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Events

Stability: 4 - API Frozen

Many objects in Node emit events: a net.Server emits an event each time a peer connects to it, a fs.readStream emits an event when the file is opened. All objects which emit events are instances of events.EventEmitter. You can access this module by doing: require("events");

Typically, event names are represented by a camel-cased string, however, there aren't any strict restrictions on that, as any string will be accepted.

Functions can then be attached to objects, to be executed when an event is emitted. These functions are called listeners. Inside a listener function, this refers to the EventEmitter that the listener was attached to.

Class: events.EventEmitter

To access the EventEmitter class, require('events').EventEmitter.

When an EventEmitter instance experiences an error, the typical action is to emit an 'error' event. Error events are treated as a special case in node. If there is no listener for it, then the default action is to print a stack trace and exit the program.

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

emitter.addListener(event, listener)

emitter.on(event, listener)

Adds a listener to the end of the listeners array for the specified event.

server.on('connection', function (stream) {
  console.log('someone connected!');
});

Returns emitter, so calls can be chained.

emitter.once(event, listener)

Adds a one time listener for the event. This listener is invoked only the next time the event is fired, after which it is removed.

server.once('connection', function (stream) {
  console.log('Ah, we have our first user!');
});

Returns emitter, so calls can be chained.

emitter.removeListener(event, listener)

Remove a listener from the listener array for the specified event. Caution: changes array indices in the listener array behind the listener.

var callback = function(stream) {
  console.log('someone connected!');
};
server.on('connection', callback);
// ...
server.removeListener('connection', callback);

Returns emitter, so calls can be chained.

emitter.removeAllListeners([event])

Removes all listeners, or those of the specified event.

Returns emitter, 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 which helps finding memory leaks. Obviously not all Emitters should be limited to 10. This function allows that to be increased. Set to zero for unlimited.

Returns emitter, so calls can be chained.

EventEmitter.defaultMaxListeners

emitter.setMaxListeners(n) sets the maximum on a per-instance basis. This class property lets you set it for all EventEmitter instances, current and future, effective immediately. Use with care.

Note that emitter.setMaxListeners(n) still has precedence over EventEmitter.defaultMaxListeners.

emitter.listeners(event)

Returns an array of listeners for the specified event.

server.on('connection', function (stream) {
  console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection'))); // [ [Function] ]

emitter.emit(event, [arg1], [arg2], [...])

Execute each of the listeners in order with the supplied arguments.

Returns true if event had listeners, false otherwise.

Class Method: EventEmitter.listenerCount(emitter, event)

Return the number of listeners for a given event.

Event: 'newListener'

  • event {String} The event name
  • listener {Function} The event handler function

This event is emitted any time someone adds a new listener. It is unspecified if listener is in the list returned by emitter.listeners(event).

Event: 'removeListener'

  • event {String} The event name
  • listener {Function} The event handler function

This event is emitted any time someone removes a listener. It is unspecified if listener is in the list returned by emitter.listeners(event).