function getData() {

  return new Promise(resolve => setTimeout(() => {

    resolve('bar');

  }, 2000));

}

​

class App extends React.Component {

  state = {

    data: 'foo'

  };

  componentDidMount() {

    getData().then(data => {

      this.setState({ data });

    })

  }

  render() {

    return <div> {this.state.data} </div>;

  }

}

​

ReactDOM.render(

  <App />,

  document.getElementById('app')

);