import React from 'react';import PropTypes from 'prop-types';class Greeting extends React.Component {
   render() {
      return (<div><h1>Hello, {this.props.name}.</h1>
                          <h2>{this.props.children}</h2></div>);
   }}Greeting.propTypes = {
   name: PropTypes.string,                         // must be a string
   children: PropTypes.element.isRequired,  // must be one element};class App extends React.Component {
   render() {
      return <Greeting name="John"><u>How do you do?</u></Greeting>;
   }}export default App;