What is the temporal dead zone?

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;

It appears from these examples that let declarations (and const, which works the same way) may not be hoisted, since aLet does not appear to exist before it is assigned a value.

That is not the case, however—let and const are hoisted (like var, class and function), but there is a period between entering scope and being declared where they cannot be accessed. This period is the temporal dead zone (TDZ).

The TDZ ends when aLet is declared, rather than assigned:

// console.log(aLet) // Would throw ReferenceError

let aLet;

console.log(aLet); // undefined
aLet = 10;
console.log(aLet); // 10

This example shows that let is hoisted:

let x = "outer value";

(function() {
  // Start TDZ for x.
  console.log(x);
  let x = "inner value"; // Declaration ends TDZ for x.
}());

Credit: Temporal Dead Zone (TDZ) demystified.

Accessing x in the inner scope still causes a ReferenceError. If let were not hoisted, it would log outer value.

The TDZ is a good thing because it helps to highlight bugs—accessing a value before it has been declared is rarely intentional.

The TDZ also applies to default function arguments. Arguments are evaluated left to right, and each argument is in the TDZ until it is assigned:

// b is in TDZ until its value is assigned.
function testDefaults(a = b, b) { }

testDefaults(undefined, 1); // Throws ReferenceError because the evaluation of a reads b before it has been evaluated.

The TDZ is not enabled by default in the babel.js transpiler. Turn on “high compliance” mode to use it in the REPL. Supply the es6.spec.blockScoping flag to use it with the CLI or as a library.

Recommended further reading: TDZ demystified and ES6 Let, Const and the “Temporal Dead Zone” (TDZ) in Depth.