How to easily listen for xhr requests with Javascript?

In javascript, you can redefine any function or object on the fly. For your problem, you can try something like this:

var original_xhr = XMLHttpRequest;
XMLHttpRequest = function () {
   /* logic that notifies your plugin goes here */
    ...
   original_xhr();
   alert ("I redefined this function");

Here is the complete version of Travis J Webb's solution. Just put this near the top of your JS or HMTL file.

(function(){
    var original_xhr = XMLHttpRequest;
    window.XMLHttpRequest = function () {
        /* logic that notifies your plugin goes here */

        original_xhr.apply(this, Array.prototype.slice.call(arguments));
        alert ("I redefined this function");
    }
}());