Apparently Steve Yegge ported much of Rails to JavaScript (on Rhino, at the time) back in 2007 at Google, but it was never open sourced AFAIK: https://steve-yegge.blogspot.com/2007/06/rhino-on-rails.html

For comparison, (according to `find . -name "*.rb" | xargs wc -l`) Rails back then was about 66K LOC (plus 38K LOC of tests) and is now 137K LOC (plus 216K LOC of tests).

However, Steve says he only ported "essentially all of ActionView, ActionController and Railties, plus a teeny bit of ActiveRecord", so maybe half of the total.