WebService Is Undefined Error In ASP.NET 4.0

1. First step is to add reference to the web service using Ajax Script Manager:

<asp:ScriptManager ID="ajaxManager" runat="server">  
     <Services>  
       <asp:ServiceReference Path="~/Services/ProductService.asmx" />  
     </Services>  
   </asp:ScriptManager> 

2. Secondly is to include the namespace in calling the webservice method
From

ProductService.GetProducts(  
            function (data) {  
              $.each(data, function (index, elem) {  
                $("<option />")  
                 .text(elem.ProductName)  
                 .val(elem.ProductID)  
                 .appendTo("#products");  
              });  
            }  
       );  

To

AccessingServerSideDataUsingClientScript.Services.ProductService.GetProducts(  
            function (data) {  
              $.each(data, function (index, elem) {  
                $("<option />")  
                 .text(elem.ProductName)  
                 .val(elem.ProductID)  
                 .appendTo("#products");  
              });  
            }  
       );  

Reference: http://omensblog.blogspot.com/2007/07/aspnet-ajax-web-service-calls-from.html
Greg