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Objects in JavaScript

JavaScript objects are containers that store related data and functionality. They contain keys, also known as properties, that are associated with values. Here, we'll cover different ways of creating objects, accessing and modifying their properties, making them immutable or extensible, and more.

Creating Objects

Object Literal Notation and Syntax

One of the simplest ways to create an object in JavaScript is using object literal notation. It involves defining an object and its properties inside curly braces {}.

let book = {
title: "1984",
author: "George Orwell",
year: 1949

In this code snippet, we create a book object with properties title, author, and year. The property names are followed by a colon : and their corresponding values.

Using the Object Constructor

The Object constructor is another way to create an object. This is a more explicit approach, and it can be used when you want to create an empty object, and then add properties to it. The blueprint for the object must be outlined first - this can be done be using object-oriented programming in JavaScript.

let book = new Object();
book.title = "1984"; = "George Orwell";
book.year = 1949;

In this snippet, we first create an empty book object using the new Object() constructor. We then add properties to the book object using dot notation.

Constructor Functions and 'new'

Constructor functions allow us to define a template for creating objects. They start with a capital letter to distinguish them from regular functions. When called with the new keyword, a new object is created and returned.

function Book(title, author, year) {
this.title = title; = author;
this.year = year;

let book = new Book("1984", "George Orwell", 1949);

Here, Book is a constructor function that takes title, author, and year as arguments. We use the this keyword to set properties on the object being created. We then create a new book object by calling Book with the new keyword.

Accessing Properties

Dot Notation

Dot notation is the most common way to access properties of an object. It involves using a dot . followed by the property name.

let title = book.title; // "1984"

In this example, we access the title property of the book object.

Bracket Notation

Bracket notation can also be used to access object properties. It can be handy when the property name is stored in a variable or the property name is not a valid identifier.

let propertyName = "author";
let author = book[propertyName]; // "George Orwell"

In this example, we use bracket notation to access the author property of the book object using a variable.

Nested Properties

Objects can contain other objects as properties. To access nested properties, we chain the property names together using dot notation or bracket notation.

let book = {
title: "1984",
author: {
firstName: "George",
lastName: "Orwell"
year: 1949

let authorLastName =; // "Orwell"

In this example, the book object contains an author object as a property. We access the lastName property of the author object by chaining the property names together.

Adding Properties

You can add properties to an existing object using dot notation or bracket notation.

book.publisher = "Secker & Warburg";
book["ISBN"] = "978-0451524935";

In this snippet, we add a publisher property and an ISBN property to the book object.

Modifying Properties

Modifying object properties is similar to adding properties. You simply assign a new value to the property using dot notation or bracket notation.

book.year = 1950;
book["title"] = "Nineteen Eighty-Four";

Here, we modify the year and title properties of the book object.

Removing Properties

The delete operator can be used to remove properties from an object.

delete book.year;
delete book["ISBN"];

In this snippet, we remove the year and ISBN properties from the book object.

Serialization and Deserialization

Serialization is the process of converting an object into a string format that can be easily stored or transmitted and then later reconstructed. In JavaScript, we use JSON.stringify() for serialization. Deserialization is the reverse process, converting the string back into an object. For this, we use JSON.parse().

let serializedBook = JSON.stringify(book);
// '{"title":"Nineteen Eighty-Four","author":"George Orwell","publisher":"Secker & Warburg"}'

let deserializedBook = JSON.parse(serializedBook);
// {title: "Nineteen Eighty-Four", author: "George Orwell", publisher: "Secker & Warburg"}

In this example, JSON.stringify() is used to serialize the book object into a string, and JSON.parse() is used to deserialize that string back into an object.

Cloning and Copying

There are various ways to clone or copy an object in JavaScript. One of the simplest ways is using the spread operator ....

let bookCopy = {};

In this example, the spread operator is used to create a shallow copy of the book object. Note that this only creates a shallow copy, so nested objects will still be references.


To enumerate, or loop over, the properties of an object, we can use a loop.

for (let property in book) {
console.log(property + ": " + book[property]);

In this snippet, a loop is used to iterate over each property in the book object and log it to the console.


Destructuring is a convenient way of extracting multiple properties from an object. It reduces the amount of code needed to access these properties.

let { title, author } = book;
console.log(title); // "Nineteen Eighty-Four"
console.log(author); // "George Orwell"

Here, we use destructuring to extract the title and author properties from the book object.

Immutability and Extensibility


The Object.freeze() method can be used to make an object immutable, meaning its properties cannot be added, deleted, or changed.

book.year = 1985; // This will be ignored

After calling Object.freeze() on the book object, any attempts to modify the object's properties will be ignored.


The Object.seal() method prevents new properties from being added to an object and marks all existing properties as non-configurable. However, the values of existing properties can still be changed.

book.year = 1985; // This will

book.ISBN = "1234567"; // This will be ignored

In this snippet, after calling Object.seal() on the book object, we can modify existing properties, but we can't add new ones.


The Object.preventExtensions() method prevents new properties from being added to an object, but it allows existing properties to be deleted or changed.

book.year = 1985; // This will work
book.ISBN = "1234567"; // This will be ignored

After calling Object.preventExtensions() on the book object, we can modify or delete existing properties, but we can't add new ones.