JavaScript的双等于（==）总是对称的吗？

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 ?

Answer 1

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

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

Answer 2

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 .

Answer 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 .