# Testing Tdd London London Vs Chicago School

> Sub-skill of testing-tdd-london: London vs Chicago School (+2).

- Skill: `vamseeachanta/testing-tdd-london-london-vs-chicago-school` (Agent Skill)
- Install (CLI): `npx skillmds@latest add vamseeachanta/testing-tdd-london-london-vs-chicago-school`
- Raw SKILL.md: https://api.skillmd.com/api/skills/vamseeachanta/testing-tdd-london-london-vs-chicago-school/raw
- Safety review: PASS (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: vamseeachanta (https://skillmd.com/u/vamseeachanta)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/vamseeachanta/testing-tdd-london-london-vs-chicago-school

---


# London vs Chicago School (+2)

## London vs Chicago School


| Aspect | London (Mockist) | Chicago (Classicist) |
|--------|------------------|----------------------|
| Focus | Behavior/Interactions | State |
| Isolation | Mock all collaborators | Use real objects |
| Direction | Outside-in | Inside-out |
| Test What | HOW objects talk | WHAT objects produce |
| Coupling | To implementation | To behavior |

## Outside-In Development Flow


```
Acceptance Test (failing)
    |
    v
Controller Test (failing)
    |
    v
Service Test (failing)
    |
    v
Repository Test (failing)
    |
    v
Implement (make tests pass from bottom up)
```

## Mock Types


```typescript
// Stub: Returns canned responses
const stubRepo = { findById: jest.fn().mockResolvedValue(user) };

// Mock: Verifies interactions
const mockNotifier = { send: jest.fn() };
// Later: expect(mockNotifier.send).toHaveBeenCalledWith(expectedArgs);

// Spy: Wraps real object, records calls
const spyLogger = jest.spyOn(logger, 'info');
```