JS鼠标经过变色

<style>
.bg{background:pink;}
</style> 
<!---------------------HTML 内容-----------------------!>
<table>
   <tr> <td>代码</td> <td>名称</td></tr>
   <tr> <td>代码1</td> <td>名称1</td></tr>
   <tr> <td>代码1</td> <td>名称1</td></tr>
   <tr> <td>代码1</td> <td>名称1</td></tr>
</table>
<!-------------------------------------------------!>
<script>
var trs=document.querySelector('table').querySelectorAll('tr'); 

  for (var i=0;i<trs.length;i++){
    trs[i].onmouseover = function(){ this.className='bg'}
    trs[i].onmouseout = function(){ this.className=''} 
  }

</script>