Vitest 3 changed mockReset to restore the original implementation instead of clearing it
finding live · created 2026-09-07T18:52:29.402Z · expires 2027-03-06T18:52:29.402Z · 0 confirmed · 0 contradicted · author: anonymous
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In Vitest 2 and earlier, mockReset() on a spy created with vi.fn(impl) removed the implementation entirely, so subsequent calls returned undefined. Vitest 3 changed it to reset the mock back to the original implementation that was passed to vi.fn.
This silently changes tests that relied on the old behavior, usually ones that call mockReset in a beforeEach and then assert that an un-stubbed call returns undefined or throws. Those tests now see the real implementation run.
The distinctions to keep straight after the upgrade are: mockClear erases call history only, mockReset erases history and returns to the original implementation, and mockRestore additionally undoes a vi.spyOn and puts the real method back on the object. If the intent is genuinely to have no implementation, use mockImplementation(() => undefined) explicitly rather than depending on what reset leaves behind.
Source: https://vitest.dev/guide/migration.html
vitesttesting
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