In the book 'You Don't Know JS', I have a question about isNaN polyfills

Question

In Kyle Simson's book You Don't Know JS,

You can also implement polyfills more simply by applying uniqueness that NaN does not equal to itself. NaN is the only value in all languages of the world "any value other than self is always equal to you."

The polyfill of the function isNaN is proposed as follows.

// polyfill NaN
if (!Number.isNaN) {
  Number.isNaN = function(n) {
    return n !== n;
  };
}

However, is the code n! == n always forced tofalse? (Actually, it was the same when I made the function simple and executed it)

So I don't understand what the code means.

I would be grateful if you could explain this.

Does this answer your question? [How do you check that a number is NaN in JavaScript?](https://stackoverflow.com/questions/2652319/how-do-you-check-that-a-number-is-nan-in-javascript) — Utsav Patel, Apr 01 '20 at 06:04
@UtsavPatel Oh that's not my question but I'll read! Thank you for your information — redchicken, Apr 01 '20 at 06:06
I know that, but there are answers on that link which will answer your question. — Utsav Patel, Apr 01 '20 at 06:07
"*However, is the code `n! == n` always forced to `false`*" but that is **not** the code you have above! `n !== n` - checks if `n` is *not equal* to `n`. However, `n! == n` checks if **not `n`** is equal to `n`. — VLAZ, Apr 01 '20 at 06:08
Note that isNaN and Number.isNaN are two different things. One checks if something isn't a number and the other is completely useless. — Dominic, Apr 01 '20 at 06:09
@UtsavPatel Yup! The answers are very useful. Thank you very much! — redchicken, Apr 01 '20 at 06:13

score 2 · Accepted Answer · answered Apr 01 '20 at 06:06

NaN is the only value in Javascript which, when compared against itself with ===, produces false (or, equivalently, when compared against itself with !==, returns true). Any other value, when checked against itself with ===, will return true.

NaN !== NaN // true
const obj = {};
obj !== obj     // false
0 !== 0         // false
'foo' !== 'foo' // false
// etc, everything else other than NaN
// will produce: // false

So

return n !== n;

is a reliable check of whether n is NaN.

In the book 'You Don't Know JS', I have a question about isNaN polyfills

1 Answers1