Monitor all AJAX requests on the page

Monitor all AJAX requests:

Do you have a problem with this: the page launches two AJAX requests, hoping that they all succeed, and then do an action?
The easy-to-think solution is to wait until one of these ends and then launch another, which is done with a callback function.
However, if one of the AJAX request code is not you write, you can not change, how to do?
Or, you just want to know when a URL request is over, don't want to call another request, how to do it?

<! DOCTYPE html>
Other related functions:
In $.ajax
Error: Called when an error occurs and can be used to escalate an incorrect request.
Complete: Both success and failure will be called
In the high version:
$.promise
$.when
Monitor all AJAX requests on the page