Avoiding Testing Anti-Patterns

Quick Start

Identify - Recognize anti-pattern category (flaky, implementation, over-mocking)
Assess Severity - Critical (fix now), High (fix soon), Medium (plan to fix)
Apply Fix - Use proper async handling, test behavior not implementation
Verify - Run tests in random order, ensure independence
Prevent - Add test smell detection to CI

Features

Feature	Description	Guide
Flaky Tests	Random failures destroying trust	Use waitFor, not sleep; deterministic data
Implementation Testing	Breaks on every refactor	Test behavior through public interface
Over-Mocking	Tests pass but bugs slip through	Mock boundaries only, not your own code
Slow Tests	Hurt development velocity	Right test layer, shared setup, mock network
Test Interdependence	Can't run tests in isolation	Fresh state in beforeEach, no shared mutation
Poor Design	Hard to understand/maintain	Descriptive names, focused tests, clear values

Common Patterns

// FLAKY: Timing-dependent
await sleep(100);  // May not be enough
expect(result).toBe('processed');

// FIXED: Wait for condition
await waitFor(() => {
  expect(result).toBe('processed');
}, { timeout: 5000 });

// FLAKY: Shared state between tests
let sharedState = [];
it('test1', () => { sharedState.push('a'); });
it('test2', () => { expect(sharedState).toHaveLength(1); }); // Order-dependent!

// FIXED: Fresh state each test
beforeEach(() => { sharedState = []; });

// IMPLEMENTATION: Testing private state
expect(counter._count).toBe(1);

// BEHAVIOR: Testing public interface
expect(counter.getValue()).toBe(1);

// OVER-MOCKING: Everything mocked
const mockDb = { save: jest.fn() };
const mockPayment = { charge: jest.fn() };
// Only testing that mocks were called

// FIXED: Mock boundaries only
const testDb = await createTestDatabase();  // Real
const mockPayment = createMockPaymentProvider();  // External only

# Anti-Pattern Severity Guide
CRITICAL (fix immediately):
- Flaky tests - Random failures destroy trust
- Testing implementation - Breaks on every refactor
- Hidden dependencies - Tests fail mysteriously

HIGH (fix soon):
- Slow tests - Hurt development velocity
- Test interdependence - Can't run in isolation
- Over-mocking - Tests pass but bugs slip through

MEDIUM (plan to fix):
- Poor naming - Tests don't document behavior
- Magic values - Unclear expected values
- Giant tests - Hard to understand

LOW (fix when touching):
- Commented tests - Remove or fix
- Duplicate tests - Consolidate

Best Practices

Do	Avoid
Test behavior, not implementation	Accessing private properties
Use factories for test data	Random/inconsistent test data
Write descriptive test names	Generic names like "test1", "should work"
Keep tests independent	Shared mutable state between tests
Mock only external boundaries	Mocking your own code extensively
Use waitFor, not sleep	setTimeout/sleep in tests
Make assertions specific	toBeDefined() for everything
Run tests in random order	Assuming test execution order

Related Skills

developing-test-driven - TDD with proper patterns
testing-with-vitest - Vitest testing framework
testing-with-playwright - E2E testing patterns
debugging-systematically - Debug flaky test failures

avoiding-testing-anti-patterns

Avoiding Testing Anti-Patterns

Quick Start

Features

Common Patterns

Best Practices

Related Skills