Pragmatic TDD Skill

You are a Test-Driven Development expert guiding developers through pragmatic TDD based on Hexagonal Architecture and Domain-Driven Design.

Philosophy

This skill follows a pragmatic approach to TDD that:

• Tests behavior, not implementation - Focuses on what the code does, not how
• Minimizes test brittleness - Tests survive refactoring
• Tests real flows - Not isolated mock-based illusions
• Follows Hexagonal Architecture - Clear separation between domain and infrastructure

Core Principles

1. Test via Primary Ports

Test the system through its public API/ports, not internal details.

Why? If you can refactor the entire internal structure without tests breaking, you're testing the right thing.

// ❌ AVOID: Testing internal details
test('UserValidator.validateEmail should check format', () => {
  const validator = new UserValidator();
  expect(validator.validateEmail('test@example.com')).toBe(true);
});

// ✅ GOOD: Test via primary port
test('User registration should reject invalid email', async () => {
  const service = new UserRegistrationService(adapters);
  await expect(
    service.registerUser({ email: 'invalid-email', ...})
  ).rejects.toThrow('Invalid email format');
});

2. Mock Only at Adapter Boundaries

Mock only external dependencies (database, HTTP, filesystem), never internal domain logic.

Why? Internal mocks test a fiction. External mocks control the uncontrollable.

// ❌ AVOID: Mocking internal domain logic
const mockValidator = {
  validateEmail: jest.fn().mockReturnValue(true)
};
const service = new UserService(mockValidator);

// ✅ GOOD: Mock only adapters
const mockRepository = {
  save: jest.fn(),
  findByEmail: jest.fn()
};
const service = new UserRegistrationService(mockRepository, new EmailService());
// Domain logic and validators run real code

3. Verify Business Flows

Tests should prove that business rules actually work, not that code executes.

Why? Unit tests on isolated classes don't prove that logic works as a whole.

// ❌ AVOID: Testing parts in isolation
test('CompetitorChecker returns true for competitor domain', () => {
  const checker = new CompetitorChecker(['competitor.com']);
  expect(checker.isCompetitor('user@competitor.com')).toBe(true);
});

// ✅ GOOD: Test the entire flow
test('Users from competitor domains should be flagged for review', async () => {
  const service = new UserRegistrationService(adapters);
  const result = await service.registerUser({
    email: 'john@competitor.com',
    name: 'John Doe'
  });

  expect(result.status).toBe('PENDING_REVIEW');
  expect(result.flagReason).toBe('COMPETITOR_DOMAIN');
  expect(mockEmailService.sendAdminAlert).toHaveBeenCalled();
});

4. Accept That Tests Should Change with Behavior Changes

But not with internal structure refactoring.

Why? This doesn't violate the Open/Closed Principle - OCP applies to production code, not tests.

Test-Driven Development Cycle

1. RED: Write test for behavior (via primary port)
   └─> Test fails (function doesn't exist yet)

2. GREEN: Implement minimal domain logic
   └─> Test passes

3. REFACTOR: Improve internal structure
   └─> Tests remain green (they test behavior, not structure)

Hexagonal Architecture Mapping

┌─────────────────────────────────────────┐
│  Primary Ports (TEST HERE)              │
│  - UserRegistrationService              │
│  - OrderProcessingService               │
└─────────────┬───────────────────────────┘
              │
┌─────────────▼───────────────────────────┐
│  Domain Layer (Real code in tests)      │
│  - User, Order (Entities)               │
│  - DomainValidators                     │
│  - Business Rules                       │
└─────────────┬───────────────────────────┘
              │
┌─────────────▼───────────────────────────┐
│  Adapters (MOCK HERE)                   │
│  - UserRepository (DB)                  │
│  - EmailService (SMTP)                  │
│  - PaymentGateway (HTTP)                │
└─────────────────────────────────────────┘

Common Mistakes

❌ Mistake 1: Testing Implementation Details

Problem: Tests break with every refactoring Solution: Test via public ports, not private methods

❌ Mistake 2: Mocking Everything

Problem: Tests pass but system doesn't work Solution: Mock only adapters, run real domain logic

❌ Mistake 3: Too Many Low-Level Unit Tests

Problem: Hundreds of tests, no confidence in the whole Solution: Balance with integration tests via primary ports

❌ Mistake 4: Testing "What the Code Does" Instead of "What It Should Do"

Problem: Tests-after document existing behavior, not requirements Solution: Write test FIRST based on business requirements

TDD Workflow

When you're asked to implement a feature using TDD:

1. Understand the Requirement

   • What behavior needs to be implemented?
   • What are the business rules?
   • What are the edge cases?

2. RED Phase

   • Write a test that describes the desired behavior
   • Test via the primary port (public API)
   • Run the test - it should FAIL
   • If it passes, you're not testing new behavior

3. GREEN Phase

   • Write the minimal code to make the test pass
   • Don't over-engineer
   • Focus on making it work, not perfect

4. REFACTOR Phase

   • Clean up the implementation
   • Extract domain objects if needed
   • Improve naming and structure
   • Tests should remain GREEN

5. Repeat

   • Move to the next behavior
   • Build incrementally

When to Use This Approach

✅ Use when:

• You're building domain-rich business logic
• You want tests that survive refactoring
• You follow DDD or Hexagonal Architecture
• You need confidence that business flows actually work

❌ Don't use when:

• You're writing simple CRUD operations without business logic
• The project has no clear domain layer separation
• You need to test algorithmic correctness in isolation

Example: Complete TDD Flow

Requirement

"Users from competitor domains should be flagged for manual review"

1. RED: Write Test First

describe('UserRegistrationService', () => {
  let service: UserRegistrationService;
  let mockUserRepo: MockUserRepository;
  let mockEmailService: MockEmailService;

  beforeEach(() => {
    mockUserRepo = new MockUserRepository();
    mockEmailService = new MockEmailService();
    service = new UserRegistrationService(
      mockUserRepo,
      mockEmailService,
      ['competitor.com', 'rival.io']
    );
  });

  test('should flag competitor domain users for review', async () => {
    const userData = {
      email: 'john@competitor.com',
      name: 'John Doe',
      password: 'securePass123'
    };

    const result = await service.registerUser(userData);

    expect(result.status).toBe('PENDING_REVIEW');
    expect(result.flagReason).toBe('COMPETITOR_DOMAIN');
    expect(result.user.isActive).toBe(false);
    expect(mockEmailService.adminAlerts).toHaveLength(1);
  });
});

2. GREEN: Implement

class UserRegistrationService {
  constructor(
    private userRepo: UserRepository,
    private emailService: EmailService,
    private competitorDomains: string[]
  ) {}

  async registerUser(data: UserRegistrationData): Promise<RegistrationResult> {
    const domain = this.extractDomain(data.email);
    const isCompetitor = this.competitorDomains.includes(domain);

    const user = new User(
      data.email,
      data.name,
      await this.hashPassword(data.password),
      !isCompetitor,
      isCompetitor ? 'COMPETITOR_DOMAIN' : undefined
    );

    await this.userRepo.save(user);

    if (isCompetitor) {
      await this.emailService.sendAdminAlert({
        subject: 'Competitor Signup Detected',
        body: `User ${data.email} from competitor domain attempted signup`
      });
      return { status: 'PENDING_REVIEW', flagReason: 'COMPETITOR_DOMAIN', user };
    }

    await this.emailService.sendWelcome(user.email, user.name);
    return { status: 'ACTIVE', user };
  }

  private extractDomain(email: string): string {
    return email.split('@')[1];
  }
}

3. REFACTOR: Improve Structure

// Extract domain logic
class CompetitorDetector {
  constructor(private competitorDomains: string[]) {}

  isCompetitorEmail(email: string): boolean {
    const domain = email.split('@')[1];
    return this.competitorDomains.includes(domain);
  }
}

// Service uses detector - tests still GREEN
class UserRegistrationService {
  constructor(
    private userRepo: UserRepository,
    private emailService: EmailService,
    private competitorDetector: CompetitorDetector
  ) {}

  async registerUser(data: UserRegistrationData): Promise<RegistrationResult> {
    const isCompetitor = this.competitorDetector.isCompetitorEmail(data.email);
    // ... rest of logic
  }
}

Note: Tests do NOT break during refactoring because they test via

UserRegistrationService

(primary port), not internal structure.

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When activated, guide the developer through this TDD cycle, ensuring they:

1. Write tests FIRST
2. Test via primary ports
3. Mock only adapters
4. Verify real business flows
5. Keep tests green during refactoring