I've been a heavy user of Flow for - two years now? I also occasionally use TypeScript. I don't contribute to the code (in the Flow case, I don't speak OCaml, I contributed a bit to the type library and the online docs; in the case of TS, I don't use it enough), but I write a bug report each time I find an issue.

I've been looking at the issues list very often, mostly out of curiosity to see what the problems are that people have (both Flow and TS) and to see which project is better at solving problems.

TypeScript seems to win hands down. Their ratio of closed tickets to open tickets is far better than that of Flow:

- TypeScript: 2,515 open, 12,716 closed -- https://github.com/Microsoft/TypeScript/issues

- Flow: 1,830 open, 2,467 closed -- https://github.com/facebook/flow/issues

I have to assume they mostly close issues after solving them - it is not unheard of that issues are closed merely because they've been lying around for a few months without any updates (definitely happens for Flow).

I prefer the approach of Flow, but I think the actual developers are not present at all on Github, most answers come from other users. There are very few actual solutions to issues that are raised. The list of issues grows ever larger. Flow does make progress, and it's not even bad. Every 10-15 days we get a new release and I'd say the progress is quite okay. What went really well the last 12 months was IDE integration for Flow in the case of Jetbrains Webstorm (for TS the situation always looked good), as of now, latest Webstorm version, it's actually usable. I still get too many Flow server crashes though, which may be due to my platform (Windows 10), not sure. However, it's now automatically restarted most of the time.

A TypeScript anecdote: I filed an issue that a "Member" closed as "works as intended" after a few comments back and forth (but it was clear right away that this would be the end). I filed it again - this time Anders Hejlsberg himself responded and acknowledged the issue (it was kind of fundamental and not just something unimportant), and it got fixed. So here too you need a bit of luck to get through to someone who actually understands the subject well enough (including knowing when they don't understand it, if that first guy had just stopped handling my issue it would have been fine). But that's the price of using a popular large project, so this is not a complaint - just an anecdote, okay?

What I do get for myself after following the two type systems for well over two years is that there are LOTS of restrictions. Just read some of the issues in both Github repos, it's really interesting.

I find the whole situation very unsatisfying. It is clear - when you actually use it heavily for a big project for a longer period - that a type system on top of Javascript is very difficult to maintain. LOTS of unsolved cases and issues. You definitely have to adapt and use only the subset of Javascript and the type system that works. I validate an object manually and I still have to do an "any" cast in Flow because Flow does not recognize that my `if` statement secures the type. Or I have to insert such `if` statements even though from the context it is perfectly clear what I get there. For example, any inner functions, e.g. when using map, filter, reduce, loses the type if you use a variable from the outer scope. Or you have to use a lot of additional `const` for object properties even though you don't change the object - but the type checker is not intelligent enough to recognize that. You definitely write different code - and while that may seem okay there are plenty of cases where it really is objectively unnecessary, but you have to do it to satisfy the type system.

On the bright side, once you get your code all "type system ready" you really do find bugs - even if it's just a few. Also good is that I now get much better autocompletion suggestions in the IDE. There's a reason why I (as project head) still insist on us using Flow even though I say "WTF?" often enough. It does cost a significant amount of time though - a very significant amount. I've been willing to do it because of the kind of project, which is going to be a very low-level building block for later larger projects, so it is worth spending a lot more resources on getting it right compared to a project that's on a top layer, on which nothing else depends.

Still, overall I have very mixed feelings and I see the type checkers (both TS or Flow, or any other that could be written) on top of a language that doesn't have types as a kludge that does not look like a long-term solution. There is too much friction between the language and the type system.