counter in React

counter.js

//reference: https://reactjs.org/docs/state-and-lifecycle.html
class Counter extends React.Component {
  constructor(props) {
    super(props);
    this.state = {count: 0};
  }

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

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

  tick() {
    this.setState({
      count: this.state.count + 1
    });
  }

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

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