temporal dead zone (TDZ)

The temporal dead zone (TDZ) is the time between entering a scope and running a let, const, or class declaration in it, during which any read or write of the declared name throws a ReferenceError.

Hoisting creates a binding for each of these declarations, the link between its name and its value, as soon as its scope starts, but leaves it uninitialized until the declaration runs. The ECMAScript specification (ECMA-262) doesn’t use the name TDZ. It says that a let or const variable may not be accessed in any way until its declaration is evaluated.

The word temporal refers to execution order. The zone ends when the declaration runs, so a function written above the declaration can read the binding if the call happens later. Even typeof, which returns "undefined" for a name that was never declared, throws on a binding in its TDZ.

function describeLimit() {
  return `Upload limit: ${uploadLimit} files`;
}

try {
  console.log(describeLimit());
} catch (error) {
  console.log(`${error.name}: ${error.message}`);
}

const uploadLimit = 5;
console.log(describeLimit());

$ node upload-limit.mjs
ReferenceError: Cannot access 'uploadLimit' before initialization
Upload limit: 5 files

> const discountCode = " SPRING10 ";
undefined

> if (discountCode) {
…   const discountCode = discountCode.trim();
… }
Uncaught ReferenceError: Cannot access 'discountCode' before initialization