I would suggest Meteor. It is not purely a front end stack, but a full stack. However, since you mention flux and rails it seems you might also be interested in

I would suggest Meteor. It is not purely a front end stack, but a full stack. However, since you mention flux and rails it seems you might also be interested in a backend. Meteor has a bit of a reputation as a "playground" stack. The sort of stack you pick when you need to implement something quickly but might want to rewrite it before production. However, I find the Meteor stack to be very production ready and have a few applications I have built and use that have high traffic built with Meteor. As for frontend heavy applications, I would consider looking into the Meteor React integration. Meteor is giving React first class support. The native blaze engine isn't bad but I do prefer React. One final note, Meteor's community is fantastic. The irc channel always has a number of helpful people in it. And there are many people actively helping to answer just about all Stackoverflow questions. I highly recommend it.