select Html Control JS opration examples

<!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 content="text/html; charset=utf-8" http-equiv="Content-Type"/>
<title>JavaScript XMl DropdownList Examples</title>
<script type="text/javascript" language="javascript">
 <!--
  var onecount;
  onecount= 0;
  subcat = new Array();
  subcat[0] = new Array("徐家汇","01","001");
  subcat[1] = new Array("嘉定区","01","002");
  subcat[2] = new Array("黄浦区","01","003");
  subcat[3] = new Array("海淀区","02","004");
  subcat[4] = new Array("昌平区","02","005");
  subcat[5] = new Array("东城区","02","006");
  subcat[6] = new Array("西城区","02","007");
  onecount = 7;
  function changelocation(locationid)
  {
     document.myform.smalllocation.length = 0;
     var locationid = locationid;
     var i;
     document.myform.smalllocation.options[0] = new Option('--所有地区--','');
     for(i=0;i<onecount;i++)
     {
        if(subcat[i][1] == locationid)
        {
          document.myform.smalllocation.options[document.myform.smalllocation.length] = new Option(subcat[i][0],subcat[i][2]);
        }
     }
  }
 //-->
</script>
</head>

<body>
<form id="myform" runat="server" name="myform" method="post">
<select id="biglocation" name="biglocation"  onchange="changelocation(document.myform.biglocation.options[document.myform.biglocation.selectedIndex].value)">
<option value="01" selected="selected">上海</option>
<option value="02">北京</option>
</select>
<select name="smalllocation">
<option selected="selected" value="">--请选择--</option>
</select>
</form>
<script language="javascript" type="text/javascript">
<!--
changelocation(document.myform.biglocation.options[document.myform.biglocation.selectedIndex].value);
//-->
</script>

</body>

</html>