react-testing: Introduce a waitFor helper

[karthikiyengar]

Overview

Relates to: https://github.com/Shopify/pos-channel/issues/9692

It looks like there is no clean way to test certain use cases that involve some state update scenarios within useEffect. Here is a minimal example.

const Component = () => {
  const [counter, setCounter] = useState(0);
  useEffect(() => {
    setTimeout(() => {
      setCounter((counter) => counter + 1);
    }, 0);
  }, []);
  return <p>{counter}</p>;
};

describe('waitFor', () => {
  it('failure', async () => {
    const root = await mount(<Component />);
    expect(root).toContainReactText('0'); // pass
    await root.act(() => {}); // ???
    expect(root).toContainReactText('1'); // fail
  });
});

react-testing-library offers a waitFor method for cases like these. Here is a naive implementation that takes us a bit further.

const waitFor = async (expectation) => {
  const timeout = 4500;
  const interval = 50;
  const maxTries = Math.ceil(timeout / interval);

  let tries = 0;
  return new Promise<void>((resolve, reject) => {
    const rejectOrRerun = (error: Error) => {
      if (tries > maxTries) {
        reject(error);
        return;
      }
      setTimeout(runExpectation, interval);
    };

    function runExpectation() {
      tries += 1;
      try {
        Promise.resolve(expectation())
          .then(() => resolve())
          .catch(rejectOrRerun);
      } catch (error) {
        rejectOrRerun(error);
      }
    }
    setTimeout(runExpectation, 0);
  });
};

And the same test using the above helper

  it('success', async () => {
    const root = await mount(<Component />);
    expect(root).toContainReactText('0'); // pass
    await waitFor(() => {
      expect(root).toContainReactText('1'); // fail
    });
  });

The above test passes, but it still feels a bit dirty considering React fires act warnings:

Warning: An update to Component inside a test was not wrapped in act(...).
When testing, code that causes React state updates should be wrapped into act(...):

Would appreciate some insights on how we could best support this.

References:

• DOM Testing Library's implementation
• wait-for-expect
• Slack discussion
• Workplace discussion

Type

• New feature
• Changes to existing features

Motivation

What inspired this feature request? What problems were you facing,
or what else makes you think this should be in quilt?

...

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[karthikiyengar]

Some use cases where we could benefit from something like this:
https://github.com/Shopify/pos-channel/blob/8c82358d1819baae14cf24a9a051ddc0a03821c6/app/ui/hooks/useLocationsQuery/test/useLocationsQuery.test.tsx#L83

[karthikiyengar]

Update: We added a stopgap version of this to another repo as part of this PR

[sircumferencee]

All links are dead