In the second problem:
typeof x; // number
typeof y; // number
isNaN(x); // false
isNaN(y); // false
I can tell you that at least one of x and y is "intuitively" a number.
This is something that ends up being quite peculiar to an algebraic intuition, so if you haven't seen it in the wild it may not be something you can logic out (unless you take quite a leap). It is, however, frequently referenced in lists of language "gotchas".
I don't think I've actually spotted it anywhere else but JS, though technically according to the IEEE 754 specification it should be applicable elsewhere. Possibly it's just that since JS lacks any additional numeric types it ends up being more common.
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