Test-Driven Development Workflow

Enforces test-driven development principles with comprehensive test coverage across all layers.

When to Activate

• Writing new features or functionality
• Fixing bugs or issues
• Refactoring existing code
• Adding API endpoints
• Creating new components
• Implementing business logic

Core Principles

1. Tests BEFORE Code

Always write tests first, then implement code to make tests pass. No exceptions.

2. Coverage Requirements

• Minimum 80% coverage (unit + integration + E2E)
• All edge cases covered
• Error scenarios tested
• Boundary conditions verified
• Happy path and sad path

3. Test Pyramid

        ┌────────┐
        │   E2E  │  (10-20%)
        ├────────┤
        │  INTEG │  (20-30%)
        ├────────┤
        │  UNIT  │  (50-70%)
        └────────┘

TDD Workflow: Red-Green-Refactor

Step 1: RED - Write Failing Test

Start with the user story:

As a [role], I want to [action], so that [benefit]

Example:
As a user, I want to search for products by category,
so that I can find relevant items quickly.

Write the test:

describe('ProductSearch', () => {
  it('returns products filtered by category', async () => {
    const products = await searchProducts({ category: 'electronics' });
    
    expect(products).toHaveLength(5);
    expect(products.every(p => p.category === 'electronics')).toBe(true);
  });
});

Step 2: Run Tests (They Should Fail)

npm test
# ✗ ProductSearch > returns products filtered by category
#   TypeError: searchProducts is not a function

This is good! Red phase confirms test is working.

Step 3: GREEN - Implement Minimal Code

Write just enough code to pass:

export async function searchProducts(filters: SearchFilters) {
  const { category } = filters;
  return db.products.findMany({
    where: { category }
  });
}

Step 4: Run Tests (They Should Pass)

npm test
# ✓ ProductSearch > returns products filtered by category (42ms)

Step 5: REFACTOR - Improve Code

Now refactor while keeping tests green:

export async function searchProducts(filters: SearchFilters) {
  const query = buildSearchQuery(filters);
  const results = await executeSearch(query);
  return transformResults(results);
}

Run tests again to ensure they still pass.

Test Types

Unit Tests

Purpose: Test individual functions in isolation

describe('calculateDiscount', () => {
  it('applies 10% discount for basic tier', () => {
    const price = calculateDiscount(100, 'basic');
    expect(price).toBe(90);
  });

  it('applies 20% discount for premium tier', () => {
    const price = calculateDiscount(100, 'premium');
    expect(price).toBe(80);
  });

  it('throws error for invalid tier', () => {
    expect(() => calculateDiscount(100, 'invalid')).toThrow();
  });
});

Integration Tests

Purpose: Test multiple components working together

describe('User Registration API', () => {
  it('creates user and sends welcome email', async () => {
    const response = await request(app)
      .post('/api/register')
      .send({ email: 'test@example.com', password: 'secret' });  // allow-secret

    expect(response.status).toBe(201);
    expect(response.body.user.email).toBe('test@example.com');
    
    // Verify email was sent
    const sentEmails = await testEmailService.getSentEmails();
    expect(sentEmails).toHaveLength(1);
    expect(sentEmails[0].to).toBe('test@example.com');
  });
});

E2E Tests (Playwright/Cypress)

Purpose: Test complete user flows through the UI

test('user can complete checkout process', async ({ page }) => {
  await page.goto('/products');
  await page.click('[data-testid="add-to-cart"]');
  await page.click('[data-testid="cart"]');
  await page.click('[data-testid="checkout"]');
  await page.fill('[name="cardNumber"]', '4242424242424242');
  await page.click('[data-testid="complete-order"]');

  await expect(page.locator('.success-message')).toBeVisible();
  await expect(page).toHaveURL(/\/order-confirmation/);
});

Coverage Verification

After implementation, check coverage:

npm test -- --coverage

# Output:
# File          | % Stmts | % Branch | % Funcs | % Lines
# --------------|---------|----------|---------|--------
# All files     |   84.2  |   78.5   |   91.3  |   85.1

Requirements:

• Statements: > 80%
• Branches: > 75%
• Functions: > 85%
• Lines: > 80%

Edge Cases Checklist

Always test:

• Empty input
• Null/undefined values
• Large datasets
• Concurrent operations
• Network failures
• Database errors
• Invalid data types
• Boundary values (0, -1, MAX_INT)
• Unauthorized access
• Rate limiting

Mocking Strategy

When to Mock

• External APIs
• Database calls (in unit tests)
• Time-dependent functions
• File system operations
• Third-party services

Example Mocks

// Mock external service
vi.mock('./emailService', () => ({
  sendEmail: vi.fn().mockResolvedValue({ success: true })
}));

// Mock database
vi.mock('./db', () => ({
  users: {
    findUnique: vi.fn().mockResolvedValue({ id: 1, name: 'Test' })
  }
}));

// Mock Date.now()
vi.spyOn(Date, 'now').mockReturnValue(1234567890000);

Test Organization

src/
  features/
    products/
      product.service.ts
      product.service.test.ts      # Unit tests
      product.integration.test.ts  # Integration tests
      product.e2e.test.ts          # E2E tests

Debugging Failed Tests

# Run single test file
npm test -- product.service.test.ts

# Run single test
npm test -- -t "calculates discount correctly"

# Run in watch mode
npm test -- --watch

# Run with coverage
npm test -- --coverage --no-cache

Integration Points

Complements:

• verification-loop: For pre-commit verification
• testing-patterns: For test design patterns
• deployment-cicd: For CI test execution
• security-implementation-guide: For security testing

Workflow Summary

1. RED: Write failing test
2. GREEN: Implement minimal code
3. REFACTOR: Improve while keeping tests green
4. VERIFY: Check coverage meets requirements
5. COMMIT: Only commit if all tests pass

Never skip steps. Never commit untested code.

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Related Skills

Complementary Skills (Use Together)

• testing-patterns - Comprehensive test patterns for unit, integration, and E2E tests
• verification-loop - Pre-commit verification workflow that validates all quality gates
• deployment-cicd - CI pipeline setup for automated test execution

Alternative Skills (Similar Purpose)

• None - TDD is a specific methodology that complements other testing approaches

Prerequisite Skills (Learn First)

• testing-patterns - Understanding test types and frameworks helps with TDD