State Management

Priority: P2 (MEDIUM)

Prefer Server State (URL) and Local State over Global Stores.

Principles

1. URL as Source of Truth: For any state that should be shareable or persistent (Search queries, Filters, Pagination, Tabs), use the URL Search Params. - Why: Keeps state syncable across refreshes and shareable links. - Tool: useSearchParams hook.
2. Colocation: Keep state as close to the component as possible. Do not lift state unless necessary to share between siblings. - Bad: App component holding searchTerm for a SearchBar nested 5 levels deep. - Good: SearchBar has internal state, or MainContent holds it if it needs to pass to siblings.
3. No Global Store Default: Avoid Redux/Zustand for simple apps. Be skeptical of adding a library. - Use Cases for Global Store: Complex interactors like a Music Player (persists across navigation), Shopping Cart (shared everywhere).

Patterns

1. Granular State (Best Practice)

Don't store large objects. Subscribe only to what you need to prevent unnecessary re-renders.

// BAD: Re-renders on any change to 'user'
const [user, setUser] = useState({ name: '', stats: {}, friends: [] });

// GOOD: Independent states
const [name, setName] = useState('');
const [stats, setStats] = useState({});

2. URL-Driven State (Search/Filter)

// Client Component
'use client';
import { useSearchParams, useRouter, usePathname } from 'next/navigation';

export function Search() {
  const searchParams = useSearchParams();
  const { replace } = useRouter();
  const pathname = usePathname();

  function handleSearch(term: string) {
    const params = new URLSearchParams(searchParams);
    if (term) params.set('q', term);
    else params.delete('q');

    // Updates URL -> Server Component re-renders with new params
    replace(`${pathname}?${params.toString()}`);
  }
}

3. Server State (TanStack Query / SWR)

If you need "Live" data on the client (e.g., polling stock prices, chat), do not implement useEffect fetch manually. Use a library.

// Automated caching, deduplication, and revalidation
const { data, error } = useSWR('/api/user', fetcher);