Microsoft JScript
isNaN Method
Language Reference
Version 1

See Also Applies To

Determines whether a value is the reserved value NaN (not a number).

isNaN(numvalue) 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 which is not equal to itself.