Frontend Patterns

Critical rules (non-obvious)

• Stale closure in `useEffect`: always list all dependencies; use useRef for values that shouldn't trigger re-run
• `useEffect` with `async`: never make the callback async directly — create inner async fn and call it
• Object/array as dependency: memoize with useMemo/useCallback or use primitive values; otherwise infinite loop
• Key prop on lists: use stable IDs, never index when list can reorder or items get deleted
• `React.memo` is not free: only wrap components with expensive renders and stable prop references

Component composition patterns

// Compound component with Context
const TabsContext = createContext<{ active: string; setActive: (v: string) => void } | null>(null);

function Tabs({ children, defaultValue }: { children: React.ReactNode; defaultValue: string }) {
  const [active, setActive] = useState(defaultValue);
  return <TabsContext.Provider value={{ active, setActive }}>{children}</TabsContext.Provider>;
}
Tabs.Trigger = function TabsTrigger({ value, children }: { value: string; children: React.ReactNode }) {
  const ctx = useContext(TabsContext)!;
  return <button onClick={() => ctx.setActive(value)} aria-selected={ctx.active === value}>{children}</button>;
};
Tabs.Content = function TabsContent({ value, children }: { value: string; children: React.ReactNode }) {
  const { active } = useContext(TabsContext)!;
  return active === value ? <>{children}</> : null;
};

State management decision

Data fetching with TanStack Query

// Fetch
const { data, isLoading, error } = useQuery({
  queryKey: ["products", filters],   // filters in key → auto-refetch on change
  queryFn: () => api.getProducts(filters),
  staleTime: 5 * 60 * 1000,          // don't refetch for 5 min
});

// Mutate with optimistic update
const mutation = useMutation({
  mutationFn: api.updateProduct,
  onMutate: async (newProduct) => {
    await queryClient.cancelQueries({ queryKey: ["products"] });
    const prev = queryClient.getQueryData(["products"]);
    queryClient.setQueryData(["products"], (old) => old.map(p => p.id === newProduct.id ? newProduct : p));
    return { prev };
  },
  onError: (_, __, ctx) => queryClient.setQueryData(["products"], ctx?.prev),
  onSettled: () => queryClient.invalidateQueries({ queryKey: ["products"] }),
});

Performance: avoid re-renders

// Memoize expensive component
const ExpensiveList = memo(({ items }: { items: Item[] }) => (
  <ul>{items.map(item => <li key={item.id}>{item.name}</li>)}</ul>
));

// Stable callback reference
const handleClick = useCallback((id: string) => {
  onSelect(id);
}, [onSelect]);  // only recreate if onSelect changes

// Expensive calculation
const sorted = useMemo(() =>
  items.sort((a, b) => b.score - a.score),
[items]);

Code splitting

const HeavyChart = lazy(() => import("./HeavyChart"));

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

Custom hooks pattern

function useDebounce<T>(value: T, delay: number): T {
  const [debounced, setDebounced] = useState(value);
  useEffect(() => {
    const timer = setTimeout(() => setDebounced(value), delay);
    return () => clearTimeout(timer);
  }, [value, delay]);
  return debounced;
}

function useLocalStorage<T>(key: string, initial: T) {
  const [value, setValue] = useState<T>(() => {
    try { return JSON.parse(localStorage.getItem(key) ?? "") ?? initial; }
    catch { return initial; }
  });
  const set = useCallback((v: T) => {
    setValue(v); localStorage.setItem(key, JSON.stringify(v));
  }, [key]);
  return [value, set] as const;
}

Error boundaries

class ErrorBoundary extends React.Component<{ fallback: React.ReactNode; children: React.ReactNode }> {
  state = { hasError: false };
  static getDerivedStateFromError() { return { hasError: true }; }
  componentDidCatch(error: Error) { console.error(error); }
  render() { return this.state.hasError ? this.props.fallback : this.props.children; }
}
// Usage: wrap async/complex sections, not entire app

Common pitfalls