Testing JavaScript and TypeScript Applications

Why test (regression/confidence/documentation), Jest setup and configuration, describe/it/expect, matchers (toBe/toEqual/toContain/toThrow/toBeNull), test organization, running tests (--watch/--verbose/--coverage)

Testing pure functions, testing async functions (resolves/rejects), jest.fn() for mock functions, jest.mock() for module mocking, jest.spyOn(), mock implementations and return values, clearing and resetting mocks

Supertest for HTTP testing, testing GET/POST/PUT/DELETE endpoints, testing with test database (in-memory SQLite), testing authentication (JWT headers), testing validation (bad input), testing error responses, beforeEach/afterEach setup

React Testing Library philosophy (test behavior not implementation), render/screen/fireEvent/waitFor, querying (getByRole/getByText/getByTestId), testing user interactions, testing async components (data fetching), testing forms, testing routing

Code coverage reports (--coverage), understanding coverage metrics (statements/branches/functions/lines), coverage thresholds in jest.config, what to test vs what not to test, snapshot testing (when useful/when harmful), CI pipeline integration basics