js-testing

Create a .test.ts file importing describe, it, and expect from vitest, then write assertions against the function's public API. Run tests with npx vitest run and add coverage with the --coverage flag.

Use MSW (Mock Service Worker) to intercept requests at the network level instead of mocking fetch directly. Define handlers with http.get, set up a server with setupServer, and start it in a Vitest setup file with server.listen().

Vitest is preferred for new projects due to native Vite integration and faster execution, while Jest suits existing projects with a mature ecosystem. The skill defaults to Vitest but supports Jest when it is already configured.

Flaky async tests usually come from setTimeout-based synchronization or real timers and network calls. Use waitFor from Testing Library, vi.useFakeTimers() for time-dependent code, and reset mocks with vi.clearAllMocks() in beforeEach.

Use Playwright for critical end-to-end user journeys like login flows that require real browser rendering and navigation. Reserve Vitest unit and component tests for isolated logic, boundaries, and error cases.