class Hello extends React.Component {

  componentWillMount() {

    console.log('Will: before mount');

  }

​

  componentDidMount() {

    console.log('Did: after mount');

  }

​

  componentWillUnmount() {

    console.log('Will: before unmount');

  }

  render() {

    return (

      <div>

        Hello world

      </div>

    );

  }

}

​

let appEl = document.getElementById('app');

ReactDOM.render(<Hello />, appEl);

​

// setTimeout(() =>{

//   ReactDOM.unmountComponentAtNode(appEl);

// }, 1000);