node/doc/api/events.markdown

EventEmitter

Many objects in Node emit events: a TCP server emits an event each time there is a stream, a child process emits an event when it exits. All objects which emit events are instances of events.EventEmitter.

Events are represented by a camel-cased string. Here are some examples: 'connection', 'data', 'messageBegin'.

Functions can be then be attached to objects, to be executed when an event is emitted. These functions are called listeners.

require('events').EventEmitter to access the EventEmitter class.

All EventEmitters emit the event 'newListener' when new listeners are added.

When an EventEmitter experiences an error, the typical action is to emit an 'error' event. Error events are special--if there is no handler for them they will print a stack trace and exit the program.

Event: 'newListener'

function (event, listener) { }

This event is emitted any time someone adds a new listener.

Event: 'error'

function (exception) { }

If an error was encountered, then this event is emitted. This event is special - when there are no listeners to receive the error Node will terminate execution and display the exception's stack trace.

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!');
});

emitter.once(event, listener)

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

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

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);

emitter.removeAllListeners(event)

Removes all listeners from the listener array for the specified event.

emitter.listeners(event)

Returns an array of listeners for the specified event. This array can be manipulated, e.g. to remove listeners.

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.