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

mahammadmansur95 — Tue, 10 Jun 2025 16:54:43 +0000

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.

DEV Community: mahammadmansur95

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

🔁 1. Scope: Global, Function, and Block

👉 var – Function Scope

👉 let and const – Block Scope

📦 2. Hoisting Behavior

👉 var is hoisted and initialized.

👉 let and const are hoisted but not initialized