How to Improve Performance in React Applications

// Without memo: re-renders every time parent renders
const ExpensiveList = ({ items }) => {
  return items.map(item => <Item key={item.id} data={item} />);
};

// With memo: only re-renders when items prop changes
const ExpensiveList = React.memo(({ items }) => {
  return items.map(item => <Item key={item.id} data={item} />);
});

function SearchResults({ query, data }) {
  // BAD: filters on every render
  const filtered = data.filter(item => item.name.includes(query));

  // GOOD: only recalculates when query or data changes
  const filtered = useMemo(
    () => data.filter(item => item.name.includes(query)),
    [query, data]
  );

  // BAD: new function reference every render breaks memo on child
  const handleClick = (id) => selectItem(id);

  // GOOD: stable reference
  const handleClick = useCallback((id) => selectItem(id), []);

  return <MemoizedList items={filtered} onClick={handleClick} />;
}

// BAD: entire admin panel loaded on home page
import AdminPanel from './AdminPanel';

// GOOD: loaded only when needed
const AdminPanel = React.lazy(() => import('./AdminPanel'));

function App() {
  return (
    <Suspense fallback={<Loading />}>
      <AdminPanel />
    </Suspense>
  );
}

// Rendering 10,000 items = 10,000 DOM nodes = slow
// Virtualization renders only visible items (~20 DOM nodes)

import { useVirtualizer } from '@tanstack/react-virtual';

function VirtualList({ items }) {
  const parentRef = useRef(null);
  const virtualizer = useVirtualizer({
    count: items.length,
    getScrollElement: () => parentRef.current,
    estimateSize: () => 50,
  });

  return (
    <div ref={parentRef} style={{ height: 400, overflow: 'auto' }}>
      <div style={{ height: virtualizer.getTotalSize() }}>
        {virtualizer.getVirtualItems().map(row => (
          <div key={row.key} style={{
            position: 'absolute',
            top: row.start,
            height: row.size,
          }}>
            {items[row.index].name}
          </div>
        ))}
      </div>
    </div>
  );
}

// BAD: all state at the top, every change re-renders everything
function App() {
  const [theme, setTheme] = useState('dark');
  const [user, setUser] = useState(null);
  const [cart, setCart] = useState([]);
  // Changing theme re-renders Cart component

  return (
    <Header user={user} />
    <Cart items={cart} />
  );
}

// GOOD: colocate state, use context for truly global state
// Or use a state manager like Zustand with selectors