Objects in JavaScript

Objects in JavaScript: A Complete Overview

In JavaScript, an object is a collection of key-value pairs where keys are strings (or Symbols) and values can be of any type.

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1. Declaring an Object

Using Object Literals (Most Common)

let person = {
  name: "Rahul",
  age: 25,
  city: "Delhi",
};

Using the Object Constructor

let person = new Object();
person.name = "Rahul";
person.age = 25;

2. Accessing & Updating Properties

console.log(person.name); // "Rahul"
console.log(person["age"]); // 25
person["age"] = 26; // Update existing
person.city = "Mumbai"; // Update existing
person.country = "India"; // Add new property

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4. Object Reference Concept

• When you create an object, JavaScript:
  • Allocates memory in the heap
  • Creates a reference in the stack
  • The variable holds this reference, not the actual object
• When you assign an object to another variable, only the reference is copied

let obj1 = { name: "A" }; // 1. Object created in heap
// 2. Reference stored in stack
// 3. obj1 points to heap location

let obj2 = obj1; // Only the reference is copied
// Both variables point to same heap location

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5. Shallow Copy vs Deep Copy

• Shallow Copy (nested objects not include)

  let objA = { name: "Rahul", details: { age: 25 } };
  let copyByAssign = Object.assign({}, objA); // Shallow copy
  let copyBySpread = { ...objA }; // Shallow copy

• Deep Copy (Copies everything)

  let objA = { name: "Rahul", details: { age: 25 } };
  let byJson = JSON.parse(JSON.stringify(objA)); // Deep copy
  let byClone = structuredClone(objA);

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6. Object Comparison (== vs ===)

Objects are compared by reference, not by value.

let obj1 = { name: "Rahul" };
let obj2 = { name: "Rahul" };

console.log(obj1 === obj2); // false (Different memory references)
console.log(obj1 == obj2); // false

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7. Freezing & Sealing Objects

• Object.freeze(obj) → Prevents adding, modifying, or deleting properties.

  let obj = { name: "Rahul" };
  Object.freeze(obj);
  obj.name = "Amit"; // No effect
  obj.age = 30; // No effect
  console.log(obj); // { name: "Rahul" }

• Object.seal(obj) → Prevents adding or deleting properties but allows modifications.

  let obj = { name: "Rahul" };
  Object.seal(obj);
  obj.name = "Amit"; // Allowed
  delete obj.name; // Not allowed
  obj.age = 30; // Not allowed
  console.log(obj); // { name: "Amit" }

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