code: function cors(req, res, next) {
    const port = process.env.NODE_ENV === 'production'
      ? null
      : new url.URL(req.query.__amp_source_origin).port;
  
    const host = process.env.NODE_ENV === 'production'
      ? `https://${req.hostname}`
      : `http://${req.hostname}:${port}`;
  
    res.header('amp-access-control-allow-source-origin', host);
  
    next();
  }
docs: cors is HTTP middleware that sets an AMP-specific cross origin resource
  sharing HTTP header on our response.
  
  @param {Object} req - The HTTP request object.
  @param {Object} res - The HTTP response object.
  @param {Function} next - The next HTTP handler to execute.