testing-strategies

Testing Strategies

Overview

Testing pyramid, patterns, and practices for building reliable software.

---

Testing Pyramid

           /\
          /  \
         / E2E\        Few, slow, expensive
        /──────\
       /        \
      /Integration\    Some, medium speed
     /──────────────\
    /                \
   /    Unit Tests    \  Many, fast, cheap
  /____________________\

Level	Speed	Scope	Quantity
Unit	Fast (ms)	Single function/class	Many (70%)
Integration	Medium (s)	Multiple components	Some (20%)
E2E	Slow (min)	Full system	Few (10%)

---

Unit Testing

Structure: Arrange-Act-Assert

describe('calculateDiscount', () => {
  it('applies 10% discount for orders over $100', () => {
    // Arrange
    const order = { items: [{ price: 150 }] };
    const discountService = new DiscountService();

    // Act
    const result = discountService.calculateDiscount(order);

    // Assert
    expect(result).toBe(15);
  });

  it('returns 0 for orders under $100', () => {
    // Arrange
    const order = { items: [{ price: 50 }] };
    const discountService = new DiscountService();

    // Act
    const result = discountService.calculateDiscount(order);

    // Assert
    expect(result).toBe(0);
  });
});

Mocking

// Mock dependencies
const mockEmailService = {
  send: jest.fn().mockResolvedValue({ success: true })
};

const mockUserRepo = {
  findById: jest.fn().mockResolvedValue({ id: '1', email: '[email protected]' })
};

describe('NotificationService', () => {
  let service: NotificationService;

  beforeEach(() => {
    jest.clearAllMocks();
    service = new NotificationService(mockEmailService, mockUserRepo);
  });

  it('sends email to user', async () => {
    await service.notifyUser('1', 'Hello!');

    expect(mockUserRepo.findById).toHaveBeenCalledWith('1');
    expect(mockEmailService.send).toHaveBeenCalledWith(
      '[email protected]',
      'Hello!'
    );
  });

  it('throws when user not found', async () => {
    mockUserRepo.findById.mockResolvedValue(null);

    await expect(service.notifyUser('999', 'Hello!'))
      .rejects.toThrow('User not found');
  });
});