Optional
options: EventEmitterOptionsStatic
Readonly
captureStatic
captureStatic
defaultStatic
Readonly
errorThis 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.
Rest
...args: Events[K]Rest
...args: Events[K]Rest
...args: Events[K]Returns an array listing the events for which the emitter has registered
listeners. The values in the array are strings or Symbol
s.
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) ]
v6.0.0
Returns the current max listener value for the EventEmitter
which is either
set by emitter.setMaxListeners(n)
or defaults to defaultMaxListeners.
v1.0.0
Rest
...args: Events[K]Rest
...args: Events[K]Rest
...args: Events[K]Rest
...args: Events[K]Rest
...args: Events[K]Optional
event: keyof EventsRest
...args: Events[K]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 toInfinity
(or 0
) to indicate an unlimited number of listeners.
Returns a reference to the EventEmitter
, so that calls can be chained.
v0.3.5
Static
getReturns a copy of the array of listeners for the event named eventName
.
For EventEmitter
s this behaves exactly the same as calling .listeners
on
the emitter.
For EventTarget
s 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]
}
v15.2.0, v14.17.0
Static
listenerA 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
The emitter to query
The event name
v0.9.12
Since v3.2.0 - Use listenerCount
instead.
Static
onconst { 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());
The name of the event being listened for
Optional
options: StaticEventEmitterOptionsthat iterates eventName
events emitted by the emitter
v13.6.0, v12.16.0
Static
onceCreates 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!
Optional
options: StaticEventEmitterOptionsv11.13.0, v10.16.0
Optional
options: StaticEventEmitterOptionsStatic
setconst {
setMaxListeners,
EventEmitter
} = require('events');
const target = new EventTarget();
const emitter = new EventEmitter();
setMaxListeners(5, target, emitter);
Optional
n: numberA non-negative number. The maximum number of listeners per EventTarget
event.
Rest
...eventTargets: (EventEmitter | _DOMEventTarget)[]v15.4.0
Sets or gets the default captureRejection value for all emitters.