var, let & const

Variable Declaration Keywords

var (ES5)

• Function-scoped or globally-scoped
• Can be redeclared and updated
• Hoisted with undefined value

var name = "John";
var name = "Jane"; // Redeclaration allowed
name = "Bob"; // Update allowed

function example() {
    var x = 1;
    if (true) {
        var x = 2; // Same variable
        console.log(x); // 2
    }
    console.log(x); // 2
}

let (ES6)

• Block-scoped
• Cannot be redeclared in same scope
• Can be updated
• Hoisted but not initialized (Temporal Dead Zone)

let age = 25;
// let age = 30; // Error: Cannot redeclare
age = 30; // Update allowed

function example() {
    let y = 1;
    if (true) {
        let y = 2; // Different variable (block scope)
        console.log(y); // 2
    }
    console.log(y); // 1
}

const (ES6)

• Block-scoped
• Cannot be redeclared or updated
• Must be initialized at declaration
• Hoisted but not initialized (Temporal Dead Zone)

const PI = 3.14159;
// const PI = 3.14; // Error: Cannot redeclare
// PI = 3.14; // Error: Cannot update

// Must initialize
// const value; // Error: Missing initializer

Important Differences

Scope Comparison

function scopeExample() {
    var varVariable = "var";
    let letVariable = "let";
    const constVariable = "const";

    if (true) {
        var varInBlock = "var in block";
        let letInBlock = "let in block";
        const constInBlock = "const in block";
    }

    console.log(varInBlock); // "var in block" - accessible
    // console.log(letInBlock); // Error: not defined
    // console.log(constInBlock); // Error: not defined
}

Hoisting Behavior

// var hoisting
console.log(hoistedVar); // undefined (not error)
var hoistedVar = "I'm hoisted";

// let/const hoisting (Temporal Dead Zone)
// console.log(hoistedLet); // Error: Cannot access before initialization
let hoistedLet = "I'm in TDZ";

// console.log(hoistedConst); // Error: Cannot access before initialization
const hoistedConst = "I'm in TDZ";

Object/Array with const

// const prevents reassignment, not mutation
const arr = [1, 2, 3];
arr.push(4); // Allowed - mutating content
console.log(arr); // [1, 2, 3, 4]
// arr = [5, 6, 7]; // Error: Cannot reassign

const obj = { name: "John" };
obj.name = "Jane"; // Allowed - mutating property
obj.age = 25; // Allowed - adding property
console.log(obj); // { name: "Jane", age: 25 }
// obj = {}; // Error: Cannot reassign

Best Practices

1. Use const by default
2. Use let when you need to reassign the variable
3. Avoid var in modern JavaScript
4. Always declare variables at the top of their scope

Quick Reference Table

Feature	var	let	const
Scope	Function/Global	Block	Block
Redeclare	✅	❌	❌
Update	✅	✅	❌
Hoisting	✅ (undefined)	✅ (TDZ)	✅ (TDZ)
Initialize	Optional	Optional	Required