prop types and default props types inside <App /> components.

AppPropTypes.jsx

class App extends Component {
  static propTypes = {
    title: propTypes.string,
  };

  static defaultProps = {
    title: 'app works from default props',
  }

  render() {
    return (
      <React.Fragment>
        <h1>{this.props.title}</h1>
      </React.Fragment>
    );
  }
}

ReactDOM.render(<App title="app works!!" />, document.getElementById("root"));