vitest-mock-implementation-persistence

Vitest tests pass in isolation but fail together due to mock implementations leaking across test boundaries. Lingering mocks cause false failures in the full suite, requiring explicit resets between tests to prevent cross-test contamination.

Prevent mock leakage in Vitest by reinitializing mock implementations within a beforeEach block. Using vi.resetAllMocks followed by appropriate re-setup ensures a clean mock state for each test boundary.

vi.clearAllMocks clears mock call history while resetting implementations requires vi.resetAllMocks. After resetting, you must re-setup full mock objects in beforeEach to prevent TypeErrors from incomplete mocks.

Fix TypeErrors from incomplete Vitest mocks by restoring a complete mock object. Reinitialize the full mockImplementation in your beforeEach setup after clearing mocks to ensure all expected properties and methods exist.

Use beforeEach to re-setup mocks in Vitest when refactoring test suites or using vi.mock to stub modules. This pattern ensures test isolation by providing each test with a fresh, complete mock implementation.

vi.mock can cause cross-test contamination in Vitest if mock implementations are not explicitly reset. Reinitializing mocks in beforeEach or using vi.resetAllMocks with appropriate re-setup prevents lingering stubs from breaking subsequent tests.