function Timer() {
   const [n,setN]= React.useState(0);   //const tid = {current: null};   // this won't work
   const tid = React.useRef();
   React.useEffect(()=>{
      tid.current = setInterval(()=>setN(n=>n+1),1000);
   },[]);
   function stopTimer(){
       clearInterval(tid.current);
   }
   return (<button onClick={stopTimer}>{n}</button>);}ReactDOM.render(<Timer/>,document.querySelector("div"));