code: function (url, callback) {
        pendingRequests[url] = true;
        var xhr = window.XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject("Microsoft.XmlHttp");
        xhr.open("HEAD", url, true);
        xhr.onreadystatechange = function () {
          delete pendingRequests[url];
          if (xhr.readyState == 4 && xhr.status != 304) {
            xhr.getAllResponseHeaders();
            var info = {};
            for (var h in headers) {
              var value = xhr.getResponseHeader(h);
              // adjust the simple Etag variant to match on its significant part
              if (h.toLowerCase() == "etag" && value) value = value.replace(/^W\//, '');
              if (h.toLowerCase() == "content-type" && value) value = value.replace(/^(.*?);.*?$/i, "$1");
              info[h] = value;
            }
            callback(url, info);
          }
        }
        xhr.send();
      }
docs: performs a HEAD request and passes the header info to the given callback