JavaScript Hoisting and Temporal Dead Zone (TDZ)

Hoisting is a fundamental JavaScript behavior that often intimidates developers. Despite its apparent complexity, it's actually a quite simple concept and for sure something that every JavaScript developer should master. In this article, we'll explore what hoisting is, how it works, why it matters and understand what is the so called 'Temporal Dead Zone (TDZ)'.

What is JavaScript Hoisting?

Hoisting in JavaScript involves moving variable and function declarations to the top of their scope. This occurs during the compilation phase, before code execution. It's important to master this concept because, if not properly understood, it can lead to unexpected results.

Let's break down some key points:

Declaration vs Initialization

One crucial aspect to understand is that hoisting applies only to declarations, not initializations. This means that while the variable or function name is hoisted (elevated) to the top of its scope, its assignment remains in its original place in the code.

var vs let/const

Variables declared with 'var' are hoisted to the top of their scope. Although, since hoisting does not apply to initializations, variables declared with 'var' are initialized with 'undefined'.

console.log(name);   // Output: undefined
var name = "Marco";        

On the other hand, variables declared with 'let' and 'const' are also hoisted to the top of their scope. But instead of being initialized with 'undefined', they are kept unitialized until their actual declaration is found. Trying to access them before declaration results in a 'ReferenceError'.

console.log(name);   // Output: undefined
var name = "Marco";

console.log(age);   // ReferenceError: Cannot access 'age'...
let age = 25;        

Functions

One of the biggest advantages of hoisting is the ability to use functions before they are declared in the code. This is very useful in situations where functions need to be called before their actual declaration in the code.

greetMarco();   // Output: "Hello, Marco!"

function greetMarco(){
   console.log("Hello, Marco!");
}        

Temporal Dead Zone (TDZ)

Temporal Dead Zone (TDZ) refers to the phase between the hoisting of a variable declaration and its actual initialization.

During this time, any attempt to access the variable, results in a 'ReferenceError'.

This concept is specific to variables declared with 'let' and 'const'. As said above, variables declared with 'var' are initialized with 'undefined' and do not cause any error.

How Hoisting Really Works?

Hoisting doesn't physically move variables and functions to the top of the code. Instead, what really happens it that during the compilation phase, JavaScript engine scans through the code, identifies all variables and function declarations and allocates memory space for them in their respective scopes.

Conclusion

By moving the function declarations to the top of the code, hoisting allows developers to declare them In summary, JavaScript hoisting is a fundamental concept that should be mastered. By doing so, you'll avoid common bugs and issues while also improving the quality of your code.

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