setSystemTime and getRealSystemTime always throw if yarn/npm does not deduplicate @jest/fake-timers

[simon-paris]

🐛 Bug Report

Jest relies on yarn/npm to deduplicate @jest/fake-timers. If it is not deduplicated, the following functions always throw the error setSystemTime is not available when not using modern/legacy timers.

• setSystemTime
• getRealSystemTime
• runAllImmediates

It's caused by the instanceof checks in this file: https://github.com/facebook/jest/blob/master/packages/jest-runtime/src/index.ts.

I've attached my real yarn lock file to prove that deduplication can fail in the wild, and not just in the contrived example below. yarn.lock.zip

To Reproduce

Run the following

yarn add [email protected] @jest/[email protected]
echo "jest.useFakeTimers('modern').setSystemTime(new Date());" > test.js
yarn jest

It will output this:

 FAIL  ./test.js
  ● Test suite failed to run

    TypeError: setSystemTime is not available when not using modern timers

    > 1 | jest.useFakeTimers('modern').setSystemTime(new Date());
        | ^
      2 |

      at Object.setSystemTime (node_modules/jest-runtime/build/index.js:1989:17)
      at Object.<anonymous> (test.js:1:1)

Expected behavior

Jest should not rely on yarn/npm deduplication behavior.

Link to repl or repo (highly encouraged)

Just run the script above in an empty folder.

envinfo

  System:
    OS: macOS 10.15.7
    CPU: (8) x64 Intel(R) Core(TM) i7-8569U CPU @ 2.80GHz
  Binaries:
    Node: 14.17.3 - /usr/local/bin/node
    Yarn: 1.22.10 - /usr/local/bin/yarn
    npm: 6.14.13 - /usr/local/bin/npm
  npmPackages:
    jest: 27.0.3 => 27.0.3 

[strunkandwhite]

I was running into this when I had both @jest/[email protected] and @jest/[email protected] in my yarn.lock. Once I updated the dependency that required @jest/[email protected], and yarn why @jest/fake-timers only yielded => Found "@jest/[email protected]", I stopped having this issue.

[FrozenKiwi]

Is there any reason the test can't just look for the existence of setSystemTime?

        if (fakeTimers instanceof _fakeTimers().ModernFakeTimers) {

becomes

        if (fakeTimers.setSystemTime) {
          fakeTimers.setSystemTime(now);
        } else {

The current check seems oddly fragile - Duck-typing seems safe enough. I haven't dug through the code all that much, but if we -need- the packages de-duped (eg, singleton access or something) then perhaps we could tweak the error message to be a bit more explicit?

[cmavelis]

I was running into this when I had both @jest/[email protected] and @jest/[email protected] in my yarn.lock. Once I updated the dependency that required @jest/[email protected], and yarn why @jest/fake-timers only yielded => Found "@jest/[email protected]", I stopped having this issue.

My yarn why only yields => Found "@jest/[email protected]", but I have the issue. Already was in that state to begin with though, so maybe that means something?

[github-actions]

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