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

Question

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

parseFloat(\'geoff\') == NaN;

parseFloat(\'ge         


        

Answer 1

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:

http://jsperf.com/own-isnan-vs-underscore-lodash-isnan

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".