The frontend was Ruby?? Are you sure?
But Rails was SSR by default, and most of the frontend was just Embedded Ruby (ERB) template files all over the place. And way back when, it was even relatively common to use Javascript supersets like CoffeeScript[1] and Opal[2]. The latter being Ruby that compiled to JS.
[1]: https://coffeescript.org/ [2]: https://opalrb.com/