<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<ul id="list">
<li>1</li>
<li>2</li>
<li>3</li>
<li>4</li>
</ul>
<p>class为on的p标签</p>
</body>
</html>
<script type="text/javascript">
window.onload=function(){
var len1= getByClass('on','list');
alert(len1.length); // 结果等于2为正确
var len2= getByClass('on');
alert(len1.length); // 结果等于3为正确
}
function getByClass(clsName, parent){
//定义函数getByClass()实现获取document或指定父元素下所有class为on的元素
var arr = [];
if(parent){
var oParent = document.getElementById(parent);
var oElements = oParent.getElementsByTagName('*');
for(var i in oElements){
if(oElements[i].className==clsName){
arr.push(oElements[i]);
}
}
}else{
var oElements = document.getElementsByTagName('*');
for(var j in oElements){
if(oElements[j].className==clsName){
arr.push(oElements[i]);
}
}
}
return arr;
}
</script>
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