Question

Is this normal? Is it a feature or a bug? (I’m using firebug):

>>> '' || true
true
>>> '' || false
false
>>> '' && false
""
>>> '' && true
""

Answer 1

It is not converting the empty string to Boolean.

With ||

It is evaluating the left hand side, of which an empty string is falsy. It then checks the right hand side (because it’s an or, and that side may be true), and returns that value.

With &&

Because && needs both sides to be true and the left hand side is falsy, it doesn’t bother checking the right hand side (short circuit evaluation). Therefore, it just returns the left hand side, which is the empty string.

JavaScript always returns the last value it evaluated.

>>> '' || 0 || undefined || null || false || NaN || 'hello'
"hello"