9-22编程练习 新手求助！
以下是某同学代码：
<!DOCTYPE html>
<html>
<head>
<title> new document </title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<script type="text/javascript">
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";
}
}
}
function addTo(){
var table = document.getElementById('table').lastChild;
var tr = document.createElement('tr');
var td = document.createElement('td');
td.innerHTML="<input type = 'text' />";
tr.appendChild(td);
td = document.createElement('td');
td.innerHTML="<input type = 'text' />";
tr.appendChild(td);
td = document.createElement('td');
td.innerHTML='<a href="#" onclick = "deleteItem(this);return false;" >删除</a>';
tr.appendChild(td);
table.appendChild(tr);
}
function deleteItem(obj){
var table = document.getElementById('table').lastChild;
var tr = obj.parentNode.parentNode;
table.removeChild(tr);
}
</script>
</head>
<body>
<table border="1" width="50%" id="table">
<tr>
<th>学号</th>
<th>姓名</th>
<th>操作</th>
</tr>
<tr>
<td>xh001</td>
<td>王小明</td>
<td><a href="#" onclick = "deleteItem(this);return false;" >删除</a></td>
</tr>
<tr>
<td>xh002</td>
<td>刘小芳</td>
<td><a href="javasript:;" onclick = "deleteItem(this);" >删除</a></td>
</tr>
</table>
<input type="button" value="添加一行" onclick = "addTo();"/>
</body>
</html>
提问：
window.onload = function(){
var trs = document.getElementsByTagName('tr');
for(var i = 1; i < trs.length; i++){
trs[i].onmouseover = function(){
this.style.backgroundColor = "#f2f2f2"; // 为什么此处不能把this换成trs[i]
}
trs[i].onmouseout = function(){
this.style.backgroundColor = "#fff";//同上？
}
}
}
