Variables in JavaScript are like containers that store information for later use. They allow you to save data, reuse it, and manipulate it dynamically in your programs.

Think of a variable as a labeled box: you can put data inside it, change it, and access it whenever you need.

1️⃣ Declaring Variables

There are three main ways to declare variables in JavaScript:

1. let (Modern, block-scoped)

let name = "Alice"; // Stores the string "Alice"
let age = 25;       // Stores the number 25

• Can change the value later
• Scope is block-level (inside { } only)

2. const (Constant, cannot be changed)

const pi = 3.1416; // Cannot reassign value later

• Used for values that should not change
• Trying to change a const value will cause an error

3. var (Older, function-scoped)

var city = "Paris";

• Older syntax, still works
• Scope is function-level, which can sometimes cause unexpected behavior
• Avoid using var in modern code

2️⃣ Variable Naming Rules

1. Names cannot start with a number
2. Names can include letters, numbers, $, or _
3. Names cannot contain spaces or special characters
4. Names are case-sensitive

Valid Examples:

let userName;
let $price;
let _age;
let totalCount;

Invalid Examples:

let 1stName; // Cannot start with number
let user-name; // Cannot use hyphen

3️⃣ Data Types Stored in Variables

JavaScript variables can store different types of data:

let name = "Alice";      // String
let age = 25;            // Number
let isStudent = true;    // Boolean
let colors = ["red","blue","green"]; // Array
let person = {name:"Alice", age:25}; // Object
let empty = null;        // Null
let notDefined;          // Undefined

• Strings → Text
• Numbers → Integer or Decimal
• Boolean → true / false
• Array → Collection of values
• Object → Key-value pairs

4️⃣ Changing Variable Values

Variables declared with let or var can be reassigned:

let age = 20;
age = 25; // New value
console.log(age); // 25

const cannot be reassigned:

const pi = 3.14;
pi = 3.1416; // ❌ Error: Cannot reassign constant

5️⃣ Using Variables in Operations

Variables can be used in mathematical operations or string concatenation:

let x = 10;
let y = 5;

console.log(x + y); // 15
console.log(x - y); // 5

let firstName = "Alice";
let lastName = "Smith";
console.log(firstName + " " + lastName); // Alice Smith

Compilation of Entire Code

Explanation:

• Variables userName, age, and pi are declared
• Clicking the button updates the paragraph dynamically using these variables
• age is incremented inside the function to show that variables can change