worker.js

addEventListener("fetch", (event) => {
  event.respondWith(
    handleRequest(event.request).catch(
      (err) => new Response(err.stack, { status: 500 })
    )
  );
});

/**
 * Many more examples available at:
 *   https://developers.cloudflare.com/workers/examples
 * @param {Request} request
 * @returns {Promise<Response>}
 */
async function handleRequest(request) {
  const url = new URL(request.url);
  const { pathname } = url;

  if (pathname.startsWith("/worker/status")) {
    return new Response("OK");
  }

  const parts = pathname.split("/", 2);
  const firstPart = parts[1];

  switch (firstPart) {
    // For /b, billing portal/customer portal
    // Also /v1 to proxy /v1/billing_portal/sessions/:session URLs
    case 'v1':
    case 'b': {
      return passthroughToOrigin(request, url, "billing.stripe.com");
    }

    // For /c, checkout
    case 'c': {
      return passthroughToOrigin(request, url, "edge-checkout.stripe.com");
    }

    default: {
      return new Response("Wrong prefix!");
    }
  }
}

async function passthroughToOrigin(request, url, origin) {
  // Override the host
  url.host = origin;

  // Clone a new request from the original to keep all headers.
  const newRequest = new Request(url.toString(), request);

  return fetch(newRequest);
}