejectionSymbol]

addListener()

addListener<K>(eventName, listener): this

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:642

Alias for emitter.on(eventName, listener).

Type Parameters

K

K

Parameters

eventName

string | symbol

listener

(…args) => void

Returns

this

Since

v0.1.26

Inherited from

EventEmitter.addListener

emit()

emit<K>(eventName, …args): boolean

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:904

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

Type Parameters

K

K

Parameters

eventName

string | symbol

args

…AnyRest

Returns

boolean

Since

v0.1.26

Inherited from

EventEmitter.emit

event()

event<N, C>(name, config): Module<AddEvent<S, N, C>>

Defined in: packages/core/src/module.ts:245

Type Parameters

N

N extends string

C

C extends Codec = Codec

Parameters

name

N

config

TypedEventConfig<C> = ...

Returns

Module<AddEvent<S, N, C>>

eventNames()

eventNames(): (string | symbol)[]

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:967

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

import { EventEmitter } from 'node:events';

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

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

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

Returns

(string | symbol)[]

Since

v6.0.0

Inherited from

EventEmitter.eventNames

executable()

executable(command, args, options?): this

Defined in: packages/core/src/module.ts:153

Set executable command.

Parameters

command

string

args

string[] = []

options?

cwd?

string

env?

Record<string, string>

Returns

this

getMaxListeners()

getMaxListeners(): number

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:819

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

Returns

number

Since

v1.0.0

Inherited from

EventEmitter.getMaxListeners

listenerCount()

listenerCount<K>(eventName, listener?): number

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:913

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

Type Parameters

K

K

Parameters

eventName

The name of the event being listened for

string | symbol

listener?

Function

The event handler function

Returns

number

Since

v3.2.0

Inherited from

EventEmitter.listenerCount

listeners()

listeners<K>(eventName): Function[]

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:832

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

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

Type Parameters

K

K

Parameters

eventName

string | symbol

Returns

Function[]

Since

v0.1.26

Inherited from

EventEmitter.listeners

maxPayloadSize()

maxPayloadSize(size): this

Defined in: packages/core/src/module.ts:266

Set max payload size.

Parameters

size

number

Returns

this

method()

Call Signature

method<N, CReq, CRes, RT>(name, config): Module<AddMethod<S, N, CReq, CRes, RT>>

Defined in: packages/core/src/module.ts:173

Use when request and response need different codecs, or when using asymmetric codecs like Arrow.

Type Parameters

N

N extends string

CReq

CReq extends Codec

CRes

CRes extends Codec

RT

RT extends ResponseType = "result"

Parameters

name

N

config

DualCodecMethodConfig<CReq, CRes> & object

Returns

Module<AddMethod<S, N, CReq, CRes, RT>>

Call Signature

method<N, C, RT>(name, config?): Module<AddMethodSymmetric<S, N, C, RT>>

Defined in: packages/core/src/module.ts:186

Type Parameters

N

N extends string

C

C extends Codec = Codec

RT

RT extends ResponseType = "result"

Parameters

name

N

config?

SingleCodecMethodConfig<C> & object

Returns

Module<AddMethodSymmetric<S, N, C, RT>>

off()

off<K>(eventName, listener): this

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:792

Alias for emitter.removeListener().

Type Parameters

K

K

Parameters

eventName

string | symbol

listener

(…args) => void

Returns

this

Since

v10.0.0

Inherited from

EventEmitter.off

on()

on<K>(eventName, listener): this

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:674

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

Type Parameters

K

K

Parameters

eventName

The name of the event.

string | symbol

listener

(…args) => void

The callback function

Returns

this

Since

v0.1.101

Inherited from

EventEmitter.on

once()

once<K>(eventName, listener): this

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:704

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

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

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

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

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

Type Parameters

K

K

Parameters

eventName

The name of the event.

string | symbol

listener

(…args) => void

The callback function

Returns

this

Since

v0.3.0

Inherited from

EventEmitter.once

onEvent()

Call Signature

onEvent<E>(eventName, handler): () => void

Defined in: packages/core/src/module.ts:656

Listen for events from module.

Type Parameters

E

E extends string

Parameters

eventName

E

Event name

handler

(data) => void

Handler function

Returns

Unsubscribe function

(): void

Returns

void

Call Signature

onEvent<T>(eventName, handler): () => void

Defined in: packages/core/src/module.ts:660

Listen for events from module.

Type Parameters

T

T = unknown

Parameters

eventName

string

Event name

handler

(data) => void

Handler function

Returns

Unsubscribe function

(): void

Returns

void

prependListener()

prependListener<K>(eventName, listener): this

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:931

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.

Type Parameters

K

K

Parameters

eventName

The name of the event.

string | symbol

listener

(…args) => void

The callback function

Returns

this

Since

v6.0.0

Inherited from

EventEmitter.prependListener

prependOnceListener()

prependOnceListener<K>(eventName, listener): this

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:947

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.

Type Parameters

K

K

Parameters

eventName

The name of the event.

string | symbol

listener

(…args) => void

The callback function

Returns

this

Since

v6.0.0

Inherited from

EventEmitter.prependOnceListener

rawListeners()

rawListeners<K>(eventName): Function[]

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:863

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

import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.once('log', () => console.log('log once'));

// Returns a new Array with a function `onceWrapper` which has a property
// `listener` which contains the original listener bound above
const listeners = emitter.rawListeners('log');
const logFnWrapper = listeners[0];

// Logs "log once" to the console and does not unbind the `once` event
logFnWrapper.listener();

// Logs "log once" to the console and removes the listener
logFnWrapper();

emitter.on('log', () => console.log('log persistently'));
// Will return a new Array with a single function bound by `.on()` above
const newListeners = emitter.rawListeners('log');

// Logs "log persistently" twice
newListeners[0]();
emitter.emit('log');

Type Parameters

K

K

Parameters

eventName

string | symbol

Returns

Function[]

Since

v9.4.0

Inherited from

EventEmitter.rawListeners

removeAllListeners()

removeAllListeners(eventName?): this

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:803

Removes all listeners, or those of the specified eventName.

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

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

Parameters

eventName?

string | symbol

Returns

this

Since

v0.1.26

Inherited from

EventEmitter.removeAllListeners

removeListener()

removeListener<K>(eventName, listener): this

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:787

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.

Type Parameters

K

K

Parameters

eventName

string | symbol

listener

(…args) => void

Returns

this

Since

v0.1.26

Inherited from

EventEmitter.removeListener

send()

Call Signature

send<M>(method, data, options?): Promise<SendReturn<S["methods"][M]["resOut"], S["methods"][M]["responseType"]>>

Defined in: packages/core/src/module.ts:440

Send a request to the module.

Type Parameters

M

M extends string

Parameters

method

M

Method name

data

S["methods"][M]["reqIn"]

Data to send

options?

Optional abort signal

signal?

AbortSignal

Returns

Promise<SendReturn<S["methods"][M]["resOut"], S["methods"][M]["responseType"]>>

Response data

Throws

if module not ready

Throws

if method unknown

Throws

if timeout

Throws

if aborted

Call Signature

send<TResponse, TRequest>(method, data, options?): Promise<TResponse>

Defined in: packages/core/src/module.ts:445

Send a request to the module.

Type Parameters

TResponse

TResponse = unknown

TRequest

TRequest = unknown

Parameters

method

string

Method name

data

TRequest

Data to send

options?

Optional abort signal

signal?

AbortSignal

Returns

Promise<TResponse>

Response data

Throws

if module not ready

Throws

if method unknown

Throws

if timeout

Throws

if aborted

setMaxListeners()

setMaxListeners(n): this

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:813

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.

Parameters

n

number

Returns

this

Since

v0.3.5

Inherited from

EventEmitter.setMaxListeners

spawnPolicy()

spawnPolicy(policy): this

Defined in: packages/core/src/module.ts:258

Set spawn policy.

Parameters

policy

SpawnPolicy

Returns

this

stream()

Call Signature

stream<M>(method, data, options?): AsyncGenerator<S["methods"][M]["resOut"]>

Defined in: packages/core/src/module.ts:535

Send a streaming request.

Type Parameters

M

M extends string

Parameters

method

M

Method name

data

S["methods"][M]["reqIn"]

Data to send

options?

Optional abort signal

signal?

AbortSignal

Returns

AsyncGenerator<S["methods"][M]["resOut"]>

AsyncGenerator yielding response chunks

Call Signature

stream<TChunk, TRequest>(method, data, options?): AsyncGenerator<TChunk>

Defined in: packages/core/src/module.ts:540

Send a streaming request.

Type Parameters

TChunk

TChunk = unknown

TRequest

TRequest = unknown

Parameters

method

string

Method name

data

TRequest

Data to send

options?

Optional abort signal

signal?

AbortSignal

Returns

AsyncGenerator<TChunk>

AsyncGenerator yielding response chunks

addAbortListener()

static addAbortListener(signal, resource): Disposable

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:403

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]();
  }
}

Parameters

signal

AbortSignal

resource

(event) => void

Returns

Disposable

Disposable that removes the abort listener.

Since

v20.5.0

Inherited from

EventEmitter.addAbortListener

getEventListeners()

static getEventListeners(emitter, name): Function[]

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:325

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

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

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

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

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

Parameters

emitter

EventEmitter<DefaultEventMap> | EventTarget

name

string | symbol

Returns

Function[]

Since

v15.2.0, v14.17.0

Inherited from

EventEmitter.getEventListeners

getMaxListeners()

static getMaxListeners(emitter): number

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:354

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
}

Parameters

emitter

EventEmitter<DefaultEventMap> | EventTarget

Returns

number

Since

v19.9.0

Inherited from

EventEmitter.getMaxListeners

listenerCount()

static listenerCount(emitter, eventName): number

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:297

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

Parameters

emitter

EventEmitter

The emitter to query

eventName

The event name

string | symbol

Returns

number

Since

v0.9.12

Inherited from

EventEmitter.listenerCount

on()

Call Signature

static on(emitter, eventName, options?): AsyncIterator<any[]>

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:270

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'

Parameters

emitter

EventEmitter

eventName

string | symbol

options?

StaticEventEmitterIteratorOptions

Returns

AsyncIterator<any[]>

An AsyncIterator that iterates eventName events emitted by the emitter

Since

v13.6.0, v12.16.0

Inherited from

EventEmitter.on

Call Signature

static on(emitter, eventName, options?): AsyncIterator<any[]>

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:275

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

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'

Parameters

emitter

EventTarget

eventName

string

options?

StaticEventEmitterIteratorOptions

Returns

AsyncIterator<any[]>

An AsyncIterator that iterates eventName events emitted by the emitter

Since

v13.6.0, v12.16.0

Inherited from

EventEmitter.on

once()

Call Signature

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

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:184

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

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

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

const ee = new EventEmitter();

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

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

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

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

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

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

const ee = new EventEmitter();

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

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

// Prints: ok boom

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

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

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

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

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

Parameters

emitter

EventEmitter

eventName

string | symbol

options?

StaticEventEmitterOptions

Returns

Promise<any[]>

Since

v11.13.0, v10.16.0

Inherited from

EventEmitter.once

Call Signature

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

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:189

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);
}

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

const ee = new EventEmitter();

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

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

// Prints: ok boom

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

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

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

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

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

Parameters

emitter

EventTarget

eventName

string

options?

StaticEventEmitterOptions

Returns

Promise<any[]>

Since

v11.13.0, v10.16.0

Inherited from

EventEmitter.once

setMaxListeners()

static setMaxListeners(n?, …eventTargets?): void

Defined in: node_modules/.pnpm/@types+node@22.19.7/node_modules/@types/node/events.d.ts:369

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

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

setMaxListeners(5, target, emitter);

Parameters

n?

number

A non-negative number. The maximum number of listeners per EventTarget event.

eventTargets?

…(EventEmitter<DefaultEventMap> | EventTarget)[]

Zero or more {EventTarget} or {EventEmitter} instances. If none are specified, n is set as the default max for all newly created {EventTarget} and {EventEmitter} objects.

Returns

void

Since

v15.4.0

Inherited from

EventEmitter.setMaxListeners