Object Definition in JavaScript

In JavaScript, an object is a data structure that stores data as key–value pairs.

Each key (property name) is usually a string or symbol, and each value can be any data type—such as numbers, strings, arrays, functions, or other objects.

Objects are used to represent real-world entities (like a user, car, or product) by grouping related data and behavior together, making them a flexible way to organize and manage information in JavaScript.

Basic Syntax

Objects are commonly defined using object literals, which use curly braces {} to contain key-value pairs separated by commas.

const person = {
  name: "John",
  age: 30,
  profession: "Engineer"
};

Accessing Object Properties

You can access an object’s properties in two ways:

1. Dot Notation

Used when the property name is known and valid.

console.log(person.name); // "John"

2. Bracket Notation

Used when property names are dynamic or contain special characters.

console.log(person["age"]); // 30
const key = "profession";
console.log(person[key]); // "Engineer"

Adding, Modifying, and Deleting Properties

JavaScript objects are dynamic, meaning you can add, modify, or delete properties at runtime.

Add a Property

person.country = "India";
console.log(person);
// { name: "John", age: 30, profession: "Engineer", country: "India" }

Modify a Property

person.age = 31;
console.log(person.age); // 31

Delete a Property

delete person.profession;
console.log(person);
// { name: "John", age: 31, country: "India" }

Computed Property Names (ES6+)

Allows defining dynamic property keys using expressions.

const prop = "id";
const obj = { [prop]: 123 };
console.log(obj.id); // 123

Objects Can Contain Functions (Methods)

When a function is defined inside an object, it’s called a method.

Methods allow objects to perform actions or define behavior.

const car = {
  brand: "Tesla",
  start: function() {
    console.log("Car started");
  }
};

car.start(); // "Car started"

ES6 Shorthand Method Syntax

You can define methods more concisely:

const car = {
  brand: "Tesla",
  start() {
    console.log("Car started");
  }
};

Using this Keyword

this refers to the current object inside a method.

const car = {
  brand: "Tesla",
  start() {
    console.log(`${this.brand} started`);
  }
};
car.start(); // "Tesla started"

Key Features of JavaScript Objects

1. Dynamic

Objects are flexible — you can add, update, or remove properties at any time.

person.hobby = "Reading";

2. Nested

Objects can contain other objects, arrays, or complex structures.

const student = {
  name: "Sara",
  marks: {
    math: 90,
    science: 85
  }
};
console.log(student.marks.math); // 90

3. Reference Type

Objects are stored and compared by reference, not by value.

const obj1 = { a: 1 };
const obj2 = obj1;
obj2.a = 2;
console.log(obj1.a); // 2

4. Foundation of JavaScript

Many JavaScript structures — such as arrays, functions, and dates — are special types of objects.

console.log(typeof []); // "object"

Example: Nested Object

Objects can represent hierarchical or structured data.

const student = {
  name: "Sara",
  marks: {
    math: 90,
    science: 85
  },
  details: {
    address: {
      city: "Mumbai",
      country: "India"
    }
  }
};

console.log(student.marks.math); // 90