Understanding let and const in JavaScript

Everything you need to know in one place!

🌟 Introduction

In JavaScript, let and const are two modern ways to declare variables, introduced in ES6 (2015). They help us write cleaner and safer code compared to the old var.

🔍 What is let?

• Block scoped
• Can be updated
• Cannot be redeclared in the same scope

let count = 1;
count = 2; // ✅ Allowed
let count = 3; // ❌ SyntaxError (already declared in same scope)

🔒 What is const?

• Block scoped
• Cannot be updated or redeclared
• Must be initialized at the time of declaration

const name = 'Aman';
name = 'Rahul'; // ❌ TypeError (Assignment to constant variable)

📦 Scope Differences

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

{
  let x = 10;
  const y = 20;
}
// console.log(x); // ❌ ReferenceError
// console.log(y); // ❌ ReferenceError

⚠️ Temporal Dead Zone

Using let or const before declaration causes a ReferenceError.

console.log(a); // ❌ ReferenceError
let a = 5;

✅ When to use what?

• Use const by default
• Use let if the variable needs to change