We did this recently with a small app that needs to be fast. The relatively simple page that you see when you first visit the site is rendered with plain old Rails, then when the user interacts with the page (based on DOM addEventListener hooks) it causes additional React components to be mounted and rendered.

Originally I’d hoped to be able to render a basic version of the page with Rails, then ‘enhance’ it with React - ie render <button>For</button> with Rails, then when React has loaded, replace it with a React component. But I couldn’t find any details on using it this way, and given time constraints choose the simplest option.