alt Hacker News* only if `x` is _an object_ (read: has methods)
To preempt the obvious: yes, I know _everything_ (nearly) in JavaScript is an object, but a module exporting a `Function` can expect the caller to use the function, not enumerate it for methods. And the function can use a declaration in the module that wasn't exported, with the caller none the wiser about it.
I think you're confusing values with types. JS modules can certainly keep a value private, but there's no way for them to expose an opaque type, because that concept simply doesn't exist in JS. The language only has a few types, and you don't get to make more of them. TypeScript adds a lot of type mechanism on top, but because it's restricted to being strippable from the actual JS code, it doesn't fundamentally change that.