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()

// ✅ 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);
  });
});

// ❌ 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 <Badge>{streak} days</Badge>;
};

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)

src/
├── feature/
│   ├── utils/
│   │   ├── calculations.ts      # Pure functions
│   │   └── calculations.test.ts # Tests here
│   ├── hooks/
│   │   └── useFeature.ts        # NO unit tests
│   └── components/
│       └── Feature.tsx          # NO unit tests

describe('FeatureName', () => {
  describe('when [condition]', () => {
    it('[expected outcome]', () => {
      // Arrange - Act - Assert (max 3-5 lines)
    });
  });
});