I use TypeScript for Web Applications and for both frontend and backend because it has a lot of tooling around it and they really got the types and type safety right. Flow (JS) on the other hand lacks tooling and most of the times I scramble to find the right way of building my contracts in which TypeScript is very intuitive and natural. Additionally TypeScript is very similar to Java so your backend engineers and full stack engineers can work with it without much of context switch.
The only time I think Flow shines is (based on probably my outdated knowledge) Flow is/was the only option if you want/wanted to build a React Native application mainly because React Native transpiler at the time I was working with it would only work with flow.
I use TypeScript because:
- incredible developer tooling and community support
- actively developed and supported by Microsoft (yes, I like Microsoft) ;)
- easier to make sense of a TS codebase because the annotations provide so much more context than plain JS
- refactors become easier (VSCode has superb support for TS)
I've switched back and forth between TS and Flow and decided a year ago to abandon Flow completely in favor of TS. I don't want to bash Flow, however, my main grievances are very poor tooling (editor integration leaves much to be desired), a slower release cycle, and subpar docs and community support.