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Advanced JavaScript
Advanced JavaScript

Advanced JavaScript: Speed up web development with the powerful features and benefits of JavaScript

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Advanced JavaScript

Chapter 2. Asynchronous JavaScript

Learning Objectives

By the end of this chapter, you will be able to:

  • Define asynchronous programming

  • Characterize the JavaScript event loop

  • Utilize callbacks and promises to write asynchronous code

  • Simplify asynchronous code with async/await syntax

In this chapter, we shall learn Asynchronous JavaScript and its uses.

Introduction


In the previous chapter, we covered many of the new and powerful features released in ES6. We discussed the evolution of JavaScript and highlighted the key additions in ES6. We discussed scope rules, variable declaration, arrow functions, template literals, enhanced object properties, destructuring assignment, classes and modules, transpiling, and iterators and generators.

In this chapter, we will learn what an asynchronous programming language is and how to write and understand asynchronous code. In the first topic, we will define asynchronous programming and show how JavaScript is an asynchronous, event driven programming language. Then, we will outline callbacks and show how to use callbacks to write asynchronous JavaScript. We will then define promises and demonstrate how to use promises to write asynchronous JavaScript. In the final topic, we will present the async/await syntax and simplify our asynchronous code using promises and this syntax.

Asynchronous Programming


JavaScript is a single threaded, event driven, asynchronous programming language. What does this mean? This means that JavaScript runs on a single thread and delays/handles certain events or function calls through an event queue. We will break down the basics of how JavaScript does this through the following topic.

Sync Versus Async

What does it mean for code to be synchronous or asynchronous? These two buzzwords get thrown around a lot in JavaScript. Synchronous is derived from the Greek root syn, meaning "with", and chronos, which means "time". Synchronous literally means "with time", or rather, code that is coordinated with time. Lines of code are run one at a time and are not started until the previous line has been handled. Asynchronous, or async, is derived from the Greek root async, meaning "not with", and chronos, hence asynchronous literally means "not with time" or rather, code that is not coordinated with time. The order code that is run is not coordinated...

Callbacks


Callbacks are the most basic form of JavaScript asynchronous programming. In the simplest terms, a callback is a function that gets called after another function completes. Callbacks are used to handle the response of an asynchronous function call.

In JavaScript, functions are treated like objects. They can be passed around as arguments, returned by functions, and saved into variables. A callback is a function object that is passed as an argument into a higher order function. A higher order function is simply a mathematics and computer science term for a function that takes one or more functions as arguments (callbacks) or returns a function. In JavaScript, a higher order function will take a callback as a parameter. Once the higher order finishes doing some form of work, such as an HTTP request or database call, it calls the callback function with the error or return values.

As mentioned in the Event Loop section in Asynchronous Programming, JavaScript is an event driven language...

Promises


In JavaScript, a promise is an object that wraps an asynchronous operation and notifies the program when the asynchronous operation completes. The promise object represents the eventual completion or failure of the wrapped operation. A promise is a proxy for a value not necessarily known. Instead of providing the value immediately, like a synchronous program, it promises to provide a value at some point in the future. Promises allow you to associate success and error handlers with an asynchronous action. These handlers are called at the completion or failure of the wrapped asynchronous process.

Promises States

Every promise has a state. A promise can only succeed with a value or fail with an error once. The state of a promise defines where the promise is in its work towards the resolution of a value.

A promise comes in three states: pending, fulfilled, or rejected. A promise starts in the pending state. This means that the async operation being done inside the promise is not complete...

Async/Await


Async/await is a new syntax form added to simplify code that uses promises. Async/await introduces two new keywords: async and await. Async is added to function declarations and await is used inside an async function. It is surprisingly easy to understand and use. In its simplest form, async/await allows us to write promise-based asynchronous code that looks almost identical to the synchronous code that does the same task. We will use async/await to simplify code using promises and make it even easier to read and understand.

Async/Await Syntax

The async keyword is added to function declarations; it must precede the function keyword. An async function declaration defines an asynchronous function. An example declaration of an async function is shown in the following snippet:

async function asyncExample( /* arguments */  ){ /* do work */ }

Snippet 2.34: Implementing promises

An async function implicitly returns a promise, no matter what the return value is specified to be. If the return...

Summary


JavaScript is an asynchronous, event-driven, single-threaded language. Instead of hanging during a long-running operation to another resource, JavaScript does work on other operations if any work is pending. JavaScript accomplishes this with the event loop. The event loop is composed of the call stack, heap, event queue, and main event loop. These four components work together to schedule when JavaScript runs different parts of the code. To leverage JavaScript's asynchronous nature, we use callbacks or promises. Callbacks are simply functions passed as arguments into other functions. Promises are special classes with event handler functions. When an asynchronous operation finishes, the JavaScript engine runs the callback or calls the promise handler attached to that operation's complete event. This is asynchronous JavaScript in its simplest form.

In the next chapter, we will learn about the Document Object Model (DOM), the JavaScript event object, and the jQuery library.

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Key benefits

  • Explore the new ES6 syntax, the event loop, and asynchronous programming
  • Learn the test-driven development approach when building apps
  • Master advanced JavaScript concepts to enhance your web developments skill

Description

If you are looking for a programming language to develop flexible and efficient applications, JavaScript is an obvious choice. Advanced JavaScript is a hands-on guide that takes you through JavaScript and its many features, one step at a time. You'll begin by learning how to use the new JavaScript syntax in ES6, and then work through the many other features that modern JavaScript has to offer. As you progress through the chapters, you’ll use asynchronous programming with callbacks and promises, handle browser events, and perform Document Object Model (DOM) manipulation. You'll also explore various methods of testing JavaScript projects. In the concluding chapters, you'll discover functional programming and learn to use it to build your apps. With this book as your guide, you'll also be able to develop APIs using Node.js and Express, create front-ends using React/Redux, and build mobile apps using React/Expo. By the end of Advanced JavaScript, you will have explored the features and benefits of JavaScript to build small applications.

Who is this book for?

This book is designed to target anyone who wants to write JavaScript in a professional environment. We expect the audience to have used JavaScript in some capacity and be familiar with the basic syntax. This book would be good for a tech enthusiast wondering when to use generators or how to use Promises and Callbacks effectively, or a novice developer who wants to deepen their knowledge on JavaScript and understand TDD.

What you will learn

  • Examine major features in ES6 and implement those features to build applications
  • Create promise and callback handlers to work with asynchronous processes
  • Develop asynchronous flows using Promise chaining and async/await syntax
  • Manipulate the DOM with JavaScript
  • Handle JavaScript browser events
  • Explore Test Driven Development and build code tests with JavaScript code testing frameworks.
  • List the benefits and drawbacks of functional programming compared to other styles
  • Construct applications with the Node.js backend framework and the React frontend framework
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Table of Contents

6 Chapters
Introducing ECMAScript 6 Chevron down icon Chevron up icon
Asynchronous JavaScript Chevron down icon Chevron up icon
DOM Manipulation and Event Handling Chevron down icon Chevron up icon
Testing JavaScript Chevron down icon Chevron up icon
Functional Programming Chevron down icon Chevron up icon
The JavaScript Ecosystem Chevron down icon Chevron up icon

Customer reviews

Rating distribution
Full star icon Full star icon Full star icon Half star icon Empty star icon 3.5
(4 Ratings)
5 star 25%
4 star 0%
3 star 75%
2 star 0%
1 star 0%
FM Jul 02, 2022
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Forget about what people are saying below about the book and the dumb reviews. This book is by far the best JS book I've ordered so far! It's modern JS, covers all the points you need and more. If you want a book about design patterns, look elsewhere. But design paterns are general and applicable to (almost) every programming language out there. The book does what says on the cover, shows you concepts both basic and advanced! 5 stars from me!
Amazon Verified review Amazon
Lee Apr 27, 2024
Full star icon Full star icon Full star icon Empty star icon Empty star icon 3
I've only almost completed the first chapter, the concepts introduced herein are clear and precise, but the amount of errata is very distracting. Most errata encountered thus far were likely caused by copying and pasting codes that are repeated throughout the same section of narrative. I wish all Packt represented books could receive stricter typo and errata screening, because this class of flaws has been around for way too long. Both authors and publishers have their responsibilities to make their books relatively joyful to read. I also think that customers should be provided with reading tools that allow the highlighting of typo and errata and the input of their reasons and correction suggestions. I requested these in multiple conversations with their reps already.
Subscriber review Packt
Mark cooper Feb 24, 2021
Full star icon Full star icon Full star icon Empty star icon Empty star icon 3
Decent overview of several core Javascript areas. A little disjointed (went from a chapter on asynchronous programming to one on the DOM and jQuery) but overall worth the money.
Amazon Verified review Amazon
Scott J Pearson Aug 15, 2021
Full star icon Full star icon Full star icon Empty star icon Empty star icon 3
In the past decade, the JavaScript language has witnessed a renaissance of new activity. Once pigeon-holed as a language for web browser eye-candy, JavaScript can now be used for the entire stack (server-side, client-side, testing, and even command-line). In this book, Shute attempts to keep developers up to date with a high-level review of the advancements. While this book is not a classic (and what modern computer-language-specific book is a classic?), it serves the needs of the IT community quite well.Shute starts by informing readers about the modernization of JavaScript in a form cryptically called ES6. This language allows for more compact and more powerful expression, including new features dealing with asynchronous functions. Most programmers who have done a deep dive into some form of JavaScript are familiar with the challenges of its asynchronous nature.In case anyone is still not familiar with the Document Object Model (DOM) and jQuery, a tutorial is provided. (Most programmers will likely not gain much other than review from this section.) A section on automated testing is provided along with a conceptual introduction to functional programming. Finally, the “ecosystem” of Node.js, Express, and React are described in broader terms.As you can see, Shute does not miss much of the recent developments. However, he stays pretty high in his overview and provides few deep dives to entertain readers as well. This is the main limitation of the book. For instance, I bought another book on Node.js to supplement while reading this one. That said, this book did inspire me to buy a book instead of merely being bored in the first place – a definite accomplishment.This book’s intended audience really consists of only the software development community – hence its higher price-tag. The materials were put together well with a sturdy cover and clear font. Due to the technical content, few pictures accompany the text. Overall, this book updated me on a high level but left me wanting to take a deep dive or two into more difficult (and interesting) topics.
Amazon Verified review Amazon
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