Monday, January 25, 2016

Book Review: JavaScript Concurrency

Adam Boduch's book JavaScript Concurrency has been published by Packt Publishing (December 2015). The subtitle of the book is, "Build better software with concurrent JavaScript programming, and unlock a more efficient and forward thinking approach to web development." JavaScript Concurrency consists of ten chapters spanning over 260 substantive pages. The author has described this book in his blog post of the same title.


The Preface of JavaScript Concurrency provides a single sentence summary of each of the book's ten chapters (find longer chapter descriptions in the author's blog post). The Preface states that the "requirements for this book" are a modern web browser, Node.js, and a code editor.

JavaScript Concurrency's Preface states that "all aspects of concurrent, asynchronous, and parallel programming are covered from first principles" and advertises that the book is "written for any JavaScript developer who wants to learn how to write more efficient, powerful, and maintainable applications that utilize the latest developments in the JavaScript language."

Chapter 1: Why JavaScript Concurrency?

The first sentence of the first chapter of JavaScript Concurrency opens with, "JavaScript is not a language associated with concurrency," but then points out that "this has changed a lot over the past few years, especially with new language features in ES 2015. The chapter introduces concepts of synchronous and asynchronous JavaScript. There is coverage of asynchronous versus parallel that uses simple juggler examples to illustrate the differences.

JavaScript Concurrency's initial chapter introduces the "JavaScript concurrency principles" of "parallelize," "synchronize," and "conserve." The chapter describes the "parallelize principle" as "taking advantage of modern CPU capabilities to compute results in less time" and explains that JavaScript parallelism is achieved in web browsers with Web Workers and in Node.js by spawning processes.

Chapter 1 describes the "synchronize principle" as "about the mechanisms used to coordinate concurrent actions and the abstractions of those mechanisms" and introduces callback functions and Promises. The chapter describes the "conserve principle" as "about saving on compute and memory resources" and introduces "lazy evaluation."

Chapter 2: The JavaScript Execution Model

JavaScript Concurrency's second chapter introduces the JavaScript execution model and related terminology: tasks, execution environment, interpreter, task queue, and event loop. The section "Creating tasks using timers" introduces and demonstrates use of setTimeout() and setInterval() "to schedule JavaScript code to run at a later time."

The second chapter of JavaScript Concurrency looks at basic ways of "responding to DOM events" and "responding to network events" using JavaScript callbacks. As part of this discussion, discussion and demonstrative code listings are provided related to use of debounce and for coordinating multiple requests. The chapter concludes with discussion of some of the challenges of using JavaScript callbacks as an introduction to the motivation for promises.

Chapter 3: Synchronizing with Promises

The third chapter of JavaScript Concurrency opens with a brief history of promises in JavaScript and defines key terms associated with promises: Promise, State, Executor, Resolver, Rejector, and Thenable. The objective of this chapter is to cover how the ES6 implementation of promises in JavaScript helps address JavaScript's so-called "callback hell."

Chapter 3 provides explanations of each of the terms related to JavaScript promises that were briefly introduced at the beginning of the chapter. The chapter uses multiple code listings and descriptive text to explain different approaches to using promises and I liked the id it presents of extending the promise prototype with an always() function that should be run for a given promise whether it resolves or rejects.

JavaScript Concurrency's third chapter reinforces that promises "are born into a pending state, and they die in either a resolved or rejected state" and then goes onto describe consequence of this. There is more in-depth coverage of use of JavaScript promises in different contexts that includes discussion and example code illustrating Promise.all() for handling multiple promises, Promise.race() for specifying that first resolved promise wins, and Promise.resolve(), and Promise.reject() for dealing with a JavaScript function that has "the potential to be both synchronous and asynchronous."

Chapter 4: Lazy Evaluation with Generators

Chapter 4 of JavaScript Concurrency opens with the sentence, "Lazy evaluation is a programming technique, which is used when we don't want to compute values until the very last second." The early portion of this chapter also explains, "The Generator is a new primitive type introduced to JavaScript as a part of the ES6 specification of the language" that helps "implement lazy evaluation techniques in [JavaScript] code." The chapter looks at the JavaScript Generator in more detail, including coverage of generator function syntax (function*), use of yield and next, and use of for .. of for iterating over a sequence of generators..

JavaScript Concurrency's fourth chapter describes with text and illustrates with code how to use generators to work with infinite and alternating/circular sequences. The chapter similarly describes and illustrates deferring between generators and interweaving generators. There are also sections in this chapter regarding passing data to generators, reusing general generators, and implementing "lightweight map/reduce." The "Coroutines" section of Chapter 4 "looks at some techniques for implementing coroutines in JavaScript using generators" and provides examples of implementing and using JavaScript coroutines based on generators.

Chapter 5: Working with Workers

The fifth chapter of JavaScript Concurrency begins with the sentence, "Web workers enable true parallelism within a web browser," and states, "In this chapter, we'll walk through the conceptual ideas of web workers, and how they relate to the concurrency principles that we're trying to achieve in our applications." The chapter introduces web workers as "at their core, ... nothing more than operating system-level threads."

Chapter 5 covers three types of web workers: dedicated (default), sub-workers, and shared workers. There is also coverage of worker environments and how these differ from the main thread's environment.

JavaScript Concurrency's fifth chapter explains and demonstrates the worker importScripts() function to import the lodash library. The chapter also covers communication between workers and related message serialization. I like that the chapter includes discussion of trade-offs and disadvantages associated with use of shared workers and sub-workers. The chapter concludes with coverage of error and exception handling in web workers.

Chapter 6: Practical Parallelism

Chapter 6 of JavaScript Concurrency introduces functional programming and related concepts such as immutability and referential transparency. A section in this chapter asks, "Do we need to go parallel?" That section briefly discusses how to decide whether to apply parallelism or not. This is complemented by a section that briefly discusses how to determine how many cores are available and use that information to decide how much parallelism is appropriate. The chapter also introduces "candidate problems" that can be addressed with parallelism such as mapping and reducing, searching collections, and "keeping the DOM responsive."

Chapter 7: Abstracting Concurrency

The seventh chapter begins by looking "at the approaches that we might use to insulate the rest of our application from tricky concurrency bits." The chapter explains how promises can be used to encapsulate the complexities of concurrent code. The explanations and code examples demonstrate "creating helper functions that wrap the worker communications" to return promises and "extending the web worker interface" postMessage() method. The chapter's section on "lazy workers" looks at "using generators in web workers."

Several of the code listings in Chapter 7 are at least partially taken from (and this is designated as code comments in those listings). After referencing these multiple code listings based on that library, the author formally introduces Parallel.js and explains that this library's purpose is "to make interacting with web workers as seamless as possible." The chapter discusses how to use Parallel.js to provide one's own functions to workers provided by Parallel.js rather than implementing one's own workers. The chapter demonstrates using Parallel.js to spawn workers and for mapping and reducing. I like that this section includes discussion of parallel slowdown and how to address that.

Chapter 7 concludes with coverage of worker pools. There are several pages devoted to explanation and demonstrative code listings related to implementing a worker "pool abstraction [that] look[s] and behave[s] like a plain dedicated worker."

Chapter 8: Evented IO with NodeJS

The purpose of Chapter 8 is to "explain how Node handles concurrency and how we need to program our NodeJS code to take full advantage of this environment." The chapter introduces evented IO, the Node.js Event Loop, and why these work well in web environments. The author introduces Dan Kegel's concept of the C10K problem and discusses how Node helps address this. The areas of specific focus are network IO and file system IO with descriptive text explanations and associated code samples.

Chapter 9: Advanced NodeJS Concurrency

Chapter 9 begins by introducing the co library as a library relying on generators and promises "to implement coroutines." The introduction to co introduces the "co() function to create a coroutine," demonstrates how co's awaiting values support is similar to proposed ES7 syntax, demonstrates resolving promises with co, and explains using "the wrap() utility to make a plain coroutine function into a reusable function."

JavaScript Concurrency's ninth chapter includes a section that explains how to fork new Node child processes to avoid issues with the blocked IO event loop. There is also a section on applying "a pool of general-purpose processes." The "Server Clusters" section discusses "scaling our Node application to several machines" and covers examples of this in the context of proxying, microservices, and load balancing.

Chapter 10: Building a Concurrent Application

The final chapter of JavaScript Concurrency uses a "simple chat application" to demonstrate concepts covered in the earlier chapters for building a concurrent application in JavaScript. The chapter begins by revisiting why it's easier to design applications to be concurrent from the beginning than to retrofit an application that was not designed to be concurrent.

The sample chat application built in Chapter 10 is based on Node.js and uses a few libraries such as commander and formidable. Although several simplifying assumptions are made to keep the example simpler, there is still a lot of code to list and describe. The chapter concludes with a section on other features that could be added to the sample chat application and with a paragraph that closes out the entire book.

General Observations

  • JavaScript Concurrency covers several modern (ES6/ES7) topics. This is very useful as many JavaScript books don't yet cover these modern abilities. However, the one drawback is that some of the covered material is not finalized and is subject to change. This is a trade-off that every book written about "cutting edge" topics faces.
  • Several JavaScript libraries with concurrent-orientation are introduced and demonstrated in JavaScript Concurrency. The most attention is paid to Parallel.js, co, formidable, and commander, probably in that order.
  • JavaScript Concurrency not only introduces syntax of new JavaScript concurrency features, but it also includes discussion on motivations for using the different constructs and trade-offs and design decisions that should be considered when selecting a JavaScript concurrent construct.
  • JavaScript Concurrency is best suited for an intermediate or advanced JavaScript developer who has little or no experience with promises, generators, workers, coroutines, and other ES6/ES7 concurrency concepts. A potential reader of this book should probably understand the material covered in Concurrency Model and Event Loop before reading this book. I wouldn't recommend this book to someone trying to learn JavaScript with no previous experience with the language, but would recommend it to someone with basic familiarity and some experience with JavaScript.
  • JavaScript Concurrency covers JavaScript concurrency both in web browser environments and on the server with Node.js.
  • There are numerous code listings in JavaScript Concurrency. These code listings are black text on white background with no color syntax and no line numbers. The code is available for download, which is probably preferable so that the code can be viewed in a favorite editor or IDE with color syntax and line numbers. The code listings are heavily commented (more so and at a lower level than I'd want in production code) and this is desirable in a book that is introducing and explaining concepts. It's nice to be able to read the code and associated comments to gain understanding and switch less between the code and associated text discussion for that understanding.
  • JavaScript Concurrency contains numerous simple diagrams to illustrate concepts. These diagrams consist of simple shapes such as boxes and arrows. In some cases, these simple diagrams make a concept easier to understand and in some cases the diagrams add very little value perhaps because they are overly simplistic.
  • There are some typos in JavaScript Concurrency that generally did not preclude me from understanding the points being made. I list a sample of them here so that readers of this post can decide if these are significant to their understanding, mildly distracting, or not an issue at all.
    • Page 100 has text introducing a code snippet that states, "Next, we'll use the loadScripts() function to bring the lodash library into our library...", but fortunately the actual code listing correctly demonstrates importScripts() rather than loadScripts().
    • Page 149, "Our application is likely filled with operations these, where concurrency simply doesn't make sense."
    • Page 233, "it will be a very simply chat application"
  • Because preferences can be subjective and opinionated, I like to include references to other reviews of the same books in many of my book reviews. Other reviews of JavaScript Concurrency are available:


JavaScript Concurrency sticks to what its two-words title describes: JavaScript concurrency. It is in many ways a "modern JavaScript" book that added clarity to my understanding of what JavaScript now supports or soon will support in many different browsers and on the server with Node.js.

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