class EventEmitterAsyncResource | Node.js events module | Bun

class EventEmitterAsyncResource | Node.js events module | Bun { 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."/> { 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."/>BuildDocsReferenceGuidesBlogDiscord/node:events/default/EventEmitterAsyncResourceM[events.captureRejectionSymbol]MaddListenerPasyncIdPasyncResourceCoconstructorMemitMemitDestroyMeventNamesMgetMaxListenersMlistenerCountMlistenersMoffMonMonceMprependListenerMprependOnceListenerMrawListenersMremoveAllListenersMremoveListenerMsetMaxListenersPtriggerAsyncId

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BuildDocsReferenceGuidesBlogDiscord/node:events/default/EventEmitterAsyncResourceM[events.captureRejectionSymbol]MaddListenerPasyncIdPasyncResourceCoconstructorMemitMemitDestroyMeventNamesMgetMaxListenersMlistenerCountMlistenersMoffMonMonceMprependListenerMprependOnceListenerMrawListenersMremoveAllListenersMremoveListenerMsetMaxListenersPtriggerAsyncId

class

events.default.EventEmitterAsyncResource { 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." data-algolia-static="false" data-algolia-merged="false" data-type="Class">class EventEmitterAsyncResourceIntegrates 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.

readonly asyncId: numberThe unique asyncId assigned to the resource.

readonly asyncResource: EventEmitterReferencingAsyncResourceThe returned AsyncResource object has an additional eventEmitter property that provides a reference to this EventEmitterAsyncResource.

readonly triggerAsyncId: numberThe same triggerAsyncId that is passed to the AsyncResource constructor.

[events.captureRejectionSymbol](error: Error,event: string | symbol,...args: any[]): void;The Symbol.for('nodejs.rejection') method is called in case a promise rejection happens when emitting an event and captureRejections is enabled on the emitter. It is possible to use events.captureRejectionSymbol in place of Symbol.for('nodejs.rejection').

import { EventEmitter, captureRejectionSymbol } from 'node:events'; class MyClass extends EventEmitter { constructor() { super({ captureRejections: true }); } [captureRejectionSymbol](err, event, ...args) { console.log('rejection happened for', event, 'with', err, ...args); this.destroy(err); } destroy(err) { // Tear the resource down here. } } addListenerE extends string | symbol>(eventName: string | symbol,listener: (...args: any[]) => void): this;Alias for emitter.on(eventName, listener).

emitE extends string | symbol>(eventName: string | symbol,...args: any[]): boolean;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 emitDestroy(): void;Call all destroy hooks. This should only ever be called once. An error will be thrown if it is called more than once. This must be manually called. If the resource is left to be collected by the GC then the destroy hooks will never be called.

{}); myEE.on('bar', () => {}); const sym = Symbol('symbol'); myEE.on(sym, () => {}); console.log(myEE.eventNames()); // Prints: [ 'foo', 'bar', Symbol(symbol) ] ```" data-algolia-static="false" data-algolia-merged="false" data-type="Method">eventNames(): string | symbol[];Returns an array listing the events for which the emitter has registered listeners.

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) ] getMaxListeners(): number;Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to events.defaultMaxListeners.

listenerCountE extends string | symbol>(eventName: string | symbol,listener?: (...args: any[]) => void): number;Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.

@param eventNameThe name of the event being listened for

@param listenerThe event handler function

{ console.log('someone connected!'); }); console.log(util.inspect(server.listeners('connection'))); // Prints: [ [Function] ] ```" data-algolia-static="false" data-algolia-merged="false" data-type="Method">listenersE extends string | symbol>(eventName: string | symbol): (...args: any[]) => void[];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] ] offE extends string | symbol>(eventName: string | symbol,listener: (...args: any[]) => void): this;Alias for emitter.removeListener().

{ 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. ```js 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 ```" data-algolia-static="false" data-algolia-merged="false" data-type="Method">onE extends string | symbol>(eventName: string | symbol,listener: (...args: any[]) => void): this;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 @param eventNameThe name of the event.

@param listenerThe callback function

{ 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. ```js 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 ```" data-algolia-static="false" data-algolia-merged="false" data-type="Method">onceE extends string | symbol>(eventName: string | symbol,listener: (...args: any[]) => void): this;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 @param eventNameThe name of the event.

@param listenerThe callback function

{ console.log('someone connected!'); }); ``` Returns a reference to the `EventEmitter`, so that calls can be chained." data-algolia-static="false" data-algolia-merged="false" data-type="Method">prependListenerE extends string | symbol>(eventName: string | symbol,listener: (...args: any[]) => void): this;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.

@param eventNameThe name of the event.

@param listenerThe callback function

{ console.log('Ah, we have our first user!'); }); ``` Returns a reference to the `EventEmitter`, so that calls can be chained." data-algolia-static="false" data-algolia-merged="false" data-type="Method">prependOnceListenerE extends string | symbol>(eventName: string | symbol,listener: (...args: any[]) => void): this;Adds a one-time listener 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.

@param eventNameThe name of the event.

@param listenerThe callback function

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'); ```" data-algolia-static="false" data-algolia-merged="false" data-type="Method">rawListenersE extends string | symbol>(eventName: string | symbol): (...args: any[]) => void[];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'); removeAllListenersE extends string | symbol>(eventName?: string | symbol): this;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.

{ 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 from `emit()` in progress. Subsequent events behave as expected. ```js 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 indexes 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: ```js 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." data-algolia-static="false" data-algolia-merged="false" data-type="Method">removeListenerE extends string | symbol>(eventName: string | symbol,listener: (...args: any[]) => void): this;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 from emit() 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 indexes 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.

setMaxListeners(n: number): this;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.

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