todoListReducer.js

const initialState = {
  1: {
    todos: [],
    visibilityFilter: 'SHOW_ALL'
  },
  2: {
    todos: [],
    visibilityFilter: 'SHOW_ALL'
  },
  3: {
    todos: [],
    visibilityFilter: 'SHOW_ALL'
  }
}

const addTodo = (state, action) => {
  const taskList = state[action.taskListId]
  const { todos } = taskList

  return {
    ...state,
    [action.taskListId]: {
      ...taskList,
      todos: [
        ...todos,
        {
          id: action.id,
          text: action.text,
          completed: false
        }
      ]
    }
  }
}

const toggleTodo = (state, action) => {
  const taskList = state[action.taskListId]
  const { todos } = taskList

  return {
    ...state,
    [action.taskListId]: {
      ...taskList,
      todos: todos.map(todo =>
        (todo.id === action.id)
          ? {...todo, completed: !todo.completed}
          : todo
      )
    }
  }
}

const setVisibilityFilter = (state, action) => {
  const taskList = state[action.taskListId]
  return {
    ...state,
    [action.taskListId]: {
      ...taskList,
      visibilityFilter: action.filter
    }
  }
}

const todoList = (state = initialState, action) => {
  const taskList = state[action.taskListId]
  const { todos, visibilityFilter } = taskList

  switch (action.type) {
    case 'ADD_TODO':
      return addTodo(state, action)

    case 'TOGGLE_TODO':
      return toggleTodo(state, action)

    case 'SET_VISIBILITY_FILTER':
      return setVisibilityFilter(state, action)

    default:
      return state
  }
}