React (JSX) - Lifecycle methods 2

import React from 'react';
import ReactDOM from 'react-dom';

class Clock extends React.Component {
  constructor(props) {
    super(props);
    this.state = { date: new Date() };
  }
  render() {
    return <div>{this.state.date.toLocaleTimeString()}</div>;
  }
  componentDidMount() {    //add this method to see the clock update every second!
    const oneSecond = 1000;
    setInterval(() => {
    this.setState({ date: new Date() });
}, oneSecond);
  }
}

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