Make maxLoops for fake timers configurable

[mxmzb]

🚀 Feature Proposal

I'd like to be able to configure maxLoops (see https://github.com/facebook/jest/blob/master/packages/jest-fake-timers/src/jestFakeTimers.ts#L88) myself when using jest.useFakeTimers()

Motivation

I am writing a timer app, which actually may count to more than 100.000ms and it would still make sense. Unable to test this though to this limitation.

Pitch

The line already expects an argument which seems to be able to come from somewhere, but the API isn't making this really publically accessible. I don't see a reason why this should not be an optional param in jest.useFakeTimers().

[mxmzb]

As a current workaround, it is possible to stack jest.advanceTimersByTime(), e.g.

jest.advanceTimersByTime(100000);
jest.advanceTimersByTime(100000);

Also, weirdly enough, the number in the code (https://github.com/facebook/jest/blob/master/packages/jest-fake-timers/src/jestFakeTimers.ts#L88) says 100000 but you actually hit the limit at 1000000.

[github-actions]

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[davidmccabe]

Another reason to make this configurable is to speed up debugging in cases where an infinite loop exists. It can take a long time for 100,000 iterations to go by!

[SimenB]

I think taking an options bag to useFakeTimers (or in timers in config) should be fine. Would also unblock e.g. #12452 (I've seen others as well, my searching is failing me, tho).

Anybody wanna work on a PR for this? I think modern or legacy today should be normalized to {strategy: 'modern'} or something.

Jest 28 is a good time to make such a breaking change to config 🙂

[mrazauskas]

Hm.. The original issue is rather puzzling. maxLoops option is mentioned as possible solution (this is the count timers that will be run), but the workaround is done through calling jest.advanceTimersByTime(100000) (here we have milliseconds, not the count of timers).

@mxmzb This is an old issue, of course. Could you to take a look, please? I am working on the implementation of timerLimit (just a different name for maxLoops) option in #12572, but can this really solve your problem?

[SimenB]

https://github.com/facebook/jest/releases/tag/v28.0.0-alpha.8

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