The shortest way to conditional insert properties into an object literal

Andrea Simone Costa on March 27, 2019

What am I ranting about? I'm talking about this: const obj = { ...condition && { prop: value }, }; Trust me, this is perfectly ac... [Read Full]
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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


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.


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.

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.

Maybe you're right. Sorry for any misunderstanding.

No problems 😃!

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

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


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.


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.


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.


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.


I completely agree with you.


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.


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 }

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},

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).

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


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


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.


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)


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!


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.


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!


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 :)


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


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


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.


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).


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


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)
>> 1
>> 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) }

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


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


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


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.


But it wouldn't be the shortest anymore ;)

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


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 } 


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


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


Google rest/spread operators 😆


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


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!


~semplicity~ simplicity

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