Root file for client side

client_index.js

import React from 'react';
import { hydrate } from 'react-dom'
import App from './App.js'
import ContextProvider from './ContextProvider.js'

const context = {
    insertCss: (...styles) => {
      const removeCss = styles.map(x => x._insertCss());
      return () => {
        removeCss.forEach(f => f());
      };
    },
  }

hydrate(<ContextProvider context={context}>
<App />
</ContextProvider>, document.getElementById('app'))