Is JavaScript's double equals (==) always symmetric?

Question

There are many cases in which JavaScript's type-coercing equality operator is not transitive. For example, see "JavaScript equality transitivity is weird."

However, are there any cases in which == isn't symmetric? That is, where a == b is true and b == a is false?

score 37 · Answer 1 · answered Apr 14 '11 at 20:52

37

It's supposed to be symmetric. However, there is an asymmetric case in some versions of IE:

window == document; // true
document == window; // false

answered Apr 14 '11 at 20:52

Jason LeBrun

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Wow, that's amazing. Could you specify which versions of IE this occurs in? – Trevor Burnham Apr 14 '11 at 21:35
Using IE10 on Win8.0 I see this behavior in "Browser Mode: IE8" (not in IE10, IE9, or *IE7* mode). – Jeroen May 13 '14 at 19:51

score 30 · Accepted Answer · edited May 16 '19 at 10:00

30

In Javascript, == is always symmetric.

The spec says:

NOTE 2 The equality operators maintain the following invariants:

A != B is equivalent to !(A == B).

A == B is equivalent to B == A, except in the order of evaluation of A and B.

edited May 16 '19 at 10:00

Damiano

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answered Apr 14 '11 at 20:48

SLaks

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Shouldn't that be *commutative*? – Stéphan Kochen Apr 14 '11 at 20:50
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@Shtééf: Maybe it should be, but it isn't. http://en.wikipedia.org/wiki/Symmetric_relation Relations aren't operators. – SLaks Apr 14 '11 at 20:52
Thanks for the correction—it's been too long since I picked up that math degree! – Trevor Burnham Apr 14 '11 at 21:32

score 11 · Answer 3 · answered Apr 14 '11 at 20:50

The answer to your actual question (is the operator symmetric) is yes. The ECMA-262 spec explicitly states:

NOTE 2 The equality operators maintain the following invariants:

A != B is equivalent to !(A == B).
A == B is equivalent to B == A, except in the order of evaluation of A and B.

Is JavaScript's double equals (==) always symmetric?

3 Answers3

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