Variables are containers for storing data values in JavaScript. They allow you to store numbers, strings, objects, or even functions for later use or manipulation. In modern JavaScript, there are three ways to declare variables: var, let, and const.
Declaring Variables
var
Older way to declare variables.
Function-scoped.
Can be redeclared and updated.
var name = "Alice";
var name = "Bob"; // Redeclaration is allowed
let
Introduced in ES6 (2015).
Block-scoped.
Can be updated, but not redeclared in the same scope.
let age = 25;
age = 30; // ✅ Allowed
// let age = 35; ❌ Not allowed in same scope
const
Block-scoped.
Cannot be updated or redeclared.
Must be initialized at the time of declaration.
const country = "India";
// country = "USA"; ❌ Error: Assignment to constant variable
Naming Rules and Conventions
Variable names must begin with a letter, $, or _.
They are case-sensitive.
Use camelCase for naming: firstName, totalAmount.
let $price = 100;
let _temp = 37;
let userName = "John";
Variable Scope
Global Scope: Variables declared outside any function or block.
Function Scope (var): Accessible anywhere inside the function.
Block Scope (let, const): Limited to { } block.
function example() {
var x = 10;
if (true) {
let y = 20;
const z = 30;
console.log(x); // ✅ 10
console.log(y); // ✅ 20
}
// console.log(y); ❌ ReferenceError
}
Hoisting
Variables declared with var are hoisted to the top of their scope, but initialized as undefined.
let and const are hoisted too, but not initialized, which leads to a temporal dead zone (TDZ).
console.log(a); // undefined (due to hoisting)
var a = 5;
console.log(b); // ❌ ReferenceError
let b = 10;
Examples
var city = "Delhi";
let temperature = 32;
const PI = 3.14;
temperature = 30; // ✅
city = "Mumbai"; // ✅
// PI = 3.1416; ❌ Error
