tdd

Write one failing test that states the expected behavior through a public interface, then implement only the minimal code to make it pass. After the test is green, refactor while keeping all tests passing, then choose the next single test.

Red-green-refactor is the TDD cycle: write a failing test (red), write just enough code to pass it (green), then improve the code structure while keeping tests passing (refactor). Each cycle covers one behavior slice at a time.

Mock only at system boundaries such as external APIs, time, and randomness. Prefer a real test database over a mocked one, and avoid mocking internal collaborators since that couples tests to implementation details.

Tests break during refactoring when they are coupled to implementation, such as testing private methods or mocking internal collaborators. Test behavior through public interfaces and stable seams so internal changes do not affect the suite.

Three common anti-patterns are implementation-coupled tests that break on refactoring, tautological tests that derive expectations from the same logic being tested, and horizontal slicing where many tests are written before any implementation exists.