/\
      /E2E\          ← few (slow, expensive)
     /______\
    /        \
   /Integration\    ← medium
  /____________\
 /              \
/   Unit Tests   \  ← many (fast, inexpensive)
/________________\

describe('calculateDiscount', () => {
  it('should apply 10% discount for orders over $100', () => {
    // Given: setup
    const order = { total: 150, customerId: '123' };

    // When: perform action
    const discount = calculateDiscount(order);

    // Then: verify result
    expect(discount).toBe(15);
  });

  it('should not apply discount for orders under $100', () => {
    const order = { total: 50, customerId: '123' };
    const discount = calculateDiscount(order);
    expect(discount).toBe(0);
  });

  it('should throw error for invalid order', () => {
    const order = { total: -10, customerId: '123' };
    expect(() => calculateDiscount(order)).toThrow('Invalid order');
  });
});

// Mock external dependencies
jest.mock('../services/emailService');
import { sendEmail } from '../services/emailService';

describe('UserService', () => {
  it('should send welcome email on registration', async () => {
    // Arrange
    const mockSendEmail = sendEmail as jest.MockedFunction<typeof sendEmail>;
    mockSendEmail.mockResolvedValueOnce(true);

    // Act
    await userService.register({ email: 'test@example.com', password: 'pass' });

    // Assert
    expect(mockSendEmail).toHaveBeenCalledWith({
      to: 'test@example.com',
      subject: 'Welcome!',
      body: expect.any(String)
    });
  });
});

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

  it('should create user with valid data', async () => {
    const response = await request(app)
      .post('/api/users')
      .send({
        email: 'test@example.com',
        username: 'testuser',
        password: 'Password123!'
      });

    expect(response.status).toBe(201);
    expect(response.body.user).toMatchObject({
      email: 'test@example.com',
      username: 'testuser'
    });

    // Verify it was actually saved to the DB
    const user = await db.user.findUnique({ where: { email: 'test@example.com' } });
    expect(user).toBeTruthy();
  });

  it('should reject duplicate email', async () => {
    // Create first user
    await request(app)
      .post('/api/users')
      .send({ email: 'test@example.com', username: 'user1', password: 'Pass123!' });

    // Attempt duplicate
    const response = await request(app)
      .post('/api/users')
      .send({ email: 'test@example.com', username: 'user2', password: 'Pass123!' });

    expect(response.status).toBe(409);
  });
});

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

test.describe('User Registration Flow', () => {
  test('should complete full registration process', async ({ page }) => {
    // 1. Visit homepage
    await page.goto('http://localhost:3000');

    // 2. Click Sign Up button
    await page.click('text=Sign Up');

    // 3. Fill out form
    await page.fill('input[name="email"]', 'test@example.com');
    await page.fill('input[name="username"]', 'testuser');
    await page.fill('input[name="password"]', 'Password123!');

    // 4. Submit
    await page.click('button[type="submit"]');

    // 5. Confirm success message
    await expect(page.locator('text=Welcome')).toBeVisible();

    // 6. Confirm redirect to dashboard
    await expect(page).toHaveURL('http://localhost:3000/dashboard');

    // 7. Confirm user info is displayed
    await expect(page.locator('text=testuser')).toBeVisible();
  });

  test('should show error for invalid email', async ({ page }) => {
    await page.goto('http://localhost:3000/signup');
    await page.fill('input[name="email"]', 'invalid-email');
    await page.fill('input[name="password"]', 'Password123!');
    await page.click('button[type="submit"]');

    await expect(page.locator('text=Invalid email')).toBeVisible();
  });
});

// 1. RED: write a failing test
describe('isPalindrome', () => {
  it('should return true for palindrome', () => {
    expect(isPalindrome('racecar')).toBe(true);
  });
});

// 2. GREEN: minimal code to pass the test
function isPalindrome(str: string): boolean {
  return str === str.split('').reverse().join('');
}

// 3. REFACTOR: improve the code
function isPalindrome(str: string): boolean {
  const cleaned = str.toLowerCase().replace(/[^a-z0-9]/g, '');
  return cleaned === cleaned.split('').reverse().join('');
}

// 4. Additional test cases
it('should ignore case and spaces', () => {
  expect(isPalindrome('A man a plan a canal Panama')).toBe(true);
});

it('should return false for non-palindrome', () => {
  expect(isPalindrome('hello')).toBe(false);
});

## Testing Strategy

### Coverage Goals
- Unit Tests: 80%
- Integration Tests: 60%
- E2E Tests: Critical user flows

### Test Execution
- Unit: Every commit (local + CI)
- Integration: Every PR
- E2E: Before deployment

### Tools
- Unit: Jest
- Integration: Supertest
- E2E: Playwright
- Coverage: Istanbul/nyc

### CI/CD Integration
- GitHub Actions: Run all tests on PR
- Fail build if coverage < 80%
- E2E tests on staging environment