TDD and Unit Tests: Driving Design and Feedback

TDD is not simply writing tests first. It compresses design feedback into a short loop: describe expected behavior, implement the minimum code, then refactor the structure.

Core Loop

stateDiagram-v2
    [*] --> Red
    Red: Write a failing test
    Red --> Green: Implement the minimum code
    Green --> Refactor: Improve design after passing
    Refactor --> Red: Next behavior

Engineering Scenario

TDD is useful when rules involve boundaries, state transitions, money calculations, or permission checks. It fits domain services, value objects, and pure business rules well.

Common Mistakes

• Writing brittle tests only for coverage.
• Testing implementation details instead of behavior.
• Creating tests so large that failures are hard to diagnose.

Best Practices

• Let each test describe one behavior.
• Make Given / When / Then visible in names or structure.
• Refactor only after tests are green.
• Cover external dependencies with contracts or integration tests.