1. run rails new rails_react_boilerplate -d=postgresql --webpack=react --skip-coffee

2. add lines to config/database.yml below line 19 (under the default database):

username: postgres
password: <%= ENV['RAILS_REACT_BOILERPLATE_DATABASE_PASSWORD'] %>

3. run export RAILS_REACT_BOILERPLATE_DATABASE_PASSWORD="<your_postgres_password>" && rails db:create

4. run rails g controller Homepage index && rm app/helpers/homepage_helper.rb app/assets/stylesheets/homepage.scss and replace line 2 from config/routes.rb file with root 'homepage#index'

5. run mv app/javascript/packs/hello_react.jsx app/javascript/packs/Index.jsx

6. add lines to app/views/layouts/application.html.erb after line 9 (inside header):

<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
<%= javascript_pack_tag 'Index' %>

7. remove all content from app/views/homepage/index.html.erb

8. run yarn add react-router-dom

9. create file app/javascript/components/Example.jsx with the following content:

import React from 'react'

const Example = props => (
  <div>This is an example!</div>
)

export default Example

10. create file app/javascript/routes/Index.jsx with the following content:

import React from "react";
import { BrowserRouter as Router, Route, Switch } from "react-router-dom";
import Example from "../components/Example";

export default (
  <Router>
    <Switch>
      <Route path="/example" exact component={Example} />
    </Switch>
  </Router>
);

11. create file app/javascript/components/App.jsx with the following content:

import React from "react";
import Routes from "../routes/Index";

export default props => <>{Routes}</>;

12. replace file app/javascript/components/App.jsx content with the following content:

import React from 'react'
import ReactDOM from 'react-dom'
import App from '../components/App'

document.addEventListener('DOMContentLoaded', () => {
  ReactDOM.render(
    <App/>,
    document.body.appendChild(document.createElement('div')),
  )
})

13. add this line to config/routes.rb file:

get '/*path' => 'homepage#index'

Based on: https://www.digitalocean.com/community/tutorials/how-to-set-up-a-ruby-on-rails-project-with-a-react-frontend