I was brushing up on some JavaScript for work and came across a problem that has bugged me before: testing for NaN¹. I’ve never dug too deeply into this one, I’ve just known that if you try something like this:

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var a = NaN
if (a == NaN){
   //a is NaN, do something
}
else{
   //a is a number, do something else
}

The code would always do something else. so I usually found a different way to solve the problem.

Today I actually dug into it and found that in JavaScript the value NaN has some odd properties:

1. It’s a number.
2. It is the one value in JavaScript that can never equal itself.

So that’s annoying, but actually lends itself to a very easy solution: if you want to test if a variable is NaN you can test it like this:

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if (a !== a){
	// a is NaN
}else{
	// a is a number
}

But people who are good at JavaScript already know this. This is not news.

What struck me about this is that Douglas Adams invented a word for this kind of value:

… a recipriversexcluson, a number whose existence can only be defined as being anything other than itself. Douglas Adams, *The Ultimate Hitchhiker’s Guide to the Galaxy (p. 345). Random House Publishing Group. Kindle Edition. *

Somehow this made me actually like JavaScript more.