JavaScript实现级联菜单的方法

<html>
<head>
 <title>test</title>
 <script type="text/javascript">
  function f()
  {
   var sel=document.getElementById("country");
   var sel2=document.getElementById("city");
   var cnarr1=new Array("bj","sh")
   var cnarr2=new Array("beijing","shanghai")
   var usaarr1=new Array("dc","ny")
   var usaarr2=new Array("Washington","NewYork")
   for(var i=sel2.length;i>=0;i--){
    sel2.removeChild(sel2.childNodes[i]);
   }
   var op=document.createElement("option");
   op.value="please";
   op.text="please";
   sel2.appendChild(op);
   if(sel.value=='USA'){
    for(i=0;i<2;i++){ 
     op.value=usaarr1[i];
     op.text=usaarr2[i];
     sel2.appendChild(op);
    }
   }
   else if(sel.value=='CN'){
    for(i=0;i<2;i++){ 
     op.value=cnarr1[i];
     op.text=cnarr2[i];
     sel2.appendChild(op);
    }
   }
  }
 </script>
</head>
<body>
<select onchange="f()" id='country'>
 <option >please</option>
 <option value='USA'>America</option>
 <option value='CN'>China</option>
</select>
<select id='city'>
 <option >please</option>
</select>
</body>
</html>