class User extends React.Component {
   render() {
      return (<b>{this.props.username}</b>);
   }}class Title extends React.Component {
   render() {
      return (<h1>Welcome, {this.props.showTitle('Alex')}!</h1>);
   }}class Header extends React.Component {
   render() {
      return (<header>
                    <Title showTitle={u => (
                        <User username={u} />)}/>
                 </header>); }}ReactDOM.render(<Header />, document.querySelector('div'));