requestMultiplexer.js

addEventListener("fetch", event => {
  event.respondWith(handleRequest(event.request));
});

/**
 * Multiplex requests to a fixed list of destinations.
 *
 * It is intentional that we only support rewriting the incoming request URL to
 * that of the destination. Everything else on the request is untouched.
 *
 * In this particular use case, we are multiplexing to hosts that want to
 * receive SendGrid's webhook payload. This Cloudflare Worker was created as a
 * workaround to SendGrid's limitation of supporting only a single webhook URL.
 * @param {Request} request
 */
async function handleRequest(request) {
  const urls = ["https://postb.in/b/ilhoNrlv", "https://postb.in/b/5yWt1wim"];

  const fetchPromises = urls.map(url =>
    fetch(new Request(url, request.clone()))
  );

  try {
    const responses = await Promise.all(fetchPromises);

    // "The Promise returned from fetch() won’t reject on HTTP error status even
    // if the response is an HTTP 404 or 500. Instead, it will resolve normally
    // (with ok status set to false), and it will only reject on network failure
    // or if anything prevented the request from completing."
    //
    // https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch
    const failedRequests = responses.filter(response => !response.ok);

    if (failedRequests.length > 0) {
      return _respondWithFailure(
        `Failed to multiplex request to ${failedRequests.length} destinations`
      );
    }

    return _respondWithSuccess(
      `Successfully multiplexed request to ${fetchPromises.length} destinations`
    );
  } catch (err) {
    return _respondWithFailure(err);
  }
}

function _respondWithSuccess(message) {
  return new Response(message, { status: 200, statusText: "OK" });
}

function _respondWithFailure(error) {
  return new Response(error, {
    status: 500,
    statusText: "Internal Server Error"
  });
}