testing-anti-patterns

Testing Anti-Patterns

Overview

Tests must verify real behavior, not mock behavior. Mocks are a means to isolate, not the thing being tested.

Core principle: Test what the code does, not what the mocks do.

These principles prevent these anti-patterns when applied consistently.

The Iron Laws

1. NEVER test mock behavior
2. NEVER add test-only methods to production classes
3. NEVER mock without understanding dependencies

Anti-Pattern 1: Testing Mock Behavior

The violation:

// ❌ BAD: Testing that the mock exists
test('renders sidebar', () => {
  render(<Page />);
  expect(screen.getByTestId('sidebar-mock')).toBeInTheDocument();
});

Why this is wrong:

• You're verifying the mock works, not that the component works
• Test passes when mock is present, fails when it's not
• Tells you nothing about real behavior

your human partner's correction: "Are we testing the behavior of a mock?"

The fix:

// ✅ GOOD: Test real component or don't mock it
test('renders sidebar', () => {
  render(<Page />);  // Don't mock sidebar
  expect(screen.getByRole('navigation')).toBeInTheDocument();
});

// OR if sidebar must be mocked for isolation:
// Don't assert on the mock - test Page's behavior with sidebar present

Gate Function

BEFORE asserting on any mock element:
  Ask: "Am I testing real component behavior or just mock existence?"

  IF testing mock existence:
    STOP - Delete the assertion or unmock the component

  Test real behavior instead

Anti-Pattern 2: Test-Only Methods in Production

The violation:

// ❌ BAD: destroy() only used in tests
class Session {
  async destroy() {
    // Looks like production API!
    await this._workspaceManager?.destroyWorkspace(this.id);
    // ... cleanup
  }
}

// In tests
afterEach(() => session.destroy());

Why this is wrong:

• Production class polluted with test-only code
• Dangerous if accidentally called in production
• Violates YAGNI and separation of concerns
• Confuses object lifecycle with entity lifecycle