Yuma-Tsushima

Posted on Jul 1, 2021

[JavaScript] 5 Interesting uses of JavaScript destructuring!

#javascript #beginners #react #redux

Looking at my regular JavaScript code, I see that destructuring assignments are everywhere.

Reading object properties and accessing array items are frequent operations. The destructuring assignments make these operations so much easier and concise.

In this post, I will describe 5 interesting uses of destructuring in JavaScript, beyond the basic usage.

1) Swap variables

The usual way to swap 2 variables requires an additional temporary variable. Let’s see a simple scenario:

let a = 1;
let b = 2;
let temp;

temp = a;
a = b;
b = temp;

a; // => 2
b; // => 1

temp is a temporary variable that holds the value of a. Then a is assigned with the value of b, and consequently b is assigned with temp.

The destructuring assignment makes the variables swapping simple, without any need of a temporary variable:

let a = 1;
let b = 2;

[a, b] = [b, a];

a; // => 2
b; // => 1

[a, b] = [b, a] is a destructuring assignment. On the right side, an array is created [b, a], that is [2, 1]. The first item of this array 2 is assigned to a, and the second item 1 is assigned to b.

Although you still create a temporary array, swapping variables using destructuring assignment is more concise.

This is not the limit. You can swap more than 2 variables at the same time. Let’s try that:

let zero = 2;
let one = 1;
let two = 0;

[zero, one, two] = [two, one, zero];

zero; // => 0
one;  // => 1
two;  // => 2

You can swap as many variables as you want! Although, swapping 2 variables is the most common scenario.

2) Access array item

You have an array of items that potentially can be empty. You want to access the first, second, or nth item of the array, but if the item does not exist, get a default value.

Normally you would use the length property of the array:

const colors = [];

let firstColor = 'white';
if (colors.length > 0) {
 firstColor = colors[0];
}

firstColor; // => 'white'

Fortunately, array destructuring helps you achieve the same way shorter:

const colors = [];
const [firstColor = 'white'] = colors;

firstColor; // => 'white'

const [firstColor = 'white'] = colors destructuring assigns to firstColor variable the first element of the colors array. If the array doesn’t have any element at the index 0, the 'white' default value is assigned.

Note the comma on the left side of the destructuring: it means that the first element is ignored. secondColor is assigned with the element at index 1 from the colors array.

3) Immutable operations

When I started using React, and later Redux, I was forced to write code that respects immutability. While having some difficulties at the start, later I saw its benefits: it’s easier to deal with unidirectional data flow.

Immutability forbids mutating objects. Fortunately, destructuring helps you achieve some operations in an immutable manner easily.

The destructuring in combination with ... rest operator removes elements from the beginning of an array:

const numbers = [1, 2, 3];

const [, ...fooNumbers] = numbers;

fooNumbers; // => [2, 3]
numbers; // => [1, 2, 3]

The destructuring [, ...fooNumbers] = numbers creates a new array fooNumbers that contains the items from numbers but the first one.

numbers array is not mutated, keeping the operation immutable.

In the same immutable manner you can delete properties from objects. Let’s try to delete foo property from the object big:

const big = {
 foo: 'value Foo',
 bar: 'value Bar'
};

const { foo, ...small } = big;

small; // => { bar: 'value Bar' }
big; // => { foo: 'value Foo', bar: 'value Bar' }

The destructuring assignment in combination with object rest operator creates a new object small with all properties from big, only without foo.

4) Destructuring iterables

In the previous sections, the destructuring was applied to arrays. But you can destructure any object that implements the iterable protocol.

Many native primitive types and objects are iterable: arrays, strings, typed arrays, sets, and maps.

const str = 'cheese';

const [firstChar = ''] = str;

firstChar; // => 'c'

You’re not limited to native types. Destructuring logic can be customized by implementing the iterable protocol.

movies holds a list of movie objects. When destructuring movies, it would be great to get the movie title as a string. Let’s implement a custom iterator:

const movies = {
  list: [
    { title: 'Skyfall' }, 
    { title: 'Interstellar' }
  ],
  [Symbol.iterator]() {
    let index = 0;
    return {
      next: () => {
        if (index < this.list.length) {
          const value = this.list[index++].title;
          return { value, done: false };
        }
        return { done: true };
      }
    };
  }
};

const [firstMovieTitle] = movies;
console.log(firstMovieTitle); // => 'Skyfall'

movies object implements the iterable protocol by defining the Symbol.iterator method. The iterator iterates over the titles of movies.

Conforming to an iterable protocol allows the destructuring of movies object into titles, specifically by reading the title of the first movie: const [firstMovieTitle] = movies.

The sky is the limit when using destructuring with iterators.

5) Destructuring dynamic properties

In my experience, the destructuring of an object by properties happens more often than arrays destructuring.

The destructuring of an object looks pretty simple:

const movie = { title: 'Skyfall' };

const { title } = movie;

title; // => 'Skyfall'

const { title } = movie creates a variable title and assigns to it the value of property movie.title.

When first reading about objects destructuring, I was a bit surprised that you don’t have to know the property name statically. You can destructure an object with a dynamic property name!

To see how dynamic destructuring works, let’s write a greeting function:

function greet(obj, nameProp) {
 const { [nameProp]: name = 'Unknown' } = obj;
 return `Hello, ${name}!`;
}

greet({ name: 'Ben' }, 'name'); // => 'Hello, Ben!'
greet({ }, 'name'); // => 'Hello, Unknown!'

greet() function is called with 2 arguments: the object and the property name.

Inside greet(), the destructuring assignment const { [nameProp]: name = 'Unknown' } = obj reads the dynamic property name using square brackets [nameProp]. The name variable receives the dynamic property value.

Even better you can specify a default value 'Unknown' in case if the property does not exist.

Conclusion

Destructuring works great if you want to access object properties and array items.

On top of the basic usage, array destructuring is convinient to swap variables, access array items, perform some immutable operations.

JavaScript offers even greater possibilities because you can define custom destructuring logic using iterators.

Question: What interesting applications of destructuring do you know? Write a comment below!

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Top comments (17)

𒎏Wii 🏳️‍⚧️ • Jul 2 '21

The usual way to swap 2 variables requires an additional temporary variable.

Yet destructuring requires a temporary array, which my intuition tells me, is harder (not impossible, mind you) to optimise and has to be garbage-collected.

let firstColor = 'white';
if (colors.length > 0) {
 firstColor = colors[0];
}

There's a much nicer way to do this without destructuring:

let firstColor = colors[0] ?? "white"

or using || on older javascript versions.

const [, ...fooNumbers] = numbers;

Again, there's a much more readable version of this without destructuring:

const fooNumbers = numbers.slice(1)

const { foo, ...small } = big;

That's actually a really good example where destructuring is the most readable way of doing it. That being said, one could always just write a function to do a similar thing:

drop = (object, drop) => Object.fromEntries(Object.entries(object).filter(([key]) => drop.indexOf(key)))

Is it pretty? no, not really. But at least it's possible :D

Personally, I really like using destructuring for function arguments. You might have already spotted it in my drop() example above: instead of indexing the array in the function body, I let the function declaration take care of that, and then just work with the variables I actually want in the body.

Getting many values out of a nested data structure is also really nice this way:

let {name: {first: firstname, last: lastname}, address: {street}} = user

Yuma-Tsushima • Jul 2 '21

To be honest I do use || for destructuring also.

let firstColor = colors[0] ?? "white"

or using || on older javascript versions.

You did make some good points here thanks :)

const fooNumbers = numbers.slice(1)

I do also like the way you did this.

:) Thanks again

GrahamTheDev • Jul 1 '21

Quick heads up - it looks like the paragraph at the end of section 2 belongs in section 3.

Great article and well explained! ❤️

Yuma-Tsushima • Jul 2 '21

Nice point out but I do like giving people a preview

GrahamTheDev • Jul 2 '21 • Edited

Note the comma on the left side of the destructuring: it means that the first element is ignored. secondColor is assigned with the element at index 1 from the colors array.

That doesn’t lead in to the next section it is referencing a code snippet that is either missing (which after rereading, looks to be the case) or further down and part of section 3 but you changed the example. Either way it is definitely not right!

Or if you were being sarcastic and I missed it then feel free to r/whoosh me 😜🤣

Yuma-Tsushima • Jul 2 '21 • Edited

ahahha

yeah I was being sarcastic , it was more of a trick

bogdanbatsenko • Jul 3 '21

It was a trick to provoke thinking over again and getting to the bottom of the topic

austinbooth • Jul 3 '21

Nice article. I have a question about the movies iterable example. Why are you using the increment operator when accessing the value?

Yuma-Tsushima • Jul 3 '21

So post increment was used in the movies example .
So it adds 1 to the operator.
So if you do index++ it will log the firstvalue.
If you do ++index it will log the secondvalue

const movies = {
  list: [
    { title: 'Skyfall' }, 
    { title: 'Interstellar' }
  ],
  [Symbol.iterator]() {
    let index = 0;
    return {
      next: () => {
        if (index < this.list.length) {
          const value = this.list[index++].title;
          return { value, done: false };
        }
        return { done: true };
      }
    };
  }
};

const [firstMovieTitle] = movies;
console.log(firstMovieTitle); // so it logs skyfall

but once we use the pre increment it will log interstellar

const movies = {
  list: [
    { title: 'Skyfall' }, 
    { title: 'Interstellar' }
  ],
  [Symbol.iterator]() {
    let index = 0;
    return {
      next: () => {
        if (index < this.list.length) {
          const value = this.list[++index].title;
          return { value, done: false };
        }
        return { done: true };
      }
    };
  }
};

const [firstMovieTitle] = movies;
console.log(firstMovieTitle); // now logs interstellar