Javascript isNAN returns false if checking Object?

Question

I have a script which behaves strangely.

console.log (typeof button);
console.log ('IsNAN ' + isNaN(button));
console.log ('isNumeric ' + jQuery.isNumeric(button));
console.log ();

when button is an object, isNaN should return true; Because object isNaN. However console shows differently:

object
IsNAN false // should be true!
isNumeric false

Why is that?

Edit: The Problematic script is http://flesler-plugins.googlecode.com/files/jquery.serialScroll-1.2.2.js

Line 126 @ function jump( e, button ).

possible duplicate of [How do you check that a number is NaN in JavaScript?](http://stackoverflow.com/questions/2652319/how-do-you-check-that-a-number-is-nan-in-javascript) — Davin Tryon, Feb 28 '14 at 12:07
Is it a blank object? It sounds like `isNaN` is type converting your falsy object to 0 — Mark Walters, Feb 28 '14 at 12:08
note that isNaN function is broken: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN#Description — AO_, Feb 28 '14 at 12:08
Firebug shows that button is: a#next-photo. so jQuery or HTMLObject if I'm not wrong. — Grzegorz, Feb 28 '14 at 12:10
@Gacek: Never just link, put the code **in** the question. Links rot. — T.J. Crowder, Feb 28 '14 at 12:17

T.J. Crowder · Accepted Answer · 2014-02-28T12:16:13.367

IsNAN false // should be true!

Not necessarily. :-) isNaN is only true if the argument to it (converted to a Number if necesssary) is NaN.

Depending on what the object is, it may or may not be NaN when converted to a Number. It depends on what happens when the object is converted to its primitive value. null, for instance, converts to 0. In many, many other cases conversion to primitive involves turning it into a string (arrays, for instance), and the default toString of some objects may return something that can be converted to a non-NaN Number.

Examples of non-NaN objects:

// null
console.log(Number(null));          // 0

// an array (which becomes "23" when converted to primitive, because
// arrays do `join` for `toString`; then the "23" becomes 23)
console.log(Number([23]));          // 23

// another object implementing toString or valueOf in a way that returns
// something convertable to a non-NaN number
console.log(Number(new Foo("47"))); // 47

...where Foo is:

function Foo(value) {
    this.value = value;
}
Foo.prototype.toString = function() {
    return this.value;
};

(In the above, toString could be valueOf as well.)

Wow! Nice anaswer! Thank you everyone for theirs. This one seems to be best one tho. Didn't know that isNaN firstly tries to cast value into a number, then checks if it's NaN. — Grzegorz, Feb 28 '14 at 12:22

score 2 · Answer 2 · answered Feb 28 '14 at 12:11

2

isNaN() called with a null object is expected to be false.

If your object is not null and cannot be converted to a number then it will return true.

answered Feb 28 '14 at 12:11

harun

1,889
18
19

score 0 · Answer 3 · answered Feb 28 '14 at 12:12

0

I think your variable button is equal to null.

typeof(null) == 'object'

isNaN(null) == false

answered Feb 28 '14 at 12:12

Andrey Bogdanov

126
2

Javascript isNAN returns false if checking Object?

3 Answers3