TDD (Test-Driven Development) Skill

Enforce the Red-Green-Refactor cycle for every feature.

The TDD Cycle

1. RED — Write a Failing Test

// Write the test BEFORE any implementation
describe('UserService', () => {
  it('should create a user with valid data', async () => {
    const result = await userService.create({
      email: 'test@example.com',
      name: 'Test User',
      password: 'SecurePass123!'
    });
    
    expect(result).toBeDefined();
    expect(result.id).toBeDefined();
    expect(result.email).toBe('test@example.com');
    expect(result.passwordHash).not.toBe('SecurePass123!');
  });

  it('should reject duplicate email', async () => {
    await userService.create({ email: 'dup@test.com', name: 'A', password: 'Pass123!' });
    await expect(
      userService.create({ email: 'dup@test.com', name: 'B', password: 'Pass123!' })
    ).rejects.toThrow('Email already exists');
  });
});

2. GREEN — Write Minimal Code to Pass

// Implement ONLY what's needed to make the test pass
// No extra features, no premature optimization
class UserService {
  async create(data: CreateUserDto): Promise<User> {
    const existing = await this.repo.findByEmail(data.email);
    if (existing) throw new Error('Email already exists');
    
    const passwordHash = await bcrypt.hash(data.password, 10);
    return this.repo.create({ ...data, passwordHash });
  }
}

3. REFACTOR — Improve Without Changing Behavior

// Clean up: extract helpers, improve naming, reduce duplication
// All tests must still pass after refactoring

Rules

• NEVER write implementation before the test
• NEVER write more code than needed to pass the current test
• ALWAYS run tests after each change
• ALWAYS refactor after green (even if small)
• One test at a time — don't batch

Test Categories Per Feature

1. Happy path — normal successful operation
2. Validation — invalid inputs, missing fields
3. Edge cases — empty data, max values, concurrent access
4. Error handling — network failure, DB error, timeout
5. Security — unauthorized access, injection attempts