frontend-patterns

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"] }),
});