function App (){
  const [count, setCount] = React.useState(0);
  
  function sayHi () {
    window.alert('Hello Ray.');
  }
  function handleClick() {
    setCount((prevState) => prevState += 1)
  }
  return (
    <div>
      <button type="button" onClick={ handleClick }>
        Count is: { count }
      </button>
      <hr />
      <button type="button" onClick={ sayHi }>打招呼</button>
    </div>
  )
}

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