Zustand

Zustand Core Knowledge

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: zustand for comprehensive documentation.

Basic Store

import { create } from 'zustand';

interface CounterStore {
  count: number;
  increment: () => void;
  decrement: () => void;
  reset: () => void;
}

const useCounterStore = create<CounterStore>((set) => ({
  count: 0,
  increment: () => set((state) => ({ count: state.count + 1 })),
  decrement: () => set((state) => ({ count: state.count - 1 })),
  reset: () => set({ count: 0 }),
}));

// Usage
function Counter() {
  const { count, increment } = useCounterStore();
  return <button
}

Async Actions

interface UserStore {
  user: User | null;
  loading: boolean;
  error: string | null;
  fetchUser: (id: string) => Promise<void>;
}

const useUserStore = create<UserStore>((set) => ({
  user: null,
  loading: false,
  error: null,
  fetchUser: async (id) => {
    set({ loading: true, error: null });
    try {
      const user = await api.getUser(id);
      set({ user, loading: false });
    } catch (err) {
      set({ error: err.message, loading: false });
    }
  },
}));

Selectors

// Select specific state (prevents unnecessary re-renders)
const count = useCounterStore((state) => state.count);
const increment = useCounterStore((state) => state.increment);

// Shallow comparison for objects
import { shallow } from 'zustand/shallow';
const { user, loading } = useUserStore(
  (state) => ({ user: state.user, loading: state.loading }),
  shallow
);

Persist Middleware

import { persist } from 'zustand/middleware';

const useStore = create(
  persist<MyStore>(
    (set) => ({
      // ... state and actions
    }),
    {
      name: 'my-store',
      partialize: (state) => ({ count: state.count }), // Only persist count
    }
  )
);

DevTools

import { devtools } from 'zustand/middleware';

const useStore = create(
  devtools<MyStore>((set) => ({
    // ... state and actions
  }), { name: 'MyStore' })
);

When NOT to Use This Skill

Anti-Patterns

Quick Troubleshooting

Production Readiness

Store Organization

// stores/userStore.ts - Typed store with slices
import { create } from 'zustand';
import { devtools, persist, subscribeWithSelector } from 'zustand/middleware';
import { immer } from 'zustand/middleware/immer';

interface UserState {
  user: User | null;
  isAuthenticated: boolean;
}

interface UserActions {
  setUser: (user: User | null) => void;
  logout: () => void;
}

type UserStore = UserState & UserActions;

const initialState: UserState = {
  user: null,
  isAuthenticated: false,
};

export const useUserStore = create<UserStore>()(
  devtools(
    persist(
      subscribeWithSelector(
        immer((set) => ({
          ...initialState,
          setUser: (user) =>
            set((state) => {
              state.user = user;
              state.isAuthenticated = !!user;
            }),
          logout: () => set(initialState),
        }))
      ),
      {
        name: 'user-store',
        partialize: (state) => ({ user: state.user }),
        // Don't persist to localStorage in SSR
        skipHydration: typeof window === 'undefined',
      }
    ),
    { name: 'UserStore', enabled: process.env.NODE_ENV === 'development' }
  )
);

Security Best Practices

// Secure persistence with encryption
import { persist, createJSONStorage } from 'zustand/middleware';
import CryptoJS from 'crypto-js';

const SECRET_KEY = process.env.NEXT_PUBLIC_STORE_KEY!;

const encryptedStorage = {
  getItem: (name: string) => {
    const encrypted = localStorage.getItem(name);
    if (!encrypted) return null;
    const decrypted = CryptoJS.AES.decrypt(encrypted, SECRET_KEY);
    return decrypted.toString(CryptoJS.enc.Utf8);
  },
  setItem: (name: string, value: string) => {
    const encrypted = CryptoJS.AES.encrypt(value, SECRET_KEY).toString();
    localStorage.setItem(name, encrypted);
  },
  removeItem: (name: string) => localStorage.removeItem(name),
};

export const useAuthStore = create(
  persist(
    (set) => ({ token: null }),
    {
      name: 'auth-store',
      storage: createJSONStorage(() => encryptedStorage),
    }
  )
);

Testing Stores

// Store testing with isolated state
import { act, renderHook } from '@testing-library/react';
import { useUserStore } from './userStore';

describe('UserStore', () => {
  beforeEach(() => {
    // Reset store before each test
    useUserStore.setState({ user: null, isAuthenticated: false });
  });

  it('should set user and authenticate', () => {
    const { result } = renderHook(() => useUserStore());

    act(() => {
      result.current.setUser({ id: '1', name: 'John' });
    });

    expect(result.current.user?.name).toBe('John');
    expect(result.current.isAuthenticated).toBe(true);
  });

  it('should logout and clear state', () => {
    useUserStore.setState({ user: { id: '1', name: 'John' }, isAuthenticated: true });

    const { result } = renderHook(() => useUserStore());

    act(() => {
      result.current.logout();
    });

    expect(result.current.user).toBeNull();
    expect(result.current.isAuthenticated).toBe(false);
  });
});

Performance Optimization

// Atomic selectors to prevent unnecessary re-renders
const userName = useUserStore((state) => state.user?.name);
const isAuthenticated = useUserStore((state) => state.isAuthenticated);

// createSelectors helper for auto-generated selectors
import { StoreApi, UseBoundStore } from 'zustand';

type WithSelectors<S> = S extends { getState: () => infer T }
  ? S & { use: { [K in keyof T]: () => T[K] } }
  : never;

const createSelectors = <S extends UseBoundStore<StoreApi<object>>>(
  _store: S
) => {
  const store = _store as WithSelectors<typeof _store>;
  store.use = {};
  for (const k of Object.keys(store.getState())) {
    (store.use as any)[k] = () => store((s) => s[k as keyof typeof s]);
  }
  return store;
};

// Usage
export const useUserStore = createSelectors(useUserStoreBase);
const userName = useUserStore.use.user()?.name;

Monitoring Metrics

Checklist

• TypeScript types for state and actions
• Devtools enabled (dev only)
• Atomic selectors for performance
• Persist sensitive data encrypted
• SSR hydration handled
• Store reset on logout
• Immer for complex updates
• subscribeWithSelector for reactions
• Comprehensive unit tests
• No sensitive data in plain localStorage

Reference Documentation

• Middleware
• Patterns