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

I just want to share another alternative, it's not necessarily better than others here, but I think it's worth looking at:

function customIsNaN(x) { return (typeof x == 'number' && x != 0 && !x); }

The logic behind this is that every number except 0 and NaN are cast to true.

I've done a quick test, and it performs as good as Number.isNaN and as checking against itself for false. All three perform better than isNan

The results

customIsNaN(NaN);            // true
customIsNaN(0/0);            // true
customIsNaN(+new Date('?')); // true

customIsNaN(0);          // false
customIsNaN(false);      // false
customIsNaN(null);       // false
customIsNaN(undefined);  // false
customIsNaN({});         // false
customIsNaN('');         // false

May become useful if you want to avoid the broken isNaN function.