I'm using Flow but I'd recommend TypeScript. I love Flow but TypeScript just seems to be more seriously maintained and less buggy. I've been waiting some bug fixes for 2 years (like all the object destructing and spread issues[1]), but they prioritize the Facebook private internal roadmap before anything else (like they improve the performances in almost every patch, but it's useful only for FB and its millions of lines of code). So yeah, the support is kind of inexistent and it's frustrating. When you find a bug, you have to rewrite your code in another way to workaround…
On the other hand, I don't agree with some of the comments here. For example, Flow is not terminal only at all, I never use the terminal to run Flow. The editor integration is totally fine, especially in Atom and VSCode.
JS with flow is way better than JS without flow. It prevents a whole class of errors, and you can basically opt-in and opt-out of the type system at any time if you're having trouble with it too. I wrote a blog post about it a while back: https://www.aria.ai/blog/posts/why-use-flow.html
That being said, while I started out using Flow, TypeScript just has way more community adoption and better tooling. So at this point, I usually recommend TS over Flow to most people. But using either of them is way better than writing just regular JS code.
Typescript has way better editor support, and is faster on large projects. Flow is mostly terminal only, and required a really annoying syntax in it's config file to exclude all node modules.
That being said, Flow is better then no typechecks, and it was half a year ago that I looked, stuff might have improved.
On the other hand, I don't agree with some of the comments here. For example, Flow is not terminal only at all, I never use the terminal to run Flow. The editor integration is totally fine, especially in Atom and VSCode.
[1] https://github.com/facebook/flow/issues/2405