When we write programs, we often need to store information so we can use it later.

For example:

• A user’s name

• A person’s age

• Whether a user is logged in or not

In JavaScript, we store information using variables.

In this article, we will learn:

• What variables are and why they are needed

• How to declare variables using var, let, and const

• Primitive data types in JavaScript

• The basic difference between var, let, and const

• What scope means (explained simply)

All examples are beginner-friendly and easy to try in the browser console or Node.js.

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What Are Variables?

Think of a variable like a box that stores information.

You can put something inside the box, and later you can take it out and use it.

Example in real life:

• A box labeled "name" might store "Alex"

• A box labeled "age" might store 25

In JavaScript, variables work in a similar way.

Example

let name = "Alex";
let age = 25;

Here:

• name is a variable storing "Alex"

• age is a variable storing 25

We can use these variables later in the program.

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

Output:

Alex
25

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Declaring Variables in JavaScript

In JavaScript, we create variables using:

• var

• let

• const

These keywords tell JavaScript that we are creating a variable.

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Using let

let is the most commonly used way to create variables today.

It allows the value of the variable to change later.

Example:

let age = 20;

console.log(age);

age = 21;

console.log(age);

Output:

20
21

Here the value changed from 20 to 21.

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Using const

const stands for constant, meaning the value cannot be changed after it is assigned.

Example:

const country = "India";

console.log(country);

Output:

India

If we try to change it:

country = "USA";

JavaScript will give an error, because const variables cannot be reassigned.

So use const when the value should stay the same.

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Using var

var was used in older JavaScript code before let and const were introduced.

Example:

var city = "Delhi";

console.log(city);

Output:

Delhi

Today, most developers prefer let and const instead of var, because they behave more predictably.

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Primitive Data Types in JavaScript

Variables can store different types of data.

The most common primitive data types are:

• string

• number

• boolean

• null

• undefined

Let’s look at each one.

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String

A string is used to store text.

Example:

let name = "Alex";
let city = "Kolkata";

Here "Alex" and "Kolkata" are strings.

Strings are usually written inside quotes.

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Number

A number is used for numeric values.

Example:

let age = 22;
let price = 199;

These are numbers.

JavaScript uses the same type for integers and decimals.

Example:

let rating = 4.5;

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Boolean

A boolean represents true or false values.

Example:

let isLoggedIn = true;
let hasPaid = false;

Booleans are often used in conditions and decision making.

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null

null represents an intentional empty value.

Example:

let selectedUser = null;

This means the variable currently has no value, but it may get one later.

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undefined

undefined means a variable has been declared but no value has been assigned yet.

Example:

let score;

console.log(score);

Output:

undefined

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Basic Difference Between var, let, and const

Here is a simple comparison.

Keyword	Can change value?	Modern usage
var	Yes	Rarely used now
let	Yes	Commonly used
const	No	Very commonly used

Example:

let age = 20;
age = 21; // allowed

const birthYear = 2002;
birthYear = 2003; // error

General rule developers follow:

• Use const by default

• Use let when the value needs to change

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What is Scope? (Simple Explanation)

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

In simple terms:

Scope defines where a variable is visible or usable.

Example:

let age = 20;

if (age > 18) {
  let message = "You are an adult";
  console.log(message);
}

Inside the block, message works fine.

But outside:

console.log(message);

This will give an error, because the variable only exists inside the block.

So its scope is limited to that block.

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Variables are one of the most fundamental concepts in JavaScript.

They allow us to store and work with information in our programs.

Key things to remember:

• Variables store data

• let, const, and var are used to declare variables

• Data types define the kind of data stored

• Scope determines where variables can be accessed