Post | mennovanslooten.nl/blog

I've neglected my duties here for a while but here's a interesting snippet of JavaScript Boolean behavior before I resume normal service. It has to do with the behavior of the && operator when the first operand evaluates to true.

The story started when I was writing a unit test for a very simple function that looked something like this:

function f( ) {
    return a && b;
}

I'm reasonably experienced in JavaScript and when I saw the function I expected it to always return a Boolean since, logically, the result of an && (AND) operation can only be true or false. I was wrong however because in the case that a evaluates to false it just returns the value of a and if it evaluates to true it returns the value of b! In code:

r = "" && true; // r = "" 
r = null && true; // r = null 
r = 0 && true; // r = 0
var UNDEF
r = UNDEF && true; // typeof r = undefined

r = true && ""; // r = ""
r = true && null; // r = null
r = true && 0; // r = 0
r = true && UNDEF; // typeof r = undefined

After my initial bafflement I can't really appreciate this curious feature. As far as I know it's so obscure or little known that if you would use it in this way practically noone would understand it just from reading the code. Take this for example:

function( a, b ) {
    // a and b are strings
    return (a && b).toUpperCase();
}

I'd probably raise an eyebrow if I saw something like that.

By the way, The ECMA Script specification explicitly mentions this in a note:

The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions.