Timer component written as a class

timer-class.jsx

class Timer extends React.Component {
  constructor(props) {
    super(props);
    this.state = { seconds: 0 };
  }

  tick = () => {
    this.setState(state => ({
      seconds: state.seconds + 1
    }));
  };

  componentDidMount() {
    this.interval = setInterval(() => this.tick(), 1000);
  }

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

  render() {
    return <div>Seconds: {this.state.seconds}</div>;
  }
}