[carlmungz]

Mithril (https://mithril.js.org) is my choice. It's React-like but very simple (10 minutes to learn about components, routing and XHR) and has a smaller footprint.

[brlewis]

Seconded. I actually like Mithril streams better than Flux for loose coupling of components. http://brlewis.github.io/2017/brlewis-rollup/