API mocking with MSW fails after upgrade to miniflare 2.2.0

Hey @mrbbot, thanks for all the work on miniflare, it’s been a game-changer! With miniflare 1, I’ve been using mock service worker to mock API responses. When attempting to upgrade to miniflare 2.2.0, my tests failed. I’ve no idea where to start looking for a solution, so I’ve put up a fairly minimal demo, hoping that’ll make it easier to pinpoint the cause. HEAD is using miniflare 2.2.0, while the miniflare-1.4.1 branch shows what it looks like when it’s working. Running npm test should show failing tests for HEAD and passing for the latter.

[mrbbot]

Hey @SupremeTechnopriest! 👋 Unfortunately, you won’t be able to use setGlobalDispatcher inside jest-environment-miniflare. Miniflare’s undici version is imported outside the Jest environment in Node. Jest and Node don’t share a module cache, so trying to import undici inside Jest will always give you a different version.

Currently, I think the best option for jest-environment-miniflare is to use Jest’s function mocks to mock the global fetch function. Something like… (untested 😅)

const fetchSpy = jest.spyOn(globalThis, "fetch");
fetchSpy.mockImplementation((input, init) => {
  const req = new Request(input, init);
  const url = new URL(req.url);
  if (req.method === "GET" && url.pathname === "/") {
    return new Response("bar");
  }
  return new Response(null, { status: 404 });
});

...

jest.restoreAllMocks();

I’d definitiely like to start a discussion on this though. Maybe a global API like getMiniflareMockAgent() that returned a correctly-setup MockAgent?

[msaiducar]

@SupremeTechnopriest and @LukePammant, I can confirm that mocking the globalThis.fetch doesn’t work with miniflare.dispatchFetch. We have some fetch() calls to some external APIs inside our worker, we wanted to test those calls, but spying fetch was not possible. We used this library to workaround. We start a local HTTP server and use this server instead of external APIs.