Why 'NaN' and 'Undefined' are not reserved keywords in JavaScript?

Question

We can do:

NaN = \'foo\'

as well as

undefined = \'foo\'

Why are they not reserved keywords?

Edit

Answer 1

Both NaN and undefined are values in JavaScript.

1. NaN is of number type. (it denotes "not a valid number").
2. undefined is of undefined type. (when there is nothing assigned to a variable, JavaScript assigned undefined particular in var declaration).

And also both are read-only values(property) of global window object.

So that's why JavaScript cannot make values as a reserved word.