function visibilityFilter(){}
function todos(){}
import { combineReducers, createStore } from 'redux';
let reducer = combineReducers({ visibilityFilter, todos });
let store = createStore(reducer);
combineReducers这个方法把传进去的那个包含两个属性的对象怎么着了?
合并为一个switch/case
把几个reducer（visibilityFilter, todos）合成一个reducer。
官方解释
combineReducers源码