Is there any value for what x === x returns false without NaN?

Question

Is there any value for what x === x returns false without NaN?

For example:

> x = 1
1
> x === x
true
> x = {}
{}
> x === x
true
> x = new Date()
Wed Nov 13 2013 15:44:22 GMT+0200 (EET)
> x === x
true
> x = NaN
NaN
> x === x
false

I see that the only value where x === x returns false is when isNaN(x) === true.

Is there another value of x for what x === x returns false? An official reference would be welcome!

And what is `x`? Maybe use `x` and `y` or something in your example. — putvande, Nov 13 '13 at 13:49
@イオニカビザウ Your profile message widget is really interesting! Is the code available anywhere? — ComFreek, Nov 13 '13 at 13:59
@ComFreek Hahaha, It's not opensource, but that's very simple. Do you want to publish it? Maybe I will... — Ionică Bizău, Nov 13 '13 at 14:02
@イオニカビザウ No, not publishing, just using it for my own profile (of course with proper attribution) ;) — ComFreek, Nov 13 '13 at 14:03

score 20 · Accepted Answer · edited May 23 '17 at 11:45

20

The strict comparison between two equal non-NaN values will always be true (SLaks's answer correctly quotes the spec). However, it's possible for the expression x to change its value during the evaluation of the the equality. This can happen with property access when using accessor property descriptors (i.e., property getters):

foo = {};
Object.defineProperty(foo, "bar", {
    get: function() {
        return Math.random();
    }
})

foo.bar === foo.bar; // false

If you do this for the global object window (or global in Node), then you can see the x === x comparison fail for a global-scope variable:

Object.defineProperty(window, "bar", {
    get: function() {
        return Math.random();
    }
})

bar === bar; // false

edited May 23 '17 at 11:45

Community

1
1

answered Nov 13 '13 at 13:53

apsillers

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@ComFreek Well, dang, I hadn't considered that. I'll add that into may answer! – apsillers Nov 13 '13 at 13:55
1

This isn't a case of the same value; it is a case of the same expression evaluating to different values. – Yann Vernier Nov 13 '13 at 13:55
@ComFreak - My eyes! The goggles do nothing! – cloudfeet Nov 13 '13 at 13:56
@YannVernier, yes, but it's worth mentioning. – zzzzBov Nov 13 '13 at 13:56
@apsillers This is so nice! Works good with `this` object also both in NodeJS and in browser. – Ionică Bizău Nov 14 '13 at 13:03

score 11 · Answer 2 · answered Nov 13 '13 at 13:51

11

The spec lists the exact rules for strict equality.
There are no other such cases, unless you count +0 and -0.

The SameValue algorithm (used for validating changes to read-only defined properties) has no such exceptions.

answered Nov 13 '13 at 13:51

SLaks

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Still, `+0 === +0` and `-0 === -0`, so even those are out. – Frédéric Hamidi Nov 13 '13 at 13:53
Per the spec, +0 and -0 do compare equal. However, the spec also does not list all cases of numbers, which is why there is the default of a false return: infinities are not considered equal, even if they share sign. – Yann Vernier Nov 13 '13 at 13:54
@Yann, `1/0 === 1/0` is `true` for me on Firefox. – Frédéric Hamidi Nov 13 '13 at 13:55
`+0 === -0` returns `true`. Why does they matter? – Ionică Bizău Nov 13 '13 at 13:55
1

To clarify: strict equality does not distinguish between `-0` and `+0`, while `SameValue` *does* distinguish. – apsillers Nov 13 '13 at 14:02

Is there any value for what x === x returns false without NaN?

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