Question

We can do:

NaN = 'foo'

as well as

undefined = 'foo'

Why are they not reserved keywords?

1. I think it should be implemented in order to be sure that when we are looking for a number, it is a number 🙂

2. If we should use IsNaN() or typeof, why are NaN or undefined needed?

Answer 1

https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/NaN

NaN is a property of the global object.

The initial value of NaN is Not-A-Number — the same as the value of
Number.NaN. In modern browsers, NaN is a non-configurable,
non-writable property. Even when this is not the case, avoid
overriding it.

https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/undefined

undefined is a property of the global object, i.e. it is a variable in global scope.

The initial value of undefined is the primitive value undefined.