JavaScript给相同class属性的元素添加背景颜色

<!DOCTYPE html>

<html>
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<script>
function getClass(oParent,oClass){
var aresult =[];
var eclass = oParent.getElementsByTagName('*');
for (var i=0;i<eclass.length;i++) {
if (eclass[i].className==oClass) {
aresult.push(eclass[i]);
}
}
return aresult;
}
window.onload = function(){
var oUl =document.getElementById("ull");
var aBox = getClass(oUl,"box");
for (var i=0;i<aBox.length;i++) {
aBox[i].style.background= "red";
}

}
</script>
<ul id="ull">
<li class="box">1</li>
<li>2</li>
<li class="box">3</li>
<li>4</li>
<li class="box">5</li>

</ul>
</body>
</html>