// ✅ Local state (single component)
function SearchBox() {
  const [query, setQuery] = useState('');
  const [isOpen, setIsOpen] = useState(false);

  return (
    <div>
      <input
        value={query}
        onChange={(e) => setQuery(e.target.value)}
        onFocus={() => setIsOpen(true)}
      />
      {isOpen && <SearchResults query={query} />}
    </div>
  );
}

// ✅ Global state (multiple components)
// User authentication info is used in Header, Profile, Settings, etc.
const { user, logout } = useAuth();  // Context or Zustand

// contexts/AuthContext.tsx
import { createContext, useContext, useState, ReactNode } from 'react';

interface User {
  id: string;
  email: string;
  name: string;
}

interface AuthContextType {
  user: User | null;
  login: (email: string, password: string) => Promise<void>;
  logout: () => void;
  isAuthenticated: boolean;
}

const AuthContext = createContext<AuthContextType | undefined>(undefined);

export function AuthProvider({ children }: { children: ReactNode }) {
  const [user, setUser] = useState<User | null>(null);

  const login = async (email: string, password: string) => {
    const response = await fetch('/api/auth/login', {
      method: 'POST',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({ email, password })
    });

    const data = await response.json();
    setUser(data.user);
    localStorage.setItem('token', data.token);
  };

  const logout = () => {
    setUser(null);
    localStorage.removeItem('token');
  };

  return (
    <AuthContext.Provider value={{
      user,
      login,
      logout,
      isAuthenticated: !!user
    }}>
      {children}
    </AuthContext.Provider>
  );
}

// Custom hook
export function useAuth() {
  const context = useContext(AuthContext);
  if (!context) {
    throw new Error('useAuth must be used within AuthProvider');
  }
  return context;
}

// App.tsx
function App() {
  return (
    <AuthProvider>
      <Router>
        <Header />
        <Routes />
      </Router>
    </AuthProvider>
  );
}

// Header.tsx
function Header() {
  const { user, logout, isAuthenticated } = useAuth();

  return (
    <header>
      {isAuthenticated ? (
        <>
          <span>Welcome, {user!.name}</span>
          <button onClick={logout}>Logout</button>
        </>
      ) : (
        <Link to="/login">Login</Link>
      )}
    </header>
  );
}

npm install zustand

// stores/cartStore.ts
import { create } from 'zustand';
import { devtools, persist } from 'zustand/middleware';

interface CartItem {
  id: string;
  name: string;
  price: number;
  quantity: number;
}

interface CartStore {
  items: CartItem[];
  addItem: (item: Omit<CartItem, 'quantity'>) => void;
  removeItem: (id: string) => void;
  updateQuantity: (id: string, quantity: number) => void;
  clearCart: () => void;
  total: () => number;
}

export const useCartStore = create<CartStore>()(
  devtools(
    persist(
      (set, get) => ({
        items: [],

        addItem: (item) => set((state) => {
          const existing = state.items.find(i => i.id === item.id);
          if (existing) {
            return {
              items: state.items.map(i =>
                i.id === item.id
                  ? { ...i, quantity: i.quantity + 1 }
                  : i
              )
            };
          }
          return { items: [...state.items, { ...item, quantity: 1 }] };
        }),

        removeItem: (id) => set((state) => ({
          items: state.items.filter(item => item.id !== id)
        })),

        updateQuantity: (id, quantity) => set((state) => ({
          items: state.items.map(item =>
            item.id === id ? { ...item, quantity } : item
          )
        })),

        clearCart: () => set({ items: [] }),

        total: () => {
          const { items } = get();
          return items.reduce((sum, item) => sum + item.price * item.quantity, 0);
        }
      }),
      { name: 'cart-storage' }  // localStorage key
    )
  )
);

// components/ProductCard.tsx
function ProductCard({ product }) {
  const addItem = useCartStore(state => state.addItem);

  return (
    <div>
      <h3>{product.name}</h3>
      <p>${product.price}</p>
      <button onClick={() => addItem(product)}>
        Add to Cart
      </button>
    </div>
  );
}

// components/Cart.tsx
function Cart() {
  const items = useCartStore(state => state.items);
  const total = useCartStore(state => state.total());
  const removeItem = useCartStore(state => state.removeItem);

  return (
    <div>
      <h2>Cart</h2>
      {items.map(item => (
        <div key={item.id}>
          <span>{item.name} x {item.quantity}</span>
          <span>${item.price * item.quantity}</span>
          <button onClick={() => removeItem(item.id)}>Remove</button>
        </div>
      ))}
      <p>Total: ${total.toFixed(2)}</p>
    </div>
  );
}

npm install @reduxjs/toolkit react-redux

// store/todosSlice.ts
import { createSlice, createAsyncThunk, PayloadAction } from '@reduxjs/toolkit';

interface Todo {
  id: string;
  text: string;
  completed: boolean;
}

interface TodosState {
  items: Todo[];
  status: 'idle' | 'loading' | 'failed';
}

const initialState: TodosState = {
  items: [],
  status: 'idle'
};

// Async action
export const fetchTodos = createAsyncThunk('todos/fetch', async () => {
  const response = await fetch('/api/todos');
  return response.json();
});

const todosSlice = createSlice({
  name: 'todos',
  initialState,
  reducers: {
    addTodo: (state, action: PayloadAction<string>) => {
      state.items.push({
        id: Date.now().toString(),
        text: action.payload,
        completed: false
      });
    },
    toggleTodo: (state, action: PayloadAction<string>) => {
      const todo = state.items.find(t => t.id === action.payload);
      if (todo) {
        todo.completed = !todo.completed;
      }
    },
    removeTodo: (state, action: PayloadAction<string>) => {
      state.items = state.items.filter(t => t.id !== action.payload);
    }
  },
  extraReducers: (builder) => {
    builder
      .addCase(fetchTodos.pending, (state) => {
        state.status = 'loading';
      })
      .addCase(fetchTodos.fulfilled, (state, action) => {
        state.status = 'idle';
        state.items = action.payload;
      })
      .addCase(fetchTodos.rejected, (state) => {
        state.status = 'failed';
      });
  }
});

export const { addTodo, toggleTodo, removeTodo } = todosSlice.actions;
export default todosSlice.reducer;

// store/index.ts
import { configureStore } from '@reduxjs/toolkit';
import todosReducer from './todosSlice';

export const store = configureStore({
  reducer: {
    todos: todosReducer
  }
});

export type RootState = ReturnType<typeof store.getState>;
export type AppDispatch = typeof store.dispatch;

// App.tsx
import { Provider } from 'react-redux';
import { store } from './store';

function App() {
  return (
    <Provider store={store}>
      <TodoApp />
    </Provider>
  );
}

// components/TodoList.tsx
import { useSelector, useDispatch } from 'react-redux';
import { RootState } from '../store';
import { toggleTodo, removeTodo } from '../store/todosSlice';

function TodoList() {
  const todos = useSelector((state: RootState) => state.todos.items);
  const dispatch = useDispatch();

  return (
    <ul>
      {todos.map(todo => (
        <li key={todo.id}>
          <input
            type="checkbox"
            checked={todo.completed}
            onChange={() => dispatch(toggleTodo(todo.id))}
          />
          <span style={{ textDecoration: todo.completed ? 'line-through' : 'none' }}>
            {todo.text}
          </span>
          <button onClick={() => dispatch(removeTodo(todo.id))}>Delete</button>
        </li>
      ))}
    </ul>
  );
}

import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';

function UserProfile({ userId }: { userId: string }) {
  const queryClient = useQueryClient();

  // GET: Fetch user info
  const { data: user, isLoading, error } = useQuery({
    queryKey: ['user', userId],
    queryFn: async () => {
      const res = await fetch(`/api/users/${userId}`);
      return res.json();
    },
    staleTime: 5 * 60 * 1000,  // Cache for 5 minutes
  });

  // POST: Update user info
  const mutation = useMutation({
    mutationFn: async (updatedUser: Partial<User>) => {
      const res = await fetch(`/api/users/${userId}`, {
        method: 'PATCH',
        body: JSON.stringify(updatedUser)
      });
      return res.json();
    },
    onSuccess: () => {
      // Invalidate cache and refetch
      queryClient.invalidateQueries({ queryKey: ['user', userId] });
    }
  });

  if (isLoading) return <div>Loading...</div>;
  if (error) return <div>Error: {error.message}</div>;

  return (
    <div>
      <h2>{user.name}</h2>
      <p>{user.email}</p>
      <button onClick={() => mutation.mutate({ name: 'New Name' })}>
        Update Name
      </button>
    </div>
  );
}

Recommended tools by scenario:

1. Simple global state (theme, language)
   → React Context API

2. Medium complexity (shopping cart, user settings)
   → Zustand

3. Large-scale apps, complex logic, middleware required
   → Redux Toolkit

4. Server data fetching/caching
   → React Query (TanStack Query)

5. Form state
   → React Hook Form + Zod