Testing

Testing Skill

Testing Pyramid

        /\
       /  \        E2E Tests (Few)
      /----\
     /      \      Integration Tests (Some)
    /--------\
   /          \    Unit Tests (Many)
  /------------\

Test Types

Unit Tests

Test individual functions in isolation.

describe('validateEmail', () => {
  it('accepts valid email', () => {
    expect(validateEmail('user@example.com')).toBe(true);
  });

  it('rejects invalid email', () => {
    expect(validateEmail('invalid')).toBe(false);
  });
});

Integration Tests

Test module interactions.

describe('user registration', () => {
  it('creates user and sends welcome email', async () => {
    const user = await registerUser({ email: 'test@example.com' });
    expect(user.id).toBeDefined();
    expect(emailService.send).toHaveBeenCalled();
  });
});

E2E Tests

Test complete user flows.

test('user can complete checkout', async ({ page }) => {
  await page.goto('/products');
  await page.click('[data-testid="add-to-cart"]');
  await page.click('[data-testid="checkout"]');
  await page.fill('#email', 'test@example.com');
  await page.click('[data-testid="pay"]');
  await expect(page).toHaveURL('/confirmation');
});

Test Structure (AAA)

it('should do something', () => {
  // Arrange
  const input = { name: 'test' };
  
  // Act
  const result = processInput(input);
  
  // Assert
  expect(result.processed).toBe(true);
});

Mocking

Function Mocks

const mockFetch = jest.fn().mockResolvedValue({ data: [] });

Module Mocks

jest.mock('./database', () => ({
  query: jest.fn().mockResolvedValue([]),
}));

Time Mocks

jest.useFakeTimers();
jest.advanceTimersByTime(1000);

Coverage Targets

Test Commands

# Run all tests
npm test

# Run specific test file
npm test -- --testPathPattern=auth

# Run with coverage
npm test -- --coverage

# Watch mode
npm test -- --watch

# E2E tests
npm run test:e2e

Testing Best Practices

Do

• One assertion per test (when possible)
• Descriptive test names
• Test behavior, not implementation
• Use factories for test data
• Clean up after tests

Don't

• Test implementation details
• Make tests depend on each other
• Use magic numbers
• Test third-party code
• Ignore flaky tests

Test File Organization

src/
├── auth/
│   ├── login.ts
│   └── __tests__/
│       ├── login.test.ts
│       └── login.integration.test.ts
└── __tests__/
    └── e2e/
        └── auth.e2e.test.ts

Fixtures

// fixtures/users.ts
export const validUser = {
  id: '1',
  email: 'test@example.com',
  name: 'Test User',
};

export const invalidUser = {
  id: '',
  email: 'invalid',
};

Snapshot Testing

it('renders correctly', () => {
  const tree = render(<Button>Click me</Button>);
  expect(tree).toMatchSnapshot();
});

Update snapshots: npm test -- -u

Performance Testing

it('completes within 100ms', async () => {
  const start = performance.now();
  await heavyOperation();
  const duration = performance.now() - start;
  expect(duration).toBeLessThan(100);
});

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