Test Isolation: Why Your Tests Should Mind Their Own Business

// 😱 BAD: Module-level shared state
let database;
let currentUser;

test('create user', () => {
  currentUser = createUser('alice');
  database = new Database();
  database.save(currentUser);
});

test('update user', () => {
  // Depends on currentUser from previous test!
  currentUser.name = 'Alice Smith';
  database.update(currentUser); // Also depends on database!
});

// 😱 BAD: Tests must run in specific order
test('1. initialize system', () => {
  System.initialize();
  expect(System.isReady).toBe(true);
});

test('2. load configuration', () => {
  // Assumes system is already initialized
  System.loadConfig('./config.json');
  expect(System.config).toBeDefined();
});

test('3. start processing', () => {
  // Needs both initialization and config
  System.startProcessing();
  expect(System.isProcessing).toBe(true);
});

// 😱 BAD: Tests pollute shared resources
test('save user to database', async () => {
  await db.query('INSERT INTO users VALUES ("test@example.com")');
  const user = await db.query('SELECT * FROM users WHERE email = "test@example.com"');
  expect(user).toBeDefined();
});

test('count users', async () => {
  // Will fail if previous test didn't run or cleanup!
  const count = await db.query('SELECT COUNT(*) FROM users');
  expect(count).toBe(1); // Assumes previous test's data exists
});

// ✅ GOOD: Fresh state for each test
describe('User Management', () => {
  let database;
  let user;
  
  beforeEach(() => {
    // Fresh instances for each test
    database = new Database();
    user = createUser('alice');
  });
  
  afterEach(() => {
    // Clean up after each test
    database.close();
  });
  
  test('create user', () => {
    database.save(user);
    expect(database.find(user.id)).toEqual(user);
  });
  
  test('update user', () => {
    database.save(user); // Set up required state
    user.name = 'Alice Smith';
    database.update(user);
    expect(database.find(user.id).name).toBe('Alice Smith');
  });
});

// ✅ GOOD: Each test sets up everything it needs
test('process order with discount', () => {
  // Complete setup within the test
  const customer = new Customer({ 
    id: 'test-123',
    loyaltyTier: 'gold' 
  });
  
  const order = new Order({
    customer,
    items: [
      { id: 1, price: 100, quantity: 2 },
      { id: 2, price: 50, quantity: 1 }
    ]
  });
  
  const discount = new DiscountCalculator();
  const total = discount.calculate(order);
  
  expect(total).toBe(225); // 250 - 10% gold discount
});

// ✅ GOOD: Consistent test data creation
class UserFactory {
  static create(overrides = {}) {
    return {
      id: Math.random().toString(36),
      email: `test-${Date.now()}@example.com`,
      name: 'Test User',
      createdAt: new Date(),
      ...overrides
    };
  }
}

test('user can update profile', () => {
  const user = UserFactory.create({ name: 'Alice' });
  const service = new UserService();
  
  service.updateProfile(user.id, { name: 'Alice Smith' });
  
  expect(service.getUser(user.id).name).toBe('Alice Smith');
});

test('user can delete account', () => {
  const user = UserFactory.create(); // Independent user
  const service = new UserService();
  
  service.deleteAccount(user.id);
  
  expect(service.getUser(user.id)).toBeNull();
});