SDD Test Generation

Generate comprehensive tests following Test-Driven Development (TDD) methodology. Write tests that serve as living documentation and ensure code correctness.

TDD Philosophy

"Write a failing test before you write the code to make it pass."

Tests are not an afterthought—they're a design tool that:

1. Document behavior - Tests show how code is intended to be used
2. Prevent regressions - Catch bugs before they ship
3. Enable refactoring - Change with confidence
4. Drive design - Writing tests first leads to better interfaces

The TDD Cycle

┌─────────────────────────────────────┐
│                                     │
│   ┌─────────┐   Write failing test  │
│   │   RED   │◄──────────────────────┤
│   └────┬────┘                       │
│        │                            │
│        ▼ Make it pass               │
│   ┌─────────┐                       │
│   │  GREEN  │                       │
│   └────┬────┘                       │
│        │                            │
│        ▼ Improve code               │
│   ┌─────────┐                       │
│   │REFACTOR │───────────────────────┘
│   └─────────┘
└─────────────────────────────────────┘

Workflow

Step 1: Identify Test Scope

/sdd-test-gen src/services/UserService.ts           # Generate tests for file
/sdd-test-gen UserService.createUser                # Generate for specific method
/sdd-test-gen src/services/ --integration           # Integration tests for module

Step 2: Analyze the Code

Before generating tests:

1. Read the source file to understand its behavior
2. Check existing tests (if any) to avoid duplication
3. Review related requirements in .spec/specs/
4. Identify dependencies that need mocking

Step 3: Test File Structure

Generate tests with this structure:

import { UserService } from '../UserService';
import { UserRepository } from '../../repositories/UserRepository';
import { EmailService } from '../../services/EmailService';

// Mock dependencies
jest.mock('../../repositories/UserRepository');
jest.mock('../../services/EmailService');

describe('UserService', () => {
  let userService: UserService;
  let mockUserRepo: jest.Mocked<UserRepository>;
  let mockEmailService: jest.Mocked<EmailService>;

  beforeEach(() => {
    jest.clearAllMocks();
    mockUserRepo = new UserRepository() as jest.Mocked<UserRepository>;
    mockEmailService = new EmailService() as jest.Mocked<EmailService>;
    userService = new UserService(mockUserRepo, mockEmailService);
  });

  describe('createUser', () => {
    it('should create a user with valid input', async () => {
      // Arrange
      const input = { email: '[email protected]', name: 'Test User' };
      mockUserRepo.save.mockResolvedValue({ id: '1', ...input });

      // Act
      const result = await userService.createUser(input);

      // Assert
      expect(result.id).toBeDefined();
      expect(mockUserRepo.save).toHaveBeenCalledWith(expect.objectContaining(input));
    });

    it('should throw error when email already exists', async () => {
      // Arrange
      mockUserRepo.findByEmail.mockResolvedValue({ id: '1', email: '[email protected]' });

      // Act & Assert
      await expect(userService.createUser({ email: '[email protected]' }))
        .rejects.toThrow('Email already exists');
    });
  });
});

Step 4: Test Categories to Generate

Unit Tests

Test individual functions in isolation:

• Mock all external dependencies
• Test one behavior per test
• Use descriptive test names

describe('calculateTotal', () => {
  it('should sum all item prices', () => { ... });
  it('should apply discount when provided', () => { ... });
  it('should handle empty cart', () => { ... });
  it('should throw error for negative quantities', () => { ... });
});

Edge Case Tests

Always test:

• Null/undefined inputs
• Empty arrays/objects
• Boundary values (0, -1, MAX_INT)
• Invalid types
• Concurrent access (if applicable)

describe('edge cases', () => {
  it('should handle null input gracefully', () => { ... });
  it('should handle empty array', () => { ... });
  it('should handle maximum allowed length', () => { ... });
  it('should reject invalid email format', () => { ... });
});

Error Handling Tests

Verify error paths:

describe('error handling', () => {
  it('should throw ValidationError for invalid input', async () => {
    await expect(service.create({})).rejects.toThrow(ValidationError);
  });

  it('should propagate database errors', async () => {
    mockRepo.save.mockRejectedValue(new DatabaseError('Connection failed'));
    await expect(service.create(validInput)).rejects.toThrow(DatabaseError);
  });
});

Integration Tests (when requested)

Test component interactions:

describe('UserService integration', () => {
  let app: Express;
  let db: Database;

  beforeAll(async () => {
    db = await createTestDatabase();
    app = createApp({ database: db });
  });

  afterAll(async () => {
    await db.close();
  });

  it('should create user and send welcome email', async () => {
    const response = await request(app)
      .post('/api/users')
      .send({ email: '[email protected]', name: 'Test' });

    expect(response.status).toBe(201);
    // Verify email was queued
    expect(await getEmailQueue()).toContainEqual(
      expect.objectContaining({ to: '[email protected]' })
    );
  });
});

Step 5: Test Naming Convention

Use the pattern: should [expected behavior] when [condition]

// Good
it('should return empty array when no users match criteria', () => {});
it('should throw AuthError when token is expired', () => {});
it('should create user and return ID when input is valid', () => {});

// Bad
it('test createUser', () => {});
it('works correctly', () => {});

Test Quality Checklist

For each generated test:

• Test name describes behavior, not implementation
• Arrange-Act-Assert pattern used
• Only one assertion concept per test
• Tests are independent (no shared state)
• Mocks are minimal (only external deps)
• Edge cases covered
• Error paths tested
• Async tests properly awaited

Integration with SDD Workflow

When generating tests for a spec:

1. Read requirements from .spec/specs/{feature}/requirements.md
2. Map each acceptance criterion to test cases
3. Check design.md for expected interfaces
4. Reference tasks.md for implementation details

Test Generation Prompt

When running /sdd-test-gen:

## Test Generation Summary

### Target: {file/function}

### Tests Generated:
- **Unit Tests**: {count}
- **Edge Cases**: {count}
- **Error Handling**: {count}
- **Integration**: {count} (if requested)

### Coverage Targets:
- Statements: 80%+
- Branches: 75%+
- Functions: 90%+
- Lines: 80%+

### Files Created:
- `src/__tests__/unit/{file}.test.ts`
- `src/__tests__/integration/{file}.integration.test.ts` (if requested)

Framework Detection

Automatically detect and use project's test framework:

• Jest (default for TypeScript/JavaScript)
• Vitest (if vite.config.ts present)
• Mocha/Chai (if mocha in dependencies)
• Pytest (for Python projects)

Example Output

For input: /sdd-test-gen src/services/AuthService.ts

import { AuthService } from '../AuthService';
import { TokenService } from '../../utils/TokenService';
import { UserRepository } from '../../repositories/UserRepository';

jest.mock('../../utils/TokenService');
jest.mock('../../repositories/UserRepository');

describe('AuthService', () => {
  // ... setup ...

  describe('login', () => {
    it('should return token when credentials are valid', async () => { ... });
    it('should throw AuthError when user not found', async () => { ... });
    it('should throw AuthError when password is incorrect', async () => { ... });
    it('should increment failed login count on failure', async () => { ... });
    it('should lock account after 5 failed attempts', async () => { ... });
  });

  describe('logout', () => {
    it('should invalidate token when called', async () => { ... });
    it('should handle already-logged-out user gracefully', async () => { ... });
  });

  describe('refreshToken', () => {
    it('should return new token when refresh token is valid', async () => { ... });
    it('should throw AuthError when refresh token is expired', async () => { ... });
  });
});