I recommend TypeScript. When used correctly, TypeScript can enable your application to be scalable, easy to refactor, safe, and stable. One of the biggest draws of working with any typed language is that it forces you to think about your functions' inputs and outputs. This is invaluable as it can lead to more declarative, functional style code that ultimately can be easier to reason about.

TypeScript is however not a silver bullet. Just like anything new it takes time to fully understand the concepts of types, interfaces, abstract classes, and enums. In my experience engineers who excel when using TypeScript are those who have experience working with a statically typed language.

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.