This is a variable jQuery uses internally, but had no reason to hide, so it’s there to use. Just a heads up, it becomes jquery.ajax.active next release. There’s no documentation because it’s exposed but not in the official API, lots of things are like this actually, like jQuery.cache (where all of jQuery.data() goes).
I’m guessing here by actual usage in the library, it seems to be there exclusively to support $.ajaxStart() and $.ajaxStop() (which I’ll explain further), but they only care if it’s 0 or not when a request starts or stops. But, since there’s no reason to hide it, it’s exposed to you can see the actual number of simultaneous AJAX requests currently going on.
When jQuery starts an AJAX request, this happens:
if ( s.global && ! jQuery.active++ ) {
jQuery.event.trigger( "ajaxStart" );
}
This is what causes the $.ajaxStart() event to fire, the number of connections just went from 0 to 1 (jQuery.active++ isn’t 0 after this one, and !0 == true), this means the first of the current simultaneous requests started. The same thing happens at the other end. When an AJAX request stops (because of a beforeSend abort via return false or an ajax call complete function runs):
if ( s.global && ! --jQuery.active ) {
jQuery.event.trigger( "ajaxStop" );
}
This is what causes the $.ajaxStop() event to fire, the number of requests went down to 0, meaning the last simultaneous AJAX call finished. The other global AJAX handlers fire in there along the way as well.