Momentum, mostly. DT/typings/@types are surprisingly comprehensive and with 2.0 typescript supports strict null checks which was the biggest flaw in my opinion. I've also unfortunately become pretty familiar with some of the obtuse concepts in typescript (e.g. extending module definitions).

I'd love to give Flow a try -- just waiting for the right opportunity.

Awesome, glad to hear they are doing such great work. Thanks for sharing your valuable insight!