> since introducing TS to you project means longer time train new employees.
From my experience, even developers who have never used statically typed languages are able top pick up the Flow / TypeScript in a matter of hours. Not everything, but enough to barely have an impact on productivity. The main hurdles in the learning process as far as I can tell have been nullable types (smaller hurdle) and declaring types for external modules (big hurdle, many of them still don't fully grasp it).
I've found Flow to have a much, much lower barrier to entry than TypeScript, and I believe it's because it's quite strict / sound as the author very well pointed out (slide 12 for example).
From my experience, even developers who have never used statically typed languages are able top pick up the Flow / TypeScript in a matter of hours. Not everything, but enough to barely have an impact on productivity. The main hurdles in the learning process as far as I can tell have been nullable types (smaller hurdle) and declaring types for external modules (big hurdle, many of them still don't fully grasp it).
I've found Flow to have a much, much lower barrier to entry than TypeScript, and I believe it's because it's quite strict / sound as the author very well pointed out (slide 12 for example).