Should jest.clearAllMocks() be called manually when automatic mock restoration is enabled?

Direct answer

No, calling jest.clearAllMocks() in afterEach is not required when automatic mock restoration is enabled, but it can reduce memory retention in watch mode when jest.spyOn is used extensively.

Likely explanation

• Automatic restoration resets public mock state after each test, but the internal mock registry, particularly entries from jest.spyOn with mockImplementationOnce, may not be fully purged.
• In watch mode or long-running Jest processes, residual references can accumulate, increasing memory retention.
• Jest 27.0.6 added explicit teardown cleanup that deletes mock registry entries, fixing a memory leak observed with repeated spyOn/mockImplementationOnce.

Confirmed facts

• Automatic mock restoration between tests has been documented since earlier Jest versions.
• Restoration behavior differs between jest.fn() and jest.spyOn(); spyOn retains spy-specific state that automatic restoration may not fully erase.
• Calling clearAllMocks() in afterEach measurably reduces memory retention in watch mode when spyOn is extensive, but is generally unnecessary for fn-based mocks or when the Jest process restarts each run.

If your project uses jest.spyOn in watch mode and observes growing memory usage between test cycles, adding jest.clearAllMocks() in afterEach is a pragmatic safeguard. Otherwise, automatic restoration alone typically suffices.

Verification step: Run the same test suite in watch mode with and without an afterEach hook that calls jest.clearAllMocks(). Use Jest internal output or a heap snapshot tool to compare mock registry size or process memory between runs. If memory growth stops or decreases with the hook, the cleanup provides measurable benefit for your setup.