For a single page app framework that encourages modularity and eschews unnecessary complexity, I highly recommend Mithril. It's pretty trivial to hook it up to a REST API which you can build in Rails or whatever.
I've seen Mithril around and it looks interesting, but the thing I'm wrestling with is whether it's better to check out this niche framework or start with something more popular like React (plus whatever for the M_C part of MVC), or Angular.
I think that Angular is the worst choice because developing an app in Angular is so radically different than developing an app in vanilla JS. For a beginning developer I think it's best to first do vanilla JS, then JQuery, then try a lightweight framework like Backbone or React or Mithril. Angular is just a bad decision, it's like patterns just for the sake of patterns.
Focusing on learning frameworks seems like a good idea for resume-building purposes, but they come and go like fashion. Learn Javascript the language very well first, and then learning the framework du jour will be trivial.