为什么不变成500px？求大神指点

Question

window.onload=function(){

var ali=document.getElementsByTagName('li');

for (var i = 0; i < ali.length; i++) {

ali[i].onmouseover=function(){

move(this,'width',500);

}

}

function move(obj,attr,target){

        obj.style.attr = target + "px";

}

}