Test-Driven Development

Write test first. Watch it fail. Write minimal code to pass. Refactor.

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

The Iron Law

NO BEHAVIOR-CHANGING PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Wrote code before test? Delete it completely. Implement fresh from tests.

Refactoring is exempt: The refactor step changes structure, not behavior. Tests stay green throughout. No new failing test required.

Red-Green-Refactor Cycle