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

As we know already, the JavaScript language is single-threaded. Long-running code on the main thread can block the thread; in the case of browsers, blocking the main thread means that browsers cannot respond to user interactions and cannot render changes on the UI. This is why the screen freezes when some long-running code blocks the main thread. In the case of NodeJs, in the context of application servers, blocking the main thread means that the server cannot handle the incoming HTTP requests until the main thread is unblocked.

To get around the limitation of a single thread where JavaScript code executes, as discussed in the previous lesson, any asynchronous operation is handled in the background, and in the meantime, the main thread can do other things.

Asynchronous operations like HTTP requests are handled by the browser in the background, and when the HTTP request is completed, our JavaScript code is executed using the callback we provided at the time of starting the HTTP request. The same is true for other asynchronous operations, like file handling in NodeJS. Every asynchronous operation in NodeJs is either handled by the internal thread pool of NodeJS or the operating system itself.

If we consider the main thread as the "JavaScript world", then the asynchronous operations actually happen outside the JavaScript world. Once the operation is completed, to get back into the JavaScript world, callbacks are used, which are invoked to execute the JavaScript code in response to the successful completion or failure of the operation.

So, in short, the code we write that executes on the main thread only initiates the asynchronous operation. Instead of waiting for the operation to complete, the main thread is free to do other things. The asynchronous operation is handled in the background by the environment in which our JavaScript code is executed. It can be a browser or a runtime like NodeJS. But how does the execution get back from the background to the main thread where our code is executed? This is where the 

The event loop in JavaScript is one of those concepts that is explained abstractly to make it easier for others to understand. This lesson will aim to create a solid understanding of the event loop.

The event loop helps to execute asynchronous operations in a non-blocking manner. When an asynchronous operation is completed in the background to execute the JavaScript callback that we provided at the time of initiating the asynchronous operation, it needs to be pushed onto the call stack.

The call stack is a stack data structure used to keep track of the currently executing code. Every function call is added to the stack as a stack frame. The frame is popped off the stack when the function execution ends.

Let us understand the event loop using the following code example:

setTimeout(() => {
  console.log("hello world");
}, 2000);

console.log("after setTimeout");

// output:
// ------
// after setTimeout
// hello world

You can run the code above in the Replit below:

---
title: Event loop
description: Explanation of event loop in JavaScript
isPublicLesson: false
useMdx: true
code: https://gitlab.com/fullstackio/books/newline-course-apps/advanced-javascript-unleashed-app/-/tree/main/module_09/lesson_09.03/event-loop-example1
---

As we know already, the JavaScript language is single-threaded. Long-running code on the main thread can block the thread; in the case of browsers, blocking the main thread means that browsers cannot respond to user interactions and cannot render changes on the UI. This is why the screen freezes when some long-running code blocks the main thread. In the case of NodeJs, in the context of application servers, blocking the main thread means that the server cannot handle the incoming HTTP requests until the main thread is unblocked.

To get around the limitation of a single thread where JavaScript code executes, as discussed in the previous lesson, any asynchronous operation is handled in the background, and in the meantime, the main thread can do other things.

Asynchronous operations like HTTP requests are handled by the browser in the background, and when the HTTP request is completed, our JavaScript code is executed using the callback we provided at the time of starting the HTTP request. The same is true for other asynchronous operations, like file handling in NodeJS. Every asynchronous operation in NodeJs is either handled by the internal thread pool of NodeJS or the operating system itself.

If we consider the main thread as the "JavaScript world", then the asynchronous operations actually happen outside the JavaScript world. Once the operation is completed, to get back into the JavaScript world, callbacks are used, which are invoked to execute the JavaScript code in response to the successful completion or failure of the operation.

So, in short, the code we write that executes on the main thread only initiates the asynchronous operation. Instead of waiting for the operation to complete, the main thread is free to do other things. The asynchronous operation is handled in the background by the environment in which our JavaScript code is executed. It can be a browser or a runtime like NodeJS. But how does the execution get back from the background to the main thread where our code is executed? This is where the **event loop** comes into the picture.

## What is an event loop?

The event loop in JavaScript is one of those concepts that is explained abstractly to make it easier for others to understand. This lesson will aim to create a solid understanding of the event loop.

The event loop helps to execute asynchronous operations in a non-blocking manner. When an asynchronous operation is completed in the background to execute the JavaScript callback that we provided at the time of initiating the asynchronous operation, it needs to be pushed onto the call stack.

The call stack is a stack data structure used to keep track of the currently executing code. Every function call is added to the stack as a stack frame. The frame is popped off the stack when the function execution ends.

Let us understand the event loop using the following code example:

```js
setTimeout(() => {
  console.log("hello world");
}, 2000);

console.log("after setTimeout");

// output:
// ------
// after setTimeout
// hello world
```

You can run the code above in the Replit below:

<ReplitEmbed src="https://replit.com/@newlineauthors/event-loop-example1" />