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Node.js v13.12.0-rc.0 Documentation
Table of Contents
-
- Passing arguments and
this
to listeners - Asynchronous vs. Synchronous
- Handling events only once
- Error events
- Capture Rejections of Promises
-
- Event:
'newListener'
- Event:
'removeListener'
EventEmitter.listenerCount(emitter, eventName)
EventEmitter.defaultMaxListeners
EventEmitter.errorMonitor
emitter.addListener(eventName, listener)
emitter.emit(eventName[, ...args])
emitter.eventNames()
emitter.getMaxListeners()
emitter.listenerCount(eventName)
emitter.listeners(eventName)
emitter.off(eventName, listener)
emitter.on(eventName, listener)
emitter.once(eventName, listener)
emitter.prependListener(eventName, listener)
emitter.prependOnceListener(eventName, listener)
emitter.removeAllListeners([eventName])
emitter.removeListener(eventName, listener)
emitter.setMaxListeners(n)
emitter.rawListeners(eventName)
emitter[Symbol.for('nodejs.rejection')](err, eventName[, ...args])
- Event:
events.once(emitter, name)
events.captureRejections
events.captureRejectionSymbol
events.on(emitter, eventName)
- Passing arguments and
Events#
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.
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 will be 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 errorMonitor
.
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#
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 EventEmitter.captureRejections = true
will change the default for all
new instances of EventEmitter
.
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
#
The EventEmitter
class is defined and exposed by the events
module:
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. Default:false
.
Event: 'newListener'
#
eventName
<string> | <symbol> The name of the event being listened forlistener
<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.
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 namelistener
<Function> The event handler function
The 'removeListener'
event is emitted after the listener
is removed.
EventEmitter.listenerCount(emitter, eventName)
#
emitter.listenerCount()
instead.emitter
<EventEmitter> The emitter to queryeventName
<string> | <symbol> The event name
A class method that returns the number of listeners for the given eventName
registered on the given emitter
.
const myEmitter = new MyEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(EventEmitter.listenerCount(myEmitter, 'event'));
// Prints: 2
EventEmitter.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 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:
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
#
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)
#
eventName
<string> | <symbol>listener
<Function>
Alias for emitter.on(eventName, listener)
.
emitter.emit(eventName[, ...args])
#
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 will be 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) ]
emitter.getMaxListeners()
#
- 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)
#
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 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.
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 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. Obviously, not all events should be limited to just 10 listeners.
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])
#
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.once(emitter, name)
#
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 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();
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.on(emitter, eventName)
#
emitter
<EventEmitter>eventName
<string> | <symbol> The name of the event being listened for- Returns: <AsyncIterator> that iterates
eventName
events emitted by theemitter
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]
}
})();
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.