import React , { useEffect , useState } from 'react' ; export function App ( ) { const [ count , setCount ] = useState ( 0 ) ; useEffect ( ( ) => { const timer = setInterval ( ( ) => { setCount ( ( c ) => c + 1 ) ; } , 1000 ) ; return ( ) => clearInterval ( timer ) ; } , [ ] ) ; return ( < div className = "padding" > < h1 > Count { count } < /h1> < div > Hello world ! < /div> < div > Fast refresh active < /div> < /div>); }