fix: stop changing the behaviour of spies on mockFn.mockReset

[feliperli]

Summary

The motivation described at this issue is to instead of returning undefined for mockReset return the original function itself.

Test plan

const myObject = {bar: () => 'bar'};

let barStub: any;

beforeAll(() => {
  barStub = jest.spyOn(myObject, 'bar');
});

test('run #1', () => {
  barStub.mockReturnValue('POTATO!');
  expect(myObject.bar()).toBe('POTATO!');
  barStub.mockReset();

  // This expect should be successful
  expect(myObject.bar()).toBe('bar');
});

[cpojer]

Can you add e2e tests that verify that this behavior is working and add a line to the CHANGELOG.md?

[SimenB]

Should probably update the docs as well to specify what we're skipping?

[feliperli]

@cpojer @SimenB e2e test added and changelog updated. not sure if we should update the docs, tho, since this is a bug and didn't changed the general idea of mockReset and resetAllMocks

[mrazauskas]

not sure if we should update the docs, tho, since this is a bug and didn't changed the general idea of mockReset and resetAllMocks

The linked issue is pointing to a note from this line of documentaion:

https://github.com/facebook/jest/blob/a325f8790d99029e45d215fcfc27790aa2bda131/docs/MockFunctionAPI.md?plain=1#L129

I guess the text in parenthesis should be deleted, or?

[feliperli]

Thanks @mrazauskas

[SimenB]

@feliperli there's a type error

[SimenB]

thanks!

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