That was a big part of my motivation in building http://LightScript.org as it was clear that was never going to happen with coffee.

Currently works with Flow syntax, but not yet the actual flow checker - would love help making that happen!

That'd be particularly difficult as CoffeeScript doesn't have var declarations.

Partly. You can define them if you want it. You can even add type support with FB Flow.

CS developers sometimes define variables because of hoisting.

Obviously, variables are defined. In CS they are simply defined for you automatically. That said, you can obviously do it yourself if you want. But you will not be able to use additional tooling around `let` and `const`

Can one use FB flow with CoffeeScript? Comments I saw on github indicated that the CS compiler mangled it, I think due to how functions are named and stripping comments?

I personally don't really like types, but I've successfully used FB Flow with CS and it was fine.

Got any docs/writeup for this? Any tips/tricks/hacks?