test-driven-development

Test-driven development begins by writing a minimal failing unit test that captures the desired behavior, confirming it fails for the expected reason, then writing the smallest code change to make it pass before refactoring.

The Red-Green-Refactor cycle is a unit testing discipline requiring a RED phase with a minimal failing test, a GREEN phase with the smallest production change to pass it, and a REFACTOR phase that keeps tests green while preventing behavior drift.

Yes, test-first development applies to bug fixes, refactoring, and behavior changes by defining expected behavior as an executable test, ensuring the implementation earns correctness rather than accidentally matching assumptions.

You should avoid the TDD workflow for throwaway prototypes and generated code unless explicitly approved, as the verification rigor of failing tests first adds unnecessary overhead to unverified or temporary exploratory code.

Verifying that a failing test fails for the expected reason ensures your implementation earns correctness through the test cycle, preventing false positives where code accidentally passes tests without meeting the actual behavior requirements.