NaN(View original on MDN)

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

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.

It is rather rare to use NaN in a program. It is the returned value when Math functions fail (like Math.sqrt(-1)) or when a function trying to parse a number fails (like parseInt("blabla")).

Testing against NaN

Equality operator (== and ===) can't be used to test a value against NaN. Use isNaN() instead:

NaN === NaN;        // false
Number.NaN === NaN; // false
isNaN(NaN);         // true
isNaN(Number.NaN);  // true