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);
                             }
                         });
                     }
                 }
             }
         };