let vs var vs const

function demo() {
  if (true) {
    var a = 1;
    let b = 2;
  }
  a;    // 1 — var ignores the block
  b;    // ReferenceError
}

const does not mean immutable

It prevents reassigning the binding, not changing the value:

const user = { name: "Mai" };
user.name = "Nam";     // fine — the object is mutable
user = {};             // TypeError — cannot rebind

Use Object.freeze if you need the contents protected too.

The loop case

let creates a fresh binding per iteration; var shares one:

for (var i = 0; i < 3; i++) setTimeout(() => console.log(i));   // 3, 3, 3
for (let i = 0; i < 3; i++) setTimeout(() => console.log(i));   // 0, 1, 2

This alone justifies dropping var.

Hoisting is not the difference people think

All three are hoisted. var is also initialised to undefined, so reading it early is legal and silently useless. let and const stay uninitialised until their declaration line — the temporal dead zone — so reading early throws at the exact point of the mistake.

What to use

const by default, let when you genuinely reassign, var never. Starting with const also makes reassignment visible in review: if a variable is let, the reader knows to look for where it changes.