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Destructuring Assignment in ES6- Arrays

Sarah Chima on October 30, 2017

Destructuring assignment is a cool feature that came along with ES6. Destructuring is a JavaScript expression that makes it possible to unpack valu...

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Christian Bewernitz • Oct 30 '17 • Edited

Tank you for posting, didn't know about this way of skipping elements.

But the following sentence might be misleading (if it wouldn't be for the headline above):

What if we want to get the first and last item on the array...?

The way you show is really only working if you know the size of the array, which obviously you don't always do, e.g. when destructing an argument inside a function.

If you don't know it, you could do the following without touching the initial array:

const input = ["Hello", "I" , "am", "Sarah"];
const [greeting, ...rest] = input;
//rest == ["I" , "am", "Sarah"]
const name = rest.pop(); // rest is modified but input isn't
//or if you prefer or need more then one from the end
const [last, secondLast] = rest.reverse(); // rest is modified!

The ...name must always be the last part, so const [greeting, ...dontCare, name] = ... does not work.

We should also mention how destructing works when encountering [] and undefined:

var [a, b, ...rest] = [];
console.log(a,b,rest);//undefined, undefined, []

var [ouch] = undefined;//Uncaught TypeError: undefined is not iterable

Hm, iterable? What happens if we destruct some other iterable?

var [a, b, ...rest] = "abc,d";
console.log(a, b, rest);//"a", "b", ["c", ",", "d"]

Happy coding!

ramachintaguntla • Nov 23 '18

I dint know that even string could be destructed.. i was under the impression that only Array and Objects can be done.. are there any others that can be destructed?(other than strings, objects, arrays)

Sayo Paul • Sep 5 '18

Wow . Thank you so much for this

ViriCruz • Mar 26 '20

Thank you for explaining this in an understandable human way!. It was very easy to digest and helpful.

Jose Quijada • Jan 31 '20 • Edited

It might be worth mentioning that the syntax used in code below is also called JavaScript rest parameter syntax, specifically the ...intro part,

var [greeting,...intro] = ["Hello", "I" , "am", "Sarah"];

Comment deleted

Sarah Chima • Aug 9 '19

Hi Ashley, I just tried the code now and it works as expected. Did you try printing the values of key and rowValues?

Ashley Oliver • Aug 9 '19 • Edited

It does for me too now. /facepalm and thank you
I'll delete my original post to avoid confusing any one

Prosper Opara • Jun 1 '18

This is an awesome piece..

I have some questions

let [greet = "hello world", name = "sarah"] = ["hello"];

console.log(greet); // returns "hello"

why is is overriding the default value assigned to hello?

Sayo Paul • Sep 5 '18

I am not sure I understand your question fully but like the article explains , in this case , the array contains one element , "hello" and thus it overrides the default value of the first variable greet whilst the second variable is its default value as the array doesn't have a second element 🙂 .

George Arthur • Oct 21 '19

I think the question isn't that clear. Variables available are 'greet' and 'name' to which they both have default values. However, "greet" is assigned a value from Destructuring and thus will now return that value i.e ("hello") instead of it's default value give at initialization.

Shrijan • May 28 '19

greet = "hello world" is just a default value , if there is the value in first element in parent array then it will get overridden for sure. That is what default value is made for.

Manish • Aug 27 '19

What if you want to swap the variables inside a function? Something like this:

var a = 3;
var b = 6;

(() => {
  [a,b] = [b,a]
})()

console.log(a)
console.log(b)

Do share your suggestions on this.

aquifzubair • Mar 17 '20

There is no need to call a function it will work without calling function.

var a = 3;
var b = 6;
[a,b] = [b,a]

Yogesh S • Mar 28 '19

I just started learning JavaScript. I was not able to follow Destructuring in JavaScript. Your explanation and the examples were very simple and easy to understand. Thank you very much for posting such a great article.

Sarah Chima • Mar 28 '19

Thank you for your kind words. I am glad you like the article.

deepankar14693 • May 1 '19 • Edited

Very nicely explained. Can you tell what is happening in below code :

case Actions.AC_UPDATE_TASK:
item = action.payload.item;
props = action.payload.props;
item.start = props.start ? props.start : item.start;
item.end = props.end ? props.end : item.end;
item.name = props.name ? props.name : item.name;
return {
data: [...state.data],
links: [...state.links],
selectedItem: state.selectedItem
};

Here item and props are coming as parameter,item is present there in data array. After manipulating item from props this item needs to be updated in data array but here its not updated in data array after manipulating it. But after destructuring syntax I can see the updated item in data array. How is this possible?

Tiruvengadam85 • Feb 29 '20

Hi I have an array item which conatains nested object, how to destructure 0: {applied_rule_ids: "XXX", base_currency_code: "AED", base_discount_amount: -310, base_grand_total: 300, base_discount_tax_compensation_amount: 14.78}
1: {applied_rule_ids: "YYY", base_currency_code: "AED", base_discount_amount: 0, base_grand_total: 170, base_discount_tax_compensation_amount: 0}
2: {base_currency_code: "AED", base_discount_amount: 0, base_discount_invoiced: 0, base_grand_total: 50, base_discount_tax_compensation_amount: 0}
3: {base_currency_code: "AED", base_discount_amount: 0, base_discount_invoiced: 0, base_grand_total: 100, base_discount_tax_compensation_amount: 0}

tai8292 • Feb 19 '19

Thank you for posting, it's very helpfull

IAMtheIAM • Jun 9 '20

This is an awesome and very thorough explanation of destructuring assignment. Thanks!

evuazeze • Jun 22 '19

You just impacted one more person with knowledge, thank you Sarah.

IamLegendChamp • Sep 16 '19

This is awesome. Thank you, Sarah :-)

Niraj Thing • Jun 19 '19

awesome..thank you sarah

Steven G. Harms • Oct 30 '17

I get into ruts of use it's nice to remember and see others' patterns. Well done.

kanavsharma1 • Apr 21 '20

great article!

Michael • Jun 16 '19

great post, thanks.

Cyril ThankGod • Jul 13 '19

Thank you for posting the article. It helped me understand Destructuring assignment.

KeithV978 • Jul 10 '21

Thanks. You've rendered to me a lot of help. God bless you.

DigitalEmma • Jan 27 '19

Thank you for the informative piece, very explanatory was using this alongside a video tutorial on the subject matter, your example were easy to understand.

Sarah Chima • Jan 30 '19

Thank you for reading it

Luqman Abdulwasii • Apr 5 '20

Thank you so much for this, it comes handy.

Anya Uchenna • May 28 '20

please I am still learning JS I tried out swapping with destructuring it didn't work i don't know why here is the code var a = 3
var b = 7
[a, b] = [b, a]

jwm63ara • Jun 9 '20

Hi Anya,

I'm still learning too, so I don't fully understand this, but it is an example of a situation where automatic semicolon insertion doesn't work the way you might expect. If you try

var a = 3
var b = 7;
[a, b] = [b, a]

you should find that you get the expected result. The following also works:

var a = 3
var b = 7
{
  [a, b] = [b, a]
}

console.log(a)   // expect 7
console.log(b)   // expect 3

However, without a semicolon after the 7, or the enclosing braces, it is somehow interpreted as

var a = 3
var b = [b, a]

console.log(a)   // expect 3
console.log(b)   // expect [undefined, 3]

Hayatu Sanusi • Mar 21 '20

Thank you Sarah, you made so easy to understand.