XMLHttpRequest of ajax

TE: the getHeaderField() methods always return nulls for me :(
      var gotContentEncoding = false;
      var gotContentLength = false;
      var gotContentType = false;
      var gotDate = false;
      var gotExpiration = false;
      var gotLastModified = false;
      for (var i = 0; ; i++)
      {
        var hdrName = conn.getHeaderFieldKey(i);
        var hdrValue = conn.getHeaderField(i);
        if (hdrName == null && hdrValue == null)
        {
          break;
        }
        if (hdrName != null)
        {
          this._headers[this._headers.length] = {h:hdrName, v:hdrValue};
          switch (hdrName.toLowerCase())
          {
            case ''content-encoding'': gotContentEncoding = true; break;
            case ''content-length'' : gotContentLength = true; break;
            case ''content-type'' : gotContentType = true; break;
            case ''date'' : gotDate = true; break;
            case ''expires'' : gotExpiration = true; break;
            case ''last-modified'' : gotLastModified = true; break;
          }
        }
      }
      // try to fill in any missing header information
      var val;
      val = conn.getContentEncoding();
      if (val != null && !gotContentEncoding) this._headers[this._headers.length] = {h:''Content-encoding'', v:val};
      val = conn.getContentLength();
      if (val != -1 && !gotContentLength) this._headers[this._headers.length] = {h:''Content-length'', v:val};
      val = conn.getContentType();
      if (val != null && !gotContentType) this._headers[this._headers.length] = {h:''Content-type'', v:val};