javascript observer object

observer.js

(function ( global, undefined ) {
    var observer = {},
        topics = {};
    observer.on = function ( topic, fn ) {
        if ( !topics[topic] ) topics[topic] = [];
        topics[topic].push( fn );
        return topics[topic].length - 1;
    };
    observer.trigger = function () {
        var args = Array.prototype.slice.call( arguments ),
            topic = args.shift();
        if ( topics[topic] && topics[topic].length )
            for ( var i = 0, l = topics[topic].length ; i < l ; i++ )
                topics[topic][i].apply( null, args );
    };
    observer.off = function ( topic, index ) {
        index === undefined ? delete topics[topic] : topics[topic].splice( index, 1 );
    };
    global.observer = global.observer || observer;
} ( this ));