AJAX调用是异步的，这意味着他们只操作完成后，返回的数据。即该方法 checkusernameavailable 不会返回任何信息（除非你告诉它该方法本身）。你需要做到以下几点：

AJAX calls are async, which means they only return data after the operation has completed. I.e. the method checkusernameavailable never returns any information (unless you tell it to within that method itself). You need to do the following:

// Just fire and forget the method
checkusernameavailable("userName");

// Change the success function to do any display you require
success: function(response) {
    alert(response);
    $("#SomeDiv").html(response);     
  },

该方法触发AJAX异步方法职位check.php。当接收到响应，你再处理与 $的成功回调关联函数的响应。AJAX 。您可以直接指定一个函数，成功的回调，以及：

The method fires the AJAX async method that posts to check.php. When the response is received, you then handle that response in function associated with the success callback of $.ajax. You can specify a function directly to that success callback as well:

// Change success to point to a function name
success: foo

// Create a function to handle the response
function foo(response)
{
   // Do something with response
}

编辑：

根据业务方案的意见，你需要改变你的AJAX调用是同步的，而不是异步（我从来没有做过一个同步调用这样的自己，所以这是未经测试）：

As per the OP's comment, you need to change your AJAX call to be synchronous, instead of asynchronous (I've never done a synchronous call like this myself, so this is untested):

var ajaxResponse;

$.ajax({
    async: false, 
    success : function (response)
              {
                  ajaxResponse = response;
              },
    // other properties
});

return ajaxResponse;

完整的API上市这里。