When does a !== a?

Asked Aug 12 '14 at 19:58

Active Aug 12 '14 at 20:33

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I ran into the expression a !== a in someone else's code:

function isEmpty(a) {
    return a === undefined || a === '' || a === null || a !== a;
}

The only case where I can think of the final expression evaluating to true is when isNaN(a). Are there any other values of a for which a !== a is true?

edited Aug 12 '14 at 20:26

asked Aug 12 '14 at 19:58

Snowball

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Unless you have some additional context, I don't see how this is reasonably on-topic on Stack Overflow. – Robert Harvey Aug 12 '14 at 20:00
This mostly covered here: http://stackoverflow.com/a/359509/1026459 – Travis J Aug 12 '14 at 20:02
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`NaN !== NaN`, but everything else is equal to itself (to my knowledge). – ckersch Aug 12 '14 at 20:04
@apsillers: Thanks, it is indeed a duplicate of that. – Snowball Aug 12 '14 at 20:06
@RobertHarvey: Is http://stackoverflow.com/questions/19955493/is-there-any-value-for-what-x-x-returns-false-without-nan also off-topic? – Snowball Aug 12 '14 at 20:11
That question at least has some context. – Robert Harvey Aug 12 '14 at 20:18
@RobertHarvey: (edited) Is this better? I'm trying to understand what you mean by context, since I thought my question had enough context before to answer it unambiguously. – Snowball Aug 12 '14 at 20:27
The best context would be the code where you saw the `a !== a`. – Robert Harvey Aug 12 '14 at 20:32
@RobertHarvey: I'll try to include code samples in future questions. Thanks for updating the reason for closure. – Snowball Aug 12 '14 at 20:42

When does a !== a?

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