My jQuery ajax doesn't work - asp.net

Question

Just started to learn jQuery ajax today, followed what tutorial said but it did not work.
HelloWorld is the method name, but it seems not be recognized as a method name but a page name based on the error message.
jQuery
$(document).ready(function () {
//alert("hello world");
$('.ordernumber').on('click', function () {
var orderNum = $(this).text();
$.ajax({
type: "POST",
url: "./OrderDetail.asmx/HelloWorld",
data: "{}",
contentType: "application/json; charset=utf-8",
dataType: "json",
success: function (msg) {
alert(msg);
// Do interesting things here.
}
});
//alert($(this).text());
});
});
OrderDetail.asmx.vb
Imports System
Imports System.Web.Services
Public Class OrderDetail
Inherits System.Web.Services.WebService
<WebMethod()> _
Public Function HelloWorld() As String
Return "Hello World"
End Function
End Class
Error Message:
POST http://localhost:64616/OrderDetail.asmx/HelloWorld 500 (Internal Server Error)

Answer 1

I think you need to add <System.Web.Script.Services.ScriptService()> to your class;
<System.Web.Script.Services.ScriptService()> _
Public Class OrderDetails
Inherits System.Web.Services.WebService
'' rest of your code
End Class
Also to return Json you need to decorate your methods with;
<ScriptMethod(ResponseFormat:=ResponseFormat.Json)> _
Update
When creating a fresh ASMX Web Service, the default code states;
' To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.
' <System.Web.Script.Services.ScriptService()> _

Answer 2

You are expecting a JSON back, but an asmx webservice returns a XML instead you need to add
<WebMethod()> _
<ScriptMethod(ResponseFormat:=ResponseFormat.Json)>_
Public Function HelloWorld() As String
Return "Hello World"
End Function
The link for more explanaiton