[nickporter]

I would start with implementing some backend in Node and Express, and with LevelUp for the database. Since you're familiar with Rails and backend development, it should be easy to build something quickly. Just make a simple API. That should show you the basics of JS!

After that, you should have enough JS knowledge to start building a client-side app that consumes your API. I honestly prefer AngularJS, but React+Flux seems like a fine choice.