---

name: frontend-state-management description: Manage application state using Redux, MobX, Zustand, and Context API. Use when centralizing state for complex applications with multiple components.

Frontend State Management

Overview

Implement scalable state management solutions using modern patterns and libraries to handle application state, side effects, and data flow across components.

When to Use

• Complex application state
• Multiple components sharing state
• Predictable state mutations
• Time-travel debugging needs
• Server state synchronization

Implementation Examples

1. Redux with Redux Toolkit (React)

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

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

interface UserState {
  items: User[];
  loading: boolean;
  error: string | null;
}

const initialState: UserState = {
  items: [],
  loading: false,
  error: null
};

export const fetchUsers = createAsyncThunk(
  'users/fetchUsers',
  async (_, { rejectWithValue }) => {
    try {
      const response = await fetch('/api/users');
      if (!response.ok) throw new Error('Failed to fetch');
      return await response.json();
    } catch (error) {
      return rejectWithValue((error as Error).message);
    }
  }
);

const userSlice = createSlice({
  name: 'users',
  initialState,
  reducers: {
    userAdded(state, action: PayloadAction<User>) {
      state.items.push(action.payload);
    },
    userRemoved(state, action: PayloadAction<number>) {
      state.items = state.items.filter(u => u.id !== action.payload);
    }
  },
  extraReducers: (builder) => {
    builder
      .addCase(fetchUsers.pending, (state) => {
        state.loading = true;
        state.error = null;
      })
      .addCase(fetchUsers.fulfilled, (state, action) => {
        state.loading = false;
        state.items = action.payload;
      })
      .addCase(fetchUsers.rejected, (state, action) => {
        state.loading = false;
        state.error = action.payload as string;
      });
  }
});

export const { userAdded, userRemoved } = userSlice.actions;
export default userSlice.reducer;

// store/index.ts
import { configureStore } from '@reduxjs/toolkit';
import userReducer from './userSlice';

export const store = configureStore({
  reducer: {
    users: userReducer
  }
});

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

// Usage in component
import { useDispatch, useSelector } from 'react-redux';

const UsersList: React.FC = () => {
  const dispatch = useDispatch<AppDispatch>();
  const { items, loading, error } = useSelector(
    (state: RootState) => state.users
  );

  React.useEffect(() => {
    dispatch(fetchUsers());
  }, [dispatch]);

  if (loading) return <p>Loading...</p>;
  if (error) return <p>Error: {error}</p>;

  return (
    <ul>
      {items.map(user => (
        <li key={user.id}>{user.name}</li>
      ))}
    </ul>
  );
};

2. Zustand (Lightweight State Management)

Copy// store/useUserStore.ts
import create from 'zustand';

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

interface UserStore {
  users: User[];
  loading: boolean;
  error: string | null;
  fetchUsers: () => Promise<void>;
  addUser: (user: User) => void;
  removeUser: (id: number) => void;
  clearError: () => void;
}

export const useUserStore = create<UserStore>((set) => ({
  users: [],
  loading: false,
  error: null,

  fetchUsers: async () => {
    set({ loading: true, error: null });
    try {
      const response = await fetch('/api/users');
      if (!response.ok) throw new Error('Failed to fetch');
      const users = await response.json();
      set({ users, loading: false });
    } catch (error) {
      set({ error: (error as Error).message, loading: false });
    }
  },

  addUser: (user) => set((state) => ({
    users: [...state.users, user]
  })),

  removeUser: (id) => set((state) => ({
    users: state.users.filter(u => u.id !== id)
  })),

  clearError: () => set({ error: null })
}));

// Usage in component
const UsersList: React.FC = () => {
  const { users, loading, error, fetchUsers } = useUserStore();

  React.useEffect(() => {
    fetchUsers();
  }, [fetchUsers]);

  if (loading) return <p>Loading...</p>;
  if (error) return <p>Error: {error}</p>;

  return (
    <ul>
      {users.map(user => (
        <li key={user.id}>{user.name}</li>
      ))}
    </ul>
  );
};

3. Context API + useReducer

Copy// context/AuthContext.tsx
import React, { createContext, useReducer, useCallback } from 'react';

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

interface AuthState {
  user: User | null;
  loading: boolean;
  error: string | null;
  isAuthenticated: boolean;
}

type AuthAction =
  | { type: 'LOGIN_START' }
  | { type: 'LOGIN_SUCCESS'; payload: User }
  | { type: 'LOGIN_ERROR'; payload: string }
  | { type: 'LOGOUT' };

const initialState: AuthState = {
  user: null,
  loading: false,
  error: null,
  isAuthenticated: false
};

function authReducer(state: AuthState, action: AuthAction): AuthState {
  switch (action.type) {
    case 'LOGIN_START':
      return { ...state, loading: true, error: null };
    case 'LOGIN_SUCCESS':
      return {
        ...state,
        user: action.payload,
        loading: false,
        isAuthenticated: true
      };
    case 'LOGIN_ERROR':
      return { ...state, error: action.payload, loading: false };
    case 'LOGOUT':
      return { ...state, user: null, isAuthenticated: false };
    default:
      return state;
  }
}

interface AuthContextType {
  state: AuthState;
  login: (email: string, password: string) => Promise<void>;
  logout: () => void;
}

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

export const AuthProvider: React.FC<{ children: React.ReactNode }> = ({ children }) => {
  const [state, dispatch] = useReducer(authReducer, initialState);

  const login = useCallback(async (email: string, password: string) => {
    dispatch({ type: 'LOGIN_START' });
    try {
      const response = await fetch('/api/login', {
        method: 'POST',
        body: JSON.stringify({ email, password })
      });
      if (!response.ok) throw new Error('Login failed');
      const user = await response.json();
      dispatch({ type: 'LOGIN_SUCCESS', payload: user });
    } catch (error) {
      dispatch({
        type: 'LOGIN_ERROR',
        payload: (error as Error).message
      });
    }
  }, []);

  const logout = useCallback(() => {
    dispatch({ type: 'LOGOUT' });
  }, []);

  return (
    <AuthContext.Provider value={{ state, login, logout }}>
      {children}
    </AuthContext.Provider>
  );
};

export const useAuth = () => {
  const context = React.useContext(AuthContext);
  if (!context) {
    throw new Error('useAuth must be used within AuthProvider');
  }
  return context;
};

4. MobX (Observable State)

Copy// store/UserStore.ts
import { makeObservable, observable, action, runInAction } from 'mobx';

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

class UserStore {
  users: User[] = [];
  loading = false;
  error: string | null = null;

  constructor() {
    makeObservable(this, {
      users: observable,
      loading: observable,
      error: observable,
      fetchUsers: action,
      addUser: action,
      removeUser: action,
      clearError: action
    });
  }

  async fetchUsers() {
    this.loading = true;
    this.error = null;

    try {
      const response = await fetch('/api/users');
      if (!response.ok) throw new Error('Failed to fetch');
      const data = await response.json();

      runInAction(() => {
        this.users = data;
        this.loading = false;
      });
    } catch (error) {
      runInAction(() => {
        this.error = (error as Error).message;
        this.loading = false;
      });
    }
  }

  addUser(user: User) {
    this.users.push(user);
  }

  removeUser(id: number) {
    this.users = this.users.filter(u => u.id !== id);
  }

  clearError() {
    this.error = null;
  }
}

export const userStore = new UserStore();

// Usage with React
import { observer } from 'mobx-react-lite';

const UsersList = observer(() => {
  const { users, loading, error, fetchUsers } = userStore;

  React.useEffect(() => {
    fetchUsers();
  }, []);

  if (loading) return <p>Loading...</p>;
  if (error) return <p>Error: {error}</p>;

  return (
    <ul>
      {users.map(user => (
        <li key={user.id}>{user.name}</li>
      ))}
    </ul>
  );
});

Best Practices

• Choose state management based on app complexity
• Keep state normalized and flat
• Separate application and UI state
• Implement proper error handling
• Use selectors to derive data
• Implement middleware for side effects
• Monitor performance and bundle size
• Document state shape and actions

Resources

• Redux Documentation
• Zustand
• MobX
• Context API
• Pinia (Vue)