unit-testing

# Unit Testing Patterns ## When to use - Setting up Vitest or Jest for a new project - Writing unit tests for business logic - Component testing with Testing Library - Mocking HTTP requests with MSW - Parametrized tests with it.each ## Core principles 1. **Test behavior, not implementation** — what does it return/do, not how 2. **MSW over jest.mock** — mock at network level, not module level 3. **One assertion per test** — single reason to fail per test 4. **Arrange-Act-Assert** — consistent structure in every test 5. **No test interdependence** — each test must run independently ## References available - `references/vitest-config.md` — globals, environment, coverage thresholds, thread pool, MSW setup - `references/business-logic-testing.md` — pure function tests, AAA pattern, it.each parametrized tests - `references/msw-http-mocking.md` — handlers setup, server lifecycle, per-test handler overrides - `references/async-testing.md` — await patterns, rejected promise assertions, try/catch error checks - `references/testing-library-react.md` — render, screen queries, userEvent vs fireEvent, role queries