Throw on incorrect property matchers usage

[thymikee]

🚀 Feature Proposal

I'd like to propose a new behavior to recently introduced "snapshot property matchers" – throwing when the object values are not asymmetric matchers.

We should also link to the guide from expect.toMatchSnapshot docs to avoid confusion (regardless this request is accepted or not).

Motivation

I've seen this code in the wild:

expect(user).toMatchSnapshot({
  createdAt: 'mocked-date',
  id: 'gen-id',
});

Which doesn't make much sense (maybe it is sometimes, but not really), because the snapshot file is going to have this very information anyway, resulting in duplicating code and reviewing effort. Such code can easily be replaced with toMatchObject or toMatchSnapshot without property matcher.

Example

We could throw something like:

Snapshot property matchers are intended to use with asymmetric matchers.
For details on how to use it properly, see: https://jestjs.io/docs/en/snapshot-testing#property-matchers

Pitch

It's an update to core matcher.

cc @rickhanlonii

[mattphillips]

I agree this is odd! I wonder if this makes more sense as an eslint rule?

[rickhanlonii]

FWIW the current behavior was what was proposed but I agree that it makes sense to only allow asymmetric matchers now that it's better understood 👌

[dortamiguel]

I like the current behaviour, if you have a big response from an api that you want to snapshot and the only thing that changes is a date, then you only need to specify the only field that changes.

[rickhanlonii]

@ellipticaldoor we would keep that but would add the requirement that you specify a matcher

const user = {
  name: 'Pete Davidson',
  createdAt: Date.now(),
  id: uuid(),
};

// BAD, properties must be a matcher
expect(user).toMatchSnapshot({
  createdAt: 'mocked-date',
  id: 'gen-id',
});

// GOOD
expect(user).toMatchSnapshot({
  createdAt: expect.any(Date),
  id: expect.any(String),
});

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