const App = () => {
  let myName = 'Ray';
  return (
    <div>
      { myName }
      <button type="button" onClick={ () => myName = 'QQ 先生' }>更改名字</button>
    </div>
  )
}

const app = document.querySelector('#app');
const root = ReactDOM.createRoot(app);
root.render(<App />);