The value of NaN is NaN. This represents the number value that's not actually a number.
NaN will return false to any comparison. To detect a NaN you should use the isNaN() global method.
global.NaN has PD{[[Writable]]:false, [[Enumerable]]:false, [[Configurable]]:false}. So you should not be able to change the value (but nothing prevents you from redeclaring it in a lower scope...).