使用AJAX和PHP检查表单字段

I am trying to write a code that will check in a php file if username/mail has already been registered. The following code doesn't work because in check_fields() when check_field() is fired for both files the return is sent immediately, without waiting for the asynchronous query to finish. I don't want to use JQuery so I thought about doing the following :I am trying to write a code that will check in