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

 keyword is among the most confusing concepts in the JavaScript language. The cause of confusion is the different ways in which the value of the 

In this module, we will try to demystify the 

 keyword. We will understand different ways in which the value of 

 keyword can be used in different contexts: inside functions, global scope, inside modules, etc. We will explore different contexts and how 

 keyword is mostly used inside functions to refer to the object using which the function was invoked. In other words, when a function is invoked as a "method" (invoked using an object), the 

 keyword becomes applicable for referencing the object used to invoke the function.

 keyword is like an implicit parameter passed to a function. Just like explicit function parameters, the value of implicit parameter 

. This is an important point. The value of 

 function in the code example above uses the 

 keyword, and looking at the code, we can tell that the 

 function will be an object with three properties: 

. But as mentioned earlier, the value of 

 object, and when a function is invoked using an object, the 

 inside that function refers to the object using which the function was invoked. So in our code example, 

 object has the three properties that are accessed using the 

 function, their values are logged to the console.

 refer to if the function is not invoked as a "method"? Consider the following code example:

function orderFood() {
  console.log("Order confirmed against the name: " + this.fullName);
}

orderFood();
// Order confirmed against the name: undefined

The answer to the question above depends on whether our code is executed in 

 inside a function, when not invoked as a method, refers to the global object, which in the case of browsers is the 

 in strict mode when not invoked as a method.

Can you guess in which mode the code was executed from the output of the code above? As 

, the code was executed in non-strict mode.

If we execute the code above in strict mode:

"use strict";

function orderFood() {
  console.log("Order confirmed against the name: " + this.fullName);
}

orderFood();
// Uncaught TypeError: this is undefined

---
title: What is this?
description: Explanation of this keyword in JavaScript
isPublicLesson: false
useMdx: true
code: https://gitlab.com/fullstackio/books/newline-course-apps/advanced-javascript-unleashed-app/-/tree/main/module_07/lesson_07.01
---

The `this` keyword is among the most confusing concepts in the JavaScript language. The cause of confusion is the different ways in which the value of the `this` keyword is set in different contexts.

In this module, we will try to demystify the `this` keyword. We will understand different ways in which the value of `this` is set. The `this` keyword can be used in different contexts: inside functions, global scope, inside modules, etc. We will explore different contexts and how `this` is set in those contexts.

## Function context

The `this` keyword is mostly used inside functions to refer to the object using which the function was invoked. In other words, when a function is invoked as a "method" (invoked using an object), the `this` keyword becomes applicable for referencing the object used to invoke the function.

The `this` keyword is like an implicit parameter passed to a function. Just like explicit function parameters, the value of implicit parameter `this` is set when the function is _invoked_. This is an important point. The value of `this` inside a function depends on _how_ that function is _called_.

Consider the following code example:

```js
const student = {
  id: 123,
  name: "John Doe",
  email: "john@email.com",
  printInfo: function () {
    console.log(`${this.id} - ${this.name} - ${this.email}`);
  }
};

student.printInfo();
// 123 - John Doe - john@email.com
```

Here's a Replit of the code above:

<ReplitEmbed src="https://replit.com/@newlineauthors/what-is-this-example1"/>

The `printInfo` function in the code example above uses the `this` keyword, and looking at the code, we can tell that the `printInfo` function assumes that the value of `this` inside the `printInfo` function will be an object with three properties: `id`, `name`, and `email`. But as mentioned earlier, the value of `this` inside a function depends on _how_ the function is called.

In the code example above, the `printInfo` function is invoked using the `student` object, and when a function is invoked using an object, the `this` inside that function refers to the object using which the function was invoked. So in our code example, `this` inside the `printInfo` refers to the `student` object. As the `student` object has the three properties that are accessed using the `this` keyword inside the `printInfo` function, their values are logged to the console.

What will `this` refer to if the function is not invoked as a "method"? Consider the following code example:

```js
function orderFood() {
  console.log("Order confirmed against the name: " + this.fullName);
}

orderFood();
// Order confirmed against the name: undefined
```

Here's a Replit of the code above:

<ReplitEmbed src="https://replit.com/@newlineauthors/what-is-this-example2"/>

What does `this` refer to inside the `orderFood` function?

The answer to the question above depends on whether our code is executed in [strict mode](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode). If non-strict mode, `this` inside a function, when not invoked as a method, refers to the global object, which in the case of browsers is the `window` object. However, the value of `this` inside a function is `undefined` in strict mode when not invoked as a method.

Can you guess in which mode the code was executed from the output of the code above? As `this.fullName` evaluated to `undefined`, the code was executed in non-strict mode.

If we execute the code above in strict mode:

```js
"use strict";

function orderFood() {
  console.log("Order confirmed against the name: " + this.fullName);
}