javascript-testing-patterns

Vitest unit tests use describe/it blocks with expect assertions, importing functions directly from source files. Configure vitest.config.ts with globals, environment, and coverage settings, then test pure functions, classes, and async code with mockResolvedValue and rejects.toThrow.

Vitest offers faster execution and native Vite integration with a Jest-compatible API, making it ideal for modern Vite projects. Jest with ts-jest remains a full-featured choice for established projects; both support coverage thresholds, mocking, and setup files.

Assign global.fetch = vi.fn() and use mockResolvedValueOnce to return fake responses with ok status and json methods. Then assert fetch was called with the expected URL and request options like method and body.

Render the component with render(), query elements using getByRole, getByPlaceholderText, or getByTestId, and simulate events with fireEvent. For hooks, use renderHook with act() to trigger state updates and assert on result.current values.

Use supertest to send HTTP requests against the app and a real PostgreSQL test database via pg Pool. Create tables in beforeAll, truncate data in beforeEach for isolation, and drop tables plus end the pool in afterAll.

Real timers make tests slow and flaky. Use vi.useFakeTimers() to control time, then vi.advanceTimersByTime() to trigger scheduled callbacks deterministically, and restore with vi.useRealTimers() after the test.