To get live asset compilation and hot-reloading, start the webpack-dev-server using yarn start, and the Rails application using bundle exec rails server. The Rails server will listen on localhost:9001, but the active content security policy and because webpack-dev-server does emit actual files, you must open localhost:9002 to get assets and hot-reloaded. The webpack-dev-server will proxy all request to localhost:9002 to the Rails app, but handle /assets and hot-reloading itself.