function MeasureExample() {
   const wRef = React.useRef(null);
   return (
    <React.Fragment>
      <h1 ref={wRef}>Hello, world</h1>
      <h2>The above header is
                {Math.round(wRef.current.getBoundingClientRect().width)}px wide.</h2>
    </React.Fragment >
  );}ReactDOM.render(<MeasureExample/>,document.querySelector("div"));