code: function saveToFile(filename, cfg) {
    // Use 'wx' flag to fails if filename exists
    fs.writeFileSync(filename, JSON.stringify(cfg, null, 2), {flag: 'wx'})
  }
docs: Save HTTP server configuration to JSON file.
  @param {string} filename - The JSON filename.
  @param {Object} cfg - The HTTP server configuration.
  @throws Will throw an error if filename exists.