[OmarIsmail]

We made the decision some months ago and Flow's ES6 support was better than TypeScript's (and we already have a lot of ES6 code).

We also have an established toolchain with gulp and browserify and babel, and again, at the time TS didn't play as nicely (vs Flow which just worked). Things are definitely improving in the TS world, and I keep tabs on it. The fortunate thing is that both Flow and TS' annotations are compatible, so it should be relatively easy to switch from one to another.

Whichever one you go with doesn't actually matter though. As long as you go with one of them you'll see a massive increase in productivity vs vanilla JS.