Yesterday, I started my JavaScript journey by understanding what JavaScript is and how it brings websites to life.

Today, I explored one of the most fundamental concepts in programming—Variables and Data Types.

Every JavaScript program, from a simple calculator to a complex web application, relies on variables to store and manage data.

Let's dive in.

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What is a Variable?

A variable is a named container used to store data.

Instead of writing the same value repeatedly, we store it in a variable and use it whenever needed.

For example:

let name = "Saurabh";

console.log(name);

Here, name is the variable, and "Saurabh" is the value stored inside it.

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Declaring Variables

JavaScript provides three ways to declare variables:

• var

• let

• const

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1. var

var age = 22;

var is the older way of declaring variables.

It is function-scoped and allows redeclaration, which can sometimes lead to unexpected behavior.

Today, it's generally avoided in modern JavaScript.

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2. let

let city = "Mumbai";

let is block-scoped and allows you to update the value later.

let score = 10;

score = 20;

This is useful when the value needs to change during program execution.

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3. const

const country = "India";

A const variable cannot be reassigned.

const pi = 3.14;

// pi = 3.14159 ❌ Error

Use const whenever the value should remain constant.

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Difference Between var, let, and const

Feature	var	let	const
Scope	Function	Block	Block
Redeclare	✅ Yes	❌ No	❌ No
Reassign	✅ Yes	✅ Yes	❌ No
Modern Usage	Rare	Common	Preferred

In modern JavaScript, developers mainly use let and const.

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What are Data Types?

A data type tells JavaScript what kind of value a variable holds.

JavaScript has two categories:

• Primitive Data Types

• Non-Primitive (Reference) Data Types

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Primitive Data Types

1. String

Stores text.

let language = "JavaScript";

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2. Number

Stores integers and decimal values.

let marks = 95;
let price = 99.99;

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3. Boolean

Represents true or false.

let isLoggedIn = true;

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4. Undefined

A variable that has been declared but not assigned a value.

let user;

console.log(user);

Output:

undefined

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5. Null

Represents the intentional absence of a value.

let selectedUser = null;

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6. BigInt

Used for very large integers.

let bigNumber = 123456789012345678901234567890n;

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7. Symbol

Used to create unique identifiers.

const id = Symbol("userId");

Symbols are commonly used in advanced JavaScript applications.

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Non-Primitive Data Types

The most common non-primitive type is the Object.

const student = {

name: "Saurabh",

age: 22

};

Arrays and functions are also objects in JavaScript.

You'll explore them in detail in upcoming lessons.

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The typeof Operator

JavaScript provides the typeof operator to check a variable's data type.

Example:

let age = 22;

console.log(typeof age);

Output:

number

More examples:

typeof "Hello";     // string

typeof true;        // boolean

typeof undefined;   // undefined

typeof {};          // object

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Naming Variables

Good variable names improve code readability.

✔ Use meaningful names:

let userName;

const totalPrice;

Avoid unclear names:

let x;

let a1;

Meaningful names make code easier to understand and maintain.

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Best Practices

✔ Prefer const by default.

✔ Use let when the value needs to change.

✔ Avoid using var in modern JavaScript.

✔ Choose descriptive variable names.

✔ Use typeof to inspect values while learning.

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My Biggest Takeaway

Today, I realized that variables are like labeled containers that help us store and organize information in a program.

Understanding the difference between let, const, and var, along with JavaScript's data types, has given me a strong foundation for writing meaningful code.

Every interactive application starts by storing and working with data—and today, I learned how JavaScript handles that data.