function GetQueryString(name) {
  var reg = new RegExp("(^|&)" + name + "=([^&]*)(&|$)");
  var r = window.location.search.substr(1).match(reg);
  if (r != null) return unescape(r[2]);
  return null;
 }
 b = GetQueryString("xxx");
 if (b == null) {
  url = window.location.host;
  a = "//" + url + "?xxx=&bbb=ccc";
  window.location.href = a;
 }