testing

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.

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.

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.

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.