jquery ajax 异步导致赋值不起作用

                        function isHaveUserName(name)
                        {
                            var res = "ok";
                            var data = { userName: name };
                            $.ajax({
                                type:"post",
                                async:false,//设置为同步，异步的话，还没有来得及赋值（success回调函数里面的赋值不起作用），就已经return。
                                url:"?t=checkUserName",
                                data:data,
                                success:function(ret)
                                   {
                                      if(ret=="False"){ res="no"; } else { res="ok"; }
                                   }
                            })
                            return res;
                        }

参考：http://www.cnblogs.com/wlmemail/archive/2010/06/22/1762765.html