Js interview #1 : var, let, and const in JavaScript – What's the Difference?

In JavaScript, variables can be declared using var, let, or const. Though they all serve the same fundamental purpose—declaring variables—they behave quite differently in terms of scope, hoisting, reassignment, and more.

Let’s break down their key differences.

🔁 1. Scope: Global, Function, and Block

Feature	var	let	const
Scope type	Function-scoped	Block-scoped	Block-scoped

👉 var – Function Scope

Variables declared with var are function-scoped, which means they are only accessible inside the function in which they are defined.

function example() {
  if (true) {
    var x = 10;
  }
  console.log(x); // ✅ 10 - accessible because var is function-scoped
}

👉 let and const – Block Scope

Both let and const are block-scoped, meaning they only exist within the enclosing {} block.

function example() {
  if (true) {
    let y = 20;
    const z = 30;
  }
  console.log(y); // ❌ ReferenceError
  console.log(z); // ❌ ReferenceError
}

📦 2. Hoisting Behavior

👉 var is hoisted and initialized.

console.log(a); // ✅ undefined
var a = 5;

👉 let and const are hoisted but not initialized

console.log(b); // ❌ ReferenceError (TDZ)
let b = 10;

🔁 3. Redeclaration & Reassignment

👉 var allows redeclaration and reassignment

var x = 1;
var x = 2; // ✅ No error
x = 3;     // ✅ No error

👉 let allows reassignment but not redeclaration

let y = 1;
let y = 2; // ❌ SyntaxError
y = 3;     // ✅ Allowed

👉 const allows neither

const z = 1;
const z = 2; // ❌ SyntaxError
z = 3;       // ❌ TypeError

⛔ 4. Temporal Dead Zone (TDZ)

The Temporal Dead Zone (TDZ) is the period between entering the scope and the actual declaration where variables declared with let and const cannot be accessed.

function test() {
  console.log(a); // ❌ ReferenceError (TDZ)
  let a = 10;
}

With var, accessing the variable before the declaration just returns undefined, but with let and const, it throws an error due to the TDZ.