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Temporal Dead Zone (TDZ) in JavaScript: let and const Explained

What is Temporal Dead Zone (TDZ)?

TDZ (Temporal Dead Zone) is the time period between the start of a variable's scope (e.g., if block, for, function, etc.) and its actual initialization, during which the variable cannot be accessed.

This zone occurs when using let or const.

Simple Example

console.log(x); // ❌ ReferenceError: Cannot access 'x' before initialization
let x = 10;

Although variable x is declared, you get an error because you accessed it before initialization — you're in the temporal dead zone.

TDZ applies to:

let
const
Function parameters with destructuring
Classes declared via class

Conclusion

Variable Type	Hoisted	Initialized	TDZ Exists
`var`	Yes	undefined	No
`let`	Yes	No	Yes
`const`	Yes	No	Yes

Important:

TDZ is the reason why let and const are safer than var: you cannot use variables before they're declared, and this prevents errors.

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What is Temporal Dead Zone (TDZ)?Simple Example TDZ applies to:Conclusion