todoReducer.js

import { createReducer } from '@reduxjs/toolkit'

const todosReducer = createReducer([], {
  ADD_TODO: (state, action) => {
    // "mutate" the array by calling push()
    state.push(action.payload)
  },
  TOGGLE_TODO: (state, action) => {
    const todo = state[action.payload.index]
    // "mutate" the object by overwriting a field
    todo.completed = !todo.completed
  },
  REMOVE_TODO: (state, action) => {
    // Can still return an immutably-updated value if we want to
    return state.filter((todo, i) => i !== action.payload.index)
  }
})

export default todosReducer;