Node.js v4.3.1-rc.2 Documentation
Table of Contents
- Events
- Passing arguments and
this
to listeners - Asynchronous vs. Synchronous
- Handling events only once
- Error events
- Class: EventEmitter
- Event: 'newListener'
- Event: 'removeListener'
- EventEmitter.listenerCount(emitter, event)
- EventEmitter.defaultMaxListeners
- emitter.addListener(event, listener)
- emitter.emit(event[, arg1][, arg2][, ...])
- emitter.getMaxListeners()
- emitter.listenerCount(event)
- emitter.listeners(event)
- emitter.on(event, listener)
- emitter.once(event, listener)
- emitter.removeAllListeners([event])
- emitter.removeListener(event, listener)
- emitter.setMaxListeners(n)
- Passing arguments and
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") periodically 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');
const util = require('util');
function MyEmitter() {
EventEmitter.call(this);
}
util.inherits(MyEmitter, EventEmitter);
const myEmitter = new MyEmitter();
myEmitter.on('event', function() {
console.log('an event occurred!');
});
myEmitter.emit('event');
Any object can become an EventEmitter
through inheritance. The example above
uses the traditional Node.js style prototypical inheritance using
the util.inherits()
method. It is, however, possible to use ES6 classes as
well:
const EventEmitter = require('events');
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
myEmitter.on('event', function() {
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. It is important to keep in mind that when an
ordinary listener function is called by the EventEmitter
, the standard this
keyword is intentionally set to reference the EventEmitter
to which the
listener is attached.
const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
console.log(a, b, this);
// Prints:
// a b MyEmitter {
// domain: null,
// _events: { event: [Function] },
// _eventsCount: 1,
// _maxListeners: undefined }
});
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 EventListener
calls all listeners synchronously in the order in which
they were registered. This is important to ensure the proper sequencing of
events and to avoid race conditions or 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();
var 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 immediately unregistered after it is called.
const myEmitter = new MyEmitter();
var 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 a special case
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, developers can either register
a listener for the process.on('uncaughtException')
event or use the
domain
module (Note, however, that the domain
module has been
deprecated).
const myEmitter = new MyEmitter();
process.on('uncaughtException', (err) => {
console.log('whoops! there was an error');
});
myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an error
As a best practice, developers should always register listeners for the
'error'
event:
const myEmitter = new MyEmitter();
myEmitter.on('error', (err) => {
console.log('whoops! there was an error');
});
myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an error
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 a listener is removed.
Event: 'newListener'#
event
String|Symbol The event namelistener
Function The event handler function
The EventEmitter
instance will emit it's own 'newListener'
event before
a listener is added to it's 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'#
event
String|Symbol The event namelistener
Function The event handler function
The 'removeListener'
event is emitted after a listener is removed.
EventEmitter.listenerCount(emitter, event)#
Stability: 0 - Deprecated: Use emitter.listenerCount()
instead.
A class method that returns the number of listeners for the given event
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.
Take caution when setting the EventEmitter.defaultMaxListeners
because the
change effects all EventEmitter
instances, including those created before
the change is made. However, calling emitter.setMaxListeners(n)
still has
precedence over EventEmitter.defaultMaxListeners
.
Note that 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));
});
emitter.addListener(event, listener)#
Alias for emitter.on(event, listener)
.
emitter.emit(event[, arg1][, arg2][, ...])#
Synchronously calls each of the listeners registered for event
, in the order
they were registered, passing the supplied arguments to each.
Returns true
if event had listeners, false
otherwise.
emitter.getMaxListeners()#
Returns the current max listener value for the EventEmitter
which is either
set by emitter.setMaxListeners(n)
or defaults to
EventEmitter.defaultMaxListeners
.
emitter.listenerCount(event)#
event
Value The type of event
Returns the number of listeners listening to the event
type.
emitter.listeners(event)#
Returns a copy of the array of listeners for the specified event
.
server.on('connection', (stream) => {
console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection')));
// Prints: [ [Function] ]
emitter.on(event, listener)#
Adds the listener
function to the end of the listeners array for the
specified event
. No checks are made to see if the listener
has already
been added. Multiple calls passing the same combination of event
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 calls can be chained.
emitter.once(event, listener)#
Adds a one time listener
function for the event
. This listener is
invoked only the next time event
is triggered, after which it is removed.
server.once('connection', (stream) => {
console.log('Ah, we have our first user!');
});
Returns a reference to the EventEmitter
so calls can be chained.
emitter.removeAllListeners([event])#
Removes all listeners, or those of the specified event
.
Note that 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 calls can be chained.
emitter.removeListener(event, listener)#
Removes the specified listener
from the listener array for the specified
event
.
var callback = function(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 event
, then removeListener
must be called
multiple times to remove each instance.
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 will means that any copies of the listener array as returned by
the emitter.listeners()
method will need to be recreated.
Returns a reference to the EventEmitter
so calls can be chained.
emitter.setMaxListeners(n)#
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. 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
)
for to indicate an unlimited number of listeners.
Returns a reference to the EventEmitter
so calls can be chained.