We use npm and webpack with rails. Have our Heroku app setup as a rails and node app, then tehre's a thing that compiles our Webpack on deploy.

Only issue is that as our app has grown, compiling has become slower and slower and has made deployments annoying.

Take a look brunch.io

What @swizec said. We use yarn and webpack for Rails on Heroku. Works fine, but deploys are a little slow. (few minutes along with all the other stuff it's doing).