Question

I read that sometimes the && operator is used to “short circuit” JavaScript into believing that a return value of 0 is 0 and not NaN because 0 is a falsy number in JavaScript. I’ve been looking around to figure out what all this means. Can someone explain it to a layman?

For example:

function sign(number) {    
    return number && number / Math.abs(number); }

Will return 0 if number is 0.

Answer 1

In JavaScript, the boolean operators && and || don’t necessarily return a boolean. Instead, they look at the “truthiness” of their arguments and might short circuit accordingly. Some values like 0, the empty string "" and null are “falsy”.

Short circuiting just means skip the evaluation of the right hand side of an expression because the left hand side is enough to provide the answer.

For example: an expression like var result = 100 / number; will give you NaN when number = 0, but:

var result = number && 100 / number;

Will give you 0 instead of a NaN since 0 is falsy. In a boolean context false && anything is false, so there’s no point in evaluating the right hand side. Similarly:

// supposed msg is a potentially empty string
var message = msg || "No message";

Will give you msg if the string msg is not empty (truthy) since true || anything is true. If msg is empty, it gives you "No message instead".