1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
|
# Events
<!--introduced_in=v0.10.0-->
> Stability: 2 - Stable
<!--type=module-->
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 will be 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.
```js
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.
```js
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:
```js
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:
```js
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 will be invoked _every time_ the named event is emitted.
```js
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.
```js
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.
```js
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.
```js
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 `errorMonitor`.
```js
const myEmitter = new MyEmitter();
myEmitter.on(EventEmitter.errorMonitor, (err) => {
MyMonitoringTool.log(err);
});
myEmitter.emit('error', new Error('whoops!'));
// Still throws and crashes Node.js
```
## Capture Rejections of Promises
> Stability: 1 - captureRejections is experimental.
Using `async` functions with event handlers is problematic, because it
can lead to an unhandled rejection in case of a thrown exception:
```js
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')`][rejection] method
if there is one, or to [`'error'`][error] event handler if there is none.
```js
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 `EventEmitter.captureRejections = true` will change the default for all
new instances of `EventEmitter`.
```js
EventEmitter.captureRejections = true;
const ee1 = new 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`
<!-- YAML
added: v0.1.26
changes:
- version: v13.4.0
pr-url: https://github.com/nodejs/node/pull/27867
description: Added captureRejections option.
-->
The `EventEmitter` class is defined and exposed by the `events` module:
```js
const EventEmitter = require('events');
```
All `EventEmitter`s emit the event `'newListener'` when new listeners are
added and `'removeListener'` when existing listeners are removed.
It supports the following option:
* `captureRejections` {boolean} It enables
[automatic capturing of promise rejection][capturerejections].
Default: `false`.
### Event: `'newListener'`
<!-- YAML
added: v0.1.26
-->
* `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 will be 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 will be inserted *before* the
listener that is in the process of being added.
```js
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'`
<!-- YAML
added: v0.9.3
changes:
- version: v6.1.0, v4.7.0
pr-url: https://github.com/nodejs/node/pull/6394
description: For listeners attached using `.once()`, the `listener` argument
now yields the original listener function.
-->
* `eventName` {string|symbol} The event name
* `listener` {Function} The event handler function
The `'removeListener'` event is emitted *after* the `listener` is removed.
### `EventEmitter.listenerCount(emitter, eventName)`
<!-- YAML
added: v0.9.12
deprecated: v4.0.0
-->
> 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`.
```js
const myEmitter = new MyEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(EventEmitter.listenerCount(myEmitter, 'event'));
// Prints: 2
```
### `EventEmitter.defaultMaxListeners`
<!-- YAML
added: v0.11.2
-->
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 `EventEmitter.defaultMaxListeners`
property can be used. If this value is not a positive number, a `TypeError`
will be thrown.
Take caution when setting the `EventEmitter.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 `EventEmitter.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:
```js
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'`.
### `EventEmitter.errorMonitor`
<!-- YAML
added: v13.6.0
-->
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.
### `emitter.addListener(eventName, listener)`
<!-- YAML
added: v0.1.26
-->
* `eventName` {string|symbol}
* `listener` {Function}
Alias for `emitter.on(eventName, listener)`.
### `emitter.emit(eventName[, ...args])`
<!-- YAML
added: v0.1.26
-->
* `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.
```js
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()`
<!-- YAML
added: v6.0.0
-->
* Returns: {Array}
Returns an array listing the events for which the emitter has registered
listeners. The values in the array will be strings or `Symbol`s.
```js
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()`
<!-- YAML
added: v1.0.0
-->
* Returns: {integer}
Returns the current max listener value for the `EventEmitter` which is either
set by [`emitter.setMaxListeners(n)`][] or defaults to
[`EventEmitter.defaultMaxListeners`][].
### `emitter.listenerCount(eventName)`
<!-- YAML
added: v3.2.0
-->
* `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)`
<!-- YAML
added: v0.1.26
changes:
- version: v7.0.0
pr-url: https://github.com/nodejs/node/pull/6881
description: For listeners attached using `.once()` this returns the
original listeners instead of wrapper functions now.
-->
* `eventName` {string|symbol}
* Returns: {Function[]}
Returns a copy of the array of listeners for the event named `eventName`.
```js
server.on('connection', (stream) => {
console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection')));
// Prints: [ [Function] ]
```
### `emitter.off(eventName, listener)`
<!-- YAML
added: v10.0.0
-->
* `eventName` {string|symbol}
* `listener` {Function}
* Returns: {EventEmitter}
Alias for [`emitter.removeListener()`][].
### `emitter.on(eventName, listener)`
<!-- YAML
added: v0.1.101
-->
* `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.
```js
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.
```js
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)`
<!-- YAML
added: v0.3.0
-->
* `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.
```js
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.
```js
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)`
<!-- YAML
added: v6.0.0
-->
* `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.
```js
server.prependListener('connection', (stream) => {
console.log('someone connected!');
});
```
Returns a reference to the `EventEmitter`, so that calls can be chained.
### `emitter.prependOnceListener(eventName, listener)`
<!-- YAML
added: v6.0.0
-->
* `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.
```js
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])`
<!-- YAML
added: v0.1.26
-->
* `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)`
<!-- YAML
added: v0.1.26
-->
* `eventName` {string|symbol}
* `listener` {Function}
* Returns: {EventEmitter}
Removes the specified `listener` from the listener array for the event named
`eventName`.
```js
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 has been emitted, all listeners attached to it at the
time of emitting will be 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 will behave as expected.
```js
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:
```js
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)`
<!-- YAML
added: v0.3.5
-->
* `n` {integer}
* Returns: {EventEmitter}
By default `EventEmitter`s 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)`
<!-- YAML
added: v9.4.0
-->
* `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()`).
```js
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])`
<!-- YAML
added: v13.4.0
-->
> Stability: 1 - captureRejections is experimental.
* `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`][capturerejections] is enabled on the emitter.
It is possible to use [`events.captureRejectionSymbol`][rejectionsymbol] in
place of `Symbol.for('nodejs.rejection')`.
```js
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.once(emitter, name)`
<!-- YAML
added: v11.13.0
-->
* `emitter` {EventEmitter}
* `name` {string}
* Returns: {Promise}
Creates a `Promise` that is fulfilled when the `EventEmitter` emits the given
event or that is rejected when the `EventEmitter` emits `'error'`.
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][WHATWG-EventTarget] interface, which has no special
`'error'` event semantics and does not listen to the `'error'` event.
```js
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();
```
## `events.captureRejections`
<!-- YAML
added: v13.4.0
-->
> Stability: 1 - captureRejections is experimental.
Value: {boolean}
Change the default `captureRejections` option on all new `EventEmitter` objects.
## `events.captureRejectionSymbol`
<!-- YAML
added: v13.4.0
-->
> Stability: 1 - captureRejections is experimental.
Value: `Symbol.for('nodejs.rejection')`
See how to write a custom [rejection handler][rejection].
## `events.on(emitter, eventName)`
<!-- YAML
added: v13.6.0
-->
* `emitter` {EventEmitter}
* `eventName` {string|symbol} The name of the event being listened for
* Returns: {AsyncIterator} that iterates `eventName` events emitted by the `emitter`
```js
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.
[WHATWG-EventTarget]: https://dom.spec.whatwg.org/#interface-eventtarget
[`--trace-warnings`]: cli.html#cli_trace_warnings
[`EventEmitter.defaultMaxListeners`]: #events_eventemitter_defaultmaxlisteners
[`domain`]: domain.html
[`emitter.listenerCount()`]: #events_emitter_listenercount_eventname
[`emitter.removeListener()`]: #events_emitter_removelistener_eventname_listener
[`emitter.setMaxListeners(n)`]: #events_emitter_setmaxlisteners_n
[`fs.ReadStream`]: fs.html#fs_class_fs_readstream
[`net.Server`]: net.html#net_class_net_server
[`process.on('warning')`]: process.html#process_event_warning
[stream]: stream.html
[capturerejections]: #events_capture_rejections_of_promises
[rejection]: #events_emitter_symbol_for_nodejs_rejection_err_eventname_args
[rejectionsymbol]: #events_events_capturerejectionsymbol
[error]: #events_error_events
|