testing

• User asks to "write tests" or "add coverage"
• New business logic is being implemented
• TDD requested for new feature
• Coverage report shows untested utils/

```

TESTABLE (Functional Core) │ NOT UNIT TESTED (Imperative Shell)

───────────────────────────────│────────────────────────────────────

Pure functions in utils/ │ React hooks (useX)

Calculations, transformations │ Components (*.tsx)

Validators, formatters │ API calls, Firebase operations

State machines, reducers │ localStorage, Date.now(), Math.random()

```

What to Test

```typescript

// ✅ TESTABLE: Pure function

export const calculateStreak = (dates: Date[], today: Date): number => {

// Pure transformation: dates → number

};

// Test with simple input/output

describe('calculateStreak', () => {

it('returns 0 for empty dates', () => {

expect(calculateStreak([], new Date('2024-01-15'))).toBe(0);

});

it('returns consecutive days count', () => {

const dates = [new Date('2024-01-14'), new Date('2024-01-15')];

expect(calculateStreak(dates, new Date('2024-01-15'))).toBe(2);

});

});

```

What NOT to Test

```typescript

// ❌ DON'T TEST: Hook with side effects

export function useStreak(userId: string) {

const { data } = useQuery(['streak', userId], () => fetchStreak(userId));

return calculateStreak(data?.dates ?? [], new Date());

}

// ❌ DON'T TEST: Component

const StreakBadge = ({ userId }) => {

const streak = useStreak(userId);

return {streak} days;

};

```

TDD Workflow

```

1. Write failing test for pure function
2. Implement pure function to pass test
3. Create thin hook/component that calls pure function
4. Skip unit tests for hook/component (E2E covers integration)

```

Test File Location

```

src/

├── feature/

│ ├── utils/

│ │ ├── calculations.ts # Pure functions

│ │ └── calculations.test.ts # Tests here

│ ├── hooks/

│ │ └── useFeature.ts # NO unit tests

│ └── components/

│ └── Feature.tsx # NO unit tests

```

| Mistake | Symptom | Fix |

|---------|---------|-----|

| Testing hooks | renderHook(), QueryClientProvider in test | Extract logic to pure function |

| Mocking time | vi.useFakeTimers(), mockDate | Inject timestamp as parameter |

| Mocking internals | vi.mock('../api/firebase') | Test pure logic, not integration |

| Testing UI | render(), screen.getByText() | Only E2E tests for UI |

Stop and refactor if your test file has:

• vi.mock() for anything except external APIs
• renderHook() or render() from testing-library
• QueryClient, Provider, or wrapper setup
• More than 5 lines of test setup

```typescript

describe('FeatureName', () => {

describe('when [condition]', () => {

it('[expected outcome]', () => {

// Arrange - Act - Assert (max 3-5 lines)

});

});

});

```