code: function toUrl(mId) {
    const parsed = parse(mId);
    let url = parsed.bareId;
    if (url[0] !== '/' && !url.match(/^https?:\/\//)) url = _baseUrl + url;
    if (!parsed.ext) {
      // no known ext, add .js
      url += '.js';
    }
    return url;
  }
docs: incoming id is already mapped