Calling Web Services from JavaScript

Calling Web Services from JavaScript allows the power of backend processing in your application without a postback, which results in better user experience. JavaScript by default uses the "JavaScript Object Notation" or JSON to communicate with a server. JSON has a shorter length compared with SOAP, and hence is more efficient. It also conforms better to the JavaScript internal object handling system.

If you open a new web site in Visual Studio 2008, the web.config would contain many new sections to enable JSON so many of the necessary framework would already be in place. To call a Web Service from JavaScript, you need to do the following three steps:

1. Modify the definition of your Web Service (say in MyWebService.asmx.vb) such that the class has an extra attribute as shown below:
   VB

   <System.Web.Script.Services.ScriptService()> _
   Public Class ClassName 
       <WebMethod()> _
       Public Sub MethodName(Parameter As TypeA)
           DoSomething()
       End Sub
   End Class

2. Use asp:scriptmanager in your ASPX file:
   XML

   <asp:scriptmanager ID="ScriptManager1" runat="server">
   <Services>
   <asp:ServiceReference Path="~/service/MyWebService.asmx" />
   </Services>
   </asp:scriptmanager>

3. Call the Web Service within a JavaScript function like this:
   JavaScript

   function CallsWebService(anyParametr ) {
     ...;
     NameSpaceName.ClassName.MethodName(anyParameter, OnWSRequestComplete);
   }

The OnWSRequestComplete() function gets called when the Web Service execution finishes. Implement this function as shown below:

function OnWSRequestComplete(results) {
  if (results != null) {
  }
}