Destructuring assignment

Introduction#

Destructuring is a pattern matching technique that is added to Javascript recently in EcmaScript 6.

It allows you to bind a group of variables to a corresponding set of values when their pattern matches to the right hand-side and the left hand-side of the expression.

Syntax#

• let [x, y] = [1, 2]
• let [first, …rest] = [1, 2, 3, 4]
• let [one, , three] = [1, 2, 3]
• let [val=‘default value’] = []
• let {a, b} = {a: x, b: y}
• let {a: {c}} = {a: {c: ‘nested’}, b: y}
• let {b=‘default value’} = {a: 0}

Remarks#

Destructuring is new in the ECMAScript 6 (A.K.A ES2015) specification and browser support may be limited. The following table gives an overview of the earliest version of browsers that supported >75% of the specification.

(Last Updated - 2016/08/18)

Destructuring function arguments

Pull properties from an object passed into a function. This pattern simulates named parameters instead of relying on argument position.

let user = {
    name: 'Jill',
    age: 33,
    profession: 'Pilot'
}    

function greeting ({name, profession}) {
    console.log(`Hello, ${name} the ${profession}`)
}

greeting(user)

This also works for arrays:

let parts = ["Hello", "World!"];

function greeting([first, second]) {
    console.log(`${first} ${second}`);
}

Renaming Variables While Destructuring

Destructuring allows us to refer to one key in an object, but declare it as a variable with a different name. The syntax looks like the key-value syntax for a normal JavaScript object.

let user = {
  name: 'John Smith',
  id: 10,
  email: 'johns@workcorp.com',
};

let {user: userName, id: userId} = user;

console.log(userName) // John Smith
console.log(userId) // 10

Destructuring Arrays

const myArr = ['one', 'two', 'three']
const [ a, b, c ] = myArr

// a = 'one', b = 'two, c = 'three'

We can set default value in destructuring array, see the example of Default Value While Destructuring.

With destructuring array, we can swap the values of 2 variables easily:

var a = 1;
var b = 3;

[a, b] = [b, a];
// a = 3, b = 1

We can specify empty slots to skip unneeded values:

[a, , b] = [1, 2, 3] // a = 1, b = 3

Destructuring Objects

Destructuring is a convenient way to extract properties from objects into variables.

Basic syntax:

let person = {
  name: 'Bob',
  age: 25
};

let { name, age } = person;

// Is equivalent to
let name = person.name; // 'Bob'
let age = person.age;   // 25

Destructuring and renaming:

let person = {
  name: 'Bob',
  age: 25
};

let { name: firstName } = person;

// Is equivalent to
let firstName = person.name; // 'Bob'

Destructuring with default values:

let person = {
  name: 'Bob',
  age: 25
};

let { phone = '123-456-789' } = person;

// Is equivalent to
let phone = person.hasOwnProperty('phone') ? person.phone : '123-456-789'; // '123-456-789'

Destructuring and renaming with default values

let person = {
  name: 'Bob',
  age: 25
};

let { phone: p = '123-456-789' } = person;

// Is equivalent to
let p = person.hasOwnProperty('phone') ? person.phone : '123-456-789'; // '123-456-789'

Destructuring inside variables

Aside from destructuring objects into function arguments, you can use them inside variable declarations as follows:

const person = {
  name: 'John Doe',
  age: 45,
  location: 'Paris, France',
};

let { name, age, location } = person;

console.log('I am ' + name + ', aged ' + age + ' and living in ' + location + '.');
// -> "I am John Doe aged 45 and living in Paris, France."

As you can see, three new variables were created: name, age and location and their values were grabbed from the object person if they matched key names.

Using rest parameters to create an arguments array

If you ever need an array that consists of extra arguments that you may or may not expect to have, apart from the ones you specifically declared, you can use the array rest parameter inside the arguments declaration as follows:

Example 1, optional arguments into an array:

function printArgs(arg1, arg2, ...theRest) {
  console.log(arg1, arg2, theRest);
}

printArgs(1, 2, 'optional', 4, 5);
// -> "1, 2, ['optional', 4, 5]"

Example 2, all arguments are an array now:

function printArgs(...myArguments) {
  console.log(myArguments, Array.isArray(myArguments));
}

printArgs(1, 2, 'Arg #3');
// -> "[1, 2, 'Arg #3'] true"

The console printed true because myArguments is an Array, also, the ...myArguments inside the parameters arguments declaration converts a list of values obtained by the function (parameters) separated by commas into a fully functional array (and not an Array-like object like the native arguments object).

Default Value While Destructuring

We often encounter a situation where a property we’re trying to extract doesn’t exist in the object/array, resulting in a TypeError (while destructuring nested objects) or being set to undefined. While destructuring we can set a default value, which it will fallback to, in case of it not being found in the object.

var obj = {a : 1};
var {a : x , b : x1 = 10} = obj;
console.log(x, x1); // 1, 10
 
var arr = [];
var [a = 5, b = 10, c] = arr;
console.log(a, b, c); // 5, 10, undefined

Nested Destructuring

We are not limited to destructuring an object/array, we can destructure a nested object/array.

Nested Object Destructuring

var obj = {
  a: {
    c: 1,
    d: 3
  },
  b: 2
};

var {
  a: {
    c: x,
    d: y
  },
  b: z
} = obj;

console.log(x, y, z);     // 1,3,2

Nested Array Destructuring

var arr = [1, 2, [3, 4], 5];

var [a, , [b, c], d] = arr;

console.log(a, b, c, d);      // 1 3 4 5

Destructuring is not just limited to a single pattern, we can have arrays in it, with n-levels of nesting. Similarly we can destructure arrays with objects and vice-versa.

Arrays Within Object

var obj = {
  a: 1,
  b: [2, 3]
};

var {
  a: x1,
  b: [x2, x3]
} = obj;

console.log(x1, x2, x3);    // 1 2 3

Objects Within Arrays

var arr = [1, 2 , {a : 3}, 4];

var [x1, x2 , {a : x3}, x4] = arr;

console.log(x1, x2, x3, x4);