for循环嵌套函数obj[i]undefined

window.onload = function(){
var tr=document.getElementsByTagName("tr");
for(var i= 0;i<tr.length;i++)
{
tr[i].onmouseover=function(){
tr[i].style.backgroundColor="#f2f2f2";//此处tr[i]导致js报错
alert(tr[i]);
}
}
}

alert（tr[i]）

tr[i]

this.