This is of course how it is supposed to work. It is useful to define NaN this way, as a simple test for NaN is (num == num) returning false.

While we're on the topic of NaN, I'd like to mention a lesson from my little book of JavaScript you should never do[1]: You should never use null in JavaScript for the value of a yet-to-determine number. Always use NaN.

Why? It will be easier to detect problems:

    var determineLater = NaN;
    var blah = 5;
    blah += determineLater;

blah is now NaN because (5 + NaN) is NaN. We want this because we want to know if there's been some kind of error in the making of blah

Better:

    var determineLater = null;
    var blah = 5;
    blah += determineLater;

blah is now 5 because (5 + null) is 5. We do not want this because it doesn't let us know that there's been an error (the error typically being that determineLater was - incorrectly - never determined!)

[1] not a real book, just a text file of mis-use cases.