Difference between let, const, and var

Question

What’s the difference between let, const, and var?

Answer

In JavaScript:

var is function-scoped, hoisted, and can be re-declared.
let and const are block-scoped and introduced in ES6.
const must be initialized and cannot be reassigned.

if (true) {
  var a = 10;
  let b = 20;
  const c = 30;
}

console.log(a); // ✅ 10
console.log(b); // ❌ ReferenceError
console.log(c); // ❌ ReferenceError

Hoisting Difference: var declarations are hoisted with undefined, while let and const are hoisted but remain in the temporal dead zone until initialized.

Real-World Example

Use const for constants, let for values that change, and never use var unless maintaining legacy code.

Quick Practice

Predict the output:

console.log(a);
var a = 5;

Now replace var with let. What happens?

Summary

Use const by default, let for mutable bindings, and avoid var. It’s 2025 — no excuses.