var myvar = '<%= MyNamespace.MyClass.MyFunction("' + $('#formvalue').val() + '") %>'

[WebMethod]
public static void AbandonSession()
 {
   HttpContext.Current.Session.Abandon();
 }

<script language="javascript" type="text/javascript">
  //<![CDATA[
  function HandleClose()
   {
     alert("Killing the session on the server!!");
      PageMethods.AbandonSession();
   }
   //]]>
</script>