JavaScript Variables and Data Types (Beginner Friendly Guide)

Every program needs a way to store information.
In JavaScript, this is done using variables.

In this guide we will learn:

• What variables are

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

• Primitive data types (string, number, boolean, null, undefined)

• The difference between var, let, and const

• What scope means (simple explanation)

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1. What Are Variables?

Think of a variable like a box that stores information.

Example:

Box label → Name
Inside box → "Tushar"

In JavaScript:

let name = "Tushar";

Here:

• name → the box

• "Tushar" → the value stored in the box

Variables store data so it can be used or changed later in the program. (GeeksforGeeks)

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2. Declaring Variables in JavaScript

JavaScript allows three ways to create variables:

• var

• let

• const

Example:

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

These keywords tell JavaScript to reserve memory to store values. (CoreLangs.org)

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3. Primitive Data Types

The type of data stored inside a variable is called a data type.

Here are the most common primitive types.

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1. String (text)

Used to store text.

let name = "Tushar";

Examples:

"Hello"
"JavaScript"
"My name is Tushar"

Strings represent text data. (Cratecode)

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2. Number

Used to store numbers.

let age = 21;
let price = 99.99;

Numbers can be:

• integers → 10

• decimals → 3.14

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3. Boolean

Boolean means true or false.

let isStudent = true;

Example uses:

isLoggedIn = true
isAdmin = false

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4. Undefined

A variable declared but not assigned a value becomes undefined.

let score;
console.log(score);

Output:

undefined

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5. Null

null means intentional empty value.

let selectedUser = null;

This means:

The variable exists
But currently has no value

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4. Difference Between var, let, and const

These keywords behave differently.

Example

var x = 10;
let y = 20;
const z = 30;

Comparison Table

Keyword	Can change value?	Scope	Notes
var	Yes	Function scope	Old way
let	Yes	Block scope	Modern JS
const	No	Block scope	Value cannot be reassigned

var variables are function-scoped, while let and const are block-scoped and preferred in modern JavaScript. (GeeksforGeeks)

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5. Changing Variable Values

Using let

Values can change.

let age = 20;
age = 21;
console.log(age);

Output

21

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Using const

Values cannot be reassigned.

const pi = 3.14;
pi = 3.1415; // Error

const variables must be assigned when created and cannot be reassigned later. (techtestlog.com)

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6. What Is Scope? (Beginner Explanation)

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

Example:

{
let message = "Hello";

  console.log(message);

}

console.log(message); // Error

Why?

Because message only exists inside the block { }.

Simple idea:

Scope = area where a variable is accessible

Types (simple view):

Global scope → accessible everywhere
Local scope → accessible only inside a block or function

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Scope Visualization

Program
│

├── Global variables

│

└── Code block

     └── Local variables

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Quick Summary

Concept	Meaning
Variable	A container for storing data
var	Old way of declaring variables
let	Variable that can change
const	Variable that cannot be reassigned
string	Text data
number	Numeric values
boolean	true or false
null	intentionally empty value
undefined	declared but not assigned
scope	where a variable can be used

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💡 Best practice in modern JavaScript

Use const by default
Use let when value needs to change
Avoid var