testing-strategies

What this skill does

# 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

```typescript
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

```typescript
// 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');
  });
});
```

### Testing Edge Cases

```typescript
describe('parseAge', () => {
  // Happy path
  it('parses valid age string', () => {
    expect(parseAge('25')).toBe(25);
  });

  // Edge cases
  it('handles zero', () => {
    expect(parseAge('0')).toBe(0);
  });

  it('handles boundary values', () => {
    expect(parseAge('1')).toBe(1);
    expect(parseAge('150')).toBe(150);
  });

  // Error cases
  it('throws on negative numbers', () => {
    expect(() => parseAge('-5')).toThrow('Age cannot be negative');
  });

  it('throws on non-numeric input', () => {
    expect(() => parseAge('abc')).toThrow('Invalid age format');
  });

  it('throws on empty string', () => {
    expect(() => parseAge('')).toThrow('Age is required');
  });

  // Null/undefined
  it('throws on null', () => {
    expect(() => parseAge(null as any)).toThrow();
  });
});
```

---

## Integration Testing

### API Testing

```typescript
import request from 'supertest';
import { app } from '../app';
import { db } from '../database';

describe('POST /api/users', () => {
  beforeEach(async () => {
    await db.users.deleteMany({});
  });

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

  it('creates a new user', async () => {
    const response = await request(app)
      .post('/api/users')
      .send({
        email: '[email protected]',
        name: 'Test User'
      })
      .expect(201);

    expect(response.body).toMatchObject({
      id: expect.any(String),
      email: '[email protected]',
      name: 'Test User'
    });

    // Verify in database
    const user = await db.users.findOne({ email: '[email protected]' });
    expect(user).not.toBeNull();
  });

  it('returns 400 for invalid email', async () => {
    const response = await request(app)
      .post('/api/users')
      .send({
        email: 'invalid-email',
        name: 'Test User'
      })
      .expect(400);

    expect(response.body.error).toBe('Invalid email format');
  });

  it('returns 409 for duplicate email', async () => {
    // Create first user
    await request(app)
      .post('/api/users')
      .send({ email: '[email protected]', name: 'First' });

    // Try to create duplicate
    const response = await request(app)
      .post('/api/users')
      .send({ email: '[email protected]', name: 'Second' })
      .expect(409);

    expect(response.body.error).toBe('Email already exists');
  });
});
```

### Database Testing with Testcontainers

```typescript
import { PostgreSqlContainer } from '@testcontainers/postgresql';
import { Pool } from 'pg';

describe('UserRepository', () => {
  let container: StartedPostgreSqlContainer;
  let pool: Pool;
  let repo: UserRepository;

  beforeAll(async () => {
    container = await new PostgreSqlContainer().start();
    pool = new Pool({ connectionString: container.getConnectionUri() });
    await runMigrations(pool);
    repo = new UserRepository(pool);
  }, 60000);

  afterAll(async () => {
    await pool.end();
    await container.stop();
  });

  beforeEach(async () => {
    await pool.query('TRUNCATE users CASCADE');
  });

  it('creates and retrieves user', async () => {
    const created = await repo.create({
      email: '[email protected]',
      name: 'Test'
    });

    const found = await repo.findById(created.id);

    expect(found).toEqual(created);
  });
});
```

---

## E2E Testing

### Playwright

```typescript
import { test, expect } from '@playwright/test';

test.describe('User Authentication', () => {
  test('successful login flow', async ({ page }) => {
    await page.goto('/login');

    // Fill form
    await page.fill('[data-testid="email-input"]', '[email protected]');
    await page.fill('[data-testid="password-input"]', 'password123');

    // Submit
    await page.click('[data-testid="login-button"]');

    // Verify redirect to dashboard
    await expect(page).toHaveURL('/dashboard');
    await expect(page.locator('[data-testid="welcome-message"]'))
      .toContainText('Welcome, [email protected]');
  });

  test('shows error for invalid credentials', async ({ page }) => {
    await page.goto('/login');

    await page.fill('[data-testid="email-input"]', '[email protected]');
    await page.fill('[data-testid="password-input"]', 'wrongpassword');
    await page.click('[data-testid="login-button"]');

    await expect(page.locator('[data-testid="error-message"]'))
      .toBeVisible()
      .toContainText('Invalid credentials');
  });
});

test.describe('Shopping Cart', () => {
  test('add item and checkout', async ({ page }) => {
    // Setup - login
    await page.goto('/login');
    await page.fill('[data-testid="email-input"]', '[email protected]');
    await page.fill('[data-testid="password-input"]', 'password');
    await page.click('[data-testid="login-button"]');

    // Browse products
    await page.goto('/products');
    await page.click('[data-testid="product-1"] [data-testid="add-to-cart"]');

    // Verify cart
    await expect(page.locator('[data-testid="cart-count"]')).toHaveText('1');

    // Checkout
    await page.click('[data-testid="cart-icon"]');
    await page.click('[data-testid="checkout-button"]');

    // Fill shipping
    await page.fill('[data-testid="address"]', '123 Test St');
    await page.click('[data-testid="place-order"]');

    // Verify success
    await expect(page).toHaveURL(/\/orders\/\d+/);
    await expect(page.locator('[data-testid="order-status"]'))
      .toContainText('Order Confirmed');
  });
});
```

### Visual Regression Testing

```typescript
import { test, expect } from '@playwright/test';

test('homepage visual regression', async ({ page }) => {
  await page.goto('/');

  // Wait for dynamic content
  await page.waitForSelector('[data-testid="hero-section"]');

  // Tak