<div id="root">
    <!-- This element's contents will be replaced with your component. -->
</div>

function ExpensiveComponent () {
  const count = React.useRef(0)

  return <div>Rerender ExpensiveComponent {count.current++}</div>
}

const MemorizedExpensiveComponent = React.memo(ExpensiveComponent)

function Component () {
  const [msg, setMsg] = React.useState('')

  return <div>
    <input onInput={e => setMsg(e.target.value)}  />
    <br />
    msg: {msg}
    <br />
    <br />
    unused React.memo:
    <ExpensiveComponent />
    <br />
    used React.memo:
    <MemorizedExpensiveComponent />
   </div>
}


ReactDOM.render(
  <Component />,
  document.getElementById('root')
);