纯JS与CSS实现的多级折叠菜单

<!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>
<title>折叠菜单</title>
<style>
#side_nav ul{display:none}
</style>
</head>
<body>
<ul id="side_nav">
 <li><span>菜单1</span>
  <ul>
   <li><a href="#">菜单1-0</a></li>
   <li><a href="#">菜单1-1</a></li>
  </ul>
 </li>
 <li><span>菜单2</span>
  <ul>
   <li><a href="#">菜单2-0</a></li>
   <li><a href="#">菜单2-1</a></li>
  </ul>
 </li>
 <li><span>菜单3</span>
  <ul>
   <li><a href="#">菜单3-0</a></li>
  </ul>
 </li>
</ul>
<script type="text/javascript">
<!--
 (function(){
  var navWrap=document.getElementById("side_nav");
  var nav1s=navWrap.getElementsByTagName("span");
  var nav2s=navWrap.getElementsByTagName("ul");
  for(var i=0,len=nav1s.length;i<len;i++){
   nav1s[i].onclick=(function(i){
    return function(){
     for(var j=0;j<len;j++){
      nav2s[j].style.display="none";
     }
     nav2s[i].style.display="block";
    }
   })(i)
  }
 })()
//-->
</script>
</body>
</html>