redux-toolkit-example3.js

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

const addTodo = createAction('ADD_TODO')
const toggleTodo = createAction('TOGGLE_TODO')

const todos = createReducer([], {
  [addTodo]: (state, action) => {
    const todo = action.payload
    return [...state, todo]
  },
  [toggleTodo]: (state, action) => {
    const index = action.payload
    const todo = state[index]
    return [
      ...state.slice(0, index),
      { ...todo, completed: !todo.completed }
      ...state.slice(index + 1)
    ]
  }
})

const store = configureStore({ reducer: todos })