Append dates to cookies to use with A/B testing

export default {
  async fetch(request) {
    const response = await fetch(request);

    // Clone the response so that it is no longer immutable
    const newResponse = new Response(response.body, response);

    // Define the dynamic expiry time. 24 h * 60 m * 60 s * 1000 ms = 86,400,000 ms
    const expiry = new Date(Date.now() + 7 * 86400000).toUTCString();
    // Define the group variable. "A" if the request header "userGroup" is "premium", "B" if otherwise.
    const group = request.headers.get("userGroup") == "premium" ? "A" : "B";

    // Append the custom header with the values
    newResponse.headers.append(
      "Set-Cookie",
      `testGroup=${group}; Expires=${expiry}; Path=/`,
    );

    return newResponse;
  },
};