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  • Ahmed Elgendy thinks this is interesting:

: If I pass isNaN a string, which isn’t a number, will it return true?

A:

A: It sure will, just as you’d expect. You can expect a variable holding the value NaN, or any other value that isn’t an actual number to result in isNaN returning true (and false otherwise). There are a few caveats to this that you’ll see when we talk about type conversion.

From

Cover of Head First JavaScript Programming

Note

What is NaN? A tip: NaN is anything that Javascript can computes however it cannot be represented by numbers.