javascript - What is the temporal dead zone?

Question

I've heard that accessing let and const values before they are initialized can cause a ReferenceError because of something called the temporal dead zone.

What is the temporal dead zone, how does it relate to scope and hoisting, and in what situations is it encountered?

Answer 1

let and const have two broad differences from var:

1. They are block scoped.
2. Accessing a var before it is declared has the result undefined; accessing a let or const before it is declared throws ReferenceError:

console.log(aVar); // undefined
console.log(aLet); // Causes ReferenceError: Cannot access 'aLet' before initialization

var aVar = 1;
let aLet = 2;