therubyrhino should just work, if not there’s a pretty solid attempt to use the engine provided with Java 8,
which is kind of a (direct) successor of Rhino, gem 'dienashorner', platform: :jruby … really depends what the JS engine is used for - if mostly for compiling with the asset pipeline than both are expected to work fine.
have also looked into xorcist, managed to run into one issue - not failing properly on frozen strings, but its pretty edge case that is easy to work-around + PR submitted.