events-mf1.js

function App() {
  const [messages, setMessages] = useState([]);

  const handleNewMessage = (event) => {
    setMessages((currentMessages) => currentMessages.concat(event.detail));
  };

  useEffect(() => {  
    window.addEventListener('message', handleNewMessage);

    return () => {
      window.removeEventListener('message', handleNewMessage)
    }
  }, [handleNewMessage]);

  ...
}