Test-Driven Development

Philosophy

Core principle: tests should verify behavior through public interfaces, not implementation details.

Good tests are integration-style and exercise real code paths through public APIs. They describe what the system does, not how it does it.

Bad tests are coupled to implementation. They mock internals, test private methods, or fail on refactors where behavior did not change.

Anti-Pattern: Horizontal Slices

Do not write all tests first and then all implementation.

Use vertical tracer bullets instead:

RED -> GREEN one behavior at a time.