multi-buildpack setup for heroku:

$ heroku buildpacks:set https://github.com/heroku/heroku-buildpack-multi.git

will set it up

.buildpacks

@@ -1 +1,2 @@
+https://github.com/heroku/heroku-buildpack-nodejs.git
 https://github.com/heroku/heroku-buildpack-ruby.git

lib/tasks/extensions.rake

@@ -0,0 +1,8 @@
+namespace :assets do
+  task :js_compile do
+    system "npm install"
+    system "npm run build"
+  end
+end
+
+Rake::Task[:'assets:precompile'].enhance([:'assets:js_compile'])