I'm trying to make a bookmarklet to download videos off of YouTube, but I've come across a little problem.

To detect the highest quality video available, I use a sort of brute force method, in which I make requests using the XMLHttpRequest object until a 404 isn't returned (I can't do it until a 200 ok is returned because YouTube redirects to a different server if the video is available, and the cross-domain policy won't allow me to access any of that data).

Once a working URL is found, I simply set window.location to the URL and the download should start, right? Wrong. A request is made, but for reasons unknown to me, the cookies are stripped and YouTube returns a 403 access denied. This does not happen if the XML requests aren't made before it, i.e. if I just set the window.location to the URL everything works fine, it's when I do the XMLHttpRequest that the cookies aren't sent.

It's hard to explain so here's the script:

var formats = ["37", "22", "35", "34", "18", ""];
var url = "/get_video?video_id=" + yt.getConfig('SWF_ARGS')['video_id'] + "&t=" + (unescape(yt.getConfig('SWF_ARGS')['t'])) + "&fmt=";
for (var i = 0; i < formats.length; i++) {
    xmlhttp = new XMLHttpRequest;
 xmlhttp.open("HEAD", url + formats[i], false);
    xmlhttp.send(null);
    if (xmlhttp.status != 404) {
        document.location = url + formats[i];
        break
    }
}

That script does not send the cookies after setting the document.location and thus does not work. However, simply doing this:

document.location = /get_video?video_id=" + yt.getConfig('SWF_ARGS')['video_id'] + "&t=" + (unescape(yt.getConfig('SWF_ARGS')['t']))

DOES send the cookies along with the request, and does work. The only downside is I can't automatically detect the highest quality, I just have to try every "fmt" parameter manually until I get it right.

So my question is: why is the XMLHttpRequest object removing cookies from subsequent requests?

This is the first time I've ever done anything in JS by the way, so please, go easy on me. ;)