index-sometimes-poly.js

import React from 'react';
import ReactDOM from 'react-dom';
import App from './App/App';
import './index.css';

const render = () => {
  ReactDOM.render(<App />, document.getElementById('root'));
}

if (
  'fetch' in window &&
  'Intl' in window &&
  'URL' in window &&
  'Map' in window &&
  'forEach' in NodeList.prototype &&
  'startsWith' in String.prototype &&
  'endsWith' in String.prototype &&
  'includes' in String.prototype &&
  'includes' in Array.prototype &&
  'assign' in Object &&
  'entries' in Object &&
  'keys' in Object
) {
  render();
} else {
  import('./polyfills').then(render);
}