Comparing with NaN in Javascript

Question

I have met this strange issue.

enter image description here

Why is so?! Should it be so?

Please add some text to the question, or ask your question at `imgur`... :) — gdoron is supporting Monica, May 28 '12 at 17:03
possible duplicate of [Javascript variable / 'NaN' behaving weirdly](http://stackoverflow.com/questions/10083937/javascript-variable-nan-behaving-weirdly) — Josh Lee, May 28 '12 at 17:12

score 12 · Accepted Answer · answered May 28 '12 at 17:05

12

From the MDN docs for isNaN:

Unlike all other possible values in JavaScript, it is not possible to rely on the equality operators (== and ===) to determine whether a value is NaN or not, because both NaN == NaN and NaN === NaN evaluate to false. Hence, the necessity of an isNaN function.

use isNaN instead.

answered May 28 '12 at 17:05

Andrew Whitaker

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Intuitively: 3 is equal to 3, 4 is equal to 4, but non-number (say, "apple") is not necessarily equal to non-number (say, "orange"). – Amadan May 28 '12 at 17:12

score 5 · Answer 2 · edited Sep 19 '19 at 18:06

5

The reason behind this is that the rules of mathematics should be preserved. Otherwise, one would have x == x + 1 if x is NaN, which is not a true relationship for any other value of x.

edited Sep 19 '19 at 18:06

ata

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answered May 28 '12 at 17:17

JohnB

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score 3 · Answer 3 · answered May 28 '12 at 17:05

3

that's why we use

isNaN(x)

it seems that x is a NaN object and it does not compare equal to another

answered May 28 '12 at 17:05

neu-rah

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score 2 · Answer 4 · edited Sep 19 '19 at 18:07

2

NaN is like an SQL null. It is never equal to anything, including itself. That's why there's the special case function isNaN() to safely test for NaN's presence.

edited Sep 19 '19 at 18:07

ata

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answered May 28 '12 at 17:04

Marc B

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Comparing with NaN in Javascript

4 Answers4

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