Temporal Dead Zone (TDZ) in JavaScript

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Introduction

A temporal dead zone is the area of a block where a variable is inaccessible until the computer completely initializes it with a value.

• Block is a pair of braces ({...}) used to group multiple statements.
• Initialization occurs when you assign an initial value to a variable.

Temporal Dead Zone is the period of time during which the let and const declarations cannot be accessed.

Example:

Before Stepping deep into this concept I would recommend you to know about Hoisting in JavaScript.

let and const have two broad differences from var:

• They are block-scoped.

• Accessing a var before it is declared has the result undefined; accessing a let or const before it is declared throws ReferenceError:

Where exactly does the TDZ begin and end?

The above example shows that let and const variables are not accessible before they are initialized with some value, and the phase between the starting of the execution of the block in which the let or const variable is declared till that variable is being initialized is called Temporal Dead Zone for the variable.
In other words, a block’s temporal dead zone starts at the beginning of the block’s local scope. It ends when the computer fully initializes your variable with a value.

And during this zone JavaScript will always through a reference error if anyone tries to access those variables.

Example:

Above Code Result: JavaScript will return a ReferenceError because we used the console.log() code to access age before its complete initialization. In other words, we invoked age within the temporal dead zone.

Start and End of TDZ: In the above example, Temporal Dead Zone starts after the opening parenthesis of the printAge function and continues until after the declaration of the age variable.

The Temporal Dead Zone is not about where, but about when the let variable or const constant is accessed. Temporal Dead Zone relates to time, not the space above the declaration of let or const.

In above example before calling to log() the age constant is declared and initialized so it is accessible in log function.

Conclusion:

The most efficient way to overcome the errors of the temporal dead zone is to initialize the variables at the top of the scope so that when our code starts running it completes the initializing part at first and then uses those variables otherwise you can run into a lot of unexpected errors in JavaScript code.

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