Understanding Variables and Data Types in JavaScript
What are Variables?
Think of a variable like a box that stores information.
The box name = variable name
The thing inside the box = value
For example:
A box labeled "name" can store "Arnab"
A box labeled "age" can store 20
Declaring Variables in JavaScript :
var
let
const
1. Using var
var name = "Arnab";
console.log(name);
2. Using let
let age = 20;
console.log(age);
The value of a let variable can be changed later:
age = 21;
console.log(age);
3. Using const
1. Using const
const country = "India";
console.log(country);
The value of a const variable cannot be changed:
country = "USA" ; // ❌ Error
Primitive Data Types
JavaScript has some basic data types you’ll use daily:
1. String (Text)
Used for names, messages, etc.
let name = "Arnab" ;
2. Number
Used for age, marks, money, etc.
let age = 20 ;
3. Boolean (True/False)
Used for yes/no type values.
let isStudent = true;
4. Null (Empty Value on Purpose)
let result = null;
5. Undefined (No Value Assigned)
let marks ;
console.log(marks) ;
Basic Difference Between var, let, and const
What Is Scope?
Scope means where you can use a variable.
Think of it like a classroom :
If you speak loudly → everyone hears (global scope)
If you whisper to a friend → only they hear (local scope)
Example:
let name = "Arnab" ; // global
function greet( ) {
let message = "Hello" ; // local
console.log(message);
}
console.log(name) ;
message works only inside the function.
name works everywhere.
How Values Change (Except const)
let score = 50;
console.log(score) ;
score = 80;
console.log(score) ;
Works because it's let.
But:
const score = 50;
score = 80; // ❌ Error
Diagram Ideas
Comparison table of var, let, and const
Simple scope visualization diagram
