redux_notes9.js

import { combineReducers } from 'redux'

// creating a composite reducer with constitute reducers via
// the `combineReducers` method.
const todoApp = combineReducers({
  visibilityFilter,
  todos,
})

// The above is equivalent to this
function todoApp(state, action) = {
  return {
    visibilityFilter: visibilityFilter(state.visibilityFilter, action),
    todos: todos(state.todos, action)
  }
}