my_react_theme/react-src/src/App.js

import React, { Component } from 'react';
import logo from './logo.svg';
import './App.css';

class App extends Component {
  render() {
    // Sample usage of the `wp.api` script.
    let wpPosts = new window.wp.api.collections.Posts();
    wpPosts.fetch({ data: { per_page: 2 } }).done( posts => {
      posts.forEach( post => {
        console.log(post.title.rendered);
      });
    });

    return (
      <div className="App">
        <header className="App-header">
          <img src={logo} className="App-logo" alt="logo" />
          <p>
            Edit <code>react-src/src/App.js</code> and save to reload.
          </p>
          <a
            className="App-link"
            href="https://reactjs.org"
            target="_blank"
            rel="noopener noreferrer"
          >
            Learn React
          </a>
        </header>
      </div>
    );
  }
}

export default App;