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6 Use Cases of Spread with Array in JavaScript

Samantha Ming on June 30, 2020

Here are 6 ways to use the Spread operator with Array in JavaScript. You can use it to merge or clone an array. Or use it to convert iterables to...

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huncyrus • Jun 30 '20

Side note: The new Set() and the spread operator could be quite dangerous, worth to check out the behaviour: it is really copy by value or just by reference... (might be handy this small detail when ppl workin' on large datasets)

Samantha Ming • Jul 1 '20

But wouldn't spread be creating a true new copy? -- do you mind expanding your thought, might be something helpful to include in my code notes 😵

Pavel Bucka • Jul 2 '20

Try this:

const set = new Set([{ id: 1, color: "blue" }, { id: 2, color: "red" }, { id: 3, color: "green" }]);

const arr = [...set];
arr[0].color = "purple";

console.log(set); // check the first entry
// or set.values().next().value.color

The conclusion is: spreading results in a shallow copy.

Source: stackoverflow.com/questions/500511...

That means, copying primitive values turns fine. What you can do in case of objects copy, is to make a copy of those underlying objects:

const arr2 = [...set].map(item => Object.assign({}, item));
arr2[0].color = "yellow";

console.log(set); // check the first entry

But how about nested objects? Try this:

const set = new Set([{ id: 1, color: "blue", details: { weight: 10 } }, { id: 2, color: "red", details: { weight: 20 } }, { id: 3, color: "green", details: { weight: 30 } }]);

const arr2 = [...set].map(item => Object.assign({}, item));
arr2[0].details.weight = 12;

console.log(set); // check the first entry

The conclusion is: Object.assign copies property values only (not a deep copy).

Source: developer.mozilla.org/en-US/docs/W...

Conclusions:

- spreading results in a shallow copy
- Object.assign copies property values only (not a deep copy)

Pavel Bucka • Jun 30 '20

I loved reading this article!! No bs, right to the point. And great analogies with Russian dolls or Eraser! I didn't understand that mud from MDN as well :D
Although it is a familiar topic to me, I found it entertaining and interesting enough to read it till the end. Great work! :-)

Samantha Ming • Jul 1 '20

Who said learning programming can't be entertaining or interesting 😆 Thanks so much for the positive feedback! Entertain-learning (yup, I just TM that 😂) is my goal for all my code notes now! 🙌

Eric Ahnell • Jun 30 '20

Fun! I can see a somewhat unusual use for this: in web games, you need arrays for various reasons. It’s usually more efficient to store only one dimension, even if the data isn’t, and simulate a multidimensional array. Spread gives a convenient way to make the simulation less annoying for consumers, from the perspective of a JS library.

Samantha Ming • Jul 1 '20

Interesting, thanks for sharing! 🎮

ponyjackal • Jun 30 '20

Nice post,
Thanks, Samantha

Samantha Ming • Jul 1 '20

Thank for reading my code notes 😄

bukazoltan • Jul 1 '20 • Edited

Lovely useful aericle.
I also often use it to get min and max of an array 😊

Samantha Ming • Jul 1 '20

min and max? please say more! 🤩

bukazoltan • Jul 1 '20 • Edited

Well, the min() and max() method cannot take an array as an input, so spreading it can solve that problem too... It saved me a lot of headaches.

array = [1, 2, 3, 4, 5];
var minimum = Math.min(...array);
var maximum = Math.max(...array);

Samantha Ming • Jul 1 '20

Ah yes! that's a fantastic use case! let me add it to my notes, thanks for sharing 💪

Eng_Mustafa • Jul 9 '20

Samantha! You just make everything simple and easy to understand. Please try to share more content. I have been addicted to yours 😭

Samantha Ming • Jul 10 '20

Thank you so much!!! Will dooo 😊😊😊

dotorimook • Jul 1 '20

Nice to read! Thank you!

Samantha Ming • Jul 1 '20

Yay! thank you for reading 🙂

Mamun Abdullah • Jul 1 '20

Good explanation.

Samantha Ming • Jul 1 '20

thanks, glad you find it helpful 👏

PenAndPapers • Jul 1 '20

I dont use spread operators much but now that I have a better understanding on how it work i'd probably use them more. Thanks for sharing this.

Samantha Ming • Jul 1 '20

great! that's the goal of the post. Spread is super awesome, but it can be tricky to understand. So glad it makes more sense to you and you're encouraged to use it more 👏

Bernard Baker • Jun 30 '20

I like reading your articles 🤸.

Samantha Ming • Jul 1 '20

Awesome happy to hear that! glad you find them helpful 👏

Glaydson V Sousa • Jul 4 '20

Thank you for sharing this, now I think I understand spread operator!