[jameslk]

I have used both TypeScript (for multiple projects) and Flow (at Facebook). TypeScript is much more feature-rich and looser in strictness. In practice, this looseness has been preferable, as I've seen many of my colleagues fighting Flow's type system vs TypeScript (prior to FB). In theory, perhaps Flow's type system prevents more bugs, but I've sometimes seen it circumvented with hacks just to get past its strictness. TypeScript's ability to infer types seems much smarter than Flow's as well.

Besides that, TypeScript has much wider adoption and community support. Most issues I have with TypeScript I can find others who have had similar issues on the web, where as I can't say the same about Flow.

[lhorie]

There's been a lot of doubt cast on Flow outside of Facebook when Jest announced it was moving to TS.

I'm curious to hear how Typescript vs Flow discussions go within Facebook, and what engineers there think of each.

[ajbogh]

I would also like to know how many projects within Facebook, if any, use Typescript. I'd be interested in the growth rate as well (how many projects over time).