let, const & Block Scope

Before ES6, JavaScript had only var. It had confusing rules — variables could “leak” outside if blocks and loops. ES6 added let and const with block scope: variables live only inside the { } where you declare them.

const — value you will not reassign

const API_URL = 'https://api.toolliyo.com';
const MAX_LOGIN_ATTEMPTS = 5;

// API_URL = 'other';  // ❌ Error — cannot reassign const

Use const by default for variables that should not point to a new value.

let — value that can change

let score = 0;
score = score + 10;  // ✅ OK
score += 5;

Block scope in action

if (user.isLoggedIn) {
  const welcome = 'Hello, ' + user.name;
  let visitCount = 1;
}
// console.log(welcome);  // ❌ Error — not visible outside block

function updateCart(items) {
  let total = 0;
  for (const item of items) {
    const lineTotal = item.price * item.qty;  // const — per iteration
    total += lineTotal;
  }
  return total;
}

updateCart([
  { name: 'Node.js ebook', price: 499, qty: 1 },
  { name: 'React course', price: 999, qty: 2 }
]); // 2497

const with objects and arrays

const prevents reassignment, not mutation:

const user = { name: 'Alex', role: 'student' };
user.role = 'admin';     // ✅ OK — editing property
// user = {};            // ❌ Error — reassigning variable

const tags = ['javascript', 'es6'];
tags.push('react');      // ✅ OK
// tags = [];            // ❌ Error