Testing Anti-Patterns

# Testing Anti-Patterns **Load this reference when:** writing or changing tests, adding mocks, or tempted to add test-only methods to production code. ## 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. **Following strict TDD prevents these anti-patterns.** ## 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:** ```typescript // ❌ 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 par