Jest

A complete roadmap for Jest, the zero-config test runner that has dominated JavaScript testing for a decade. The series covers first tests, matchers, lifecycle hooks, async testing, and the full mocking system—jest.fn(), jest.spyOn(), and jest.mock()—including the three distinct cleanup methods that are easy to confuse. It moves into snapshot testing, configuration, TypeScript and ESM setup, coverage, parameterized tests, and custom matchers, then into Jest 30’s breaking changes: removed aliases, –testPathPatterns, globalsCleanup, and the performance gains from unrs-resolver. It closes with React Testing Library integration, DOM environments, migration from Jest 29, and an honest comparison with Vitest.