When learning JavaScript, one of the first things you encounter is variables and data types.

Variables allow us to store and manage information inside a program.

For example:

• A user’s name

• A person’s age

• Whether a student is enrolled or not

Understanding these concepts is essential because almost every program relies on variables.

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What Are Variables?

A variable is like a box used to store information.

Imagine you have labeled boxes:

Box Label	Stored Value
name	Viral
age	22
city	Ahmedabad

In programming, we store these values using variables.

Example:

let name = "Viral";
let age = 22;

Here:

• name stores "Viral"

• age stores 22

The program can use or modify these values later.

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Declaring Variables in JavaScript

In JavaScript, variables can be declared using:

• var

• let

• const

Example:

var city = "Ahmedabad";
let age = 22;
const country = "India";

Each keyword behaves slightly differently.

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var

var is the older way to declare variables.

Example:

var language = "JavaScript";

console.log(language);

Output:

JavaScript

However, modern JavaScript developers prefer let and const.

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let

let is used when the value may change later.

Example:

let score = 10;

score = 20;

console.log(score);

Output:

20

Here the value changed from 10 → 20.

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const

const is used when the value should not change.

Example:

const pi = 3.14;

console.log(pi);

If we try to change it:

pi = 3.1415;

JavaScript will produce an error.

So const is used for fixed values.

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Primitive Data Types in JavaScript

Data types define what type of value a variable holds.

Here are the most common primitive data types.

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String

Strings represent text values.

Example:

let name = "Viral Jain";

Examples of strings:

• Names

• Cities

• Messages

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Number

Numbers represent numeric values.

Example:

let age = 22;
let price = 199.99;

Numbers can represent:

• age

• marks

• price

• quantity

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Boolean

Booleans represent true or false values.

Example:

let isStudent = true;

Boolean values are often used in conditions.

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null

null means intentional empty value.

Example:

let selectedUser = null;

It means the variable currently has no value assigned intentionally.

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undefined

undefined means a variable exists but has not been assigned a value yet.

Example:

let phoneNumber;

console.log(phoneNumber);

Output:

undefined

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Difference Between var, let, and const

Feature	var	let	const
Modern JavaScript	No	Yes	Yes
Can change value	Yes	Yes	No
Recommended	No	Yes	Yes
Used for constants	No	No	Yes

In modern JavaScript:

• Use let for variables that change

• Use const for fixed values

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What is Scope? (Beginner Explanation)

Scope means where a variable can be accessed in your code.

Example:

{
  let message = "Hello World";
  console.log(message);
}

Inside the block { }, the variable is accessible.

But outside:

console.log(message);

This would cause an error because the variable exists only inside that block.

So scope defines where variables live and can be used.

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Assignment Practice

Try the following example in your browser console.

Step 1: Declare Variables

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

Step 2: Print Them

console.log(name);
console.log(age);
console.log(isStudent);

Output:

Viral
22
true

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Experiment With let and const

Changing a let value

let age = 22;
age = 23;

console.log(age);

Output:

23

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Changing a const value

const country = "India";

country = "USA";

This will produce an error because const values cannot change.

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