What is the temporal dead zone (TDZ)?

The temporal dead zone is the period between entering a scope and the line where a let or const binding is initialized. During the TDZ, accessing the variable throws a ReferenceError.

console.log(a) // undefined — var is hoisted and initialized
var a = 1

console.log(b) // ReferenceError — TDZ
let b = 2

Why it exists

• Prevents using block-scoped variables before they are declared (catching bugs var would silently allow).
• let/const are hoisted to the top of their block, but unlike var they are not initialized until the declaration is evaluated.

let x = 1
{
  console.log(x) // ReferenceError — inner `x` is in TDZ
  let x = 2
}

Interview contrast with var: var is function-scoped, hoisted, and initialized to undefined. let/const are block-scoped, hoisted to the block, and trapped in TDZ until initialized. const additionally requires an initializer and disallows reassignment (the binding is fixed, though object contents can mutate).