Variables var, let, and const

In JavaScript, variables are used to store data values. There are three ways to declare a variable: var, let, and const.

var – Old way (pre‑ES6). Function‑scoped, can be redeclared, and is hoisted.

var name = "Alice";
var name = "Bob"; // redeclaration allowed
console.log(name); // Bob

let – Introduced in ES6. Block‑scoped, cannot be redeclared in the same scope, but can be updated.

let age = 25;
age = 26; // allowed

// let age = 30; // Error: Identifier 'age' has already been declared

const – Also ES6. Block‑scoped, cannot be updated or redeclared. Must be initialized at declaration.

const PI = 3.14159;
// PI = 3.14; // Error: Assignment to constant variable
// const GRAVITY; // Error: Missing initializer in const declaration

For objects and arrays declared with const, the reference cannot change, but properties/elements can be modified.

const person = { name: "Alice" };
person.name = "Bob"; // allowed
// person = { name: 'Charlie' }; // Error

Best practice: Use const by default, let when you need to reassign, and avoid var in modern code.