// Unknown for external data
function parse(input: unknown): User { ... }

// Union for specific options
type Status = 'loading' | 'success' | 'error';

// Generics for reusable code
function first<T>(arr: T[]): T | undefined { ... }

// Type assertion as last resort (with runtime check)
if (isUser(data)) { return data as User; }

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true
  }
}

src/
  features/
    auth/
      components/
      hooks/
      api.ts
      types.ts
      index.ts  # Barrel: controlled exports
    orders/
      ...
  shared/
    ui/
    utils/

const { data, isLoading, error } = useQuery({
  queryKey: ['user', userId],
  queryFn: () => fetchUser(userId),
});

type AuthState =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'authenticated'; user: User }
  | { status: 'error'; error: AppError };

async function fetchUser(id: string): Promise<User> {
  try {
    const res = await fetch(`/api/users/${id}`);
    if (!res.ok) throw new ApiError(res.status, await res.text());
    return res.json();
  } catch (error) {
    throw new AppError('Failed to fetch user', { cause: error });
  }
}