Destructuring

The two most used data structures in JavaScript are Object and Array. The destructuring assignment introduced in ECMAScript 2015 is a shorthand syntax that allows us to extract array values or object properties into variables. In this lesson, we go through the syntax of both data structures and give examples of when you can use them.

Destructuring arrays

In ES5, you could access items in an array by index:

var fruit = ['apple', 'banana', 'kiwi'];

var apple = fruit[0];
var banana = fruit[1];
var kiwi = fruit[2];

With ES6 destructuring, the code becomes simpler:

const [apple, banana, kiwi] = fruit;

Sometimes, you might want to skip over items in the array being destructured:

const [,,kiwi] = ['apple', 'banana', 'kiwi'];

Or, you can save the other items in an array with the rest pattern:

const [apple, ...rest] = ['apple', 'banana', 'kiwi'];

console.log(rest); // -> ["banana", "kiwi"]

Note that the destructuring assignment pattern works for any iterable.

Destructuring objects

Object destructuring lets you extract properties from objects and bind them to variables. Instead of accessing width through rectangle.width, we can access the property and assign it to a variable:

const rectangle = { width: 5, height: 8 };

const { width } = rectangle;

Even better, object destructuring can extract multiple properties in one statement. This makes the code clearer. The order of the properties does not matter.

const { width, height } = rectangle;

If we want to change the name of the property, we can do it using a colon:

const { width: w, height: h } = rectangle;

console.log(w); // -> 5

Here, we assign the width property from rectangle to a variable named w.

Default values

When you destructure on properties that are not defined, you get undefined:

const { password } = {};

console.log(password); // -> undefined

You can provide default values for when the property you are destructuring is not defined:

const [favorite = 'apple'] = [];
console.log(favorite); // -> apple

const { width = 100 } = {};
console.log(width); // -> 100

Examples

There are many places where we can use destructuring to make the code terser. Let’s see some examples!

String manipulation

We could, for example, do some string manipulation on a name and directly assign the firstName and lastName variables to it:

const [firstName, lastName] = 'Max Best'.split(' ');

// firstName = 'Max', lastName = 'Best'

Swapping variable values

Let’s first do the ES5 swap:

var me = 'happy', you = 'sad';

var temp = me;
me = you;
you = temp;

// me = 'sad', you = 'happy'

Now, let’s swap back to ES6 destructuring:

[me, you] = [you, me];

// me = 'happy', you = 'sad'

No more temporary variables needed!

Function parameter objects

When you are passing multiple parameters, it often makes sense to do it as an object. We can use destructuring whenever we want to unpack its properties:

function calculateArea({ width, height }) {
  console.log('Area is ' + width * height);
}

calculateArea({ width: 5, height: 6 });
// -> Area is 30

We can also set default values and change variable names:

function calculateArea({ width: w = 10, height: h = 5 }) {
  console.log('Area is ' + w * h);
}

calculateArea({ height: 6 });
// -> Area is 60

Multiple return values

If you need to return multiple values from a function, you can return an array or object and destructure the result:

function returnWidthAndHeight() {
  return [5, 10];
}

const [width, height] = returnWidthAndHeight();
// width = 5, height = 10

Conclusion

Destructuring is a JavaScript expression that allows us to extract data from arrays and objects. In this lesson, we have explored syntax and the various ways we can use it in our code. Destructuring arrays and objects is something that will make your code a bit shorter and cleaner all over the place.

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Quiz on destructuring