call c-sharp function in JavaScript

$.ajax({
 url: '<%=ResolveUrl("~/URL of the page where your method is kept") %>'+ '&MethodName=name of the method defined code behind',
                      dataType: "json",
                    success: function (msg) {
                        "What to do is sucess"
                    }
                });

if (Request["MethodName"] == "YourMethod")
                {
                    FillDetail();//call ur method here
                }

$.ajax({
 url: '<%=ResolveUrl("~/event.aspx") %>'+ '&MethodName=Event',
                      dataType: "json",
                    success: function (msg) {
                        "What to do is sucess"
                    }
                });

protected void Page_Load(object sender, EventArgs e)
   {
    if (!Page.IsPostBack)
    {
         if (Request["MethodName"] == "Event")//Event same as in JQuery
                {
                    FillDetail();//call ur method here
                }


}
}