TDD London School (Mockist)

Outside-in, mock-driven development focusing on object collaborations and behavior verification

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

Core Concepts

London vs Chicago School

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

// 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');

Implementation Pattern

1. Outside-In Development

// Start with acceptance test (outermost layer)
describe('User Registration Feature', () => {
  it('should register new user successfully', async () => {
    // Mock all collaborators
    const mockRepository = {
      save: jest.fn().mockResolvedValue({ id: '123', email: 'test@example.com' }),
      findByEmail: jest.fn().mockResolvedValue(null)
    };

    const mockNotifier = {
      sendWelcome: jest.fn().mockResolvedValue(true)
    };

    const userService = new UserService(mockRepository, mockNotifier);
    const result = await userService.register({
      email: 'test@example.com',
      password: 'secure123'
    });

    // Verify the conversation between objects
    expect(mockRepository.findByEmail).toHaveBeenCalledWith('test@example.com');
    expect(mockRepository.save).toHaveBeenCalledWith(
      expect.objectContaining({ email: 'test@example.com' })
    );
    expect(mockNotifier.sendWelcome).toHaveBeenCalledWith('123');
    expect(result.success).toBe(true);
  });
});

2. Interaction Testing

describe('Order Processing', () => {
  it('should follow proper workflow interactions', async () => {
    const mockPayment = { charge: jest.fn().mockResolvedValue({ success: true }) };
    const mockInventory = { reserve: jest.fn().mockResolvedValue(true) };
    const mockShipping = { schedule: jest.fn().mockResolvedValue({ trackingId: 'ABC' }) };

    const service = new OrderService(mockPayment, mockInventory, mockShipping);
    await service.processOrder(order);

    // Verify call order matters
    const callOrder = [];
    mockInventory.reserve.mockImplementation(() => {
      callOrder.push('reserve');
      return Promise.resolve(true);
    });
    mockPayment.charge.mockImplementation(() => {
      callOrder.push('charge');
      return Promise.resolve({ success: true });
    });
    mockShipping.schedule.mockImplementation(() => {
      callOrder.push('schedule');
      return Promise.resolve({ trackingId: 'ABC' });
    });

    await service.processOrder(order);
    expect(callOrder).toEqual(['reserve', 'charge', 'schedule']);
  });
});

3. Contract Definition Through Mocks

// Define contracts for collaborators
const userServiceContract = {
  register: {
    input: { email: 'string', password: 'string' },
    output: { success: 'boolean', id: 'string' },
    collaborators: ['UserRepository', 'NotificationService'],
    interactions: [
      { method: 'findByEmail', args: ['email'], returns: 'null|User' },
      { method: 'save', args: ['User'], returns: 'User' },
      { method: 'sendWelcome', args: ['userId'], returns: 'boolean' }
    ]
  }
};

// Generate mocks from contract
function createMockFromContract(contract) {
  return Object.fromEntries(
    contract.interactions.map(i => [i.method, jest.fn()])
  );
}

Configuration

london_tdd_config:
  testing:
    framework: jest
    mock_library: jest  # or sinon, testdouble
    strict_mocks: true  # fail on unexpected calls

  coverage:
    interaction_coverage: true
    verify_all_mocks: true

  swarm_coordination:
    share_contracts: true
    sync_mock_definitions: true

  patterns:
    verify_call_order: true
    verify_call_count: true
    verify_call_args: true

Usage Examples

Example 1: Service Orchestration Test

describe('Service Collaboration', () => {
  let mockServiceA: jest.Mocked<ServiceA>;
  let mockServiceB: jest.Mocked<ServiceB>;
  let mockServiceC: jest.Mocked<ServiceC>;
  let orchestrator: ServiceOrchestrator;

  beforeEach(() => {
    mockServiceA = {
      prepare: jest.fn().mockResolvedValue({ data: 'prepared' })
    };
    mockServiceB = {
      process: jest.fn().mockResolvedValue({ result: 'processed' })
    };
    mockServiceC = {
      finalize: jest.fn().mockResolvedValue({ status: 'complete' })
    };

    orchestrator = new ServiceOrchestrator(
      mockServiceA,
      mockServiceB,
      mockServiceC
    );
  });

  it('should coordinate dependencies in correct order', async () => {
    await orchestrator.execute(task);

    // Verify coordination sequence
    expect(mockServiceA.prepare).toHaveBeenCalledBefore(mockServiceB.process);
    expect(mockServiceB.process).toHaveBeenCalledBefore(mockServiceC.finalize);

    // Verify data flow between services
    expect(mockServiceB.process).toHaveBeenCalledWith(
      expect.objectContaining({ data: 'prepared' })
    );
    expect(mockServiceC.finalize).toHaveBeenCalledWith(
      expect.objectContaining({ result: 'processed' })
    );
  });
});

Example 2: Error Handling Verification

describe('Error Handling', () => {
  it('should handle repository failure gracefully', async () => {
    const mockRepository = {
      save: jest.fn().mockRejectedValue(new Error('Connection failed'))
    };
    const mockLogger = {
      error: jest.fn()
    };
    const mockRetry = {
      attempt: jest.fn().mockResolvedValue(false)
    };

    const service = new UserService(mockRepository, mockLogger, mockRetry);

    await expect(service.register(userData)).rejects.toThrow('Registration failed');

    // Verify error handling interactions
    expect(mockLogger.error).toHaveBeenCalledWith(
      'Repository save failed',
      expect.objectContaining({ error: expect.any(Error) })
    );
    expect(mockRetry.attempt).toHaveBeenCalledTimes(3);
  });
});

Example 3: Swarm Coordination Testing

describe('Swarm Test Coordination', () => {
  let swarmCoordinator: SwarmCoordinator;

  beforeAll(async () => {
    // Signal other swarm agents
    await swarmCoordinator.notifyTestStart('unit-tests');
  });

  afterAll(async () => {
    // Share test results with swarm
    await swarmCoordinator.shareResults(testResults);
  });

  it('should share mock contracts across swarm', () => {
    const sharedMocks = {
      userRepository: createSwarmMock('UserRepository', {
        save: jest.fn(),
        findByEmail: jest.fn()
      }),
      notificationService: createSwarmMock('NotificationService', {
        sendWelcome: jest.fn()
      })
    };

    // Other swarm agents can verify against these contracts
    swarmCoordinator.publishContracts(sharedMocks);
  });
});

Execution Checklist

• Write failing acceptance test (outside)
• Define mock contracts for all collaborators
• Write failing unit test for next layer
• Implement minimal code to pass test
• Verify all mock interactions
• Refactor while keeping tests green
• Move to next inner layer
• Share contracts with swarm if coordinating

Best Practices

Mock Management

• Keep mocks simple and focused on single behavior
• Verify interactions, not implementation details
• Use jest.fn() for behavior verification
• Avoid over-mocking internal details
• Reset mocks between tests

Contract Design

• Define clear interfaces through mock expectations
• Focus on object responsibilities and collaborations
• Use mocks to DRIVE design decisions
• Keep contracts minimal and cohesive

Common Pitfalls

// BAD: Over-mocking internal details
const mock = {
  _internalState: {},
  _privateMethod: jest.fn()  // Don't mock private methods
};

// GOOD: Mock only public interface
const mock = {
  publicMethod: jest.fn().mockReturnValue(expectedResult)
};

// BAD: Verifying too many implementation details
expect(mock.method).toHaveBeenCalledTimes(3);  // Fragile

// GOOD: Verify essential behavior
expect(mock.method).toHaveBeenCalledWith(expectedArgs);

Error Handling

Missing Mock Verification

// Always verify mocks were called as expected
afterEach(() => {
  // Fail if any mock was called unexpectedly
  expect(unexpectedCallsDetected()).toBe(false);
});

// Use strict mocks
const strictMock = jest.fn().mockImplementation(() => {
  throw new Error('Unexpected call');
});

Mock Leakage Between Tests

// Always reset mocks
beforeEach(() => {
  jest.clearAllMocks();  // Clears call history
  // or
  jest.resetAllMocks();  // Also resets implementation
});

Metrics & Success Criteria

Integration Points

MCP Tools

// Store successful test patterns
mcp__claude-flow__memory_usage({
  action: "store",
  namespace: "test-patterns",
  key: "order_processing_mocks",
  value: JSON.stringify(mockDefinitions)
});

// Share contracts across swarm
mcp__claude-flow__memory_usage({
  action: "store",
  namespace: "test-contracts",
  key: "user_service_contract",
  value: JSON.stringify(userServiceContract)
});

Hooks

# Pre-test: Coordinate with swarm
npx claude-flow@alpha hooks pre-task --description "TDD London: $TEST_SUITE"

# Post-test: Share results
npx claude-flow@alpha hooks post-task --task-id "tdd-$SESSION_ID"

Related Skills

• testing-production - Production validation
• planning-code-goal - TDD integration in SPARC
• webapp-testing - Web application testing

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