[jQuery] return value from JQuery ajax custom method

[Sara]

hi everybody
I'm using jquery ajax within a custom method and try to return its
result to that original method but I get nothing while it works fine
if I show it in alert window

here is my code:
jQuery(document).ready(function() {
jQuery.Test = function(){
  jQuery.ajax({
    type: "POST",
    url: "WebService.asmx/HelloWorld",
    success: function(msg) {
    // alert($("string", msg).text()); //it works fine
     return $("string", msg).text();   //it doesn't works
    },
    error: function(XMLHttpRequest, textStatus, errorThrown) {
      alert(XMLHttpRequest.responseText); }
  });
 };
 alert(jQuery.Test());//it returns 'undefined'
 });

Thanks