function Example() {
   const [count, setCount] = React.useState(0);
      React.useEffect(() => {  // similar to componentDidMount() & componentDidUpdate()
      document.title = `You clicked ${count} times`;
      return ()=>{   // similar to componentWillUnmount() with []; optional
          alert("Sorry to see you go!");
      }
   },[]);   // prevents unintentional alert() call since no change in array;, optional
   return (<div><p>You clicked {count} times</p>
                       <button onClick={() => setCount(count + 1)}> Click me </button>
              </div>);}ReactDOM.render(<Example/>,document.querySelector("div"));setTimeout(()=>{    // simulating unmounting after 5 seconds
   ReactDOM.unmountComponentAtNode(document.querySelector('div'));},5000);