index.js

import React from 'react'
import { render } from 'react-dom'
import { Provider } from 'react-redux'
import { createStore } from 'redux'
import App from './components/App' // New

let store = createStore()

render(
  <Provider store={store}>
    {/* Appコンポーネントを追記 */}
    <App />
  </Provider>,
  document.getElementById('root')
)