Question

I don’t understand why one scenario evaluates false and the other true.

Scenario 1:

> '' == ''
true

Scenario 2:

> '' == ('' || undefined)
false

Is scenario 2 not asking if (empty string) is equal to: (empty string) OR undefined?

I’m sure I’m missing something fundamental here, which is really what I’m trying to figure out. I can easily code around this, but I’d like to learn why it’s happening… for next time, ya know?

Answer 1

'' == ( '' || undefined )

Is not the same as

( '' == '' ) || ( '' == undefined )

It’s more along the lines of:

var foo = '' || undefined; // Results in undefined

And then comparing foo to an empty string:

foo == ''; // undefined == '' will result in false

Explanation

The logical || is a short-circuiting operator. If the argument to its left is truthy, the argument to the right is not even evaluated. In JavaScript, '' is not considered to be truthy:

if ( '' ) console.log( 'Empty Strings are True?' );

As such undefined is returned and compared against an empty string. When you perform this logic within a the parenthesis, the '' and the undefined don’t know about the impending equality check that is waiting to happen – they just want to know which of them is going to survive this evaluation.