Simple EventEmitter for browsers

EventEmitter.js

// usage: 
// EventEmitter.mixin(myObject)
// myObject.emit('eventname', param1, param2, etc)
// myObject.on('eventname', function(param1, param2, etc) {})

var EventEmitter = function () {};
EventEmitter.prototype = {

	// add event handler
	on: function (event, fn)
	{
		this._events = this._events ||
		{};
		this._events[event] = this._events[event] || [];
		this._events[event].push(fn);
	},

	// add event handler that executes once and then removes itself
	once: function (event, fn)
	{
		var fired = false;

		function g()
		{
			this.off(event, g);

			if (!fired)
			{
				fired = true;
				fn.apply(this, arguments);
			}
		}

		g.fn = fn;
		this.on(event, g);
	},

	// remove event handler
	off: function (event, fn)
	{
		this._events = this._events ||
		{};
		if (event in this._events === false)
		{
			return;
		}
		this._events[event].splice(this._events[event].indexOf(fn), 1);
	},

	// emit event
	emit: function (event)
	{
		this._events = this._events ||
		{};
		if (event in this._events === false)
		{
			return;
		}
		for (var i = 0; i < this._events[event].length; i++)
		{
			this._events[event][i].apply(this, Array.prototype.slice.call(arguments, 1));
		}
	},

	// get listeners
	listeners: function (type)
	{
		return !type || !this._events || !this._events[type] ? [] : this._events[type];
	}
};

EventEmitter.mixin = function (target)
{
	var props = ['on', 'once', 'off', 'emit', 'listeners'];
	for (var i = 0; i < props.length; i++)
	{
		if (typeof target === 'function')
		{
			target.prototype[props[i]] = EventEmitter.prototype[props[i]];
		}
		else
		{
			target[props[i]] = EventEmitter.prototype[props[i]];
		}
	}
};