testing-patterns

Structure Vitest unit tests with the AAA pattern: arrange inputs, act by calling the function, then assert the result. Use describe/it blocks, vi.fn() for mocks, and it.each for parameterized cases across multiple inputs.

Mock API calls with MSW by defining request handlers that return JSON responses, then starting the server in your test setup file. For simpler cases, stub global.fetch with vi.fn() and mockResolvedValueOnce per test.

Vitest is the default choice in this skill for Vite-based projects because it shares configuration with the build pipeline and runs faster. Jest patterns like module mocking and spies translate directly via the vi namespace.

Yes, Playwright supports saving browser state with storageState after a one-time login setup project. Subsequent test projects load the saved state file, skipping repeated logins and speeding up E2E suites.

Target 70-80% for lines and branches, and 80%+ for functions, with 90%+ on critical code paths. Chasing 100% coverage does not guarantee good tests, bug-free code, or covered edge cases.

Mock the database in unit tests to keep them fast and isolated, but use a real test database in integration tests to verify actual queries and constraints. The rule of thumb is to mock only at system boundaries.