Combines two URLs. The baseUrl shall be absolute URL. If the url is an absolute URL, it will be returned as is.
function combineUrl(baseUrl, url) {
  if (!url) {
    return baseUrl;
  }
  if (/^[a-z][a-z0-9+\-.]*:/i.test(url)) {
    return url;
  }
  var i;
  if (url.charAt(0) === '/') {
    // absolute path
    i = baseUrl.indexOf('://');
    if (url.charAt(1) === '/') {
      ++i;
    } else {
      i = baseUrl.indexOf('/', i + 3);
    }
    return baseUrl.substring(0, i) + url;
  } else {
    // relative path
    var pathLength = baseUrl.length;
    i = baseUrl.lastIndexOf('#');
    pathLength = i >= 0 ? i : pathLength;
    i = baseUrl.lastIndexOf('?', pathLength);
    pathLength = i >= 0 ? i : pathLength;
    var prefixLength = baseUrl.lastIndexOf('/', pathLength);
    return baseUrl.substring(0, prefixLength + 1) + url;
  }
}