`clearTimeout` will not clear timers created prior to clock installation

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• FakeTimers version : 8.0.1
• Environment : node12, macOS Catalina 10.15.7
• Other libraries you are using: several, this is happening in tests for a VS Code extension

What did you expect to happen? clearTimeout to clear timers even if they weren’t created with the mock.

What actually happens The timer that should have been cleared still fires.

How to reproduce

Describe with code how to reproduce the faulty behaviour or link to code on JSBin or similar

const timer = setTimeout(() => console.log('This should not be called'), 1);
const clock = FakeTimers.install();
clearTimeout(timer);
clock.uninstall();
// ... wait 1 ms + next event loop
// > 'This should not be called'

I can make a PR to fix this if the current behavior is not intended. I added a patch to my own source, though a patch here would be much more robust (need to consider things like duplicate handles, clearInterval, clearImmediate, etc.)

[JadenSimon]

I made this issue more as a feature request rather than necessarily a bug report. I think respecting the expectations of older timers is useful functionality, though it might not quite fit within this library. I was leaning more towards an opt-in flag to enable this behavior rather than it be the default. Also, a warning at the very least serves to make debugging less obnoxious, otherwise things can randomly break with no clear reason why (this was the most frustrating part).

Just a little more about the context in which I’m using Sinon in:

I work on an extension for VS Code where many of our ‘unit tests’ are more like mini-integration tests. It is cumbersome to stub/mock every dependency since we essentially live inside our dependency. So these APIs still run in the background, occasionally breaking during tests because of clearTimeout being replaced. There’s ways around this, but for us the cleanest way is to preserve clearTimeout behavior for pre-existing timers. In truth our tests should’ve been written with more separation in mind, but that was before my time.

[fatso83]

Closed by #407