test-quality-analysis

What this skill does

# Test Quality Analysis

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

## Core Dimensions

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

## Test Smells

### Overmocking

**Problem**: Mocking too many dependencies makes tests fragile.

```typescript
// ❌ BAD: Overmocked
test('calculate total', () => {
  const mockAdd = vi.fn(() => 10)
  const mockMultiply = vi.fn(() => 20)
  // Testing implementation, not behavior
})

// ✅ GOOD: Mock only external dependencies
test('calculate order total', () => {
  const mockPricingAPI = vi.fn(() => ({ tax: 0.1 }))
  const total = calculateTotal(order, mockPricingAPI)
  expect(total).toBe(38)
})
```

**Detection**: More than 3-4 mocks, mocking pure functions, complex mock setup.

**Fix**: Mock only I/O boundaries (APIs, databases, filesystem).

### Fragile Tests

**Problem**: Tests break with unrelated code changes.

```typescript
// ❌ BAD: Tests implementation details
await page.locator('.form-container > div:nth-child(2) > button').click()

// ✅ GOOD: Semantic selector
await page.getByRole('button', { name: 'Submit' }).click()
```

### Flaky Tests

**Problem**: Tests pass or fail non-deterministically.

```typescript
// ❌ BAD: Race condition
test('loads data', async () => {
  fetchData()
  await new Promise(resolve => setTimeout(resolve, 1000))
  expect(data).toBeDefined()
})

// ✅ GOOD: Proper async handling
test('loads data', async () => {
  const data = await fetchData()
  expect(data).toBeDefined()
})
```

### Poor Assertions

```typescript
// ❌ BAD: Weak assertion
test('returns users', async () => {
  const users = await getUsers()
  expect(users).toBeDefined() // Too vague!
})

// ✅ GOOD: Strong, specific assertions
test('creates user with correct attributes', async () => {
  const user = await createUser({ name: 'John' })
  expect(user).toMatchObject({
    id: expect.any(Number),
    name: 'John',
  })
})
```

## Analysis Tools

```bash
# Vitest coverage (prefer bun)
bun test --coverage
open coverage/index.html

# Check thresholds
bun test --coverage --coverage.thresholds.lines=80

# pytest-cov (Python)
uv run pytest --cov --cov-report=html
open htmlcov/index.html
```

## Best Practices Checklist

### Unit Test Quality (FIRST)
- [ ] **Fast**: Tests run in milliseconds
- [ ] **Isolated**: No dependencies between tests
- [ ] **Repeatable**: Same results every time
- [ ] **Self-validating**: Clear pass/fail
- [ ] **Timely**: Written alongside code

### Mock Guidelines
- [ ] Mock only external dependencies
- [ ] Don't mock business logic or pure functions
- [ ] Use real implementations when possible
- [ ] Limit to 3-4 mocks per test maximum

### Coverage Goals
- [ ] 80%+ line coverage for business logic
- [ ] 100% for critical paths (auth, payment)
- [ ] All error paths tested
- [ ] Boundary conditions tested

### Test Structure (AAA Pattern)

```typescript
test('user registration', async () => {
  // Arrange
  const userData = { email: '[email protected]' }

  // Act
  const user = await registerUser(userData)

  // Assert
  expect(user.email).toBe('[email protected]')
})
```

## Code Review Checklist

- [ ] Tests verify behavior, not implementation
- [ ] Assertions are specific and meaningful
- [ ] No flaky tests (timing, ordering issues)
- [ ] Proper async/await usage
- [ ] Test names clearly describe behavior
- [ ] Minimal code duplication
- [ ] Critical paths have tests
- [ ] Both happy path and error cases covered

## Common Anti-Patterns

### Testing Implementation Details

```typescript
// ❌ BAD
const spy = vi.spyOn(Math, 'sqrt')
calculateDistance()
expect(spy).toHaveBeenCalled() // Testing how, not what

// ✅ GOOD
const distance = calculateDistance({ x: 0, y: 0 }, { x: 3, y: 4 })
expect(distance).toBe(5) // Testing output
```

### Mocking Too Much

```typescript
// ❌ BAD
const mockAdd = vi.fn((a, b) => a + b)

// ✅ GOOD: Use real implementations
import { add } from './utils'
// Only mock external services
const mockPaymentGateway = vi.fn()
```

## See Also

- `vitest-testing` - TypeScript/JavaScript testing
- `playwright-testing` - E2E testing
- `mutation-testing` - Validate test effectiveness