testing-strategy

🧪 Testing Strategy Skill

Philosophy: If it's not tested, it's broken. You just don't know it yet.

When to Use

Use this skill when:

• Writing new code (TDD approach)
• Adding tests to existing code
• Improving test coverage
• Fixing flaky tests
• Setting up testing infrastructure
• Debugging test failures

Do NOT use this skill when:

• Just running existing tests
• Quick syntax check

---

The Testing Pyramid

                 ╱╲
                ╱  ╲
               ╱ E2E╲           Few, slow, expensive
              ╱──────╲          Full system tests
             ╱        ╲
            ╱Integration╲       Medium amount
           ╱────────────╲       Component interaction
          ╱              ╲
         ╱   Unit Tests   ╲     Many, fast, cheap
        ╱──────────────────╲    Single unit isolation

---

TDD Workflow: RED → GREEN → REFACTOR

Step 1: RED (Write Failing Test)

// Write the test BEFORE the implementation
describe('calculateDiscount', () => {
  it('should apply 10% discount for orders over $100', () => {
    // This test will FAIL because function doesn't exist yet
    const result = calculateDiscount(150);
    expect(result).toBe(135);
  });
});

Run test → Should FAIL (RED)

Step 2: GREEN (Minimal Implementation)

// Write the MINIMUM code to pass the test
function calculateDiscount(amount: number): number {
  if (amount > 100) {
    return amount * 0.9;
  }
  return amount;
}

Run test → Should PASS (GREEN)

Step 3: REFACTOR (Improve)

// Improve code while keeping tests green
const DISCOUNT_THRESHOLD = 100;
const DISCOUNT_RATE = 0.1;

function calculateDiscount(amount: number): number {
  if (amount > DISCOUNT_THRESHOLD) {
    return amount * (1 - DISCOUNT_RATE);
  }
  return amount;
}

Run test → Should still PASS

---

Unit Testing Patterns

Basic Structure (AAA Pattern)

describe('UserService', () => {
  describe('createUser', () => {
    it('should create user with valid data', async () => {
      // Arrange
      const userData = { email: '[email protected]', name: 'Test' };
      const mockRepo = { create: jest.fn().mockResolvedValue({ id: '1', ...userData }) };
      const service = new UserService(mockRepo);
      
      // Act
      const result = await service.createUser(userData);
      
      // Assert
      expect(result.id).toBe('1');
      expect(result.email).toBe(userData.email);
      expect(mockRepo.create).toHaveBeenCalledWith(userData);
    });
  });
});