Testing Anti Patterns

// BAD - Always passes because it tests nothing meaningful
it('should process data', () => {
  const result = processData(input);
  expect(result).toBeDefined(); // Too weak
});

// GOOD - Actually verifies behavior
it('should transform input to uppercase', () => {
  const result = processData({ text: 'hello' });
  expect(result.text).toBe('HELLO');
});

// BAD - Tests multiple things
it('should handle user registration', async () => {
  const user = await register(userData);
  expect(user.id).toBeDefined();
  expect(user.email).toBe(userData.email);
  expect(user.password).toBeUndefined();
  expect(sendEmail).toHaveBeenCalled();
  expect(createProfile).toHaveBeenCalled();
  // ... 20 more assertions
});

// GOOD - Focused tests
it('should create user with provided email', async () => {
  const user = await register(userData);
  expect(user.email).toBe(userData.email);
});

it('should send welcome email on registration', async () => {
  await register(userData);
  expect(sendEmail).toHaveBeenCalledWith(
    expect.objectContaining({ type: 'welcome' })
  );
});

// BAD - Tests internal implementation
it('should use QuickSort for sorting', () => {
  const sorter = new Sorter();
  const spy = jest.spyOn(sorter, '_quickSort');
  sorter.sort([3, 1, 2]);
  expect(spy).toHaveBeenCalled();
});

// GOOD - Tests behavior/output
it('should return sorted array', () => {
  const sorter = new Sorter();
  expect(sorter.sort([3, 1, 2])).toEqual([1, 2, 3]);
});

// BAD - Everything is mocked, test proves nothing
it('should calculate price', () => {
  const mockProduct = { getPrice: jest.fn().mockReturnValue(100) };
  const mockDiscount = { apply: jest.fn().mockReturnValue(80) };
  const mockTax = { calculate: jest.fn().mockReturnValue(8) };

  const total = calculateTotal(mockProduct, mockDiscount, mockTax);
  expect(total).toBe(88); // Just testing mock arithmetic
});

// GOOD - Use real objects where feasible
it('should apply 20% discount to price', () => {
  const product = new Product({ price: 100 });
  const discount = new PercentageDiscount(20);

  const total = calculateTotal(product, discount);
  expect(total).toBe(80);
});

// BAD - Depends on current time
it('should show recent items', () => {
  const item = { createdAt: new Date() };
  expect(isRecent(item)).toBe(true);
});

// GOOD - Control the time
it('should show items from last 24 hours', () => {
  const now = new Date('2024-01-15T12:00:00Z');
  jest.setSystemTime(now);

  const recent = { createdAt: new Date('2024-01-15T00:00:00Z') };
  const old = { createdAt: new Date('2024-01-13T00:00:00Z') };

  expect(isRecent(recent)).toBe(true);
  expect(isRecent(old)).toBe(false);
});

// BAD - Real network call
it('should fetch user data', async () => {
  const response = await fetch('https://api.example.com/users/1');
  const user = await response.json();
  expect(user.name).toBeDefined();
});

// GOOD - Mocked network
it('should parse user response', async () => {
  mockFetch.mockResolvedValue({
    json: () => Promise.resolve({ id: 1, name: 'Test User' })
  });

  const user = await fetchUser(1);
  expect(user.name).toBe('Test User');
});