It is almost impossible for the Haskell metadata widget I described to have bugs. But I've seen production bugs in the corresponding JS code twice in the last few months. You simply cannot get the kind of correctness guarantees in JS that you have in Haskell.
I described a very specific example of something Haskell can do that JS cannot in this reddit thread:
https://www.reddit.com/r/haskell/comments/4ujg5i/what_are_yo...
It is almost impossible for the Haskell metadata widget I described to have bugs. But I've seen production bugs in the corresponding JS code twice in the last few months. You simply cannot get the kind of correctness guarantees in JS that you have in Haskell.