Javascript to convert Youtube video page url to video url

The following Javascript to convert Youtube video page url to video url,
function GetVideoUrl()
    {
        var url = "http://www.youtube.com/watch?v=8tbh4K0wsJ8&feature=fvst";
        var startoffset = url.indexOf("v=")+2;
        url = url.substring(startoffset);
        var endoffset = url.indexOf("&");
        if (endoffset != -1)
        url = url.substring(0, endoffset);
        url="http://www.youtube.com/v/"+url;
        alert(url);
    }