Class MongoClient

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

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

  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