Temporal Dead Zone (TDZ)

TL;DR

Explanation

TDZ is the period between entering a scope and reaching a variable's declaration. Unlike var (hoisted and initialised to undefined), let/const are hoisted but not initialised — accessing them in the TDZ throws ReferenceError: Cannot access 'x' before initialization. This is intentional — it prevents the var hoisting confusion. TDZ affects: let/const in blocks, class declarations (classes are in TDZ unlike function declarations), default parameter values that reference earlier parameters. typeof is the only operation that doesn't throw in the TDZ.

Common Misconception

Why It Matters

Common Mistakes

• Using a let variable before its declaration in the same block.
• Expecting class expressions to behave like function declarations (they're in TDZ).
• Not knowing typeof variable still throws in TDZ for let/const.

Code Examples

console.log(x); // ReferenceError — TDZ
let x = 5;

// Class TDZ:
const obj = new MyClass(); // ReferenceError
class MyClass {}

// Always declare before use:
let x = 5;
console.log(x); // 5

// Classes too:
class MyClass {}
const obj = new MyClass(); // Fine

// Safe check without TypeError:
if (typeof possiblyUndeclared !== 'undefined') {}

References

• ↗ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let#temporal_dead_zone_tdz

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