How do you check that a number is NaN in JavaScript?

0 votes
asked Apr 16, 2010 by paul-d-waite

I’ve only been trying it in Firefox’s JavaScript console, but neither of the following statements return true:

parseFloat('geoff') == NaN;

parseFloat('geoff') == Number.NaN;

26 Answers

0 votes
answered Apr 16, 2010 by jerome-wagner

You should use the global isNaN(value) function call, because:

  • It is supported cross-browser
  • See isNaN for documentation


 isNaN('geoff'); // true
 isNaN('3'); // false

I hope this will help you.

0 votes
answered Apr 16, 2010 by chiborg

Try this code:


For checking whether any value is NaN, instead of just numbers, see here: How do you test for NaN in Javascript?

0 votes
answered Apr 16, 2010 by rahul

Use this code:


See isNaN() docs on MDN.

alert ( isNaN('abcd'));  // alerts true
alert ( isNaN('2.0'));  // alerts false
alert ( isNaN(2.0));  // alerts false
0 votes
answered Apr 18, 2012 by d2vid

I use underscore's isNaN function because in JavaScript:

-> true

At the least, be aware of that gotcha.

0 votes
answered Apr 6, 2013 by stephan-ahlf

It seems that isNaN() is not supported in Node.js out of the box.
I worked around with

var value = 1;
if (parseFloat(stringValue)+"" !== "NaN") value = parseFloat(stringValue);
0 votes
answered Apr 7, 2013 by jazzy

I just came across this technique in the book Effective JavaScript that is pretty simple:

Since NaN is the only JavaScript value that is treated as unequal to itself, you can always test if a value is NaN by checking it for equality to itself:

var a = NaN;
a !== a; // true 

var b = "foo";
b !== b; // false 

var c = undefined; 
c !== c; // false

var d = {};
d !== d; // false

var e = { valueOf: "foo" }; 
e !== e; // false

Didn't realize this until @allsyed commented, but this is in the ECMA spec:

0 votes
answered Apr 8, 2013 by jonathan-azulay

NaN is a special value that can't be tested like that. An interesting thing I just wanted to share is this

var nanValue = NaN;
if(nanValue !== nanValue) // Returns true!
    alert('nanValue is NaN');

This returns true only for NaN values and Is a safe way of testing. Should definitely be wrapped in a function or atleast commented, because It doesnt make much sense obviously to test if the same variable is not equal to each other, hehe.

0 votes
answered Apr 24, 2013 by ryan-griffith

While @chiborg 's answer IS correct, there is more to it that should be noted:

parseFloat('1.2geoff'); // => 1.2
isNaN(parseFloat('1.2geoff')); // => false
isNaN(parseFloat('.2geoff')); // => false
isNaN(parseFloat('geoff')); // => true

Point being, if you're using this method for validation of input, the result will be rather liberal.

So, yes you can use parseFloat(string) (or in the case of full numbers parseInt(string, radix)' and then subsequently wrap that with isNaN(), but be aware of the gotcha with numbers intertwined with additional non-numeric characters.

0 votes
answered Apr 8, 2014 by muratsplat
NaN === NaN;        // false
Number.NaN === NaN; // false
isNaN(NaN);         // true
isNaN(Number.NaN);  // true

Equality operator (== and ===) cannot be used to test a value against NaN.

Look at Mozilla Documentation The global NaN property is a value representing Not-A-Numbe

The best way is using 'isNaN()' which is buit-in function to check NaN. All browsers supports the way..

0 votes
answered Apr 17, 2014 by marksyzm

To fix the issue where '1.2geoff' becomes parsed, just use the Number() parser instead.

So rather than this:

parseFloat('1.2geoff'); // => 1.2
isNaN(parseFloat('1.2geoff')); // => false
isNaN(parseFloat('.2geoff')); // => false
isNaN(parseFloat('geoff')); // => true

Do this:

Number('1.2geoff'); // => NaN
isNaN(Number('1.2geoff')); // => true
isNaN(Number('.2geoff')); // => true
isNaN(Number('geoff')); // => true

EDIT: I just noticed another issue from this though... false values (and true as a real boolean) passed into Number() return as 0! In which case... parseFloat works every time instead. So fall back to that:

function definitelyNaN (val) {
    return isNaN(val && val !== true ? Number(val) : parseFloat(val));

And that covers seemingly everything. I benchmarked it at 90% slower than lodash's _.isNaN but then that one doesn't cover all the NaN's:

Just to be clear, mine takes care of the human literal interpretation of something that is "Not a Number" and lodash's takes care of the computer literal interpretation of checking if something is "NaN".

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