Modular Slice Creation with createReducer

Explain how to construct a modular user slice that can manage real-time user session data with `createReducer` for better scalability.

import { createAction, createReducer } from '@reduxjs/toolkit';

// Action creators
export const userLoggedIn = createAction('user/loggedIn');
export const userLoggedOut = createAction('user/loggedOut');
export const userSessionUpdated = createAction('user/sessionUpdated');

// Initial state
const initialState = {
    currentUser: null,
    sessionData: {},
    isLoggedIn: false
};

// Modular user slice reducer created with createReducer
export const userReducer = createReducer(initialState, (builder) => {
    builder
        .addCase(userLoggedIn, (state, action) => {
            state.currentUser = action.payload;
            state.isLoggedIn = true;
        })
        .addCase(userLoggedOut, (state) => {
            state.currentUser = null;
            state.sessionData = {};
            state.isLoggedIn = false;
        })
        .addCase(userSessionUpdated, (state, action) => {
            state.sessionData = { ...state.sessionData, ...action.payload };
        });
});

Defines actions and a reducer for a user slice managing user sessions using createReducer for enhanced modularity and scalability. The reducer updates state for user login, logout, and session data updates.