class.jsx

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

  componentDidMount() {
    this.interval = setInterval(() => {
      this.setState(({ count }) => ({ count: count + 1 }));
    }, 1000);
  }
  componentWillUnmount() {
    clearInterval(this.interval);
  }
  render() {
    return (
      <div>
        <h1>Count: {this.state.count}</h1>
      </div>
    );
  }
}