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

Enter your name and email address below, then click the **"Send my free chapter"** button to get started

300+ page PDF

EPub

newline Access with Replits

All Code Samples

Discord Community Access

One-time price of $69

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

In this module, we will cover asynchronous programming in JavaScript. We will learn what asynchronous programming means and how it was traditionally done in JavaScript. We will also discuss the problems with the traditional way of handling asynchronous code in JavaScript and how promises, introduced in ES2015, changed the way we handle asynchronous code in JavaScript. We will discuss promises in detail and also learn about the 

In the context of programming, "asynchronous" means that the program starts a potentially long-running task and is free to do other tasks while the long-running task is executed in the background. The key point to understand here is that the program doesn't have to wait for the long-running task to be completed; it is free to do other tasks. Once the task is complete, the program is notified and presented with the result of the task.

Asynchronous programming is needed because of the problems with traditional synchronous programming. In synchronous programs, each instruction is executed one after the other in a sequential manner. Instructions are executed in the order they appear in the program. As a result, synchronous programs are easier to reason about, but they also present problems that are solved by asynchronous programming.

The problem with synchronous programs is that a potentially long-running task will block the program's execution until its completion. This presents problems like poor program performance, poor user experience, and inefficient resource utilization.

Although asynchronous programs solve the problems presented by synchronous programs, asynchronous programs come with their own set of challenges, like error handling, managing the shared state and resources, managing the coordination between different parts of the program, etc.

Before we dive into how asynchronous code can be written in JavaScript and how it is handled behind the scenes, let's take a step back and see the problem we face in JavaScript if we execute some long-running code, like a loop in the following example:

function block() {
  const start = new Date();

  while (new Date() - start < 3000) {
    // simulate long running operation
    // that takes approximately 3 seconds
  }
}

console.log("Before long running operation");
// gets logged immediately

block();

console.log("After long running operation");
// gets logged after approximately 3 seconds

---
title: Overview
description: Overview of asynchronous behavior of JavaScript
isPublicLesson: false
useMdx: true
code: https://gitlab.com/fullstackio/books/newline-course-apps/advanced-javascript-unleashed-app/-/tree/main/module_09/lesson_09.01
---

In this module, we will cover asynchronous programming in JavaScript. We will learn what asynchronous programming means and how it was traditionally done in JavaScript. We will also discuss the problems with the traditional way of handling asynchronous code in JavaScript and how promises, introduced in ES2015, changed the way we handle asynchronous code in JavaScript. We will discuss promises in detail and also learn about the `async-await` syntax that simplifies using promises.

## What does asynchronous mean?

In the context of programming, "asynchronous" means that the program starts a potentially long-running task and is free to do other tasks while the long-running task is executed in the background. The key point to understand here is that the program doesn't have to wait for the long-running task to be completed; it is free to do other tasks. Once the task is complete, the program is notified and presented with the result of the task.

Asynchronous programming is needed because of the problems with traditional synchronous programming. In synchronous programs, each instruction is executed one after the other in a sequential manner. Instructions are executed in the order they appear in the program. As a result, synchronous programs are easier to reason about, but they also present problems that are solved by asynchronous programming.

The problem with synchronous programs is that a potentially long-running task will block the program's execution until its completion. This presents problems like poor program performance, poor user experience, and inefficient resource utilization.

Although asynchronous programs solve the problems presented by synchronous programs, asynchronous programs come with their own set of challenges, like error handling, managing the shared state and resources, managing the coordination between different parts of the program, etc.