Why parseInt(key) === NaN always evaluates to false

Question

I'm doing decision based on dictionary key, i have only two types of keys, numeric and alpha,

var _dict = { 'a':'one', 'b':'two', '1':'three' };

$.each( _dict, function( key, value ){

  if( parseInt( key ) === NaN ) {

     // this statement always evalute to false

    } else {

    }

});

if i print console.log(parseInt('a')), it will also return NaN

I alreay found the solution from this question javascript parseInt return NaN for empty string

But i was wondering why it always evaluates to false.

`NaN != NaN`, use `isNaN(parseInt( key ))` or in your case `!parseInt( key )` — A. Wolff, Sep 01 '16 at 11:28
and NaN !== NaN because so it was in the beginning and to change it now may break the www's (or parts of it) — Jaromanda X, Sep 01 '16 at 12:39

score 3 · Accepted Answer · answered Sep 01 '16 at 11:35

3

That's because NaN is defined to be not equal to anything (including itself). Check it:

>> NaN === NaN
False

You should use isNaN() function instead:

>> isNaN(NaN)
true
>> isNaN(0/0)
true
>> isNaN(parseInt('a'))
true

answered Sep 01 '16 at 11:35

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Why parseInt(key) === NaN always evaluates to false

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