Custom Asymmetric Matchers

[aymericbouzy]

Summary

In response to issues #4711 and #5490, this PR attempts to extend the power of expect.extend.

It would now be possible to do such thing :

const period = {
  start: moment("2018-05-17"),
  end: moment("2018-05-18"),
}

// Current Behavior
expect.extend({
  sameDate(received, expected) {
    ...
  },
})

expect(period.start).sameDate("2018-05-17")

// Added Behavior
expect(period).toEqual({
  start: expect.sameDate("2018-05-17"),
  end: expect.sameDate("2018-05-18"),
})

Test plan

here is my test :

jestExpect.extend({
  toBeDivisibleBy(actual, expected) {
    const pass = actual % expected === 0;
    const message = pass
      ? () => `expected ${actual} not to be divisible by ${expected}`
      : () => `expected ${actual} to be divisible by ${expected}`;

    return {message, pass};
  },
});

it('defines asymmetric matchers', () => {
  jestExpect({value: 2}).toEqual({value: expect.toBeDivisibleBy(2)});
  jestExpect({value: 3}).toEqual({value: expect.not.toBeDivisibleBy(2)});
  expect(() =>
    jestExpect({value: 3}).toEqual({value: expect.toBeDivisibleBy(2)}),
  ).toThrowErrorMatchingSnapshot();
});

[aymericbouzy]

There are a lot of new technologies I don't know about so much 😰

I've already managed to get a working example. Can someone tell me if I'm on the right track and what might be the next step for me?

[pedrottimark]

@aymericbouzy Thank you for providing specific example of moment objects. That helps a lot!

It might be until Monday before I can compare your PR to what I wrote in #4711. Hang in there :)

[aymericbouzy]

Hello, any news on this?

[aymericbouzy]

@pedrottimark I see #5517 has been closed : should I resolve conflicts and make sure it works for custom matchers too?

[SimenB]

Yes please 🙂

[aymericbouzy]

Hi, I think I might be close, but I'm struggling to fix flow errors (I have very very little knowledge of Flow to be honest) : can somebody give me a hint?

EDIT: It's fixed!

[codecov-io]

Codecov Report

Merging #5503 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5503   +/-   ##
=======================================
  Coverage   63.65%   63.65%           
=======================================
  Files         216      216           
  Lines        7918     7918           
  Branches        3        3           
=======================================
  Hits         5040     5040           
  Misses       2877     2877           
  Partials        1        1

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[aymericbouzy]

@pedrottimark @SimenB waiting for your review for the next steps!

[pedrottimark]

@aymericbouzy You have my apology for delay. This turtle is coming up for air after diving deep :(

[aymericbouzy]

Haha it's OK I'm sure you had plenty other things to do! I'm just hoping my PR will eventually get merged 😉

[SimenB]

This needs docs and a changelog, but the implementation LGTM 🙂

[cpojer]

I went ahead and merged it because we need this at FB soon. @aymericbouzy thanks so much for building this feature! Would you mind following up with a PR to add the changelog entry and the documentation for it? Thank you!

[github-actions]

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