jquery ajax的async使用

$.ajax(...{   
           type:"POST",  
           url:'url',  
           dataType:"html",  
           success:function(result)...{   //function1()  
              f1();  
              f2();  
           }  
            failure:function (result) ...{   
               alert('Failed');   
           },  
   }  
  function2();  

		

   function CheckUserName()...{  
        var username = $("#username").val();  
        var result=false;  
        $.ajax(...{async:false  //要设置为同步的，要不CheckUserName的返回值永远为false  
                ,url:'b.php',data:...{name:username}  
                ,success:function(data)...{  
                if(data == 1)...{  
                    $("#warnning").html("<font color=#FF3300>Account is used.</font>");  
                    result=false;  
                } else ...{  
                    $("#warnning").html("<font color=#00CC66>You can register.</font>");  
                   result=true;  
                }  
        }});  
        return result; 
    }