index.js.diff

 import React from 'react';
-import ReactDOM from 'react-dom';
+import { hydrate, render } from 'react-dom';
 import './index.css';
 import App from './App';
-import registerServiceWorker from './registerServiceWorker';
 
-ReactDOM.render(<App />, document.getElementById('root'));
-registerServiceWorker();
+const rootElement = document.getElementById('root');
+if (rootElement.hasChildNodes()) {
+  hydrate(<App />, rootElement);
+} else {
+  render(<App />, rootElement);
+}