[kevincennis]

But what are the own properties of a number? There's nothing to iterate, and therefore nothing to map.

WeakMap is a new-ish type whose keys are objects. The references to those objects are weakly held, which means that an object `x` whose only reference is a WeakMap key will still be eligible for garbage collection. They are non-iterable for exactly that reason.

Also: how do you iterate over `false` or a function? What about user-created classes?

There are so many cases where the behavior would either be nonsensical or at least unintuitive that (in my personal and completely subjective opinion) they outweigh the slight convenience of being able to "map" a plain old JavaScript object.

[radarsat1]

No answer for WeakMap, and like I said, not sure about integers, but if you take a look at Haskell monads, you'll see that surprising things like Maybe (booleans, essentially, or rather container classes that either contain something or don't) can be iterated over. It's a very useful concept that abstracts somewhat the idea of "iterable", to "do something with all the things", where in the case of Maybe, all the things is either one thing or nothing.

Granted, in JavaScript maybe this wouldn't make sense.