fix(jest-mock): restore withImplementation after errors

[fallintoplace]

Summary

Fixes mockFn.withImplementation(...) so it restores the previous mock state when the callback throws synchronously or rejects asynchronously.

Closes #16218.

Motivation

withImplementation is documented as a temporary override, but the current implementation only restores state on the normal synchronous path and on fulfilled promises. That leaves the temporary implementation installed after exceptional exits.

How this works

• restore the previous mock state in a shared helper
• restore immediately when the callback throws
• adopt thenables through the environment Promise and restore on both fulfillment and rejection
• add regressions for sync throws and async rejections, including restoration of queued mockImplementationOnce state and whenCalledWith fallback behavior

Validation

• yarn jest packages/jest-mock/src/__tests__/index.test.ts --runInBand
• yarn jest packages/jest-mock/src/__tests__ --runInBand
• yarn build:ts
• yarn typecheck:tests

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commit: 6bbc503

[github-actions]

This PR is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 30 days.

[soltonigiri]

isPromise(returnedValue) still runs outside the restoration try block. isPromise reads .then. If that property access throws, the error escapes before restore() and leaves the temporary implementation installed. The documentation explicitly supports callbacks that return thenables, so reading .then is part of this documented code path.

const mock = jest.fn(() => 'outside callback');

const thenable = Object.defineProperty({}, 'then', {
  get() {
    throw new Error('boom');
  },
}) as Promise<unknown>;

expect(() =>
  mock.withImplementation(() => 'inside callback', () => thenable),
).toThrow('boom');

expect(mock()).toBe('outside callback'); // Receives "inside callback"

Could the try block also cover isPromise(returnedValue) and the Promise.resolve(...).then(...) setup? I reproduced this on the PR head 55ce3234b303562ceb63b41ad44a8832d9b0ba61 and after applying the PR to current main be425a0b0e3bd60a74e4a7e350aa38c63a2d25ef. Moving those operations inside the try restores the original implementation. Locally, all 261 jest-mock tests across 6 suites pass.

[soltonigiri]

Thanks for fixing the restoration issue. I rechecked 6bbc5036, and an exception thrown while reading .then no longer skips restore(). I left two inline comments: the withImplementation tests don't exercise the changed source, and the changelog entry is under the released 30.5.0 section.

[soltonigiri]

30.5.0 is already out. Could you move this entry to ## main under Fixes? yarn check-changelog 16219 exits 1 because it expects the PR link there.

[fallintoplace]

Thanks a lot for the comments.

Are you a maintainer though? I feel like this PR is not getting any attention.

Once again, I appreciate the reviews.

[soltonigiri]

No, I'm not a maintainer, just a fellow contributor 😅 I don't have merge permissions.

This PR has been open for a while, so I just wanted to help get it into tip-top shape for when a maintainer gets a chance to review it.

Thanks for being so receptive to the feedback!

[soltonigiri]

These tests create the target mocks with the runner's global jest.fn, so those mocks execute packages/jest-mock/build/index.js instead of the package source imported by this unit test. A focused coverage run of the full file left the changed index.ts:947-978 block uncovered. The three new error cases exercise those lines when switched to moduleMocker.fn, but the synchronous-success and fulfilled-promise restore cases need the same change. Could all withImplementation target mocks use moduleMocker.fn?