Cloudflare Worker

// 1. Register a FetchEvent listener that sends a custom
//    response for the given request.
addEventListener('fetch', event => {
  event.respondWith(handleRequest(event.request))
})

// 2. Return a custom request object
async function handleRequest(request) { 

  // Forward request to origin, get response.
  let response = await fetch(request);

  // Copy Response object so that we can edit headers.
  response = new Response(response.body, response);

  const country = request.headers.get('cf-ipcountry');

  let expiryDate = new Date();
  expiryDate.setTime(expiryDate.getTime() + (7*24*60*60*1000));
  let expires = expiryDate.toUTCString();

  const countryCookie = `universal_visitor_country=${country}; Expires=${expires}; Path=/; HttpOnly; SameSite=Lax; Secure`;

  // Set cookie so that we don't add the headers
  // next time.
  response.headers.append('Set-Cookie', countryCookie);

  // Return on to client.
  return response;
}