JavaScript Object

What is an Object in JavaScript?

An object in JavaScript is a collection of key-value pairs, where:

  • Keys are called properties
  • Values can be any type: string, number, array, function, even another object

Objects are used to store structured data and model real-world entities.


Creating an Object


1. Object Literal Syntax (most common) 

let person = {
  name: "Alice",
  age: 30,
  isStudent: false
};

console.log(person); // Output: { name: "Alice", age: 30, isStudent: false }

2. Using new Object() 

let user = new Object();
user.name = "Bob";
user.age = 25;

console.log(user); // Output: { name: "Bob", age: 25 }

Accessing Object Properties


1. Dot Notation 

console.log(person.name); // Output: Alice

2. Bracket Notation 

console.log(person["age"]); // Output: 30

Bracket notation is useful when the property name is dynamic or contains spaces/special characters.


Modifying Object Properties 

person.age = 31;            // Update
person.country = "India";   // Add new property
delete person.isStudent;    // Delete a property

console.log(person);
// Output: { name: "Alice", age: 31, country: "India" }

Object Methods


An object can have functions as values — these are called methods. 

let car = {
  brand: "Toyota",
  start: function () {
    return "Car is starting...";
  }
};

console.log(car.start()); // Output: Car is starting...

Shorter method syntax (ES6+): 

let car = {
  brand: "Toyota",
  start() {
    return "Car is starting...";
  }
};

this Keyword in Objects


Inside an object method, this refers to the object itself. 

let user = {
  name: "Emma",
  greet() {
    return "Hello, " + this.name;
  }
};

console.log(user.greet()); // Output: Hello, Emma

Looping Through Object Properties


Using for...in loop 

let student = { name: "Max", grade: "A" };

for (let key in student) {
  console.log(key + ": " + student[key]);
}
// Output:
// name: Max
// grade: A

Useful Object Methods

Method Description
Object.keys(obj) Returns array of keys
Object.values(obj) Returns array of values
Object.entries(obj) Returns array of [key, value] pairs
Object.assign() Copies properties from one or more objects to another

Example

let book = { title: "JS Basics", pages: 100 };

console.log(Object.keys(book));   // Output: ["title", "pages"]
console.log(Object.values(book)); // Output: ["JS Basics", 100]
console.log(Object.entries(book));// Output: [["title", "JS Basics"], ["pages", 100]]

Nested Objects


Objects can contain other objects: 

let user = {
  name: "John",
  address: {
    city: "New York",
    zip: 10001
  }
};

console.log(user.address.city); // Output: New York

Objects vs Arrays

Feature Object Array
Key Type Strings or Symbols Indexed (0, 1, 2...)
Use Case Named data Ordered list
Syntax { key: value } [value1, value2]

Quick Recap

  • Objects store key-value pairs.
  • Keys are strings (or Symbols), values can be any data type.
  • Methods are functions stored in object properties.
  • Use this inside methods to refer to the object.
  • Use for...in, Object.keys(), Object.values() for accessing properties.

Example Summary 

let person = {
  name: "Liam",
  age: 28,
  greet() {
    return `Hi, I'm ${this.name}`;
  }
};

console.log(person.greet()); // Output: Hi, I'm Liam