Yeah, Flow had the ambition to be sound but has never accomplished it.
If you read the Flow codebase and its Git history, you can see that it's not for lack of trying, either — every couple of years there's an ambitious new engineer with a new plan for how to make it happen. But it's a real tough migration problem — it only works if they can provide a credible, appealing migration path to the other engineers across Facebook/Meta's giant JS codebase. Starting from a language like JS with all the dynamic tricks people use there, that's a tough job.
(And naturally it'd be even harder if they were trying to get any wider community to migrate, outside their own employer.)
Flow doesn't even check that array access is in-bounds, contrast to TypeScript with noUncheckedIndexedAccess on. They're clearly equally willing to make a few trade-offs for developer convenience (a position I entirely agree with FWIW)
Neat example, thanks! I hadn't known TS had that option. Array access was actually exactly the example that came to mind for me in a related discussion:
https://news.ycombinator.com/item?id=41076755
I wonder how widely used that option is. As I said in that other comment, it feels to me like the sort of thing that would produce errors all over the place, and would therefore be a real pain to migrate to. (It'd be just fine if the language semantics were that out-of-bounds array access throws, but that's not the semantics JS has.) I don't have a real empirical sense of that, though.
If you read the Flow codebase and its Git history, you can see that it's not for lack of trying, either — every couple of years there's an ambitious new engineer with a new plan for how to make it happen. But it's a real tough migration problem — it only works if they can provide a credible, appealing migration path to the other engineers across Facebook/Meta's giant JS codebase. Starting from a language like JS with all the dynamic tricks people use there, that's a tough job.
(And naturally it'd be even harder if they were trying to get any wider community to migrate, outside their own employer.)