Setting timers config property to modern does not use modern mode by default in test files

[blimmer]

🐛 Bug Report

My understanding of the timers config property (docs) is that setting the value to modern should automatically use "modern" mode everytime I call jest.useFakeTimers(). However, that doesn't appear to be the case.

I receive this error when trying to call modern-specific methods:

  ● bug report › should allow using setSystemTime because timers is set to 'modern' in config

    TypeError: setSystemTime is not available when not using modern timers

      1 | describe('bug report', () => {
      2 |   it("should allow using setSystemTime because timers is set to 'modern' in config", () => {
    > 3 |     jest.useFakeTimers().setSystemTime(0); // This does not work, but should based on the jest.config.js `timers` setting
        |                          ^
      4 |     jest.useFakeTimers('modern').setSystemTime(0); // This does work
      5 |   })
      6 | })

      at Object.setSystemTime (node_modules/jest-runtime/build/index.js:1856:17)
      at Object.<anonymous> (bug-report.test.js:3:26)

To Reproduce

Steps to reproduce the behavior:

1. Set timers in jest.config.js to modern
2. Try to call jest.useFakeTimers().setSystemTime(0); in a test file.

An error is thrown when running the test file.

Expected behavior

Because I set timers to modern in jest.config.js, I expected to be able to call jest.useFakeTimers().setSystemTime(0); because modern should be the default based on my configuration.

Link to repl or repo (highly encouraged)

https://replit.com/@blimmer/jest-playground#bug-report.test.js

envinfo

  System:
    OS: macOS 11.2.3
    CPU: (8) x64 Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
  Binaries:
    Node: 12.21.0 - ~/.nvm/versions/node/v12.21.0/bin/node
    Yarn: 1.22.10 - /usr/local/bin/yarn
    npm: 6.14.11 - ~/.nvm/versions/node/v12.21.0/bin/npm
  npmPackages:
    jest: ^26.6.3 => 26.6.3

[SimenB]

Setting timer in docs means you're always using fake timers. If you later call jest.useFakeTimers() you "override" that setting, and if you want to use modern timers you need to pass 'modern'. I agree this is no clear in the docs - would you be up to sending a PR specifying?

Note that in Jest 27 the default is flipped, so wording in docs/Configuration.md should be different from the ones in versioned_docs.

[blimmer]

Oh, interesting. Is there a way to call setSystemTime when you've faked timers via the config?

The only way I know how to do it is by chaining the call to useFakeTimers.

[SimenB]

Just calling jest.setSystemTime should be fine - the chain is jest, not a returned fake timer implementation

[blimmer]

Cool - thanks. I opened #11376 with a documentation update.

[github-actions]

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