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Justen Hintz

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What is JavaScript Hoisting?

Learn to code better with a clearer understanding of JS hoisting

Hoisting is JS’s default behavior of defining all the declarations at the top of the scope before code execution. One of the benefits of hoisting is that it enables us to call functions before they appear in the code. JavaScript only hoists declarations, not initializations. You can find the official documentation here.

Understanding what exactly is JavaScript Hoisting

Variable declaration and initialization occur in the following sequence:

Declaration –> Initialization/Assignment –> Usage

// Variable lifecycle
let x;                 // Declaration
x = “hoisting”;        // Assignment
console.log(x);        // Usage

However, since JavaScript permits both declaration and initialization of variables simultaneously, this is the most used pattern:

let x = “hoisting”;

Most importantly, you should always remember that the JavaScript declares the variable first in the background. Then, initializing them. Thus, it is also good to know that the processing of variable declarations takes place before the execution of any code. However, until the execution of code assigning them takes place the undeclared variables do not exist in JavaScript.

Therefore, when the assignment is executed, a value assigned to an undeclared variable implicitly creates it as a global variable. This specifies that all undeclared variables are global variables.

// hoisting
function Hoisting(){
  x = 100;
  let y = 200;
}
Hoisting();
console.log(x); // 100
console.log(y); // Reference Error: y is not defined

In the above code sample, there is a function called Hoisting(). Thus, we have a variable which we didn’t declare using let/var/const and a let variable* y*. The assigning of the undeclared variable to the global scope is done by JavaScript. But for variable y, we get a ReferenceError.

Hosting in ES5

In ES5, we consider the *var *keyword. Hoisting with var is somewhat different in comparison to let/const. The example with var to see how hoisting works:

var num (global)
console.log(car);    // undefined
var car = ‘Lamborgini’;

In the above code, when logging the variable name which was declared and assigned later than using it, the compiler gives an “undefined” result. This was not expected as we should have got ReferenceError as trying to use the car variable even before declaring it.

But the interpreter sees this differently, which is as follows:

//how interpreter sees the above code
var car;
console.log(car); // undefined
car = ‘Lamborgini’;

Hosting in Function scoped variable

Let’s learn how **hoisting **of function-scoped variables takes place with the following example:

//function scoped
function myFunc(){
  console.log(car);
  var car = ‘Lamborgini’;
}
myFunc();                // undefined

However, there is no difference as in comparison to the code where the declaration of the global variable takes place. The interpreter provides the result as “undefined”.

//function scoped
function myFunc(){
 var car;
 console.log(car);
 car = ‘Lamborgoni’;
}
myFunc();                // undefined

In order to mitigate this, we can make sure to declare and assign the variable at the same time, before using it. Thus, the code below shows the declaration at the top:

function myFunc(){
  var car = ‘Lamborgini’;
  console.log(car)         // Lamborgini
}  
myFunc();

Hoisting in ES6

We can see the example below:

let num(global)
console.log(num);
let num = 003;         // ReferencError: num is not defined

In the var keyword, we expect the output of the log to be undefined. However, since the *let *in es6 doesn’t take allow using undeclared variables, the interpreter throws a Reference error explicitly. This makes sure that we always declare our variable first.

The let and const Keywords.

Variables and constants declared with let or const are not hoisted!

**JavaScript **Initializations are Not Hoisted

JavaScript only hoists declarations, not initializations.

var a = “volkswagon”; // Initialize a
var b = “Lamborgini”; // Initialize b
elem = document.getElementById("car"); // Find an element
elem.innerHTML = a + " " + b;       // Display a and b as volkswagon and lamborgini

In the above code, since the declaring of variables takes place before the results. As a result, the execution of the code prints the result of variable a and b.

var a = “i10”;  // Initialize a
elem = document.getElementById("car");      // Find an element
elem.innerHTML = "a is " + a + “ and b is " + b;  // Display a and b
var b = “Lamborgini”;  // Initialize b
Result:
a is i10 and b is undefined

Hence, this is because only hoisting of the declaration (var b) takes place, not the initialization (= “Lamborgini”) to the top. Because of hoisting, b has been declared before it is used, but because initializations are not hoisted, the value of b is undefined.

Hoisting classes

JavaScript classes can be classified into two classes:

  • Class declarations
  • Class expressions

In Class declarations

They are much like function counterparts. Therefore, this means no hoisting of JavaScript class declarations. However, they remain uninitialized until evaluation. Thus, this effectively means that you have to declare a class before you can use it.

var car1 = new car();
car1.height = 5;
car1.weight = 500;
console.log(car1);   // Output: ReferenceError: car is not defined
class car{
  constructor(height, weight) {
    this.height = height;
    this.weight = weight;
  }
}

In the above code, the reference error occurs. This is because, after initialization of the car1 variable, the defining of car class is takes place. To solve this error, we just need to define the car class before car1 initialization. This is hosting in the class declaration.

class car{
  constructor(height, weight) {
    this.height = height;
    this.weight = weight;
  }
}
var car1 = new car();
car1.height = 5;
car1.weight = 500;
console.log(car1);

Therefore, this gives the proper result.

In Class expressions

They are much like their function counterparts. Therefore, this means no hoisting of class expression.

Therefore, below is an example with the un-named or anonymous variant of the class expression:

var rect = new shapes();
rect.height = 10;
rect.width = 20;
console.log(rect); // Output: TypeError: shapes is not a constructor
var shapes = class {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }
};

Thus, the correct way to do it is like this:

var shapes = class {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }
};
var rect = new shapes();
rect.height = 10;
rect.width = 20;
console.log(rect);

Summary

To many developers, Hoisting is an unknown behavior of JavaScript. Many developers overlook it’s importance as well. Moreover, if a developer doesn’t understand hoisting, programs may contain bugs (errors). In order to avoid bugs, always declare all variables at the beginning of every scope. Thus, this is how JavaScript interprets the code, it is always a good rule.

#javascript #web-development

What is GEEK

Buddha Community

What is JavaScript Hoisting?

Understanding Hoisting in JavaScript

In JavaScript, Hoisting is the default behavior where variables and function declarations are moved to the top of their scope before code execution.

No Matter where function and variable are declared, it moved up top on their own scope. Doing this, it allows us to call functions before even writing them in our code.

Image for post

How interpreter sees the above code:

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We Know, In JavaScript, when we have a variable that is not defined then it occurs an undefined error. So in the above example, JavaScript only hoists the declarations part and we got an undefined error.

It’s important to keep in mind that, JavaScript only hoists declarations, not the initializations.

let us take another example,

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why this time we got a ReferenceError? Because of trying to access a previously undeclared variable And remember JavaScript only hoists declarations. So Initialisation can’t be hoisted and we got an error.

ES6: Let Keyword

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Like before, for the var keyword, we expect the output to be undefined. But this time we got a reference error. That Means let and const variables not hoisted? The answer is Variables declared with let are still hoisted, but not initialized, inside their nearest enclosing block. If we try to access it before initializing will throw ReferenceError due being into Temporal Dead Zone.

Hoisting functions

Like variables, the JavaScript engine also hoists the function declarations. And it allows us to call functions before even writing them in our code.

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Rahul Jangid

1622207074

What is JavaScript - Stackfindover - Blog

Who invented JavaScript, how it works, as we have given information about Programming language in our previous article ( What is PHP ), but today we will talk about what is JavaScript, why JavaScript is used The Answers to all such questions and much other information about JavaScript, you are going to get here today. Hope this information will work for you.

Who invented JavaScript?

JavaScript language was invented by Brendan Eich in 1995. JavaScript is inspired by Java Programming Language. The first name of JavaScript was Mocha which was named by Marc Andreessen, Marc Andreessen is the founder of Netscape and in the same year Mocha was renamed LiveScript, and later in December 1995, it was renamed JavaScript which is still in trend.

What is JavaScript?

JavaScript is a client-side scripting language used with HTML (Hypertext Markup Language). JavaScript is an Interpreted / Oriented language called JS in programming language JavaScript code can be run on any normal web browser. To run the code of JavaScript, we have to enable JavaScript of Web Browser. But some web browsers already have JavaScript enabled.

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JavaScript Hello World Program

In JavaScript, ‘document.write‘ is used to represent a string on a browser.

<script type="text/javascript">
	document.write("Hello World!");
</script>

How to comment JavaScript code?

  • For single line comment in JavaScript we have to use // (double slashes)
  • For multiple line comments we have to use / * – – * /
<script type="text/javascript">

//single line comment

/* document.write("Hello"); */

</script>

Advantages and Disadvantages of JavaScript

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Niraj Kafle

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The essential JavaScript concepts that you should understand

As a JavaScript developer of any level, you need to understand its foundational concepts and some of the new ideas that help us developing code. In this article, we are going to review 16 basic concepts. So without further ado, let’s get to it.

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Ajay Kapoor

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