Create New Asmx Page in asp.net - asp.net

Question

I am creating an asmx web service.
In visual studio 2012 on the project name I right clicked and added an web service name UserDetails.asmx
Now I am trying to use the web service in the js file. like this
$.ajax({
type: "POST",
url: "UserDetails.asmx/HelloWorld",
contentType: "application/json; charset=utf-8",
dataType: "json",
success: function (data) {
alert('success');
},
error: function () {
alert("Error");
}
});
Its showing error that POST http: //192.168.9.185/GPS/UserDetails.asmx/HelloWorld 500 (Internal Server Error)
Is there any fault in my code or I am missing something so that its showing error.
My Asmx Page
<%# WebService Language="C#" CodeBehind="~/App_Code/UserDetails.cs" Class="UserDetails" %>
My Code Behind
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Services;
/// <summary>
/// Summary description for UserDetails
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.
// [System.Web.Script.Services.ScriptService]
public class UserDetails : System.Web.Services.WebService {
public UserDetails () {
//Uncomment the following line if using designed components
//InitializeComponent();
}
[WebMethod]
public string HelloWorld() {
return "Hello World";
}
}

Answer 1

Edit
Short Answer: just uncomment [System.Web.Script.Services.ScriptService]
Digging deep in the error message by using
error: function (xhr,status, error) {
alert("Error");
}
and inspecting xhr revealed that the actual error was something else (Only Web services with a [ScriptService] attribute on the class definition can be called from script).
I used [WebMethod] in my aspx code behind and whenever I received such message, I found out that there was an error in the data I am passing. So try adding data:"{}", in your $.ajax()
as described in this post
When making a read-only request, the empty data parameter is the key. For reasons unknown, jQuery does not properly set the specified content-type when there is no data included.