CODEWITHSUNDEEP
node.jsjest-fetch-mock
Eric
Eric

Reputation: 2275

node.js: jest unit tests: do mocks need to be drained?

At https://github.com/rancher-sandbox/rancher-desktop/blob/22f94cb2c3a8cd0734f2f4f505b10ee660e94041/pkg/rancher-desktop/backend/tests/k3sHelper.spec.ts#L194 we have a sequence of mocks for fetch, structured like so:

jest.mocked(fetch).mockImplementationOnce((url) => {
// Fake fetching from a channel
}).mockImplementationOnce((url) => {
// Fake fetching release data
}).mockImplementationOnce((url) => {
// Fake hitting a 403
}).mockImplementationOnce((url) => {
// Fake getting the end of the data
}).mockImplementationOnce((url) => {
  // Shouldn't call this one!
  throw new Error(`Unexpected fetch call to ${ url }`);
});
// Do the thing ...
expect(fetch).toHaveBeenCalledTimes(4);

I added a new test, that runs immediately after the one I described, with its own sequence of mocked fetches. But the first call to fetch hits the fifth mocked function in the previous test.

We have a beforeEach that we think clears the mocked fetch:

beforeEach(() => {
  jest.mocked(fetch).mockClear();
});

If I add a fifth call to try { fetch('blah') } catch{} in the first test, the second test runs as expected.

Currently the first test is the only test where fetch is getting mocked.

So my question is, do we still need to ensure that all the mocked calls are actually hit before trying to write a new test that also mocks fetch? The docs at https://jestjs.io/docs/mock-function-api only say what happens if you call a mocked function too many times, but there's no mention of what happens when you don't consume all the mocked implementations.

Upvotes: 0

Views: 24

Answers (1)

Eric
Eric

Reputation: 2275

The solution is to change the call to mockClear to in that beforeEach to mockReset. I'm dense, because the docs for mockReset say:

Does everything that mockFn.mockClear() does, and also replaces the mock implementation with an empty function, returning undefined.

I suppose if I write 10,000 more jest tests one day I'll understand why I would not want mockClear to clear the unused function mocks. But todaay I'm baffled....

Upvotes: 0

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