Vitest

Reliable Vitest unit and integration tests require behavior-focused assertions with deterministic state. You achieve this by mirroring source structure for unit tests, isolating integration setups, and using correct mocking patterns to ensure refactors remain risk-free.

Correct Vitest mocking uses vi.mock() for module replacement, vi.fn() for controllable mocks, and vi.spyOn() for spying. Maintaining this strict separation prevents mixing toolchains and avoids flaky, misleading tests that fail to validate actual behavior.

Async Vitest assertions silently pass when you fail to await expect(...).rejects or expect(...).resolves. Properly awaiting these async assertions ensures that test failures are never silently ignored during your JavaScript or TypeScript test runs.

Enforce coverage thresholds in vitest.config.ts to establish guardrails that block coverage regressions. Configuring these thresholds ensures your test suite maintains strict behavior validation standards across your JavaScript and TypeScript codebase.

Jest mocking APIs should not be used with Vitest. This skill actively enforces anti-pattern avoidance, specifically targeting Jest API misuse, and standardizes correct vi.mock, vi.fn, and vi.clearAllMocks hygiene for deterministic test state.

The best way to structure integration tests is to isolate them from unit behavior by placing them in tests/integration, while unit tests mirror the source under src/__tests__. This organization ensures clean, deterministic state for both test types.