React Patterns

Modern React 19+ patterns and best practices.

Component Patterns

Composition Over Prop Drilling

// Compose with children instead of passing many props
<Card>
  <Card.Header>
    <Card.Title>User Profile</Card.Title>
  </Card.Header>
  <Card.Body>
    <UserInfo user={user} />
  </Card.Body>
  <Card.Footer>
    <Button onClick={save}>Save</Button>
  </Card.Footer>
</Card>

Render Props for Flexibility

interface ListProps<T> {
  items: T[];
  renderItem: (item: T, index: number) => ReactNode;
  keyExtractor: (item: T) => string;
}

function List<T>({ items, renderItem, keyExtractor }: ListProps<T>) {
  return (
    <ul>
      {items.map((item, i) => (
        <li key={keyExtractor(item)}>{renderItem(item, i)}</li>
      ))}
    </ul>
  );
}

Hook Patterns

Custom Data Fetching Hook

function useQuery<T>(key: string, fetcher: () => Promise<T>) {
  const [state, setState] = useState<{
    data: T | null;
    error: Error | null;
    isLoading: boolean;
  }>({ data: null, error: null, isLoading: true });

  useEffect(() => {
    let cancelled = false;
    setState(prev => ({ ...prev, isLoading: true }));

    fetcher()
      .then(data => { if (!cancelled) setState({ data, error: null, isLoading: false }); })
      .catch(error => { if (!cancelled) setState({ data: null, error, isLoading: false }); });

    return () => { cancelled = true; };
  }, [key]);

  return state;
}

useCallback for Stable References

const handleSubmit = useCallback(
  async (data: FormData) => {
    await api.submit(userId, data);
    onSuccess();
  },
  [userId, onSuccess]
);

useMemo for Expensive Computations

const sortedItems = useMemo(
  () => [...items].sort((a, b) => a.name.localeCompare(b.name)),
  [items]
);

State Management

Local State First

// Start simple
const [isOpen, setIsOpen] = useState(false);

// Graduate to useReducer for complex state
const [state, dispatch] = useReducer(cartReducer, initialCart);

// Context for cross-component state
const ThemeContext = createContext<Theme>('light');

Server State (React Query / SWR)

const { data, isLoading } = useQuery({
  queryKey: ['users', userId],
  queryFn: () => fetchUser(userId),
  staleTime: 5 * 60 * 1000,
});

Performance

Code Splitting

const Dashboard = lazy(() => import('./Dashboard'));

function App() {
  return (
    <Suspense fallback={<Skeleton />}>
      <Dashboard />
    </Suspense>
  );
}

Virtualization for Long Lists

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

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

  return (
    <div ref={parentRef} style={{ overflow: 'auto', height: 400 }}>
      {virtualizer.getVirtualItems().map(row => (
        <div key={row.key} style={{ transform: `translateY(${row.start}px)` }}>
          {items[row.index].name}
        </div>
      ))}
    </div>
  );
}

Error Boundaries

class ErrorBoundary extends Component<Props, { hasError: boolean }> {
  state = { hasError: false };

  static getDerivedStateFromError() {
    return { hasError: true };
  }

  render() {
    if (this.state.hasError) return this.props.fallback;
    return this.props.children;
  }
}

Checklist

• Functional components only (no class components for new code)
• Custom hooks for reusable stateful logic
• Correct dependency arrays in useEffect/useCallback/useMemo
• Error boundaries at route level
• Loading and error states handled for async data
• Accessibility: ARIA labels, keyboard navigation, focus management
• Code splitting for route-level components
• Keys use stable, unique identifiers (not array index)