💡 Day 2 — Variables, Constants & Data Types

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💡 Day 2 — Variables, Constants & Data Types

Welcome back, Rahul! 🎯 Now that you know what JavaScript is, let’s explore the **core foundation of programming** — storing and managing data using variables, constants, and data types.

📦 1️⃣ What Are Variables?

Variables are containers that store data in memory so you can use or modify it later. In JavaScript, variables are declared using:

  • var — old way (function-scoped)
  • let — modern way (block-scoped)
  • const — for values that never change
var name = "Krishna";
let age = 23;
const country = "India";

console.log(name, age, country);

🔍 2️⃣ Naming Rules

  • Must start with a letter, underscore (_), or $.
  • Cannot start with a number.
  • Case-sensitive (Name ≠ name).

🧮 3️⃣ Data Types in JavaScript

JavaScript has **primitive** and **non-primitive** data types.

Primitive Types:

  • String → text ("Hello")
  • Number → numeric values (42, 3.14)
  • Boolean → true or false
  • Undefined → declared but not assigned
  • Null → intentional empty value
let user = "krishna";
let score = 95;
let isPassed = true;
let grade;
let certificate = null;

console.log(typeof user);       // string
console.log(typeof score);      // number
console.log(typeof isPassed);   // boolean
console.log(typeof grade);      // undefined
console.log(typeof certificate); // object

🧩 4️⃣ String Interpolation (Template Literals)

Use backticks (`) and placeholders (${}) to embed variables directly inside strings. 

let name = "Rahul";
let age = 23;
console.log(`My name is ${name} and I am ${age} years old.`);

🧠 5️⃣ Dynamic Typing

JavaScript is a dynamically typed language — meaning you don’t need to specify data types; they’re assigned automatically. 

let value = 10;
console.log(typeof value); // number
value = "Ten";
console.log(typeof value); // string

🧮 6️⃣ Practice Task

  1. Create three variables for your name, age, and city.
  2. Use template literals to print them in a full sentence.
  3. Try changing variable values and observe output.

🎯 Summary

Today, you learned how to store, change, and describe data in JavaScript. Next, we’ll explore **Operators & Expressions** — where you start performing real logic and calculations.

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