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https://jestjs.io/docs/cli#--seednum

Description

https://jestjs.io/docs/cli#--seednum describes the CLI seed option, but only mentions it's usage within getSeed (with a link to https://jestjs.io/docs/jest-object#jestgetseed ).

As far as I can tell, the seed option is also used to initialze the shuffle that applies to children of a describe block here: https://github.com/jestjs/jest/blob/654dbd6f6b3d94c604221e1afd70fcfb66f9478e/packages/jest-circus/src/run.ts#L88-L90

If that is correct, it would help a lot to mention that test flakyness might be related to this shuffling, and reusing the same seed of a failed run can help reproduce that failure.

Good point! Wanna send a PR with this addition?

Yeah, I would do that, though I still need some clarification. I've only now noticed the randomize option: https://jestjs.io/docs/cli#--randomize Which does mention seed, too.

Since randomize is an option, does it mean this shuffling is disabled by default?

If so, should a random seed (not setting it explicitly) make any difference if randomize isn't used?

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Still looking for clarification on how the seed and randomize options interact. With that (see previous comment) answered, I could create a docs PR.

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Shuffling of test suites happens by default.

Shuffling of tests within each describe block only happens when randomize is on.

Seed is also used for any other parts of your system that can accept a seed.

For example if you rely on fuzz testing like fast-check then getSeed can be used to specify the seed of the randomizer there and ensure replicable tests.

As an example from my usage of fast-check.

  it('throws error for random string', () => {
    fc.assert(
      fc.property(
        fc.string().filter(s => s !== 'True' && s !== 'False'),
        s => {
          expect(() => parseBoolean(s)).toThrow();
        },
      ),
      { seed: jest.getSeed() },
    );
  });

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