Path: csiph.com!eternal-september.org!reader02.eternal-september.org!.POSTED!not-for-mail
From: Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups: comp.lang.javascript
Subject: Re: Understanding &&
Date: Fri, 16 Apr 2021 11:00:19 +0100
Organization: A noiseless patient Spider
Lines: 30
Message-ID: <878s5ift1o.fsf@bsb.me.uk>
References: <U_CdnQWSXulHj-T9nZ2dnUU7-evNnZ2d@westnet.com.au>
Mime-Version: 1.0
Content-Type: text/plain
Injection-Info: reader02.eternal-september.org; posting-host="bcf9a544088c93a28385d943b82a88e7"; logging-data="13801"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX19Iz5V+QLFpLLR/PVgMUT/usx4dy4oVsVM="
Cancel-Lock: sha1:mcP8PElGKEU5PXoO/EvpbOjjdCg= sha1:MzXa+Zf2MS61w3oLlEhx6C5NnXA=
X-BSB-Auth: 1.672ff4eabf8721ea7cf3.20210416110019BST.878s5ift1o.fsf@bsb.me.uk
Xref: csiph.com comp.lang.javascript:39950

Andrew Poulos <ap_prog@hotmail.com> writes:

> I've been looking at moveable.js
> <https://daybrush.com/moveable/release/latest/doc/index.html>
> when I noticed this (fragment of) code:
>
> on("resize", ({ target, width, height, dist, delta, clientX, clientY }) => {
>   delta[0] && (target.style.width = `${width}px`);
> })
>
> I don't understand what the double ampersand in
>   delta[0] && (target.style.width = `${width}px`);
> is doing.
>
> Does it mean that if delta[0] is falsey then the width of target is
> set? That is if (!delta[0]) target.style.width = `${width}px`; ?

It's the other way round.  In x && y, only if x is "truthy" must y be
evaluated.  When x is false, the whole expression must be false and the
language guarantees that the right-hand operand won't be evaluated.

The same (with the appropriate switch between truth and falsehood) is
true for ||.  There is also the much newer ?? operator that evaluates
and returns the right-hand operand only when the left operand is
null or undefined (sometimes called "nullish").

Such operators are often said to have "short circuit" evaluation.

-- 
Ben.