opal-rails VS isomorfeus-project

Opal itself aims just at creating a Ruby runtime and a couple of life improvements, but there exist a couple of helper gems, for instance Opal-Browser (for an idiomatic API to interact with DOM and other browser APIs), Opal-Rails (for integrating with a Sprockets pipeline of Rails), Opal-RSpec (to test your frontend application). There exist a number of libraries and bindings to JS libraries for Opal.

For Rails integration we provide opal-rails, a Gem, that after inclusion will transparently handle .rb files in your JavaScript asset directory (via Sprockets). To interface with a web browser, you would also need to use opal-browser. As an alternative you can use Hyperstack, which deeply integrates React, Rails and Opal.

Opal itself is a low-level thing, not depending on Rails whatsoever. We provide an official Gem for integration with Rails that integrates everything nicely and allows you to have .rb files in your JavaScript assets directory. Unlike Rails, Opal is not opinionated, to interface with web browser APIs properly you need to either use a library that wraps DOM directly, use a similar one that wraps jQuery or use Hyperstack, a fully-fledged Rails-integrated framework based on React that also allows you to share your models between frontend and backend. It's also possible to not use any of those and interface JavaScript APIs directly using an API that looks like this: $$[:document][:location].replace("https://opalrb.com/") or simply embed JavaScript with a backtick notation: `document.location.replace(#{@url})`.

Opal itself is quite low level. It just gives you a compiler and Ruby runtime. There exist a couple of frameworks using Opal: two most developed are Hyperstack (deeply integrates Rails, Opal and React) and Isomorfeus (integrates Roda, Opal and Preact)