window.onload = function(){
           var trs = document.getElementsByTagName('tr');
           for(var i = 1; i < trs.length; i++){
                trs[i].onmouseover = function(){
                     this.style.backgroundColor = "#f2f2f2";
                }
                trs[i].onmouseout = function(){
                     this.style.backgroundColor = "#fff";
                }
          }
        }

这个函数为什么要时候for循环遍历一次?