Javascript Observable

Observable

 var Observable = function(){
        var that = this,
            listeners = {},
            callHandler = function(args){
                this.handler.apply(this.scope, args);
            },
            on = function(eventName, handlerFun, scope){
                var listenerList = [], handlerObj;
                if(!eventName){
                    throw "cannot call 'on' with empty eventName";
                }
                if(!handlerFun){
                    throw "cannot call 'on' with empty handlerFun";
                }
                listenerList = listeners[eventName] = listeners[eventName] || listenerList;
                listenerList.push(handlerObj = {
                    handler: handlerFun,
                    call: callHandler,
                    scope: scope || that
                });
                return function(){
                    var index = listenerList.indexOf(handlerObj);
                    if(index != -1){
                        listenerList.splice(index,1);
                    }
                };
            },
            fireEvent = function(eventName, rest){
                var args = [].splice.call(arguments,0),
                    name = args.splice(0,1),
                    listenerList = [], index;
                if(!name){
                    throw "cannot call 'fireEvent' with no eventName";
                }
                if(listeners[name]){
                    listenerList = listeners[name];
                }
                for(index = 0;index < listenerList.length; index += 1){
                    listenerList[index].call(args);
                }

            };

        that.on = on;
        that.fireEvent = fireEvent;
    };