React Patterns

React Patterns

Idiomatic React 18/19 patterns for building robust, accessible, performant component trees.

When to Activate

• Writing or modifying React function components, custom hooks, or component trees
• Reviewing JSX/TSX files
• Designing state shape or component composition
• Migrating class components or older forwardRef/useEffect-heavy code
• Choosing between local state, lifted state, context, and external stores
• Working with Server Components / Client Components (Next.js App Router, RSC)
• Implementing forms with React 19 actions or controlled inputs
• Wiring data fetching with TanStack Query / SWR / RSC

Core Principles

1. Render is a Pure Function of Props and State

// Good: derive during render
function Cart({ items }: { items: CartItem[] }) {
  const total = items.reduce((sum, i) => sum + i.price * i.qty, 0);
  return <span>{formatMoney(total)}</span>;
}

// Bad: derived state stored separately
function Cart({ items }: { items: CartItem[] }) {
  const [total, setTotal] = useState(0);
  useEffect(() => {
    setTotal(items.reduce((sum, i) => sum + i.price * i.qty, 0));
  }, [items]);
  return <span>{formatMoney(total)}</span>;
}