React元素变量

<div id="example"></div>
<script type="text/babel">
function UserGreeting(props) {
  return <h1>欢迎回来!</h1>;
}
function GuestGreeting(props) {
  return <h1>请先注册。</h1>;
}
function Greeting(props) {
  const isLoggedIn = props.isLoggedIn;
  if (isLoggedIn) {
    return <UserGreeting />;
  }
  return <GuestGreeting />;
}
ReactDOM.render(
  // Try changing to isLoggedIn={true}:
  <Greeting isLoggedIn={false} />,
  document.getElementById('example')
);
</script>