text:    'use strict';
  
      class Clock extends React.Component {
          constructor(props) {
              super(props);
              this.state = {date: new Date()};
          }
  
          componentDidMount() {
              this.timerID = setInterval(
              () => this.tick(),
              1000
              );
          }
  
          componentWillUnmount() {
              clearInterval(this.timerID);
          }
  
          tick() {
              this.setState({
              date: new Date()
              });
          }
  
          render() {
              return (
              <div>
                  <h1>Hello, world!</h1>
                  <h2>It is {this.state.date.toLocaleTimeString()}.</h2>
              </div>
              );
          }
      }
  
      ReactDOM.render(
          <Clock />,
          document.getElementById('app')
      );
  