JavaScript Interview Question: var vs let vs const and Hoisting

Short answer: var is function-scoped and hoisted with the value undefined. let and const are block-scoped and can't be used before their declaration (the temporal dead zone). const can't be reassigned.

Scope

if (true) {
  var a = 1;
  let b = 2;
}
console.log(a); // 1 - var ignores blocks
console.log(b); // ReferenceError - b only exists inside the block

Hoisting and the temporal dead zone

console.log(x); // undefined - declaration hoisted, value not
var x = 5;

console.log(y); // ReferenceError: Cannot access 'y' before initialization
let y = 5;

All declarations are hoisted, but let/const stay uninitialised until their line runs. Function declarations are hoisted completely, which is why you can call them before they appear.

const doesn't mean immutable

const user = { name: 'Asha' };
user.name = 'Ravi';     // fine - the object can change
user = {};              // TypeError - the binding can't be reassigned

const frozen = Object.freeze({ name: 'Asha' }); // shallow immutability

Other differences

• var can be re-declared in the same scope; let/const can't.
• Top-level var creates a property on the global object in scripts; let/const don't.
• let in a for loop gets a fresh binding per iteration (see the closure loop question).

What to use

const by default, let when you need to reassign, and no var in modern code.