JS EventEmitter

"use strict";

function EventEmitter() {
    this.events = {};
}

EventEmitter.prototype = {
    constructor: EventEmitter,

    listeners: function(event) {
        if (!this.events.hasOwnProperty(event)) {
            return this.events[event] = [];
        } else {
            return this.events[event];
        }
    },

    on: function (event, fn) {
        this.listeners(event).push(fn);
    },

    off: function (event, fn) {
        var listeners = this.listeners(event);
        var index = listeners.indexOf(fn);
        if (index > -1) {
            listeners.splice(index, 1);
        }
    },

    emit: function () {
        var args = arguments;
        var event = args.shift();
        var listeners = this.listeners(event);
        var len = listeners.length;
        for (var i = 0; i < len; ++i) {
            listeners[i].apply(null, args);
        }
    }
};