State Management

State Management Skill

Patterns for choosing and implementing state management strategies in frontend applications.

State Categories

1. Local UI State

State that only affects a single component.

// Form input values
const [value, setValue] = useState('');

// Toggle states
const [isOpen, setIsOpen] = useState(false);

// Loading/error states
const [status, setStatus] = useState<'idle' | 'loading' | 'error'>('idle');

2. Shared UI State

State shared between components (lift state up or use context).

// Theme context
const ThemeContext = createContext<Theme>('light');

function ThemeProvider({ children }) {
  const [theme, setTheme] = useState<Theme>('light');

  return (
    <ThemeContext.Provider value={{ theme, setTheme }}>
      {children}
    </ThemeContext.Provider>
  );
}

3. Server State

Data from external sources (use TanStack Query, SWR, or similar).

// TanStack Query
const { data, isLoading, error } = useQuery({
  queryKey: ['users'],
  queryFn: fetchUsers,
  staleTime: 5 * 60 * 1000, // 5 minutes
});

// Mutations
const mutation = useMutation({
  mutationFn: createUser,
  onSuccess: () => {
    queryClient.invalidateQueries({ queryKey: ['users'] });
  },
});

4. URL State

State that should be shareable/bookmarkable.

// Next.js App Router
import { useSearchParams, useRouter } from 'next/navigation';

function FilteredList() {
  const searchParams = useSearchParams();
  const router = useRouter();

  const filter = searchParams.get('filter') || 'all';

  const setFilter = (value: string) => {
    const params = new URLSearchParams(searchParams);
    params.set('filter', value);
    router.push(`?${params.toString()}`);
  };

  return <Filters value={filter} />;
}

State Management Libraries

When to Use What

Zustand (Recommended for Global State)

import { create } from 'zustand';

interface AppState {
  user: User | null;
  setUser: (user: User | null) => void;
  notifications: Notification[];
  addNotification: (notification: Notification) => void;
  removeNotification: (id: string) => void;
}

const useAppStore = create<AppState>((set) => ({
  user: null,
  setUser: (user) => set({ user }),
  notifications: [],
  addNotification: (notification) =>
    set((state) => ({
      notifications: [...state.notifications, notification],
    })),
  removeNotification: (id) =>
    set((state) => ({
      notifications: state.notifications.filter((n) => n.id !== id),
    })),
}));

TanStack Query (Server State)

// Query configuration
const queryClient = new QueryClient({
  defaultOptions: {
    queries: {
      staleTime: 60 * 1000, // 1 minute
      gcTime: 5 * 60 * 1000, // 5 minutes
      retry: 3,
      refetchOnWindowFocus: false,
    },
  },
});

// Custom hook for domain logic
function useUsers(filters: UserFilters) {
  return useQuery({
    queryKey: ['users', filters],
    queryFn: () => fetchUsers(filters),
    select: (data) => data.users,
  });
}

State Patterns

Derived State

Compute values instead of storing them.

// Bad: Synced state
const [items, setItems] = useState([]);
const [total, setTotal] = useState(0);

useEffect(() => {
  setTotal(items.reduce((sum, item) => sum + item.price, 0));
}, [items]);

// Good: Derived state
const [items, setItems] = useState([]);
const total = useMemo(
  () => items.reduce((sum, item) => sum + item.price, 0),
  [items]
);

State Machines

For complex state transitions.

type Status = 'idle' | 'loading' | 'success' | 'error';

function useAsync<T>() {
  const [state, setState] = useState<{
    status: Status;
    data: T | null;
    error: Error | null;
  }>({
    status: 'idle',
    data: null,
    error: null,
  });

  const execute = async (promise: Promise<T>) => {
    setState({ status: 'loading', data: null, error: null });
    try {
      const data = await promise;
      setState({ status: 'success', data, error: null });
    } catch (error) {
      setState({ status: 'error', data: null, error: error as Error });
    }
  };

  return { ...state, execute };
}

Optimistic Updates

const mutation = useMutation({
  mutationFn: updateTodo,
  onMutate: async (newTodo) => {
    // Cancel outgoing refetches
    await queryClient.cancelQueries({ queryKey: ['todos'] });

    // Snapshot previous value
    const previousTodos = queryClient.getQueryData(['todos']);

    // Optimistically update
    queryClient.setQueryData(['todos'], (old) =>
      old.map((t) => (t.id === newTodo.id ? newTodo : t))
    );

    return { previousTodos };
  },
  onError: (err, newTodo, context) => {
    // Rollback on error
    queryClient.setQueryData(['todos'], context.previousTodos);
  },
  onSettled: () => {
    // Always refetch after error or success
    queryClient.invalidateQueries({ queryKey: ['todos'] });
  },
});

Anti-Patterns

Avoid These

1. Prop Drilling: Use context or state management library
2. State Syncing: Derive state instead of syncing
3. Storing Derived Data: Compute on render
4. Global State for Local Concerns: Keep state close
5. Mutating State Directly: Always use setter functions

Decision Framework

Is the state...

1. Used by only this component?
   → useState/useReducer

2. Shared by a few nearby components?
   → Lift state up to common ancestor

3. Used across many unrelated components?
   → Context or global store

4. From an external API?
   → TanStack Query/SWR

5. Should be in the URL?
   → Router state (params/search)

6. Complex with many transitions?
   → State machine (XState, useReducer)

Integration

Used by:

• frontend-developer agent
• React/Next.js stack skills