class Clock extends React.Component {
  constructor(props) {
    super(props);
    this.state = {date: new Date()};   // may have more fields
  }
  componentDidMount() {
    this.timerID = setInterval(
      () => this.tick(),
      1000
    );
  }
  componentWillUnmount() {
    clearInterval(this.timerID);
  }
  tick() {    // this.state.date = new Date();   // changing a state directly won’t work
    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('root'));