Add an ability to clear pending interceptors in MockAgent

[stanislav-halyn]

This would solve...

I'm migrating from nock to undici mocks and after each test, I clear all active mocks, so that they do not interact with other test cases. This solves the problem when mocks from a failed test might interact with other test cases, making debugging difficult.

The implementation should look like...

It would be great to be able to call a cleanMocks method on MockPool or MockAgent to clean all pending mocks.

I have also considered...

I have considered creating a separate MockAgent instance for every test case, but I'm not sure it's a good idea since it can have a performance overhead and a lof of boilerplate code for cleaning up the mock agent.

I also considered using .assertNoPendingInterceptors() after each test case, but it leads to all tests failing if any of them haven't used a mocked request.

Additional context

It's the same feature nock has.

[stanislav-halyn]

As far as I understand the code, the implementation would be quite straightforward: I need to clean the this[kDispatches] variable in MockPool and MockClient.
If this feature gets approved, I can try to implement it.

[mcollina]

Thanks for reporting! Would you like to send a Pull Request to address this issue? Remember to add unit tests.

[stanislav-halyn]

Sure, I will give it a try

[metcoder95]

I have considered creating a separate MockAgent instance for every test case, but I'm not sure it's a good idea since it can have a performance overhead and a lof of boilerplate code for cleaning up the mock agent.

Not to disregard, I agree the feature might be beneficial; but I wouldn't discard this option as adds the most isolation to your tests cases, reducing the surface of side-effects between them than can lead to flaky or weak tests.

[stanislav-halyn]

@metcoder95 thanks for your answer!
My main concerns were:

• If I call setGlobalDispatcher(mockAgent) with newly created mockAgent in each test case, won't it interact with other test cases? What if we want to call the tests concurrently?
• won't mockAgent leave any memory leaks of I don't mockAgent.close after each test case ?

For context, I use setGlobalDispatcher in my app fetcher.
Also, I have a test file for each fastify router I have and for each test file I generate a mock agent

[mcollina]

If I call setGlobalDispatcher(mockAgent) with newly created mockAgent in each test case, won't it interact with other test cases? What if we want to call the tests concurrently?

Yes.

won't mockAgent leave any memory leaks of I don't mockAgent.close after each test case ?

I don't think so

[stanislav-halyn]

@mcollina thanks for the answer!
I guess it still would be beneficial to have this feature in the mock agent

[mcollina]

Agreed!

[mouhannad-sh]

@stanislav-halyn I came across the same issue and I ended up writing this into my test suite setup

export const TestSuiteMockAgent = new MockAgent();
export const clearInterceptors = () => {
  if (!TestSuiteMockAgent.pendingInterceptors()?.length) return;
  const kClients = Object.getOwnPropertySymbols(TestSuiteMockAgent).find((s) => s.toString() === "Symbol(clients)");
  if (!kClients) return;
  // @ts-ignore
  const clients = [...TestSuiteMockAgent[kClients].values()];
  for (const client of clients) {
    const kDispatches = Object.getOwnPropertySymbols(client).find((s) => s.toString() === "Symbol(dispatches)");
    client[kDispatches] = [];
  }
};

I'm happy to open a PR soon and extend the mockAgent.

I'm basically clearing the dispatches array in the agent's clients, @mcollina are there any other properties / internals that also need updating ?

Potentially we could also add a selector as an argument to pick which interceptors to clear. can be useful in some sophisticated testing scenarios