Mocking Stubbing

This handles test isolation by replacing dependencies with mocks, stubs, and spies so you can unit test without hitting real databases, APIs, or file systems. It covers Jest for JavaScript/TypeScript, unittest.mock for Python, and Mockito for Java, plus patterns for things like simulating errors and verifying interactions. The guidance on mocking at boundaries (repositories, services) rather than mocking everything you own is solid advice that will keep your tests maintainable. Best used when you need fast, deterministic tests and want to avoid the brittleness that comes from over-mocking internal implementation details.