JavaScript实现多个重叠层点击切换效果的方法

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>三个层重叠，点击可切换</title>
<script type="text/javascript">
function $(obj){
return document.getElementById(obj);
}
function change(n){
for (var i=1;i<4;i++){
if(n==i){
$("a"+i).style.zIndex="100";
}else{
$("a"+i).style.zIndex="0";
}
}
}
</script>
</head>
<body>
<div id="a1" 
style="position:absolute;left:100px;top:100px;width:100px;height:100px;
background-color:#ddeeff;z-index:1" onclick="change('1')" >
</div>
<div id="a2" 
style="position:absolute;left:150px;top:130px;width:100px;height:100px;
background-color:#eeffdd;z-index:2" onclick="change('2')">
</div>
<div id="a3" 
style="position:absolute;left:200px;top:160px;width:100px;height:100px;
background-color:#ffddee;z-index:3" onclick="change('3')">
</div>
<div>https://www.jb51.net/</div>
</body>
</html>