node/doc/api/events.markdown

## Events

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


### 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.

#### 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!');
    });

#### 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.

#### Event: 'newListener'

`function (event, listener) { }`

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