Microsoft® JScript®
isNaN Method
 Language Reference 
Version 1 

See Also                  Applies To

Returns a Boolean value that indicates whether a value is the reserved value NaN (not a number).

The numvalue argument is the value to be tested against NaN.

The isNaN function returns true if the value is NaN, and false otherwise. You typically use this function to test return values from the parseInt and parseFloat methods.

Alternatively, a variable could be compared to itself. If it compares as unequal, it is NaN. This is because NaN is the only value that is not equal to itself.