Test Quality Analysis

Expert knowledge for analyzing and improving test quality - detecting test smells, overmocking, insufficient coverage, and other testing anti-patterns.

When to Use This Skill

Use this skill when...	Use another skill instead when...
Reviewing test quality and smells	Writing new unit tests (use vitest-testing)
Detecting overmocking or flaky tests	Setting up E2E tests (use playwright-testing)
Analyzing test coverage gaps	Validating tests via mutation (use mutation-testing)
Improving test maintainability	Generating test data (use property-based-testing)
Auditing test suites for anti-patterns	Writing Python tests (use python-testing)

Core Expertise

Test Quality Dimensions

Correctness: Tests verify the right behavior
Reliability: Tests are deterministic and not flaky
Maintainability: Tests are easy to understand and modify
Performance: Tests run quickly
Coverage: Tests cover critical code paths
Isolation: Tests don't depend on external state

Test Smells - Quick Reference

Smell	Symptom	Fix
Overmocking	3+ mocks per test; mocking pure functions	Mock only I/O boundaries; use real implementations
Fragile tests	Break on refactor without behavior change	Test public APIs; use semantic selectors
Flaky tests	Non-deterministic pass/fail	Proper async/await; mock time; ensure isolation
Test duplication	Copy-pasted setup across tests	Extract to `beforeEach()`, fixtures, helpers
Slow tests	Suite > 10s for unit tests	`beforeAll()` for expensive setup; parallelize
Poor assertions	`toBeDefined()`, no assertions, mock-only assertions	Specific matchers; assert outputs and state
Insufficient coverage	Critical paths untested	80%+ on business logic; test error paths and boundaries

Analysis Tools

TypeScript/JavaScript

vitest --coverage                              # Coverage report
vitest --coverage --coverage.thresholds.lines=80  # Threshold check
vitest --reporter=verbose                      # Find slow tests

Python

uv run pytest --cov --cov-report=term-missing  # Coverage with missing lines
uv run pytest --cov --cov-fail-under=80        # Threshold check
uv run pytest --durations=10                   # Find slow tests

Key Anti-Patterns

Testing Implementation vs Behavior

// BAD: Testing how
test('uses correct algorithm', () => {
  const spy = vi.spyOn(Math, 'sqrt')
  calculateDistance({ x: 0, y: 0 }, { x: 3, y: 4 })
  expect(spy).toHaveBeenCalled()
})

// GOOD: Testing what
test('calculates distance correctly', () => {
  const distance = calculateDistance({ x: 0, y: 0 }, { x: 3, y: 4 })
  expect(distance).toBe(5)
})

Weak Assertions

// BAD
expect(users).toBeDefined()     // Too vague
expect(mockAPI).toHaveBeenCalled() // Testing mock, not behavior

// GOOD
expect(user).toMatchObject({
  id: expect.any(Number),
  name: 'John',
  email: 'john@example.com',
})

Missing Coverage

// BAD: Only tests happy path
test('applies discount', () => {
  expect(calculateDiscount(100, 'SAVE20')).toBe(80)
})

// GOOD: Tests all paths
describe('calculateDiscount', () => {
  it('applies SAVE20', () => expect(calculateDiscount(100, 'SAVE20')).toBe(80))
  it('applies SAVE50', () => expect(calculateDiscount(100, 'SAVE50')).toBe(50))
  it('invalid coupon', () => expect(calculateDiscount(100, 'INVALID')).toBe(100))
  it('no coupon', () => expect(calculateDiscount(100)).toBe(100))
})

Test Structure (AAA Pattern)

test('user registration flow', async () => {
  // Arrange
  const userData = { email: 'user@example.com', password: 'secure123' }
  const mockEmailService = vi.fn()

  // Act
  const user = await registerUser(userData, mockEmailService)

  // Assert
  expect(user).toMatchObject({ email: 'user@example.com', emailVerified: false })
  expect(mockEmailService).toHaveBeenCalledWith('user@example.com', expect.any(String))
})

Agentic Optimizations

Context	Command
Coverage check (TS)	`vitest --coverage --reporter=dot`
Coverage check (Python)	`uv run pytest --cov --cov-fail-under=80 -q`
Find slow tests (TS)	`vitest --reporter=verbose`
Find slow tests (Python)	`uv run pytest --durations=10 -q`
Missing lines (Python)	`uv run pytest --cov --cov-report=term-missing`

For detailed examples, advanced patterns, and best practices, see REFERENCE.md.

References

Test Smells: https://testsmells.org/
Test Double Patterns: https://martinfowler.com/bliki/TestDouble.html
Testing Best Practices: https://kentcdodds.com/blog/common-mistakes-with-react-testing-library

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