Javascript: false || undefined vs undefined || false

Question

What is the explanation for behavior of the "||" operator (logical OR), when using it with false and undefined on both sides in JavaScript?

1)

> false || undefined
undefined

2)

> undefined || false
false

Answer 1

The logical OR operator isn't commutative like +, *, etc. It returns the first expression which can be converted into true. (Source Mozilla Doc)

1. In false || undefined, false can't be converted to true by definition (since it's the opposite), so it returns the second operand (undefined)

2. In undefined || false, undefined is a value, but considered as false in Javascript, so the logical operator evaluate the second operand and returns false (because both operands are false).