testing

Jest provides jest.fn() for mock functions, jest.mock() for modules, and jest.spyOn() for spying on methods. For API calls, mock fetch/axios or use libraries like MSW. Mocks isolate code under test and allow controlling dependencies.

React Testing Library tests components from the user's perspective. Use render() to mount components, query methods (getByRole, getByText, etc.) to find elements, fireEvent or userEvent for interactions, and waitFor for async updates.

Unit tests test individual functions/components in isolation with mocked dependencies. Integration tests verify multiple units work together correctly, testing real interactions between components, APIs, or databases with fewer mocks.

Code coverage measures how much source code is executed by tests. Metrics include lines, statements, branches, and functions. High coverage doesn't guarantee quality - focus on testing critical paths and edge cases rather than chasing 100%.

Use @testing-library/react's renderHook to test custom hooks in isolation. The function returns result.current with the hook's return value, and rerender to trigger updates. For hooks with state, use act() to wrap state updates.