import React, { Component } from 'react';

class DigitalClock extends Component {
  constructor(props) {
    super(props);
    this.state = {
      time: 0
    };
  }

  componentDidMount() {
    this.timer = setInterval(() => {
      this.setState(prevState => ({
        time: prevState.time + 1
      }));
    }, 1000);
  }

  componentWillUnmount() {
    clearInterval(this.timer);
  }

  render() {
    return (
      <div>
        <h1>{this.state.time}</h1>
      </div>
    );
  }
}

export default DigitalClock;

import React from 'react';
import ReactDOM from 'react-dom';
import DigitalClock from './DigitalClock';

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