A JavaScript object is a collection of key–value pairs, used to store structured data.

1. Creating Objects

Object Literal (Most Common)

const user = {
  name: "John",
  age: 25,
  isAdmin: true
};

Using new Object()

const user = new Object();
user.name = "John";
user.age = 25;

2. Accessing Object Properties

Dot Notation

console.log(user.name);

Bracket Notation

console.log(user["age"]);

Use bracket notation when:

  • Property name has spaces
  • Property name is dynamic
const key = "name";
console.log(user[key]);

3. Adding, Updating, and Deleting Properties

user.city = "Dhaka";      // Add
user.age = 26;            // Update
delete user.isAdmin;      // Delete

4. Objects with Methods

Objects can contain functions (methods).

const user = {
  name: "John",
  greet() {
    console.log("Hello " + this.name);
  }
};

user.greet();

this refers to the current object.

5. Nested Objects

const user = {
  name: "John",
  address: {
    city: "Dhaka",
    zip: 1200
  }
};

console.log(user.address.city);

6. Looping Through Objects

for...in Loop

for (let key in user) {
  console.log(key, user[key]);
}

Object.keys()

Object.keys(user); // ["name", "age"]

Object.values()

Object.values(user); // ["John", 25]

Object.entries()

Object.entries(user); // [["name","John"],["age",25]]

7. Copying Objects

Shallow Copy

const user2 = Object.assign({}, user);

or using spread operator:

const user2 = { ...user };

Deep Copy

const deepCopy = JSON.parse(JSON.stringify(user));

⚠ Does not copy methods or undefined values.

8. Object Destructuring (ES6)

const user = { name: "John", age: 25 };

const { name, age } = user;

9. Object Properties Check

"user" in user; // false
"name" in user; // true

user.hasOwnProperty("name"); // true

10. Object Freezing & Sealing

Object.freeze()

Object.freeze(user);
user.age = 30; // ❌ Not allowed

Object.seal()

Object.seal(user);
user.age = 30; // allowed
user.city = "Dhaka"; // ❌ Not allowed

11. Reference Behavior of Objects

Objects are stored by reference.

const obj1 = { value: 10 };
const obj2 = obj1;

obj2.value = 20;
console.log(obj1.value); // 20

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