Sys.Application.addload(load)

I am creating web using JavaScript the below my code man

  <script type="text/javascript">
        var departments = null;
     
        Sys.Application.add_load(page_load);
        Sys.Application.add_uload(page_uload);
        function page_load(sender,e){
        departments=$get("Departments");
        $addHandler(departments,"change",departments_onchange);
        }
    function page_uload(sender, e){

    $removeHandler(departments,"change",department_onchange);

    }
    function department_onchange(sender,e)
    {
    $get("ResultDisplay").innerHTML="";
    $get("loading").style.display="block";
    var selectedvalue=departments.value;
    WebService1.GetEmployeeCount(selectedvalue,onSuccess);
    }
    function onSuccess(result){
    $get("loading").style.display="none";
    $get("ResultDisplay").innerHTML="Employee count:"+result;


    }
    
    </script>

when i run it .It give sys undefine error please help on this

thanks