How do you test for NaN in JavaScript?

Question

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.

Rheumatic

"You don't want do know how I learned about this flaw." - I wonder why you would call isNaN on an array? – Undergrowth

@Undergrowth -- I was writing a date-picker. I almost punched the screen when ["2015", "05", "05"] was marked as NaN. – Rheumatic

But why are you expecting an array to behave like a number at all? – Undergrowth

@Undergrowth -- I am not. Angular is giving me NaN to mean "the data isn't available yet", and the actual data (which in this case is an array) when it is. I need a way to know whether the data is there. – Rheumatic

OK, then I'd rather use Array.isArray to detect the data, instead of detecting its absence. – Undergrowth

@Undergrowth -- if you look at the original question, you can see the issue: Angular calls ngModel.$render with NaN if the data isn't ready and with the data if it is. The data is an array in one case, but there are lots of other cases. Of course, there is no reason that NaN itself couldn't be a legal value... – Rheumatic

As regards the answer to my question, I was asking how to check that a number is NaN, not how to check whether any value is NaN. So I guess this question actually isn’t answered by the answers to mine, and thus isn’t a duplicate. – Mcneely

Answer 1

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;
};

Rheumatic

After reading MDN docs for Number.NaN, it suggests to also check if typeof x === "number". isNaN is different in that it coerces the value to a number before comparing (+[] == 0, wat?) – Ehling

@Ehling When you say +[] == 0 what happens is that javascript tries to convert the first value of the array to integer. Try to call array[0] un an empty array and it will return undefined. This conversion is not equal that of parseInt and undefined will be converted to 0, try +[1] and + ['1223fsds']. Finally 0 == 0. That's why you get that result. – Blennioid

@Ehling Yes I saw the video. It was hilarious. Check this post #30326396 – Blennioid

@Ehling - wat is not wat. – Rheumatic

Please keep calm and remember that Java, C#, Python, Scala, C++ (under clang, g++ and msvc) all have the same NaN behaviour as JavaScript. In all of those languages 0/0 is NaN and in all of them NaN is not equal to itself. It is simply what the standard requires – Flipflop

@BenjaminGruenbaum — Standards are not moral imperatives. The usual arguments for adhering to the standards are (1) they represent Best Practices in the considered opinion of a group of experts and (2) they are what the user is likely to expect. Neither of those obtain in this case. – Rheumatic

Perhaps, but this is more of a CPU architecture problem than a JavaScript one - JavaScript is simply doing what every other language in the world is doing in this case. – Flipflop

What if x is "2"? This would fail. Because x="2"; x !==x; // is false – Considerable

Answer 2

Short Answer

For ECMAScript-5 Users:

#1
if(x !== x) {
    console.info('x is NaN.');
}
else {
    console.info('x is NOT a NaN.');
}

For people using ECMAScript-6:

#2
Number.isNaN(x);

And For consistency purpose across ECMAScript 5 & 6 both, you can also use this polyfill for Number.isNan

#3
//Polyfill from MDN
Number.isNaN = Number.isNaN || function(value) {
    return typeof value === "number" && isNaN(value);
}
// Or
Number.isNaN = Number.isNaN || function(value) {     
    return value !== value;
}

Note: I prefer to test using #1 which works same all places and does not have dependency on latest JS also. (It always gives me correct result. No surprises!)

---

Detailed Explanation:

Here is our awesome NaN

NaN == NaN; // false
NaN === NaN; // false

Please don't blame JavaScript for this, it is NaN which is supposed to behave this way in other languages also Which is fine as per rationale for all comparisons returning false NaN values

So comes isNaN as our savior, but wait it acts little differently in some scenarios like

isNaN(undefined); // true
isNaN({});        // true
isNaN("lorem ipsum"); // true

I had some strange faces by seeing the results above. And here comes reason from MDN

When the argument to the isNaN function is not of type Number, the value is first coerced to a Number. The resulting value is then tested to determine whether it is NaN.

So how should we test NaN for the non-numbers variables at all? I always go by the following

if(x !== x) {
    console.info('Is a NaN');
}
else {
    console.info('Not a NaN');
}

ECMAScript-6/JavaScript-2015 Updates

Do we have anything in ECMAScript-6 for the same. Yup we do...

Number.isNaN(x); // true

The ES6 implementation will also be helpful for the above cases like

Number.isNaN(undefined); // false
Number.isNaN({}); // false    
Number.isNaN("lorem ipsum"); // false

whereas ECMAScript-5 global function isNaN outputs in true for the above cases, which sometimes may not align with our expectation.

Clean

like isNaN( "" ) = false and isNaN( {} ) = true while Number.isNaN( "" ) = false and Number.isNaN( {} ) = false – Adequacy

Polyfill could also be as simple as Number['isNaN'] || (Number.isNaN = function(a) { return a !== a }); Basically the same, minus a handful or so of chars and with the variable set moved to the other side of the operator. – Hiller

Read more about isNaN() here: w3schools.com/jsref/jsref_isnan.asp – Wetnurse

Answer 3

It looks like an overkill but how about trying this?:

if (Number(dataInput) !== Number(dataInput))
{
    ...
}

Philanthropy

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