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);

                             }

                         });

                     }

                 }

             }

         };