Understanding Variables and Data Types in JavaScript

Think of variables as box that stores information,so it you multiple box then you need to label the box to understand what is stored inside.

In programming varibale stores data and vairable name is label

let name = "John";

Here name is label and John is value

How to declare varaibles

we can use let,var & const to declare variable

let age = 25;
const country = "USA"
var isStudent=true

Primitives data type

• String

let name = "Alice";

• number

let age = 20;

• Boolean

let isStudent = true;

• null

let middleName = null;

• undefined

let score;

When to user var,let & const:
var : declares function-scoped or globally-scoped variables
let : declares a block-scoped variable
const: declares a block-scoped variable, assigning a value that cannot be re-assigned.

What is block-scopewhere
Scope means where your variable is used.

Think of variable as scope in your room:

• if you put your box in living room everyone can see it
• if you put your box in your bedroom only you can see it

{
  let message = "Hello";
  console.log(message); // ✅ Works here
}

console.log(message); // ❌ Error (not accessible outside)

Declare Variables

let name = "Emma";
let age = 22;
let isStudent = true;

Try Changing

age = 23; // ✅ Allowed
isStudent = false; // ✅ Allowed

Now try with const:

const country = "Canada";
country = "USA"; // ❌ What happens?

Scope Visulization

Global Scope
  ├── name
  ├── age
  └── Block Scope
        └── message