Jest has changed its timer implementation causing waitFor() not to work properly

• @testing-library/dom version: 7.5.2
• Testing Framework and version: 10.0.4
• DOM Environment: Jest Jest version: 25.1.0 Node v12

We had some code which uses fakeTimers, I noticed it was screwing up waitFor(). So i needed to run jest.useRealTimers() just before calling waitFor() to fix the issue.

This shouldn’t be needed, as testing-library tries to safe-guard against fakeTimer use, you can see here: https://github.com/testing-library/dom-testing-library/blob/master/src/helpers.js#L5-L8

However, i noticed this line will always return undefined regardless of whether you have fakeTimers on or not. This is due to the fact that globalObj.setTimeout._isMockFunction is always undefined. My guess is Jest used to mock setTimeout using its own utilities and this property once existed, but now Jest uses @Sinon/MockTimers where this property is not added.

When did this happen?

Looks like the change happened in Jest v26 https://github.com/facebook/jest/releases/tag/v26.0.0

there were also implementation changes in v25.1 https://github.com/facebook/jest/releases?after=v25.5.1

_isMockFunction is added here: https://github.com/facebook/jest/blob/master/packages/jest-fake-timers/src/legacyFakeTimers.ts#L370

It was swapped out in v25, plan here: https://github.com/facebook/jest/pull/7776#issuecomment-513552169

Solution

The fix is to find another way to see if the timers are being mocked or not, as the current heuristics don’t work.

[kgregory]

I’d love to

[kentcdodds]

🎉 This issue has been resolved in version 7.15.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀