Class Plug

Static Readonly captureRejectionSymbol

Static captureRejections

Static defaultMaxListeners

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));
});
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Static Readonly errorMonitor

Optional [captureRejectionSymbol]

• [captureRejectionSymbol](error, event, ...args): void

• Parameters

  • error: Error

  • event: string

  • Rest ...args: any[]

  Returns void

addListener

• addListener(eventName, listener): Plug

• Alias for emitter.on(eventName, listener).

  Parameters

  • eventName: string | symbol

  • listener: ((...args) => void)

    • • (...args): void

      • Parameters

        • Rest ...args: any[]

        Returns void

  Returns Plug

  Since

  v0.1.26

eventNames

• eventNames(): (string | symbol)[]

• 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) ]
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  Returns (string | symbol)[]

  Since

  v6.0.0

getMaxListeners

• getMaxListeners(): number

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

  Returns number

  Since

  v1.0.0

listenerCount

• listenerCount(eventName, listener?): 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.

  Parameters

  • eventName: string | symbol

    The name of the event being listened for

  • Optional listener: Function

    The event handler function

  Returns number

  Since

  v3.2.0

listeners

• listeners(eventName): Function[]

• 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] ]
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  Parameters

  • eventName: string | symbol

  Returns Function[]

  Since

  v0.1.26

off

• off(eventName, listener): Plug

• Alias for emitter.removeListener().

  Parameters

  • eventName: string | symbol

  • listener: ((...args) => void)

    • • (...args): void

      • Parameters

        • Rest ...args: any[]

        Returns void

  Returns Plug

  Since

  v10.0.0

once

• once(eventName, listener): Plug

• 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!');
  });
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  Returns a reference to the EventEmitter, so that calls can be chained.

  By default, event listeners are invoked in the order they are added. Theemitter.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
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  Parameters

  • eventName: string | symbol

    The name of the event.

  • listener: ((...args) => void)

    The callback function

    • • (...args): void

      • Parameters

        • Rest ...args: any[]

        Returns void

  Returns Plug

  Since

  v0.3.0

prependListener

• prependListener(eventName, listener): Plug

• 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 eventNameand listener will result in the listener being added, and called, multiple times.

  server.prependListener('connection', (stream) => {
    console.log('someone connected!');
  });
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  Returns a reference to the EventEmitter, so that calls can be chained.

  Parameters

  • eventName: string | symbol

    The name of the event.

  • listener: ((...args) => void)

    The callback function

    • • (...args): void

      • Parameters

        • Rest ...args: any[]

        Returns void

  Returns Plug

  Since

  v6.0.0

prependOnceListener

• prependOnceListener(eventName, listener): Plug

• 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!');
  });
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  Returns a reference to the EventEmitter, so that calls can be chained.

  Parameters

  • eventName: string | symbol

    The name of the event.

  • listener: ((...args) => void)

    The callback function

    • • (...args): void

      • Parameters

        • Rest ...args: any[]

        Returns void

  Returns Plug

  Since

  v6.0.0

rawListeners

• rawListeners(eventName): Function[]

• 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');
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  Parameters

  • eventName: string | symbol

  Returns Function[]

  Since

  v9.4.0

removeAllListeners

• removeAllListeners(event?): Plug

• 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

  • Optional event: string | symbol

  Returns Plug

  Since

  v0.1.26

removeListener

• removeListener(eventName, listener): Plug

• Removes the specified listener from the listener array for the event namedeventName.

  const callback = (stream) => {
    console.log('someone connected!');
  };
  server.on('connection', callback);
  // ...
  server.removeListener('connection', callback);
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  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 anyremoveListener() 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
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  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');
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  Returns a reference to the EventEmitter, so that calls can be chained.

  Parameters

  • eventName: string | symbol

  • listener: ((...args) => void)

    • • (...args): void

      • Parameters

        • Rest ...args: any[]

        Returns void

  Returns Plug

  Since

  v0.1.26

setMaxListeners

• setMaxListeners(n): Plug

• 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 toInfinity (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 Plug

  Since

  v0.3.5

Static addAbortListener

• addAbortListener(signal, resource): Disposable
• Experimental

  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]();
    }
  }
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  Parameters

  • signal: AbortSignal

  • resource: ((event) => void)

    • • (event): void

      • Parameters

        • event: Event

        Returns void

  Returns Disposable

  Disposable that removes the abort listener.

  Since

  v20.5.0

Static getEventListeners

• getEventListeners(emitter, name): Function[]

• 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] ]
  }
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  Parameters

  • emitter: EventEmitter | _DOMEventTarget

  • name: string | symbol

  Returns Function[]

  Since

  v15.2.0, v14.17.0

Static getMaxListeners

• getMaxListeners(emitter): number

• 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
  }
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  Parameters

  • emitter: EventEmitter | _DOMEventTarget

  Returns number

  Since

  v19.9.0

Static listenerCount

• listenerCount(emitter, eventName): number

• A class method that returns the number of listeners for the given eventNameregistered 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
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  Parameters

  • emitter: EventEmitter

    The emitter to query

  • eventName: string | symbol

    The event name

  Returns number

  Since

  v0.9.12

  Deprecated

  Since v3.2.0 - Use listenerCount instead.

Static on

• on(emitter, eventName, options?): AsyncIterableIterator<any>

• 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
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  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());
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  Parameters

  • emitter: EventEmitter

  • eventName: string

    The name of the event being listened for

  • Optional options: StaticEventEmitterOptions

  Returns AsyncIterableIterator<any>

  that iterates eventName events emitted by the emitter

  Since

  v13.6.0, v12.16.0

Static once

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

• 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);
  }
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  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
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  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!
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  Parameters

  • emitter: _NodeEventTarget

  • eventName: string | symbol

  • Optional options: StaticEventEmitterOptions

  Returns Promise<any[]>

  Since

  v11.13.0, v10.16.0

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

• Parameters

  • emitter: _DOMEventTarget

  • eventName: string

  • Optional options: StaticEventEmitterOptions

  Returns Promise<any[]>

Static setMaxListeners

• setMaxListeners(n?, ...eventTargets): void

• import { setMaxListeners, EventEmitter } from 'node:events';
  
  const target = new EventTarget();
  const emitter = new EventEmitter();
  
  setMaxListeners(5, target, emitter);
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  Parameters

  • Optional n: number

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

  • Rest ...eventTargets: (EventEmitter | _DOMEventTarget)[]

  Returns void

  Since

  v15.4.0