jQuery and ASP.NET Custom Validator

Question

I'm trying to learn jQuery and it occurred to me that existing JS in some of my sites could be replaced with just a few lines of jQuery code. In the following code, I'm trying to set the value of a custom validator by making an AJAX call. The first block of code does not work as it should, whereas the second block works fine. The whole "if it ain't broke don't fix it" answer isn't helpful, I really want to learn jQuery. For the record, I've placed alerts in the code and they both return the exact same result, just one is setting the args and the other is not for some reason.

Why does this code NOT work:

    function CheckForUserName(sender, args)
{
    args.IsValid = true;

    var url = "/somepage.aspx";

    MakeCall(url, function(txt) {
    if (txt == "false") {
        args.IsValid = false;
    }
    });
}

function MakeCall(url,callback) { 
    $.ajax({
        url: url,
        dataType: "text",
        success: callback
    });
}

This code DOES work:

    function CheckForUserName(sender, args)
{
     args.IsValid = true;

     var call = MakeCall();
     if (call == "false")
     {
      args.IsValid = false;
     }

}

function MakeCall()
{ 
    var xmlHttp;
    var validation=true;

    xmlHttp=GetXmlHttpObject();
    if (xmlHttp==null)
    {
     alert ("Your browser does not support AJAX!");
     return;
    } 

    var url="/somepage.aspx";
    xmlHttp.onreadystatechange=function () 
    {
     if (xmlHttp.readyState==4)
     { 
      if (xmlHttp.status==200)
      {
       return xmlHttp.responseText;
      }
      else
      {
       alert(xmlHttp.status);
      }
     }
    };
    xmlHttp.open("GET",url,false);
    xmlHttp.send(null);
    return xmlHttp.responseText;
}

function GetXmlHttpObject()
{
var xmlHttp=null;
try
  {
  // Firefox, Opera 8.0+, Safari
  xmlHttp=new XMLHttpRequest();
  }
catch (e)
  {
  // Internet Explorer
  try
    {
    xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
    }
  catch (e)
    {
    xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
  }
return xmlHttp;
}

Answer 1

In order to make it work, you need to specify the async option as false:

function MakeCall(url,callback) { 
    $.ajax({
        async: false,
        url: url,
        dataType: "text",
        success: callback
    });
}

Answer 2

This works fyi.. ignore my custom javascript namespace functions, but you should get the concept.

<script type="text/javascript">
        function VerifyCustomerNumber(s, a) {
            var r = ProcessCustomerNumber(a.Value);
            a.IsValid = r;
        }
        function ProcessCustomerNumber(n) {
            var u = '/Services/WebServices/Customer.asmx/CountByCustomerNumber';
            var d = '{"Number": "' + n + '"}';
            $j.ajax({
                type: "POST",
                contentType: "application/json; charset=utf-8",
                url: u,
                cache: false,
                async: false,
                data: d,
                dataType: "json",
                success: function(r) {
                    var v = Data.JS.Ajax.ParseJSON(r);
                    return v;
                }
            });
        }
    </script>