Tag: objects

Before getting into destructuring, lets have a quick recap on arrays and objects.

• Objects allows us to create a single entity that gather data items by key.

const emp = {

  name: “Anna”,

  id: “101”,

};

• Array allows us to gather data items into an ordered list.

const emp = [“Anna”,”Tina”,”Tim”];

Destructuring introduced in ES6 allows us to unpack elements in an array or elements. It extracts multiple properties, items and values from an array or object into a separate variable.

Object Destructuring:

Syntax:

let { var1,var2} ={var1:val1, var2:val2}

To destruct object we use curly braces { }

Consider you have a student object with properties firstname and lastname.

let student = {

    firstname: "Anna",

    lastname: "Hills"

};

Prior to ES6:

For assigning properties and values to variables we have,

let student = {

    firstname: “Anna”,

    lastname: “Hills”

};

let firstName = student.firstname;

let lastName = student.lastname;

console.log(firstName);

console.log(lastName);

From ES6:

// assigning object attributes to variables
let student = {
    firstname: "Anna",
    lastname: "Hills"
};
// destructuring assignment
let { firstname, lastname} = student;
console.log(firstname); // Anna
console.log(lastname); // Hills
In the above example we have used the same variable names.
How to assign new variable name?
// assigning object attributes to variables
let student = {
    firstname: "Anna",
    lastname: "Hills"
};
// destructuring assignment to new variable
let { firstname:fname, lastname:lname} = student;
console.log(fname); // Anna
console.log(lname); // Hills

The order of the name in the object is not important we can also write as,

let {lastname, firstname} = student;

How to assign default values?

We can pass default values as follows,

let student = {
    firstname: "Anna",
    lastname: "Hills"
};
let { firstname, lastname, age = 22} = student;
console.log(age); //22

Output:

22

How to assign multiple object properties to single variable?

const student = {
    firstname: 'Anna',
    lastname: 'Hills',
    age: 25,
    gender: 'female'    
}
// destructuring assignment
// assigning remaining properties to rest
let { firstname, ...details } = student;

console.log(firstname); // Anna
console.log(details); // {lastname: 'Hills', age: 25, gender: 'female'}

Output:

Anna

{ lastname: ‘Hills’, age: 25, gender: ‘female’ }

Nested destructuring in objects:

In order to execute the nested destructuring assignment for objects, you have to enclose the variables in an object structure (by enclosing inside {}).

const student = {
    name: 'Anna',
    age: 26,
    details: {
        course: 'Javascript',
        fee: 2500
    }
}
// nested destructuring 
const {name, details: {course, fee}} = student;
console.log(name); // Anna
console.log(course); // Javascript
console.log(fee); // 2500

Output:

Anna

Javascript

2500

Array destructuring:

To destruct arrays we use square brackets [ ].

Syntax:

const [var1,var2,…] = arrayName;

Example:

const arrValue = ['Anna', 'Tina', 'Lena'];

// destructuring assignment in arrays
const [x, y, z] = arrValue;

console.log(x); 
console.log(y); 
console.log(z); 

Output:

Anna

Tina

Lena

How to assign default values?

let arr1 = [20];
// assigning default value 5 and 7
let [x = 10,  y = 9] = arr1;
console.log(x); // 20
console.log(y); // 9

In the above array we have one element so when we assign default values only y=9 is assigned and x=20 remains the same.

Output:

20

9

How to swap variables using destructuring assignment?

// program to swap variables
let x = 10;
let y = 20;
// swapping variables
[x, y] = [y, x];
console.log(x); // 20
console.log(y); // 10

Output:

20

10

How to skip assigning unwanted items in an array?

We can skip assigning unwanted items in an array to a local variable using comma ( , )

const arr1 = ['one', 'two', 'three'];
// destructuring assignment in arrays
const [x, , z] = arr1;
console.log(x); // one
console.log(z); // three

Output:

one

three

How to assign multiple elements to a single variable?

We can assign multiple elements to a single variable using spread syntax …

const arr1 = ['one', 'two', 'three', 'four'];
// destructuring assignment in arrays
// assigning remaining elements to y
const [x, ...y] = arr1;
console.log(x); // one
console.log(y); // ["two", "three", "four"]

Output:

one

[ ‘two’, ‘three’, ‘four’ ]

one is assigned to x variable. Rest of the elements are assigned to y variable.

Nested Destructuring Assignment in arrays:

In order to execute the nested destructuring assignment, you have to enclose the variables in an array structure (by enclosing inside []).

Nested destructuring in arrays is

// nested array elements
const arr1 = ['one', ['two', 'three', 'four']];
// nested destructuring assignment in arrays
const [a, [b, c, d]] = arr1;
console.log(a); // one
console.log(b); // two
console.log(c); // three
console.log(d); // four

Output:

one

two

three

four

Destructuring allows for instantly mapping an object or array to many variables.