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Events

Stability: 2 - Stable

Much of the Node.js core API is built around an idiomatic asynchronous event-driven architecture in which certain kinds of objects (called "emitters") emit named events that cause Function objects ("listeners") to be called.

For instance: a net.Server object emits an event each time a peer connects to it; a fs.ReadStream emits an event when the file is opened; a stream emits an event whenever data is available to be read.

All objects that emit events are instances of the EventEmitter class. These objects expose an eventEmitter.on() function that allows one or more functions to be attached to named events emitted by the object. Typically, event names are camel-cased strings but any valid JavaScript property key can be used.

When the EventEmitter object emits an event, all of the functions attached to that specific event are called synchronously. Any values returned by the called listeners are ignored and discarded.

The following example shows a simple EventEmitter instance with a single listener. The eventEmitter.on() method is used to register listeners, while the eventEmitter.emit() method is used to trigger the event.

const EventEmitter = require('events');

class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();
myEmitter.on('event', () => {
  console.log('an event occurred!');
});
myEmitter.emit('event');

Passing arguments and this to listeners

The eventEmitter.emit() method allows an arbitrary set of arguments to be passed to the listener functions. Keep in mind that when an ordinary listener function is called, the standard this keyword is intentionally set to reference the EventEmitter instance to which the listener is attached.

const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
  console.log(a, b, this, this === myEmitter);
  // Prints:
  //   a b MyEmitter {
  //     domain: null,
  //     _events: { event: [Function] },
  //     _eventsCount: 1,
  //     _maxListeners: undefined } true
});
myEmitter.emit('event', 'a', 'b');

It is possible to use ES6 Arrow Functions as listeners, however, when doing so, the this keyword will no longer reference the EventEmitter instance:

const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  console.log(a, b, this);
  // Prints: a b {}
});
myEmitter.emit('event', 'a', 'b');

Asynchronous vs. synchronous

The EventEmitter calls all listeners synchronously in the order in which they were registered. This ensures the proper sequencing of events and helps avoid race conditions and logic errors. When appropriate, listener functions can switch to an asynchronous mode of operation using the setImmediate() or process.nextTick() methods:

const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  setImmediate(() => {
    console.log('this happens asynchronously');
  });
});
myEmitter.emit('event', 'a', 'b');

Handling events only once

When a listener is registered using the eventEmitter.on() method, that listener is invoked every time the named event is emitted.

const myEmitter = new MyEmitter();
let m = 0;
myEmitter.on('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Prints: 2

Using the eventEmitter.once() method, it is possible to register a listener that is called at most once for a particular event. Once the event is emitted, the listener is unregistered and then called.

const myEmitter = new MyEmitter();
let m = 0;
myEmitter.once('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Ignored

Error events

When an error occurs within an EventEmitter instance, the typical action is for an 'error' event to be emitted. These are treated as special cases within Node.js.

If an EventEmitter does not have at least one listener registered for the 'error' event, and an 'error' event is emitted, the error is thrown, a stack trace is printed, and the Node.js process exits.

const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));
// Throws and crashes Node.js

To guard against crashing the Node.js process the domain module can be used. (Note, however, that the domain module is deprecated.)

As a best practice, listeners should always be added for the 'error' events.

const myEmitter = new MyEmitter();
myEmitter.on('error', (err) => {
  console.error('whoops! there was an error');
});
myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an error

It is possible to monitor 'error' events without consuming the emitted error by installing a listener using the symbol events.errorMonitor.

const { EventEmitter, errorMonitor } = require('events');

const myEmitter = new EventEmitter();
myEmitter.on(errorMonitor, (err) => {
  MyMonitoringTool.log(err);
});
myEmitter.emit('error', new Error('whoops!'));
// Still throws and crashes Node.js

Capture rejections of promises

Using async functions with event handlers is problematic, because it can lead to an unhandled rejection in case of a thrown exception:

const ee = new EventEmitter();
ee.on('something', async (value) => {
  throw new Error('kaboom');
});

The captureRejections option in the EventEmitter constructor or the global setting change this behavior, installing a .then(undefined, handler) handler on the Promise. This handler routes the exception asynchronously to the Symbol.for('nodejs.rejection') method if there is one, or to 'error' event handler if there is none.

const ee1 = new EventEmitter({ captureRejections: true });
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);

const ee2 = new EventEmitter({ captureRejections: true });
ee2.on('something', async (value) => {
  throw new Error('kaboom');
});

ee2[Symbol.for('nodejs.rejection')] = console.log;

Setting events.captureRejections = true will change the default for all new instances of EventEmitter.

const events = require('events');
events.captureRejections = true;
const ee1 = new events.EventEmitter();
ee1.on('something', async (value) => {
  throw new Error('kaboom');
});

ee1.on('error', console.log);

The 'error' events that are generated by the captureRejections behavior do not have a catch handler to avoid infinite error loops: the recommendation is to not use async functions as 'error' event handlers.

Class: EventEmitter

The EventEmitter class is defined and exposed by the events module:

const EventEmitter = require('events');

All EventEmitters emit the event 'newListener' when new listeners are added and 'removeListener' when existing listeners are removed.

It supports the following option:

Event: 'newListener'

  • eventName {string|symbol} The name of the event being listened for
  • listener {Function} The event handler function

The EventEmitter instance will emit its own 'newListener' event before a listener is added to its internal array of listeners.

Listeners registered for the 'newListener' event are passed the event name and a reference to the listener being added.

The fact that the event is triggered before adding the listener has a subtle but important side effect: any additional listeners registered to the same name within the 'newListener' callback are inserted before the listener that is in the process of being added.

class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();
// Only do this once so we don't loop forever
myEmitter.once('newListener', (event, listener) => {
  if (event === 'event') {
    // Insert a new listener in front
    myEmitter.on('event', () => {
      console.log('B');
    });
  }
});
myEmitter.on('event', () => {
  console.log('A');
});
myEmitter.emit('event');
// Prints:
//   B
//   A

Event: 'removeListener'

  • eventName {string|symbol} The event name
  • listener {Function} The event handler function

The 'removeListener' event is emitted after the listener is removed.

emitter.addListener(eventName, listener)

  • eventName {string|symbol}
  • listener {Function}

Alias for emitter.on(eventName, listener).

emitter.emit(eventName[, ...args])

  • eventName {string|symbol}
  • ...args {any}
  • Returns: {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.

const EventEmitter = require('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

emitter.eventNames()

  • Returns: {Array}

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

const EventEmitter = require('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) ]

emitter.getMaxListeners()

  • Returns: {integer}

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

emitter.listenerCount(eventName)

  • eventName {string|symbol} The name of the event being listened for
  • Returns: {integer}

Returns the number of listeners listening to the event named eventName.

emitter.listeners(eventName)

  • eventName {string|symbol}
  • Returns: {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] ]

emitter.off(eventName, listener)

  • eventName {string|symbol}
  • listener {Function}
  • Returns: {EventEmitter}

Alias for emitter.removeListener().

emitter.on(eventName, listener)

  • eventName {string|symbol} The name of the event.
  • listener {Function} The callback function
  • Returns: {EventEmitter}

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.

const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a

emitter.once(eventName, listener)

  • eventName {string|symbol} The name of the event.
  • listener {Function} The callback function
  • Returns: {EventEmitter}

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.

const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a

emitter.prependListener(eventName, listener)

  • eventName {string|symbol} The name of the event.
  • listener {Function} The callback function
  • Returns: {EventEmitter}

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.

emitter.prependOnceListener(eventName, listener)

  • eventName {string|symbol} The name of the event.
  • listener {Function} The callback function
  • Returns: {EventEmitter}

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.

emitter.removeAllListeners([eventName])

  • eventName {string|symbol}
  • Returns: {EventEmitter}

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.

emitter.removeListener(eventName, listener)

  • eventName {string|symbol}
  • listener {Function}
  • Returns: {EventEmitter}

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.

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:

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.

emitter.setMaxListeners(n)

  • n {integer}
  • Returns: {EventEmitter}

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.

emitter.rawListeners(eventName)

  • eventName {string|symbol}
  • Returns: {Function[]}

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

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');

emitter[Symbol.for('nodejs.rejection')](err, eventName[, ...args])

  • err Error
  • eventName {string|symbol}
  • ...args {any}

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').

const { EventEmitter, captureRejectionSymbol } = require('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.
  }
}

events.defaultMaxListeners

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 all EventEmitter 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:

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'.

events.errorMonitor

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.

events.getEventListeners(emitterOrTarget, eventName)

  • emitterOrTarget {EventEmitter|EventTarget}
  • eventName {string|symbol}
  • Returns: {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.

const { getEventListeners, EventEmitter } = require('events');

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

events.once(emitter, name[, options])

  • emitter {EventEmitter}
  • name {string}
  • options {Object}
    • signal {AbortSignal} Can be used to cancel waiting for the event.
  • Returns: {Promise}

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.

const { once, EventEmitter } = require('events');

async function run() {
  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.log('error happened', err);
  }
}

run();

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:

const { EventEmitter, once } = require('events');

const ee = new EventEmitter();

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

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

// Prints: ok boom

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

const { EventEmitter, once } = require('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!

Awaiting multiple events emitted on process.nextTick()

There is an edge case worth noting when using the events.once() function to await multiple events emitted on in the same batch of process.nextTick() operations, or whenever multiple events are emitted synchronously. Specifically, because the process.nextTick() queue is drained before the Promise microtask queue, and because EventEmitter emits all events synchronously, it is possible for events.once() to miss an event.

const { EventEmitter, once } = require('events');

const myEE = new EventEmitter();

async function foo() {
  await once(myEE, 'bar');
  console.log('bar');

  // This Promise will never resolve because the 'foo' event will
  // have already been emitted before the Promise is created.
  await once(myEE, 'foo');
  console.log('foo');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo().then(() => console.log('done'));

To catch both events, create each of the Promises before awaiting either of them, then it becomes possible to use Promise.all(), Promise.race(), or Promise.allSettled():

const { EventEmitter, once } = require('events');

const myEE = new EventEmitter();

async function foo() {
  await Promise.all([once(myEE, 'bar'), once(myEE, 'foo')]);
  console.log('foo', 'bar');
}

process.nextTick(() => {
  myEE.emit('bar');
  myEE.emit('foo');
});

foo().then(() => console.log('done'));

events.captureRejections

Value: {boolean}

Change the default captureRejections option on all new EventEmitter objects.

events.captureRejectionSymbol

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

See how to write a custom rejection handler.

events.listenerCount(emitter, eventName)

Stability: 0 - Deprecated: Use emitter.listenerCount() instead.

  • emitter {EventEmitter} The emitter to query
  • eventName {string|symbol} The event name

A class method that returns the number of listeners for the given eventName registered on the given emitter.

const { EventEmitter, listenerCount } = require('events');
const myEmitter = new EventEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(listenerCount(myEmitter, 'event'));
// Prints: 2

events.on(emitter, eventName[, options])

  • emitter {EventEmitter}
  • eventName {string|symbol} The name of the event being listened for
  • options {Object}
    • signal {AbortSignal} Can be used to cancel awaiting events.
  • Returns: {AsyncIterator} that iterates eventName events emitted by the emitter
const { on, EventEmitter } = require('events');

(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')) {
    // 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:

const { on, EventEmitter } = require('events');
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());

events.setMaxListeners(n[, ...eventTargets])

  • n {number} A non-negative number. The maximum number of listeners per EventTarget event.
  • ...eventsTargets {EventTarget[]|EventEmitter[]} 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.
const {
  setMaxListeners,
  EventEmitter
} = require('events');

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

setMaxListeners(5, target, emitter);

Class: events.EventEmitterAsyncResource extends EventEmitter

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.

const { EventEmitterAsyncResource } = require('events');
const { notStrictEqual, strictEqual } = require('assert');
const { executionAsyncId } = require('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.

new events.EventEmitterAsyncResource(options)

  • options {Object}
    • captureRejections {boolean} It enables automatic capturing of promise rejection. Default: false.
    • name {string} The type of async event. Default:: new.target.name.
    • triggerAsyncId {number} The ID of the execution context that created this async event. Default: executionAsyncId().
    • requireManualDestroy {boolean} If set to true, disables emitDestroy when the object is garbage collected. This usually does not need to be set (even if emitDestroy is called manually), unless the resource's asyncId is retrieved and the sensitive API's emitDestroy is called with it. When set to false, the emitDestroy call on garbage collection will only take place if there is at least one active destroy hook. Default: false.

eventemitterasyncresource.asyncId

  • Type: {number} The unique asyncId assigned to the resource.

eventemitterasyncresource.asyncResource

  • Type: The underlying {AsyncResource}.

The returned AsyncResource object has an additional eventEmitter property that provides a reference to this EventEmitterAsyncResource.

eventemitterasyncresource.emitDestroy()

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.

eventemitterasyncresource.triggerAsyncId

  • Type: {number} The same triggerAsyncId that is passed to the AsyncResource constructor.

EventTarget and Event API

The EventTarget and Event objects are a Node.js-specific implementation of the EventTarget Web API that are exposed by some Node.js core APIs.

const target = new EventTarget();

target.addEventListener('foo', (event) => {
  console.log('foo event happened!');
});

Node.js EventTarget vs. DOM EventTarget

There are two key differences between the Node.js EventTarget and the EventTarget Web API:

  1. Whereas DOM EventTarget instances may be hierarchical, there is no concept of hierarchy and event propagation in Node.js. That is, an event dispatched to an EventTarget does not propagate through a hierarchy of nested target objects that may each have their own set of handlers for the event.
  2. In the Node.js EventTarget, if an event listener is an async function or returns a Promise, and the returned Promise rejects, the rejection is automatically captured and handled the same way as a listener that throws synchronously (see EventTarget error handling for details).

NodeEventTarget vs. EventEmitter

The NodeEventTarget object implements a modified subset of the EventEmitter API that allows it to closely emulate an EventEmitter in certain situations. A NodeEventTarget is not an instance of EventEmitter and cannot be used in place of an EventEmitter in most cases.

  1. Unlike EventEmitter, any given listener can be registered at most once per event type. Attempts to register a listener multiple times are ignored.
  2. The NodeEventTarget does not emulate the full EventEmitter API. Specifically the prependListener(), prependOnceListener(), rawListeners(), setMaxListeners(), getMaxListeners(), and errorMonitor APIs are not emulated. The 'newListener' and 'removeListener' events will also not be emitted.
  3. The NodeEventTarget does not implement any special default behavior for events with type 'error'.
  4. The NodeEventTarget supports EventListener objects as well as functions as handlers for all event types.

Event listener

Event listeners registered for an event type may either be JavaScript functions or objects with a handleEvent property whose value is a function.

In either case, the handler function is invoked with the event argument passed to the eventTarget.dispatchEvent() function.

Async functions may be used as event listeners. If an async handler function rejects, the rejection is captured and handled as described in EventTarget error handling.

An error thrown by one handler function does not prevent the other handlers from being invoked.

The return value of a handler function is ignored.

Handlers are always invoked in the order they were added.

Handler functions may mutate the event object.

function handler1(event) {
  console.log(event.type);  // Prints 'foo'
  event.a = 1;
}

async function handler2(event) {
  console.log(event.type);  // Prints 'foo'
  console.log(event.a);  // Prints 1
}

const handler3 = {
  handleEvent(event) {
    console.log(event.type);  // Prints 'foo'
  }
};

const handler4 = {
  async handleEvent(event) {
    console.log(event.type);  // Prints 'foo'
  }
};

const target = new EventTarget();

target.addEventListener('foo', handler1);
target.addEventListener('foo', handler2);
target.addEventListener('foo', handler3);
target.addEventListener('foo', handler4, { once: true });

EventTarget error handling

When a registered event listener throws (or returns a Promise that rejects), by default the error is treated as an uncaught exception on process.nextTick(). This means uncaught exceptions in EventTargets will terminate the Node.js process by default.

Throwing within an event listener will not stop the other registered handlers from being invoked.

The EventTarget does not implement any special default handling for 'error' type events like EventEmitter.

Currently errors are first forwarded to the process.on('error') event before reaching process.on('uncaughtException'). This behavior is deprecated and will change in a future release to align EventTarget with other Node.js APIs. Any code relying on the process.on('error') event should be aligned with the new behavior.

Class: Event

The Event object is an adaptation of the Event Web API. Instances are created internally by Node.js.

event.bubbles

  • Type: {boolean} Always returns false.

This is not used in Node.js and is provided purely for completeness.

event.cancelBubble()

Alias for event.stopPropagation(). This is not used in Node.js and is provided purely for completeness.

event.cancelable

  • Type: {boolean} True if the event was created with the cancelable option.

event.composed

  • Type: {boolean} Always returns false.

This is not used in Node.js and is provided purely for completeness.

event.composedPath()

Returns an array containing the current EventTarget as the only entry or empty if the event is not being dispatched. This is not used in Node.js and is provided purely for completeness.

event.currentTarget

  • Type: {EventTarget} The EventTarget dispatching the event.

Alias for event.target.

event.defaultPrevented

  • Type: {boolean}

Is true if cancelable is true and event.preventDefault() has been called.

event.eventPhase

  • Type: {number} Returns 0 while an event is not being dispatched, 2 while it is being dispatched.

This is not used in Node.js and is provided purely for completeness.

event.isTrusted

  • Type: {boolean}

The {AbortSignal} "abort" event is emitted with isTrusted set to true. The value is false in all other cases.

event.preventDefault()

Sets the defaultPrevented property to true if cancelable is true.

event.returnValue

  • Type: {boolean} True if the event has not been canceled.

This is not used in Node.js and is provided purely for completeness.

event.srcElement

  • Type: {EventTarget} The EventTarget dispatching the event.

Alias for event.target.

event.stopImmediatePropagation()

Stops the invocation of event listeners after the current one completes.

event.stopPropagation()

This is not used in Node.js and is provided purely for completeness.

event.target

  • Type: {EventTarget} The EventTarget dispatching the event.

event.timeStamp

  • Type: {number}

The millisecond timestamp when the Event object was created.

event.type

  • Type: {string}

The event type identifier.

Class: EventTarget

eventTarget.addEventListener(type, listener[, options])

  • type {string}
  • listener {Function|EventListener}
  • options {Object}
    • once {boolean} When true, the listener is automatically removed when it is first invoked. Default: false.
    • passive {boolean} When true, serves as a hint that the listener will not call the Event object's preventDefault() method. Default: false.
    • capture {boolean} Not directly used by Node.js. Added for API completeness. Default: false.

Adds a new handler for the type event. Any given listener is added only once per type and per capture option value.

If the once option is true, the listener is removed after the next time a type event is dispatched.

The capture option is not used by Node.js in any functional way other than tracking registered event listeners per the EventTarget specification. Specifically, the capture option is used as part of the key when registering a listener. Any individual listener may be added once with capture = false, and once with capture = true.

function handler(event) {}

const target = new EventTarget();
target.addEventListener('foo', handler, { capture: true });  // first
target.addEventListener('foo', handler, { capture: false }); // second

// Removes the second instance of handler
target.removeEventListener('foo', handler);

// Removes the first instance of handler
target.removeEventListener('foo', handler, { capture: true });

eventTarget.dispatchEvent(event)

  • event {Event}
  • Returns: {boolean} true if either event’s cancelable attribute value is false or its preventDefault() method was not invoked, otherwise false.

Dispatches the event to the list of handlers for event.type.

The registered event listeners is synchronously invoked in the order they were registered.

eventTarget.removeEventListener(type, listener)

  • type {string}
  • listener {Function|EventListener}
  • options {Object}
    • capture {boolean}

Removes the listener from the list of handlers for event type.

Class: NodeEventTarget

  • Extends: {EventTarget}

The NodeEventTarget is a Node.js-specific extension to EventTarget that emulates a subset of the EventEmitter API.

nodeEventTarget.addListener(type, listener[, options])

  • type {string}

  • listener {Function|EventListener}

  • options {Object}

    • once {boolean}

  • Returns: {EventTarget} this

Node.js-specific extension to the EventTarget class that emulates the equivalent EventEmitter API. The only difference between addListener() and addEventListener() is that addListener() will return a reference to the EventTarget.

nodeEventTarget.eventNames()

  • Returns: {string[]}

Node.js-specific extension to the EventTarget class that returns an array of event type names for which event listeners are registered.

nodeEventTarget.listenerCount(type)

  • type {string}

  • Returns: {number}

Node.js-specific extension to the EventTarget class that returns the number of event listeners registered for the type.

nodeEventTarget.off(type, listener)

  • type {string}

  • listener {Function|EventListener}

  • Returns: {EventTarget} this

Node.js-specific alias for eventTarget.removeListener().

nodeEventTarget.on(type, listener[, options])

  • type {string}

  • listener {Function|EventListener}

  • options {Object}

    • once {boolean}

  • Returns: {EventTarget} this

Node.js-specific alias for eventTarget.addListener().

nodeEventTarget.once(type, listener[, options])

  • type {string}

  • listener {Function|EventListener}

  • options {Object}

  • Returns: {EventTarget} this

Node.js-specific extension to the EventTarget class that adds a once listener for the given event type. This is equivalent to calling on with the once option set to true.

nodeEventTarget.removeAllListeners([type])

  • type {string}

  • Returns: {EventTarget} this

Node.js-specific extension to the EventTarget class. If type is specified, removes all registered listeners for type, otherwise removes all registered listeners.

nodeEventTarget.removeListener(type, listener)

  • type {string}

  • listener {Function|EventListener}

  • Returns: {EventTarget} this

Node.js-specific extension to the EventTarget class that removes the listener for the given type. The only difference between removeListener() and removeEventListener() is that removeListener() will return a reference to the EventTarget.