Question

({}).toString.call(Number.prototype) === "[object Number]"

The Number prototype object is itself a Number object (its [[Class]] is "Number") whose value is +0.

15.7.4

Why would it be useful for Number.prototype to be a Number? (the same goes for every other built-in prototype which has the [[Class]] set to not Object)

I’m picking on Number.prototype specifically because I can imagine sensible legacy reasons for Array.prototype and Date.prototype.

Answer 1

In general, Constructor.prototype is an exemplar of the “type” defined by Constructor. Although things seem to get hairy for immutable primitives, and especially once you involve the boxing stuff, this exemplar concept still makes sense, with 0 being the “exemplar” of Number.