With the inclusion of webpack and Yarn, the Rails team is showing a great deal of maturity and humility, recognizing that the web is no longer rendered only from the server, but also dynamically from the client. Not only has the team embraced these changes, but they are incorporating them in such a way as to make it feel like there’s a “Rails Way” to do JavaScript.

Source: Embracing Change: Rails 5.1 Adopts Yarn, Webpack, and the JS Ecosystem – Pixelated