Data Types

 1. Primitive Types

Stored by value

Types:

• String → "hello"
• Number → 10
• Boolean → true / false
• Undefined → variable declared but no value
• Null → intentionally empty

Example:

let name = "blackbox";   // String
let age = 20;           // Number
let isStudent = true;   // Boolean
let x;                  // Undefined
let y = null;           // Null

2. Reference Types

👉 Stored by reference (memory address)

Types:

• Object
• Array
• Function

Example:

let user = {
  name: "blackbox",
  age: 20
};

let arr = [1, 2, 3];

Key Difference (VERY IMPORTANT)

Primitive:

let a = 10;
let b = a;

b = 20;

console.log(a); // 10

👉 Copy created → separate memory

Reference:

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

obj2.name = "Alex";

console.log(obj1.name); // Alex 😱

👉 Same memory → both change