JS Puzzles · Const

Temporal Dead Zone

The temporal dead zone is the interval after a lexical binding's scope starts but before execution initializes its let, const, or class declaration. Reading the binding during that interval throws a ReferenceError.

How it works

A let, const, or class binding exists from the beginning of its block, but it is uninitialized until execution reaches its declaration. Reading it during that interval throws a ReferenceError; the relevant boundary is execution order, not merely where the declaration appears in the source.

Key points

For let, const, and class, the TDZ begins at block entry and ends when execution reaches the initialized declaration.
The binding exists conceptually but has no usable value during the TDZ.
Initialization happens at runtime when control reaches the declaration, not simply when the scope is created.
A let declaration without an initializer becomes undefined once execution reaches it.
The term applies to all three lexical declaration forms documented on the page.

Common mistakes

Reading a binding during its TDZ throws instead of producing undefined.
Moving execution order or calling code earlier can trigger the TDZ even when the declaration is visibly present below.

Example: Access after initialization

{
  try {
    console.log(value);
  } catch (error) {
    console.log(error instanceof ReferenceError);
  }

  const value = 42;
  console.log(value); // 42
}

The first access runs before initialization and throws; after the declaration executes, the binding can be read normally.

When to use it

Find reads before initialization

Debugging an early ReferenceError

Detect lexical shadowing hazards

Reviewing shadowed variables

Make runtime dependencies explicit

Designing initialization order

Interview questions

QWhat is the temporal dead zone?

A TDZ is the interval from the start of a block until execution reaches and initializes a let, const, or class declaration. Accessing the binding in that interval throws a ReferenceError.

Follow-up: Does source position alone determine the error? No; the ordering of execution and the declaration's initialization is what matters.

QHow does TDZ behavior differ from var?

let and const bindings are available only after execution reaches their declaration, so an earlier read fails. A var binding instead receives undefined when hoisted, so this particular TDZ error does not occur.

Follow-up: What about typeof? It still throws for a lexical binding inside its TDZ.

QWhen does a TDZ end?

The binding is initialized when execution reaches the declaration. With let and no initializer, that initialization gives the variable undefined; with const, an initializer is required.

Follow-up: Why must const have an initializer? It cannot be assigned later, so leaving it uninitialized would make it unusable.

let declarations
const declarations
class declarations
ReferenceError
block scope
variable hoisting

Source: let. Checked 2026-10-15.