class App extends React.Component {
  sayHi(e) {
    e.preventDefault()
    window.alert('Hello！');
  }
  
  render() {
    return (
      <div>
        <a href="https://israynotarray.com/" onClick={ this.sayHi }>點我</a>
      </div>
    );
  }
}

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