testing

How to reliably test async operations including promises, async/await, timers, and error rejections using Jest and Vitest patterns that prevent false positives and flaky tests.

Why testing matters, what unit tests are, what to test versus what to skip, and how automated tests improve code quality, enable safe refactoring, and serve as living documentation.

Understanding the four types of code coverage (line, branch, statement, function), interpreting coverage reports, setting meaningful targets, and knowing why 100% coverage does not guarantee bug-free code.

End-to-end testing verifies complete user flows through a real browser, comparing Cypress and Playwright on architecture, cross-browser support, debugging, and performance to choose the right tool for your project.

How to test custom React hooks using renderHook, handling state updates with act(), testing hook return values, and avoiding common pitfalls when hooks depend on context or async operations.

The difference between unit tests and integration tests, when to use each, the testing pyramid versus testing trophy, and how integration tests provide the best return on investment for most applications.

The essential Jest and Vitest matchers (toBe, toEqual, toMatch, toContain, toThrow) and how to choose the right assertion for each scenario to write clear, expressive test expectations.

Test-Driven Development (Red-Green-Refactor cycle) versus Behavior-Driven Development (Given-When-Then specifications), when each methodology shines, and how to apply them in practice for JavaScript projects.

How to use jest.fn(), jest.mock(), and jest.spyOn() (or their Vitest equivalents) to isolate code under test, plus API mocking with MSW for realistic network request testing.

How to test application performance with benchmarks, Web Vitals monitoring, Lighthouse CI integration, and performance budgets that prevent regressions in bundle size and runtime metrics.

Testing React components the way users interact with them using React Testing Library's query priority hierarchy, userEvent for realistic interactions, and the philosophy of testing behavior over implementation details.

Using beforeAll, beforeEach, afterEach, and afterAll hooks for test setup and teardown, understanding execution order, scope with describe blocks, and preventing test pollution.

Designing a comprehensive testing strategy using the testing trophy, defining what to test at each level, setting coverage targets, integrating tests into CI/CD, and making pragmatic trade-offs for large-scale applications.

How to organize test files, structure tests with describe and it blocks, write descriptive test names, apply the Arrange-Act-Assert pattern, and choose between co-located and centralized test directories.

Catching unintended UI changes with visual regression testing using screenshot comparison tools like Percy, Chromatic, and Playwright, including how pixel diffing works and when visual tests add value.