react_clock.js

class Clock extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      date: new Date(),
    }
  }
  
  comoponentDidMount() {
    this.timerID = setInterval(
      () => this.tick(),
      1000
    );
  }
  
  tick() {
    this.setState({
      date: new Date(),
    });
  }
  
  
  render() {
    return (
      <div>
        <h3>Clock</h3>
        <h3>It is now {this.state.date.toLocaleTimeString()}.</h3>
      </div>
    );
  }
}

ReactDOM.render(
  <Clock />,
  document.getElementById('root')
);