Why does "0 && true" return 0 in javascript instead of a boolean?

Question

I was convinced that any logical expression in Javascript will return a boolean value, yet this expression returns a number 0 instead of a bool:

0 && true
> 0

Why is that so? How do I approach logical expressions in Javascript in this case to prevent this kind of mistake in the future?

Background story - I was baffled by this statement in jQuery:

$('.something').toggle(0 && true);

It doesn't toggle the element because '0' is returned, and not a boolean!

Maybe there are some clever reasons why this is so, but can't say I like it.

Answer 1

The documentation about the && operator says:

expr1 && expr2: Returns expr1 if it can be converted to false; otherwise, returns expr2.

This is why is returns the first value: 0

Source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_Operators#Description

You expected as a result false (a boolean), however the boolean value of the resulting value is falsy too. This means that you can use it in a condition (if) and have the expected behavior.

If you need to store it in a variable and prefer to have a boolean value, you can convert it. This can be done using a double negation: !!

!!(0 && true)

Or using Boolean:

Boolean(0 && true)