Firstly, I suck at JavaScript, so take this for what it's worth ...

I'm surprised React.js is so popular. It is a very large library, which seems to be mostly used for doing fairly simple tasks. For most instances, I'd have thought a much lighter library would have been preferred.

For my own purposes, I've found a very small custom script was more than ample (I admittedly haven't attempted anything as large as Calypso before though).