Andrea Simone Costa

Posted on Mar 27, 2019 • Updated on Apr 13, 2021

The shortest way to conditional insert properties into an object literal

#javascript #objects #beginners #specification

Hi, nice to meet you 😄!
You can find this article in my personal blog here.

Top comments (48)

Why should I choose the shortest way if it needs an article to be explained? In my opinion, the best code is the self-explanatory one, not the shortest one

Andrea Simone Costa • Mar 28 '19 • Edited on

I completely disagree because lot of JS syntax and features need an explaination. Think about coercion, generators, arrow functions, async await, the asynchronous iteration, proxies, ecc.
More generally each concept in every programming language, also the basic ones, may need an explanation.

And, with your line of thinking, people risk to never learn anything and never improve themselves.

The article is explaining why the syntax is valid. But the main concept, the optional insertion of a property, is understandable in 10 seconds.
After a little example, if you spend 5 mins with it, you can already understand what's going on:

...condition && {prop:value}

If the condition is true a prop will be added. No side effects in the other case. Simple.

It's something with which one can become familiar in no time, it's something immediately teachable to others and I've demostrated that it works not because an undefined, untrustable behaviour but thanks to a precise logic put black on white in the spec.

Andrea Chiarelli • Mar 28 '19 • Edited on

I don't criticize the use of idiomatic code (I often use it, too) but the emphasis on the shortest way.
It seems to encourage the writing of compact or idiomatic code at any cost, and it is not always the best choice.
As with natural language, if you use very strict jargon, you reduce the number of people who are able to understand it with little effort.

Andrea Simone Costa • Mar 28 '19

The shortest way means only one thing: fewer characters are required.
The second part you deduced is your personal interpretation of the title, which obviously does not reflect my thinking at all.

Andrea Chiarelli • Mar 29 '19

Maybe you're right. Sorry for any misunderstanding.

Andrea Simone Costa • Mar 29 '19

No problems 😃!

See you at the DevFest, with peaceful intentions of course 😝

Andrea Chiarelli • Mar 29 '19

I've never had any non-peaceful intentions :-)
See you at the DevFest!

Charles F. Munat • Mar 28 '19

This is referred to as the "either/or fallacy", also referred to as the "false dilemma": en.wikipedia.org/wiki/False_dilemma

This is a perfectly self-explanatory technique. If you understand the spread syntax, and how short-circuted Boolean expressions work, then you can see at a glance what this does. The text above is not an explanation of the code, but proof that it works without surprises. Hence, this is both the shortest and quite self-explanatory.

I would even go so far as to say that this is a lot more readable than the alternatives. Bravo, Mr. Costa.

At no point does the author claim that short code should supersede readable code.

Andrea Simone Costa • Mar 29 '19 • Edited on

You centered the point, thank you for intervening!

P.S. I think it's very readable too, but I'm realizing, reading a great number of opinions and comments even on a small thing like the one I presented in the article, how readability is a subjective thing.

Benny Powers 🇮🇱🇨🇦 • Mar 29 '19

I like this pattern not just because it's the "shortest" but because it's the most declarative. This way, I can construct and assign my object in one place, without going back later to mutate it's contents. I think that leads to better code because it reduces the need to mentally execute each line.

Bob Myers • Jun 10 '19

It's called an "idiom". Like any other idiom, including idioms in naturally language, you have to learn it. That doesn't mean it's not useful. If people run into it and are not familiar with it, they'll figure it out quickly and then know it themselves.

Andrea Simone Costa • Jul 3 '19

I completely agree with you.

Konstantin Rouda • Mar 29 '19

Interesting trick, but what if our value is 0 which can be a valid value to use? So our code:

𝐯𝐚𝐫 𝐜𝐨𝐢𝐧𝐬 = 𝟎;
𝐯𝐚𝐫 𝐨𝐛𝐣 = {
  𝐧𝐚𝐦𝐞: "𝐉𝐨𝐡𝐧",
  ...𝐜𝐨𝐢𝐧𝐬 && { 𝐜𝐨𝐢𝐧𝐬𝐀𝐦𝐨𝐮𝐧𝐭: 𝐜𝐨𝐢𝐧𝐬 }
}

won't copy anything.

Reply guy 🤷‍♂️ • Apr 1 '19 • Edited on

Just make sure to internalize what JavaScript considers truthy and you won't think this is a "gotcha" anymore.

So in your specific case you'd probably want to do something like

const coins = 0;
const obj = {
  name: "John",
  ...typeof coins === "number" && !isNaN(coins) && {coins},
};

Konstantin Rouda • Apr 1 '19

AFAIK, this is considered as a bad practice to use typeof to check the number, as NaN is also has type number (e.g. typeof NaN === "number"; // -> true). Moreover, typeof when used against number will return number, as a result, in lower case (i.e. not Number but number).

Reply guy 🤷‍♂️ • Apr 2 '19

Yeah the casing of number was just a typo. Adjusted it to check for NaN.

Andrea Simone Costa • Mar 29 '19 • Edited on

So there is no point to use the trick in this way if 0 is a valid amount as well.

Remember that on the left you put the condition, so you may write like the following:

𝐯𝐚𝐫 𝐜𝐨𝐢𝐧𝐬 = 𝟎;
𝐯𝐚𝐫 𝐨𝐛𝐣 = {
  𝐧𝐚𝐦𝐞: "𝐉𝐨𝐡𝐧",
  ...𝐜𝐨𝐢𝐧𝐬>=0 && { 𝐜𝐨𝐢𝐧𝐬𝐀𝐦𝐨𝐮𝐧𝐭: 𝐜𝐨𝐢𝐧𝐬 }
}

tkane2000 • Mar 29 '19

you could pass coins to a method that checks for false, null, and undefined only (or something along those lines)

Dale Anderson • Mar 29 '19

It's a useful trick! There are a bunch of others at blog.bitsrc.io/6-tricks-with-resti... (not my post), including a concise way to remove properties from objects.

theodoros klikas • Mar 28 '19 • Edited on

I believe in a quote, attributed to Brian Kernigan that goes like:

"Everyone knows that debugging is twice as hard as writing a program in the first place. So if you're as clever as you can be when you write it, how will you ever debug it?"

In languages like javascript, there is a minifier anyway (and a transpiler most probably) in front of our code and the production code that gets served in the browser (unless of course your write code for the server-side).

To be fair, the above is not the worst example of "smart" code that I have seen and the article is a nice explanation of why this works so thanks for that!

Reply guy 🤷‍♂️ • Apr 1 '19

To me, more terse code is almost always easier to parse. Of course, everyone's different. There's not really a one size fits all solution. You'll have to agree with your team on a working standard.

Andrea Simone Costa • Mar 28 '19

I've already expressed mine opinion here around, and to be technically precise the code is easily testable as any if construct.
Thanks for sharing your opinion!

theodoros klikas • Mar 29 '19

I do find the article helpful but to be honest with you, it was the first time I saw something like this.

I guess someone that is more experienced with the spread operator and augmenting objects in this way might have seen it more than me.

As you said, it is most probably a matter of opinion as a lot of things in javascript are those days, for example, I find something like this pretty hard to read:

let adder = (x) => (y, z) => x + y + z;

compared to the old-style alternative but there are people who love it.

So thank you again for going deep in your explanation :)

Andrea Simone Costa • Mar 29 '19

You're welcome!

David Sharp • Apr 1 '19

This seems much nicer than my current const obj = { ...{ condition? { prop: value }: {} }}; 🤔

While it's not entirely clear at first glance how it works, I think the intent is clear enough for someone stumbling across it (excusing any bugs caused by gotchas)

willsmart • Mar 28 '19 • Edited on

That's awesome. Thanks for posting.
I didn't know that the rest operator could take an expression (though it does make sense).

I think your example could be better though. As said you'd quickly run into unexpected behaviour with falsey primitive values, which you mention but downplay. It's a showstopper imo.
As a shortened, silly example showing what could happily ruin your day in longer, serious code...

wrappedStringLength  =  string => ({...string&&{string}}.string.length)
wrappedStringLength('a')
>> 1
wrappedStringLength('')
>> Uncaught TypeError: Cannot read property 'length' of undefined

Most coders would assume a method would treat an empty string like any other.

I'd just code the example object as...

{
    state, priority,
    collection: 'Cats',  
    sort: 'asc',
}

Just simpler and less brittle.

A better example might use something where the inserted properties are computed. a la...

userObject  =  ({id, name, type, age}) => ({
  name, age, type,
  ... type=='prof' && { gradStudents: fetchGradStudentsForProf(id)  }
  ... type=='roofer' && { jobs: fetchJobsByRoofer(id) }
})

Andrea Simone Costa • Mar 28 '19

I think you will understand that this is beyond the scope of the article.
My first point was to show a nice, little know js fact. The second was to explain why such code is allowed.
All the rest is left to the reader's good will and curiosity :D

Felix Becker • Mar 28 '19

In most cases, just defaulting the property value to undefined is the easier choice.

Bob Myers • Jun 10 '19

In the case of Firebase, to take one example, a value of undefined is treated differently than a missing property, and causes an error.

Bruno Donizetti • Mar 29 '19 • Edited on

This makes total sense. I usually just build the object then run a sanitize function on it. No wonder why I never stoped to think about this.

Thanks for posting, simple trick but mind opening (mainly if you are biased by different approaches like me).

Andrea Simone Costa • Mar 29 '19

I do love find alternative ways to do common things 😆, I'm honored to have opened your mind 😝

Stan Lindsey • Mar 29 '19

Basically, it evaluates like this:
...(condition && { prop: value }),

Carlos • Apr 1 '19

Very helpful what Stan posted here. Parenthesis really help. Actually they are a default lint recommendation in this specific case. I think adding them to the code in the article would help a lot.

Andrea Simone Costa • Apr 2 '19

But it wouldn't be the shortest anymore ;)

Carlos • Apr 2 '19

hey man! I'm trying to help you here! lol...great article!

Jeff Lindholm • Apr 2 '19

Personally, I would do it like ...(obj && { props: foo}) so it is obvious how the operators work, people who don't immediately know that && has precedence over ...won't read the code as easily.

Tim Bendt • Mar 28 '19

This is a great explanation. Thanks for digging in and explaining what was happening.

Andrea Simone Costa • Mar 28 '19

Thank you!

Tom Pegler • Apr 5 '19

For added fun with this, I tried a couple of things:

var state = 'tom';
var o = {
  name: 'test',
  state: state,
  ...( ( true && ( this.state = 'jamie' ) ) && false ) && { state } 

}

Chad Kirby • Mar 29 '19

Mind blown!

guico33 • Mar 28 '19

Interesting article, I had always found that behavior a bit eery and never bothered looking into the why. Thanks for sharing 🙏

Andrea Simone Costa • Mar 28 '19

You're welcome!

Cees Timmerman • Mar 30 '19

I know how && works, but wtf does ... do?

Andrea Simone Costa • Mar 30 '19

Google rest/spread operators 😆

Cees Timmerman • Mar 31 '19 • Edited on

It unpacks stuff, like * in Python, which "spread" could be a synonym for, but "rest operator"? googles Ah, as in "remaining parameters".

Marc Hanisch • Feb 20 '20 • Edited on

I'm a year to late at the party, but thank you very much for the good explanation. I was in search for exactly this feature and it is an elegant and modern solution!