Definition: NaN is a global property (variable) with a constant value. NaN is an abbreviation for "Not a Number". Comparison of any object to this property will return true if the object is a number and false if it is not. This is not a reserved word so you can declare your own variable or function called NaN but if you do then you will not be able to reference the global property to test for if a field contains a numeric value.
Examples:
if (x == NaN)
