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

问题

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?

回答1:

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.

回答2:

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

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

回答3:

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.

来源：https://stackoverflow.com/questions/5669440/is-javascripts-double-equals-always-symmetric