Testing Tdd London

Quick Start

// 1. Start with acceptance test (outside)
describe('User Registration', () => {
  it('should register new user successfully', async () => {
    const mockRepository = { save: jest.fn().mockResolvedValue({ id: '123' }) };
    const mockNotifier = { sendWelcome: jest.fn() };

    const service = new UserService(mockRepository, mockNotifier);
    await service.register({ email: 'test@example.com' });

    // 2. Verify behavior (interactions)
    expect(mockRepository.save).toHaveBeenCalledWith(
      expect.objectContaining({ email: 'test@example.com' })
    );
    expect(mockNotifier.sendWelcome).toHaveBeenCalledWith('123');
  });
});

When to Use

• Testing object collaborations and message passing
• Contract-driven development with clear interfaces
• Outside-in development starting from user behavior
• When isolation of units is critical
• Service orchestration testing
• Testing HOW objects work together (not WHAT they contain)

Prerequisites

• Understanding of mock objects vs stubs
• Jest, Vitest, or similar testing framework with mocking support
• Clear separation of concerns in architecture
• Dependency injection pattern in codebase

References

• London School TDD
• Mocks Aren't Stubs
• Growing Object-Oriented Software

Version History

• 1.0.0 (2026-01-02): Initial release - converted from tdd-london-swarm agent

Sub-Skills

• Configuration
• Example 1: Service Orchestration Test (+2)
• Mock Management (+2)

Sub-Skills

• Execution Checklist
• Error Handling

Sub-Skills

• London vs Chicago School (+2)
• 1. Outside-In Development (+2)
• Metrics & Success Criteria
• MCP Tools (+2)