windowed-mf1.js

import { Observable } from 'windowed-observable';

const observable = new Observable('messages');

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

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

  useEffect(() => {  
    observable.subscribe(handleNewMessage);

    return () => {
      observable.unsubscribe(handleNewMessage)
    }
  }, [handleNewMessage]);

  ...
}