Print jest mock fuctions as MockFunction in jest-snapshot

[SimenB]

Summary
Fixes #4835. /cc @mjesun

Test plan
Changed snapshots

[SimenB]

It feels really convoluted to do it this way. @pedrottimark is this necessary to just change the header of the object?

[pedrottimark]

FYI, a solution for a built-in plugin is to call printObjectProperties for example:

https://github.com/facebook/jest/blob/master/packages/pretty-format/src/plugins/asymmetric_matcher.js#L40-L58

But AsymmetricMatcher will soon change not to do that for the reason that I missed when I first read #4835 (comment)

which will make impossible to accidentally match with an object.

Our currently proposed serialization matches instance constructed from ordinary class:

class MockFunction {
  constructor(calls = [], name = 'jest.fn()') {
    this.calls = calls;
    this.name = name;
  }
}

When I read Mock Functions page to borrow an example for alternative, a question hit me:

Should the serialization also include instances property?

http://facebook.github.io/jest/docs/en/mock-function-api.html#mockfnmockinstances

[SimenB]

I originally had instances as well, but it didn't really provide anything of value.

3514516

[codecov-io]

Codecov Report

Merging #4836 into master will decrease coverage by 0.07%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #4836      +/-   ##
==========================================
- Coverage   59.24%   59.17%   -0.08%     
==========================================
  Files         200      200              
  Lines        6633     6670      +37     
  Branches        4        3       -1     
==========================================
+ Hits         3930     3947      +17     
- Misses       2703     2723      +20

Impacted Files	Coverage Δ
packages/jest-snapshot/src/mock_serializer.js	50% <0%> (ø)	⬆️
packages/jest-haste-map/src/worker.js	93.93% <0%> (-1.9%)	⬇️
...ckages/jest-cli/src/reporters/coverage_reporter.js	54.45% <0%> (-1.11%)	⬇️
packages/jest-haste-map/src/index.js	96.8% <0%> (-0.71%)	⬇️
packages/jest-runner/src/test_worker.js	0% <0%> (ø)	⬆️
packages/jest-cli/src/generate_empty_coverage.js	85.71% <0%> (ø)	⬆️
packages/jest-runner/src/index.js	38.29% <0%> (+0.52%)	⬆️
packages/jest-worker/src/index.js	100% <0%> (+15.71%)	⬆️
packages/jest-cli/src/reporters/coverage_worker.js	81.81% <0%> (+31.81%)	⬆️

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[pedrottimark]

To continue thought from inline comment with a question for me to learn: are there any situations when the expected result for assertion about mock function has both calls and instances non-empty, or would that only occur in an incorrect received value?

[pedrottimark]

You mean, because invoking mock function with new is rare?

My mental slow cooker is working on a format which only displays neither, either, or both properties which are non-empty arrays.

A format without calls before mock has been called might solve original problem with mocked props in snapshot of JavaScript object or React element.

But I need to sketch what some realistic diffs would look like when snapshot fails.

[cpojer]

Nice!

[github-actions]

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