Test-Driven Development: London & Chicago Schools

<default_to_action> When implementing TDD or choosing testing style:

1. IDENTIFY code type: domain logic → Chicago, external deps → London
2. WRITE failing test first (Red phase)
3. IMPLEMENT minimal code to pass (Green phase)
4. REFACTOR while keeping tests green (Refactor phase)
5. REPEAT cycle for next functionality

Quick Style Selection:

• Pure functions/calculations → Chicago (real objects, state verification)
• Controllers/services with deps → London (mocks, interaction verification)
• Value objects → Chicago (test final state)
• API integrations → London (mock external services)
• Mix both in practice (London for controllers, Chicago for domain)

Critical Success Factors:

• Tests drive design, not just verify it