class Counter extends React.Component { constructor() { super() this.state = { count: 0 } } componentDidMount() { this.interval = setInterval(() => { this.setState(state => ({ count: state.count + 1 })) }, 1000) } componentWillUnmount() { clearInterval(this.interval) } render() { return ( <center> <h3> {this.state.count} </h3> </center> ) } }