Question

84

How do you test for NaN in JavaScript?

rated 0 times [ 85] [ 1] / answers: 1 / hits: 63293 / 9 Years ago, mon, may 18, 2015, 12:00:00

I have a variable x and I want to test if x is set to NaN. How do I do that?

My first instinct is probably to, you know, test it, like this:

if (x === NaN) {  ...

Silly rabbit, no, that would be far too easy. NaN is like NULL in SQL, it is not equal to anything, even itself.

But look, there is a function called isNaN() -- maybe that will do it!

No, so far as I can tell, isNaN() is utterly worthless.

For example, isNaN([]) properly returns false, but isNaN([.]) returns true. You don't want do know how I learned about this flaw.

How do I do this?

Turns out, my question is a duplicate of this one, but the selected answer is wrong. The right answer has 20% as many upvotes.

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lelasamiraa

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3 Years ago

answered 9 Years ago dylondaytond · Accepted Answer

The question has the answer if you read closely enough. That is the way I found it: I was typing out the question and... bingo.

You remember when I wrote NaN is like NULL in SQL, it is not equal to anything, even itself? So far as I know, NaN is the only value in Javascript with this property. Therefore you can write:

var reallyIsNaN = function(x) {

   return x !== x;

};