TDD Workflow Skill

Overview

Test-Driven Development methodology for Node.js/TypeScript projects.

The RED-GREEN-REFACTOR Cycle

RED Phase: Design Failing Tests

Write tests BEFORE implementation:

1. Identify Behavior: What should the code do?
2. Design Test Cases: Cover all scenarios
3. Write Tests: Use AAA pattern
4. Run Tests: Confirm they FAIL
5. Verify Failure: Tests fail for the RIGHT reason

GREEN Phase: Minimal Implementation

Make tests pass with minimal code:

1. Focus: One failing test at a time
2. Implement: Just enough to pass
3. Verify: Run tests, confirm GREEN
4. Iterate: Next failing test
5. Complete: All tests passing

REFACTOR Phase: Improve Design

Improve code while keeping tests green:

1. Review: Identify code smells
2. Plan: Choose refactoring
3. Apply: Make the change
4. Verify: Tests still GREEN
5. Repeat: Until quality gates met

AAA Pattern

describe('Calculator', () => {
  it('should add two numbers correctly', () => {
    // Arrange - Set up test conditions
    const calculator = createCalculator();

    // Act - Execute the behavior
    const result = calculator.add(2, 3);

    // Assert - Verify the outcome
    expect(result).toBe(5);
  });
});

Test Naming Convention

Format:

should {expectedBehavior} when {scenario}

Examples:

it('should return empty array when input is empty', ...);
it('should throw ValidationError when email is invalid', ...);
it('should emit event when state changes', ...);

Test Categories

Unit Tests

• Test pure functions and logic
• No I/O, no side effects
• Fast execution
• High isolation

describe('validateEmail', () => {
  it('should return true for valid email', () => {
    expect(validateEmail('user@example.com')).toBe(true);
  });
});

Integration Tests

• Test module boundaries
• Include I/O operations
• Test with real (or fake) dependencies

describe('UserService', () => {
  it('should persist user to database', async () => {
    const db = createTestDatabase();
    const service = createUserService({ db });

    await service.createUser({ email: 'test@example.com' });

    const user = await db.users.findFirst();
    expect(user.email).toBe('test@example.com');
  });
});

Contract Tests

• Verify API contracts
• Type safety at boundaries
• Response shape validation

describe('API Contract', () => {
  it('should return user with expected shape', async () => {
    const response = await api.getUser('1');

    expect(response).toMatchObject({
      id: expect.any(String),
      email: expect.any(String),
      createdAt: expect.any(Date),
    });
  });
});

Test Doubles

Stub

Returns canned data:

const stubApi = {
  getUser: () => Promise.resolve({ id: '1', name: 'Test' }),
};

Mock

Verifies interactions:

const mockLogger = {
  info: jest.fn(),
  error: jest.fn(),
};
// Later: expect(mockLogger.info).toHaveBeenCalledWith('message');

Fake

Working implementation:

const createFakeDatabase = () => {
  const store = new Map();
  return {
    save: (entity) => store.set(entity.id, entity),
    findById: (id) => store.get(id),
  };
};

Spy

Records calls:

const spy = jest.spyOn(service, 'notify');
await service.process();
expect(spy).toHaveBeenCalledTimes(1);

Test Organization

src/
  services/
    user-service.ts
    user-service.test.ts      # Co-located unit tests
  api/
    handlers.ts
    handlers.test.ts
tests/
  integration/                 # Integration tests
    user-flow.test.ts
  fixtures/                    # Shared test data
    users.ts
  helpers/                     # Test utilities
    test-context.ts

Anti-Patterns

Testing Implementation Details

// Bad - testing internal state
expect(service._cache.size).toBe(1);

// Good - testing behavior
expect(service.getCachedValue('key')).toBe('value');

Overly Specific Assertions

// Bad - brittle
expect(result).toEqual({
  id: '123',
  name: 'Test',
  createdAt: new Date('2024-01-01'),
  updatedAt: new Date('2024-01-01'),
});

// Good - flexible
expect(result).toMatchObject({
  id: expect.any(String),
  name: 'Test',
});

Test Interdependence

// Bad - tests depend on order
let user;
it('should create user', () => { user = createUser(); });
it('should update user', () => { updateUser(user); }); // Depends on previous

// Good - independent tests
it('should update user', () => {
  const user = createUser();
  updateUser(user);
});