Get request url from xhr object

With the following hack no wrapper is required:

var xhrProto = XMLHttpRequest.prototype,
    origOpen = xhrProto.open;

xhrProto.open = function (method, url) {
    this._url = url;
    return origOpen.apply(this, arguments);
};

Usage:

var r = new XMLHttpRequest();
r.open('GET', '...', true);
alert(r._url); // opens an alert dialog with '...'