How to handle jQuery ajax post error when navigating away from a page

It looks like the answer to this is to examine the jqXHR.status. The XMLHttpRequest spec outlines these steps to set the status:

The status attribute must return the result of running these steps:

1. If the state is UNSENT or OPENED, return 0 and terminate these steps.

2. If the error flag is set, return 0 and terminate these steps.

3. Return the HTTP status code.¹

NOTE also:
The error flag indicates some type of network error or request abortion. It is initially unset and is used during the DONE state.

From what I understand therefore, this code check should fix the issue:

    if (xhr.status == 0)
        alert('error');

¹_{https://web.archive.org/web/20120204040047/http://www.w3.org/TR/XMLHttpRequest/#the-status-attribute}