tdd

Write one failing test for a single behavior (red), write the minimal code to make it pass (green), then refactor while tests stay green. Repeat one test at a time as vertical slices rather than writing all tests upfront.

Unit tests check one function in isolation, integration tests exercise real components working together through a seam, and acceptance tests verify public behavior against spec criteria. The taxonomy also includes contract and property-based layers, with a decision matrix for which layers each change type requires.

Mock only at system boundaries such as external APIs, time, randomness, and sometimes databases or file systems. Never mock your own classes or internal collaborators, since that couples tests to implementation and makes them break during refactors.

Tests break during refactoring when they assert on implementation details like private methods, call counts, or internal collaborators instead of observable behavior. Rewrite them against the public interface so they describe what the system does, not how.

A tautological test recomputes its expected value using the same logic as the code under test, so it passes by construction and catches nothing. Expected values must come from an independent source such as a known-good literal, worked example, or the specification.