I recently underwent a very similar experience at MemSQL[1] but for only 30K LOC. The pain points that led us away from Flow to TypeScript seem to be the same as the ones as Quizlet's .

Overall, I'm very happy to see more and more teams move to TypeScript as a direct consequence of its Babel 7 support. TypeScript makes more sense as a pure type checker and not as a full blown transpiler.

It's also interesting to see that their type coverage increased from 66% to 86% (ours increased from 88% to 96%). This pattern is probably because of better third party type definitions.

I still prefer Flow's Type Inference and philosophy over TypeScript's, but in practice TypeScript is much better pretty much any use case I can think of.

[1]: https://davidgomes.com/porting-30k-lines-of-code-from-flow-t...