Why reactDom does not apply a Twig file

Symfony5 inside twig file:

{% block body %}
     <div id="root"></div>
 {% endblock %}
{% block javascripts %}
    {{ encore_entry_script_tags('app') }}
{% endblock %}

AND assets/js/app.js:

import React from 'react';
import ReactDOM from 'react-dom';
import { BrowserRouter as Router } from 'react-router-dom';
class App extends React.Component {
  render() {
    return (
      <div>
        <p>Hello</p>
      </div>
    )
  }
} 
ReactDOM.render(<Router><App /></Router>, document.getElementById('root'));

inside "webpack.config.js":

.addEntry('app', './assets/js/app.js')

in console appear: http://localhost/build/runtime.js http://localhost/build/vendors-node_modules_axios_index_js-node_modules_core-js_modules_es_array_map_js-node_modules-0497df.js http://localhost/build/app.js

the project is accessed as: http://localhost/symfony1/public/shop