Implement JavaScript Testing Patterns

Covers modern JavaScript and TypeScript testing across five layers with working code. Framework setup for Jest (ts-jest preset, coverage thresholds at 80%) and Vitest (v8 coverage provider, globals enabled). Unit testing patterns for pure functions (a calculator's add and divide with error-case assertions), classes (a UserService with create, findById, update, and delete and their error paths), and async functions (an ApiService whose fetchUser and createUser are tested against a mocked global fetch). Mocking patterns covering module mocking (vi.mock for nodemailer, asserting the wrapped transporter.sendMail call), dependency-injection testing (a UserService built against an IUserRepository interface so tests inject a mock repository instead of touching the real one), and spying (vi.spyOn on a shared logger to assert call arguments and count without replacing the whole module). Integration testing with supertest against a real Express app and test database (create and get endpoints, validation errors, duplicate-email conflicts, and authenticated-route access with a real login token) and a UserRepository integration suite against a live PostgreSQL test database with setup and teardown of tables. Frontend testing with React Testing Library (rendering a UserForm, firing change and click events, asserting onSubmit payloads) and hook testing (renderHook and act against a useCounter hook's increment, decrement, and reset). Closes with test fixtures via @faker-js/faker factories, snapshot testing with toMatchSnapshot, fake-timer testing with vi.useFakeTimers and advanceTimersByTime, and a 15-item best-practices list covering the AAA pattern, descriptive test names, beforeEach and afterEach setup, mocking external dependencies, edge-case coverage, avoiding implementation-detail tests, an 80%+ coverage target, and preferring semantic queries over data-testid.

Use it when setting up test infrastructure for a new project, writing unit tests for functions and classes, creating integration tests for APIs and services, implementing end-to-end tests for user flows, mocking external dependencies, testing React or Vue components, practicing TDD, or setting up continuous testing in CI/CD. Skip it for anything unrelated to JavaScript testing or outside this scope.

Input is the code under test and its layer - a pure function, a class with dependencies, an async API call, an HTTP endpoint, a database repository, or a UI component; output is the matching Jest or Vitest test file, including the mocking, fixture, or rendering setup that layer needs.

Jest with ts-jest, or Vitest with its native Vite integration; @testing-library/react and its renderHook helper for components and hooks; supertest for HTTP integration tests; node-postgres's Pool for direct PostgreSQL integration tests; @faker-js/faker for fixture data. Links to the Jest, Vitest, and Testing Library documentation plus Kent C. Dodds's testing blog.

JavaScript and TypeScript engineers setting up or extending a test suite who need concrete, runnable patterns across unit, mocking, integration, and frontend layers rather than framework documentation alone.