Problems with xmlhttprequest status 302

I am trying to get real path in link megaupload but always but this dont work.

function getRealURL(){

    var st = new String(""); 
    var req = new XMLHttpRequest();
    req.open("GET","http://www.megaupload.com/?d=6CKP1MVJ",true);
    req.send(null);
    req.send(null);
    req.onreadys开发者_如何学Gotatechange = function (aEvt) {
     if (req.readyState == 4) {
        if(req.status == 302){
          //SUCESSO
           st = req.responseText;
        }
      }
    };//funcao

    element.getElementById("id").setAttribute("value", st);

}

i need this link:

Redirect to: http://www534.megaupload.com/files/c2c36829bc392692525f5b7b3d9d81dd/Coldplay - Warning Sign.mp3

insted of this:

http://www.megaupload.com/?d=6CKP1MVJ

XMLHttpRequest follows the redirect automatically by default so you don't see the 302 response. You need to set nsIHttpChannel.redirectionLimit property to zero to prevent it:

req.open("GET","http://www.megaupload.com/?d=6CKP1MVJ",true);
req.channel.QueryInterface(Components.interfaces.nsIHttpChannel).redirectionLimit = 0;
req.send(null);

Not that the link you use here redirects anywhere but this is the general approach. Btw, instead of looking at the response text for redirects you should look at req.getResponseHeader("Location").