isNaN

[Michael Brockington]

I (think) I understand the subtle difference between the result of isNaN( someString )  and actually checking if a variable holds the value  NaN  but I notice that your example doesn't show how this causes an issue, and for the life of me I cannot think of any "real world" situation where this would be an issue.
Sure, you might lose a point in an exam, but in my experience, I only ever want to know whether something is a workable number or not - I've NEVER actually cared whether a particular value is NaN itself.

Mike