A/B testing with same-URL direct access

This version passes through requests for /test/* and /control/* URI paths to the origin server, bypassing random assignment.

const NAME = "myExampleABTest";

export default {
  async fetch(request) {
    // Clone the original URL
    const url = new URL(request.url);

    // Enable Passthrough to allow direct access to control and test routes.
    if (url.pathname.startsWith("/control") || url.pathname.startsWith("/test"))
      return fetch(request);

    // Determine which group this requester is in.
    const cookie = request.headers.get("cookie");

    if (cookie && cookie.includes(`${NAME}=control`)) {
      url.pathname = "/control" + url.pathname;
    } else if (cookie && cookie.includes(`${NAME}=test`)) {
      url.pathname = "/test" + url.pathname;
    } else {
      // If there is no cookie, this is a new client. Choose a group and set the cookie.
      const group = Math.random() < 0.5 ? "test" : "control"; // 50/50 split
      if (group === "control") {
        url.pathname = "/control" + url.pathname;
      } else {
        url.pathname = "/test" + url.pathname;
      }
      // Reconstruct response to avoid immutability
      let response = await fetch(url);
      response = new Response(response.body, response);
      // Set cookie to enable persistent A/B sessions.
      response.headers.append("Set-Cookie", `${NAME}=${group}; path=/`);
      return response;
    }
    return fetch(url);
  },
};