const { useState } = React;

const root = ReactDOM.createRoot(document.getElementById('app'));

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

root.render(<App />);