code:

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

serviceWorker.unregister();

class MyClass extends React.Component {
    render() {
        return <h1>Hello world!</h1>;
    }
}
  
export default App;

ReactDOM.render(
    <MyClass />,
    document.getElementById('root')
);