javascript模拟jQuery封装委托事件，兼容IE

var $ = function(id){
            var dom = document.getElementById(id);
            return {
                on:function(eventType,element,callback){
                    if(document.addEventListener){
                        dom.addEventListener(eventType,function(e){
                            var ev = e || window.event;
                            var target = ev.target || ev.srcElement
                            if(target.tagName.toLowerCase()===element){
                                callback.call(target,ev);
                            }
                        },false);    
                    }else{
                        //兼容IE浏览器
                        document.attachEvent("on"+eventType,function(e){
                            var ev = e || window.event;
                            var target = ev.target || ev.srcElement
                            if(target.tagName.toLowerCase()===element){
                                callback.call(target,ev);
                            }
                        });
                    }
                }
            }
        };