Always recreate mocks in FakeTimers.useFakeTimers()

[itszero]

Summary

If the flag resetMocks or jest.resetAllMocks is used in the test, all the timer-related tests following that will fail with no way of recovering.

Cause

jest.resetAllMocks resets everything, including mocks created by fakeTimers. There are also no public methods that we can use to re-instantiate a new FakeTimer object to mitigate the problem.

Solution

Given that both mock and fake timers are part of the jest, they should be working together out-of-the box without extra code gluing them together. We change the useFakeTimers method to always create new mocks so the mocks will not be reverted by resetAllMocks. If there is a better way to manage the lifecycle of those mocks, please let me know. I will be happy to make changes.

Test plan

I have provided a new integration test suite named timer_after_resetAllMocks. It fails on jest 17.0.2 and is passing after this patch.

Note

This patch makes flow unhappy and I'm confused as to why it fails. I would love to hear some feedbacks on how to fix this.

> @ typecheck /Users/zcho/src/jest-work/jest
> flow check

packages/jest-util/src/FakeTimers.js:90
 90:       this._timerAPIs.nextTick = global.process.nextTick;
                           ^^^^^^^^ property `nextTick`. Property not found in
 90:       this._timerAPIs.nextTick = global.process.nextTick;
           ^^^^^^^^^^^^^^^ object literal

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[cpojer]

Thanks for the PR! I just pushed an update that should fix your Flow issue :)

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