没有URL的Ajax调用

I want to hide a dropdown list when user click radio button. For this am going to use AJAX instead of JS. Here, my code's :

<script type="text/javascript">
function loadXMLDoc()
{
   var xmlhttp;
   if (window.XMLHttpRequest)
   {
      xmlhttp=new XMLHttpRequest();
   }
   else
   {
      xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
   }
   xmlhttp.onreadystatechange=function()
   {
     if (xmlhttp.readyState==4 && xmlhttp.status==200)
     {
        var element = document.getElementById("myDiv");
        element.style.display = "";
        **element.innerHTML=xmlhttp.responseText;**
     }
   }
   xmlhttp.open("POST","**URL**",true);
   xmlhttp.send();
}    
</script>

Above element.innerHTML=xmlhttp.responseText & URL should i want to use ? there's no reason to pass ajax call to some other page. i done my work here. without URL script not worked. plz sort out me from this issue.

How about his,

function loadXMLDoc()   // your suitable method name
{

        var element = document.getElementById("myDiv");
        element.style.display = "";
}   

You dont need an ajax call ;)