Welcome back to our JavaScript tutorial series! 🚀Today, we will learn Variables and Data Types in JavaScript with clear examples.
So far:

• Day 1 – We introduced JavaScript and wrote our first program.
• Day 2 – We learned about adding JavaScript to HTML and structuring our code.

Now, on Day 3, we will cover variables and data types – the building blocks that allow us to store and work with data in JavaScript.

By the end of this blog, you’ll understand:

• What variables are and why we use them
• How to declare variables (var, let, const)
• The different data types in JavaScript

What is a Variable?

A variable is like a container that stores data.
Think of it as a box with a label: you can put something inside (like a number or text), and later open the box to use it.

Example:

let name = "Anand";
let age = 25;

console.log(name); // Anand
console.log(age);  // 25

let name = "Anand";
let age = 25;

console.log(name); // Anand
console.log(age);  // 25

Here, name and age are variables.

Declaring Variables in JavaScript

JavaScript gives us three ways to declare variables:

(a) var (Old way)

var city = "Jaipur";

var city = "Jaipur";

• Used in older JavaScript versions.
• Not recommended today because it has problems with scope.

(b) let (Modern way)

let country = "India";
country = "USA"; // allowed

let country = "India";
country = "USA"; // allowed

• Introduced in ES6 (2015).
• Can be reassigned.
• Preferred for variables that may change.

(c) const (Constant values)

const pi = 3.14;
pi = 3.14159; // ❌ Error, cannot reassign

const pi = 3.14;
pi = 3.14159; // ❌ Error, cannot reassign

• Value cannot be changed after declaration.
• Best for values that should stay fixed (like configuration, constants).

Naming Rules for Variables

When naming variables, keep these rules in mind:
✔ Must start with a letter, _, or $
✔ Cannot start with a number
✔ Case-sensitive (age and Age are different)
✔ Use meaningful names (not x, y, unless for short math codes)

✅ Good:

let userName = "Anand";
let totalMarks = 95;

let userName = "Anand";
let totalMarks = 95;

❌ Bad:

let a = "Anand";
let b = 95;

let a = "Anand";
let b = 95;

Data Types in JavaScript

JavaScript has two categories of data types:

(A) Primitive Data Types

These are basic building blocks:

1. String – Text data

let fruit = "Mango";

let fruit = "Mango";

2. Number – Integers and decimals

let price = 99.99;

let price = 99.99;

3. Boolean – True or False values

let isLoggedIn = true;

let isLoggedIn = true;

4. Undefined – A variable declared but not assigned a value

let x;
console.log(x); // undefined

let x;
console.log(x); // undefined

5. Null – Represents “nothing” or empty value

let emptyBox = null;

let emptyBox = null;

6. Symbol – Unique values (advanced, used for special identifiers)

let id = Symbol("id");

let id = Symbol("id");

7. BigInt – For very large numbers

let bigNumber = 1234567890123456789012345678901234567890n;

let bigNumber = 1234567890123456789012345678901234567890n;

(B) Non-Primitive Data Type

1. Object – Stores collections of data

let person = {
  name: "Anand",
  age: 25
};
console.log(person.name); // Anand

let person = {
  name: "Anand",
  age: 25
};
console.log(person.name); // Anand

2. Array – Stores multiple values in a list

let colors = ["red", "green", "blue"];
console.log(colors[0]); // red

let colors = ["red", "green", "blue"];
console.log(colors[0]); // red

Mini Project – User Profile

Let’s combine variables and data types into a small project.

const name = "Anand";
let age = 25;
let isStudent = true;
let skills = ["JavaScript", "React", "Node.js"];

let userProfile = {
  name: name,
  age: age,
  student: isStudent,
  skills: skills
};

console.log(userProfile);

const name = "Anand";
let age = 25;
let isStudent = true;
let skills = ["JavaScript", "React", "Node.js"];

let userProfile = {
  name: name,
  age: age,
  student: isStudent,
  skills: skills
};

console.log(userProfile);

👉 Output:

{
  name: "Anand",
  age: 25,
  student: true,
  skills: ["JavaScript", "React", "Node.js"]
}

{
  name: "Anand",
  age: 25,
  student: true,
  skills: ["JavaScript", "React", "Node.js"]
}

Here we used strings, numbers, booleans, arrays, and objects all together.

Key Takeaways

• Variables are containers for storing data.
• Use let and const (avoid var).
• JavaScript has 7 primitive types (string, number, boolean, null, undefined, symbol, bigint) and objects/arrays for complex data.
• Always give meaningful names to variables.

What’s Next?

In Day 4 of our JavaScript tutorial series, we’ll explore Type Conversions & Operators. You’ll learn how to convert data from one type to another (e.g., string to number) and use operators to perform calculations.