Course Overview

Overview of history of the JavaScript language

History of JavaScript

Overview of the relationship between JavaScript and Java

JavaScript vs Java

Overview of how JavaScript is evolved as a programming language

How is it evolved?

Overview of how we can track upcoming features in the JavaScript language

How to track upcoming features?

Overview of how the ECMAScript is versioned

How is ECMAScript versioned?

Explanation of whether JavaScript is interpreted or a compiled language

Is it interpreted or compiled?

Explanation of how JavaScript is executed

How is it executed?

Execution Context

Explanation of what is a callstack and how it is used in JavaScript

Callstack

Overview of memory management in JavaScript

Memory

What is JavaScript

What is hoisting?

Explanation of common misconception about hoisting in JavaScript

Misconception about hoisting

Hoisting

What is scope?

Explanation of global scope in JavaScript

Global scope

Explanation of function scope in JavaScript

Function scope

Block scope

Explanation of module scope in JavaScript

Module scope

Scope chain

Scope

What is coercion?

Diving into inner details of  coercion in JavaScript

Deeper look at coercion

Explanation of what abstract operations are and common types of abstract operations in JavaScript

Abstract operations

Explanation of how abstract or loose equality works in JavaScript

Abstract equality operator

Explanation of how the addition operator works in JavaScript

Addition operator

Explanation of how relational operators work in JavaScript

Relational operators

Coercion exercise for testing and practicing the knowledge learned in this module

Coercion exercise

Coercion

What is a closure?

Explanation of a common misconception about closures in JavaScript

Misconception about closures

Explanation of the problem of closures inside loops and how to fix it

Closures inside loops

Explanation of closures can be used to hide data

Data hiding using closures

Closures

What is prototypal inheritance?

Prototype chain

__proto__ property

Explanation of how we can create objects with custom prototypes in JavaScript

Custom prototypes

Explanation of how prototype chain is set with ES6 classes

ES6 classes and prototypes

Prototypes

Explanation of this keyword in JavaScript

What is this?

Explanation of how "this" works inside arrow functions

Arrow functions and "this"

Explanation of how value of "this" can be set explicitly

Explicitly binding "this"

globalThis

Summarizing "this"

'this' keyword

What is a symbol?

Explanation of why symbols were added in JavaScript

Why symbols?

Symbols and privacy

Explanation of global symbols in JavaScript

Global symbols

Explanation of what well-known symbols are in JavaScript

Well-known symbols

Symbol

Overview of asynchronous behavior of JavaScript

Overview

Overview of how callbacks help us write asynchronous code in JavaScript

Callbacks

Event loop

Promise

Explanation of promise chaining in JavaScript

Promise chaining

Explanation of some static methods of promises and their use cases

Use cases for static Promise methods

Explanation of async-await syntax in JavaScript

async await

Explanation of microtasks, how they are processed, and how they are linked to promises

Microtasks

Explanation of common anti-patterns to avoid when using promises in JavaScript

Promise anti-patterns

Asynchronous JavaScript

Explanation of what is an iterator in JavaScript

Iterators

Explanation of what is a generator in JavaScript

Generators

Explanation of how we can create asynchronous iterators in JavaScript

Asynchronous iterators

Explanation of how we can create asynchronous generators in JavaScript

Asynchronous generators

Iterators and Generators

Overview of what is debugging and different ways to debug JavaScript

Debugging

Overview of using the "debugger" statement to debug JavaScript

Debugger statement

Overview of using the breakpoints in browser developers' tools to debug JavaScript

Breakpoints in browser

Overview of using the VS Code debugger to debug JavaScript

VS Code debugger

Debugging JavaScript

Final thoughts and advice regarding how to continue the exciting journey of learning JavaScript

Conclusion

Wrap up

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Become a confident JavaScript pro by unraveling the most challenging aspects of the language. With easy-to-understand explanations and code examples, you’ll gain a deeper and solid understanding of JavaScript's most complex features.

Advanced JavaScript Unleashed is for beginner and intermediate developers alike, who want to elevate their JavaScript skills and become senior engineers by gaining a deeper understanding of the core topics of the language.

Advanced JavaScript Unleashed

 works inside arrow functions, let us first explore the problem with using the 

 keyword inside regular functions. Consider the following code example:

You can run the above code in the Replit below:

 as an output? The reason for unintended output is the incorrect value of 

 gets set inside a function. It depends on 

 the function is called. In the code example above, how is 

 function called? Is it called as a "method" or as a standalone function? It is called a "function", so the value of 

 depends on whether our code is in strict mode or not. Assuming that our code is in non-strict mode, the value of 

 function is the global object, i.e., the 

 is added to a number, i.e., the value of the 

How can we fix this problem? How can we ensure that the value of 

 function is what we want it to be? There are multiple ways to handle this problem. One way is to save the value of 

 function before returning a function, and inside the returned function, use the variable containing the value of 

. The following code example shows this approach in action:

The approach shown above was commonly used to fix similar problems where the value of 

 from the surrounding context was needed instead of the one from the current function where 

 was actually used. In the code example above, we needed the value of 

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title: Arrow functions and "this"
description: Explanation of how "this" works inside arrow functions
isPublicLesson: false
useMdx: true
code: https://gitlab.com/fullstackio/books/newline-course-apps/advanced-javascript-unleashed-app/-/tree/main/module_07/lesson_07.02
---

Before diving into how `this` works inside arrow functions, let us first explore the problem with using the `this` keyword inside regular functions. Consider the following code example:

```js
function Counter(startingValue) {
  this.value = startingValue;
}

Counter.prototype.incrementFactory = function (incrementStep) {
  return function () {
    this.value += incrementStep;
    console.log(this.value);
  };
};

const counter = new Counter(0);
const increment5 = counter.incrementFactory(5);
increment5(); // NaN
increment5(); // NaN
increment5(); // NaN
```

You can run the above code in the Replit below:

<ReplitEmbed src="https://replit.com/@newlineauthors/arrow-functions-and-this-example1" />

Why did we get `NaN` as an output? The reason for unintended output is the incorrect value of `this` inside the function returned from the `incrementFactory` function.

Recall how the value of `this` gets set inside a function. It depends on _how_ the function is called. In the code example above, how is `increment5` function called? Is it called as a "method" or as a standalone function? It is called a "function", so the value of `this` depends on whether our code is in strict mode or not. Assuming that our code is in non-strict mode, the value of `this` inside the `increment5` function is the global object, i.e., the `window` object in the case of browsers. So, `this.value` is actually `window.value`, and it is `undefined` because the `window` object, by default, doesn't have a `value` property. As a result, we get the `NaN` value when `undefined` is added to a number, i.e., the value of the `incrementStep` parameter.

How can we fix this problem? How can we ensure that the value of `this` inside the `increment5` function is what we want it to be? There are multiple ways to handle this problem. One way is to save the value of `this` inside the `incrementFactory` function before returning a function, and inside the returned function, use the variable containing the value of `this` instead of directly using `this`. The following code example shows this approach in action:

```js
function Counter(startingValue) {
  this.value = startingValue;
}

Counter.prototype.incrementFactory = function (incrementStep) {
  const thisVal = this; // save `this` value
  return function () {
    // use `thisVar` variable instead of `this`
    thisVal.value += incrementStep;
    console.log(thisVal.value);
  };
};

const counter = new Counter(0);
const increment5 = counter.incrementFactory(5);
increment5(); // 5
increment5(); // 10
increment5(); // 15
```

You can run the above code in the Replit below:

<ReplitEmbed src="https://replit.com/@newlineauthors/arrow-functions-and-this-example2" />

The approach shown above was commonly used to fix similar problems where the value of `this` from the surrounding context was needed instead of the one from the current function where `this` was actually used. In the code example above, we needed the value of `this` from the surrounding context of the `incrementFactory` function and not the one inside the `increment5` function.

## Arrow functions to the rescue