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Refactoring JavaScript turning bad code into good code [First edition]

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Evan Burchard

Refactoring JavaScript

Turning Bad Code into Good Code

Evan Burchard


Boston Farnham Sebastopol


Refactoring JavaScript by Evan Burchard Copyright © 2017 Evan Burchard. All rights reserved. Printed in the United States of America. Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472. O’Reilly books may be purchased for educational, business, or sales promotional use. Online editions are also available for most titles ( For more information, contact our corporate/insti‐ tutional sales department: 800-998-9938 or [email protected]

Editors: Nan Barber and Allyson MacDonald Production Editor: Kristen Brown Copyeditor: Rachel Monaghan Proofreader: Rachel Head

Indexer: Ellen Troutman-Zaig Interior Designer: David Futato Cover Designer: Karen Montgomery Illustrator: Rebecca Demarest

First Edition

March 2017:

Revision History for the First Edition 2017-03-10:

First Release

See for release details. The O’Reilly logo is a registered trademark of O’Reilly Media, Inc. Refactoring JavaScript, the cover image, and related trade dress are trademarks of O’Reilly Media, Inc. While the publisher and the author have used good faith efforts to ensure that the information and instructions contained in this work are accurate, the publisher and the author disclaim all responsibility for errors or omissions, including without limitation responsibility for damages resulting from the use of or reliance on this work. Use of the information and instructions contained in this work is at your own risk. If any code samples or other technology this work contains or describes is subject to open source licenses or the intellectual property rights of others, it is your responsibility to ensure that your use thereof complies with such licenses and/or rights.

978-1-491-96492-7 [LSI]

For Jade, again and always.

Table of Contents

Foreword. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii 1. What Is Refactoring?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 How Can You Guarantee Behavior Doesn’t Change? Why Don’t We Care About Details of Implementation? Why Don’t We Care About Unspecified and Untested Behavior? Why Don’t We Care About Performance? What Is the Point of Refactoring if Behavior Doesn’t Change? Balancing Quality and Getting Things Done What Is Quality and How Does It Relate to Refactoring? Refactoring as Exploration What Is and Isn’t Refactoring Wrapping Up

1 3 4 5 6 7 7 9 9 10

2. Which JavaScript Are You Using?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Versions and Specifications Platforms and Implementations Precompiled Languages Frameworks Libraries What JavaScript Do You Need? What JavaScript Are We Using? Wrapping Up

12 13 15 16 17 18 18 19

3. Testing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 The Many Whys of Testing

24 v

The Many Ways of Testing Manual Testing Documented Manual Testing Approval Tests End-to-End Tests Unit Tests Nonfunctional Testing Other Test Types of Interest Tools and Processes Processes for Quality Tools for Quality Wrapping Up

25 26 26 27 29 30 32 33 33 34 39 43

4. Testing in Action. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 New Code from Scratch New Code from Scratch with TDD Untested Code and Characterization Tests Debugging and Regression Tests Wrapping Up

46 54 72 77 86

5. Basic Refactoring Goals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Function Bulk Inputs Outputs Side Effects Context Part 1: The Implicit Input this in Strict Mode Context Part 2: Privacy Is There Privacy in JavaScript? Wrapping Up

90 93 99 102 103 104 109 121 122

6. Refactoring Simple Structures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 The Code Our Strategy for Confidence Renaming Things Useless Code Dead Code Speculative Code and Comments Whitespace Do-Nothing Code Debugging/Logging Statements Variables



Table of Contents

127 130 132 137 137 137 139 139 143 143

Magic Numbers Long Lines: Part 1 (Variables) Inlining Function Calls Introducing a Variable Variable Hoisting Strings Concatenating, Magic, and Template Strings Regex Basics for Handling Strings Long Lines: Part 2 (Strings) Working with Arrays: Loops, forEach, map Long Lines: Part 3 (Arrays) Which Loop to Choose? Better Than Loops Wrapping Up

143 145 146 147 149 151 152 153 154 156 157 158 160 161

7. Refactoring Functions and Objects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 The Code (Improved) Array and Object Alternatives Array Alternative: Sets Array Alternative: Objects Object Alternative: Maps Array Alternative: Bit Fields Testing What We Have Our Setup Test Characterization Tests for classify Testing the welcomeMessage Testing for labelProbabilities Extracting Functions Getting Away from Procedural Code Extracting and Naming Anonymous Functions Function Calls and Function Literals Streamlining the API with One Global Object Extracting the classifier Object Inlining the setup Function Extracting the songList Object Handling the Remaining Global Variables Making Data Independent from the Program Scoping Declarations: var, let, and const Bringing classify into the classifier Untangling Coupled Values Objects with Duplicate Information Bringing the Other Functions and Variables into classifier

163 166 166 167 169 173 174 176 177 178 179 180 180 185 186 187 191 192 192 193 195 195 196 207 212 214

Table of Contents



Shorthand Syntax: Arrow, Object Function, and Object Getting New Objects with Constructor Functions Constructor Functions Versus Factory Functions A class for Our Classifier Choosing Our API Time for a Little Privacy? Adapting the Classifier to a New Problem Domain Wrapping Up

220 226 229 233 235 237 239 242

8. Refactoring Within a Hierarchy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 About “CRUD Apps” and Frameworks Let’s Build a Hierarchy Let’s Wreck Our Hierarchy Constructor Functions Object Literals Factory Functions Evaluating Your Options for Hierarchies Inheritance and Architecture Why Do Some People Hate Classes? What About Multiple Inheritance? Which Interfaces Do You Want? Has-A Relationships Inheritance Antipatterns Hyperextension Goat and Cabbage Raised by a Wolf Wrapping Up

243 244 252 253 256 258 260 261 261 262 265 267 268 269 272 277

9. Refactoring to OOP Patterns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 Template Method A Functional Variant Strategy State null Object Wrapper (Decorator and Adapter) Facade Wrapping Up

280 283 284 287 293 301 309 311

10. Asynchronous Refactoring. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315 Why Async? Fixing the Pyramid of Doom Extracting Functions into a Containing Object Testing Our Asynchronous Program



Table of Contents

315 318 318 321

Additional Testing Considerations Callbacks and Testing Basic CPS and IoC Callback Style Testing Promises The Basic Promise Interface Creating and Using Promises Testing Promises Wrapping Up

323 326 327 328 331 331 332 335 336

11. Functional Refactoring. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 The Restrictions and Benefits of Functional Programming Restrictions Benefits The Future (Maybe) of Functional Programming The Basics Avoiding Destructive Actions, Mutation, and Reassignment Don’t return null Referential Transparency and Avoiding State Handling Randomness Keeping the Impure at Bay Advanced Basics Currying and Partial Application (with Ramda) Function Composition Types: The Bare Minimum Burritos Introducing Sanctuary The null Object Pattern, Revisited! Functional Refactoring with Maybe Functional Refactoring with Either Learning and Using Burritos Moving from OOP to FP Return of the Naive Bayes Classifier Rewrites Wrapping Up

340 340 342 345 346 346 354 355 358 358 361 361 364 367 371 373 375 380 383 385 387 387 391 392

12. Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395 A. Further Reading and Resources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397 Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 401

Table of Contents




I still remember reading Martin Fowler’s book Refactoring: Improving the Design of Existing Code when it came out in 1999. It was a revelation: I had never before seen code being considered to be something malleable. Programmers tend to have the urge to rewrite code bases from scratch, but this book argued that it is possible to evolve and clean up existing code via small, principled, and comparatively safe steps. While doing so, tests provide an additional safety net and enable you to move for‐ ward with confidence. One piece of advice from the book will always stick with me— whenever you code, always keep two kinds of activity completely separate: imple‐ menting new functionality and refactoring existing code. If you do that, you’ll avoid doing too many things at the same time and will produce less bugs. Refactoring JavaScript takes the ideas of refactoring and applies them to the world of JavaScript. JavaScript’s dynamic nature means that you need different techniques compared to more static languages such as Java. In Java, you have static typing. And inheritance and polymorphism are used quite often. For JavaScript, you often rely on static checking tools (such as ESLint and Flow) and can adapt objects flexibly accord‐ ing to your needs. Functional programming techniques are also more popular. Addi‐ tionally, tests play an even more important role, but they also tend to be more lightweight. With all of these issues and more (e.g., asynchronous code), this book has you covered! Happy reading! — Axel Rauschmayer November 2016



Welcome to Refactoring JavaScript. Throughout this book, we’ll be looking at ways to write better JavaScript, drawing inspiration from classical refactoring techniques while exploring various styles of coding.

Why This Book Exists Like it or not, JavaScript is not going away. No matter what framework or “compilesto-JS” language or library you use, bugs and performance concerns will always be an issue if the underlying quality of your JavaScript is poor. Rewrites, including porting to the framework of the month, are terribly expensive and unpredictable. The bugs won’t magically go away, and can happily reproduce themselves in a new context. To complicate things further, features will get dropped, at least temporarily. This book provides clear guidance on how best to avoid these pathological approaches to writing JavaScript. Bad code doesn’t have to stay that way. And making it better doesn’t have to be intimidating or unreasonably expensive.

Who This Book Is For This book is meant for programmers who have some experience writing bad code, and an interest in writing better code. It’s for those writing JavaScript on the frontend or the backend. It’s for those writing JavaScript by choice as well as those who are “stuck with it” due to JavaScript’s monopoly of the browser platform. If you’re an absolute beginner, you might want to write some bad code for a couple of months first. If you’re not interested in writing better code, you might not have the patience for this book. If neither of those situations describes you, we’re good to go. Interestingly enough, there are numerous efforts working to make JavaScript better, while at the same time others aim to make it obsolete. The number of ways to write good and bad JavaScript continues to expand. Frameworks can go a long way toward xiii

handling complexity, but programmers constrained by frameworks will be limited. If you find that you (or your codebase) are struggling to work outside of a framework (or at some of the more confusing edges of it), this book should give you new ideas for how to approach your work. If you have trouble testing, debugging, or having confidence in your codebase, this book should be helpful. Most of us don’t work on perfect codebases, especially in JavaScript, where engineers might primarily use Ruby, Python, Java, and so on. What this book does is help you identify what specific parts of a codebase are bad, while providing a multitude of options for improvement.

How To Use This Book Chapters 1–5 describe the interplay between JavaScript, refactoring, quality, confi‐ dence, and testing. In many books, it is common to tack on testing at the end. In this book, for the types of code we are exploring, this wouldn’t be appropriate. Testing is essential for confidence. Confidence is essential to refactoring. Refactoring is essential to quality, which is the goal: testing -> confidence -> refactoring -> quality

JavaScript (and its ecosystem) happens to provide the space in which that transfor‐ mation takes place, so these opening chapters necessarily include an exploration of the language itself. If you’re completely comfortable with the transformation just described, you might want to skim or skip these chapters. Although that is not rec‐ ommended, it is your book, so you can use it however you want. If you think it’s best used as a doorstop or to start a fire for warmth or a sacrifice of some sort, go for it. If you do find an unconventional use for the book, email me a picture or video. I’m at or @evanburchard on Twitter and GitHub.

Can I burn or doorstopify digital copies too? Unfortunately, no. However, since this book is under a Creative Commons license, you’re free to share links to the HTML version and any other files available at, for example.

After Chapter 5, things get harder, especially if you skipped 1–5. There’s more code to write and follow along with. In Chapters 6 and 7, we go through refactoring functions and objects, and we don’t shy away from some of the more complicated bits of Java‐ Script. Generally, the goal of these chapters is to provide options for improving code without radically changing the interface. Through applying techniques found in these two chapters, you’ll be able to turn a mess of a codebase into one that has a decent baseline of quality. xiv



Chapter 8 expands our view of architecture to those that include (or avoid) hierar‐ chies. Chapters 9, 10, and 11 are dedicated to specific topics (design patterns, asynchronous programming, and functional programming, respectively) that can take your code beyond that baseline, but necessarily involve more aggressive changes. With the design patterns in Chapter 9, we recognize ways to extend and draw from JavaScript’s object-oriented side, and cover the historical connection between refactoring and object-oriented programming (OOP). In Chapter 10, we deal with the reality that many JavaScript codebases have more than one thing to do at once. In Chapter 11, we take a tour of functional programming through a couple of libraries that provide use‐ ful interfaces that go beyond those that our standard Array functions (forEach, map, reduce, etc.) give us. In some sense, those last three chapters in particular break away from our initial goal of refactoring by changing the implementation details without changing the interface. On the other hand, these interfaces are both useful and sometimes unavoidable. We may easily find ourselves wanting to write code that is necessarily asynchronous for performance reasons. Or we could find ourselves “stuck” in a codebase that has much invested in good or bad attempts at OOP or functional programming (FP). So, either through choice or code we inherit, these are parts of JavaScript that we should pay attention to and be able to improve upon. If you adopt a completely different para‐ digm to a codebase, it is unlikely that you’ll be “refactoring” in the sense that we mean throughout most of the book. If we want to be rigid about it, these chapters still refactor within their paradigms (OOP to better OOP, async to better async, and FP to better FP), and if we wish to think in the broadest terms about the execution of our program (e.g., “running node myprogram.js” as input and “being satisfied with how it ran” as output), then we can be refactoring even while jumping from paradigm to paradigm. I encourage you to first work with smaller, incremental changes that are easy to test and be confident in. To quote William Opdyke’s original thesis on refactoring: This definition of semantic equivalence allows changes throughout the program, as long as this mapping of input to output values remains the same. Imagine that a circle is drawn around the parts of a program affected by a refactoring. The behavior as viewed from outside the circle does not change. For some refactorings, the circle sur‐ rounds most or all of the program. For example, if a variable is referenced throughout a program, the refactoring that changes its name will affect much of the program. For other refactorings, the circle covers a much smaller area;




for example, only part of one function body is effected when a particular function call contained in it is inline expanded. In both cases, the key idea is that the results (includ‐ ing side effects) of operations invoked and references made from outside the circle do not change, as viewed from outside the circle.1

Although we’re free to draw “the circle” as large as we’d like, it’s very common for the term refactoring to get thrown around as though it simply meant “changing code.” As we discuss in Chapter 1, it does not. That is easier to see on a small scale, like the ones that we spend the most pages on. Think of Chapters 8, 9, and 10 first presenting as architectural options, and second possibilities for creating better code (safely and incrementally) within those options. As an example, if someone says something about “refactoring to use asynchronous code” it is likely too broad of a problem to execute in a safe and incremental way. But if you want to think of Chapter 9 as giving you the power to do so, I can’t stop you. It’s your book now. You can draw the circle as big as you want. If you find any of the tools or concepts confusing, you will probably find the appen‐ dix helpful. If you are looking for code samples and other information, visit the book’s website. You can also find an HTML version of the book there if you prefer to read that way. So in summary, use this book to learn about: • • • • • •

Refactoring Testing JavaScript Refactoring and testing JavaScript A few JavaScript paradigms Refactoring and testing within those JavaScript paradigms

Alternatively (under adult supervision, of course), the paper version of the book can be set on fire and used for: • Warmth • Protest • Ritual tech book sacrifices The digital files from can get passed around in accordance with the Creative Commons license restrictions. If you have any problems, questions, complaints, or compliments, feel free to reach out to me through my website.

1 William Opdyke, “Refactoring Object-Oriented Frameworks” (PhD thesis, University of Illinois at Urbana-

Champaign, 1992), 40.




Some Words in This Book App, Application, Program Some words in this book are imprecise. App, application, program, and website are interchangeable much of the time. In case there is any confusion, this book describes general principles for improving the quality of JavaScript, so none of those terms should be taken too literally. Maybe your code is a library or framework? In any case, the techniques in this book should apply just fine.

Inclusivity Through Words and Diagrams Some words in this book may not feel inclusive to everyone. I tried to balance the use of he and she, which I realize isn’t everyone’s ideal. Although I’d prefer to use the sin‐ gular they, that’s not within publisher guidelines at the moment. Additionally, I am realizing (too late) that my reliance on diagrams, especially those in Chapter 5, may do a terrible disservice to readers with a visual impairment. If you feel like you’ve missed out on any content for this reason, please feel free to reach out to me with any questions.

Users There’s also one word in this book I really hate, and that’s users. It’s imprecise and also creates some distance between the creators (developers/designers) and consumers (users). More precise and charitable words are often specific to the problem domain, or else we’re stuck with terms like “people” or “people who use the program/website.” If there is no more specific term than person or user (even including customer), it might be a hint that the business model is based purely on selling people as data, but that’s another discussion. The point is that the term user is used in this book to convey a familiar concept: a person who uses the program/website. Also, there are not yet magnanimous and accurate terms to supplant the related terms of user experience (UX) or user interface (UI). Rather than explaining this in several places or using nonstandard or specific terms for frequently abstract concepts, I chose to save the effort and just talk about it here. In any case, I fully endorse the following quote (and its implications) by “the Leo‐ nardo da Vinci of Data,” Edward Tufte: There are only two industries that refer to their customers as users: illegal drugs, and software houses.

There is a movement called “ethical design” that hopefully will help the industry shed this term (and the inconsiderate practices that stem from it) at some point. Preface



Third-Party Libraries and Communities Although I tried very hard to present the best tools to demonstrate the fundamentals of refactoring and testing in JavaScript, there may be times where you find that a par‐ ticular tool isn’t working for you. The great news here is that JavaScript has a rich eco‐ system of options. I have a preference for tools that are simple, flexible, and atomic, but you may feel differently. Large frameworks in particular are not explored in this text, as they tend to come with their own ecosystems of other tools (often themselves quite active and varied). I would absolutely recommend a framework when you’re starting out, but they are most useful when combined with facility in the underlying language, which I believe this book will teach you very well. Additionally, every tool, framework, and library will come with some community and history. Just as I don’t believe in any one true way for tooling, I also don’t endorse the community behind any given third-party code or project. Many projects will come with a code of conduct that will let you know if participating in them will be an enjoyable use of your time.

API, Interface, Implementation, “Client Code” This gets a little murky, but one thing I wish I could highlight more is the hierarchy not in terms of objects, but in the interface of a well-designed codebase. When code is a simple script, we expect it to run top to bottom, as a procedure. As a codebase matures (through design, not butchery mixed with entropy), we expect it to develop in three main layers (although this is obviously extended in more complex codebases). The first layer—the code behind the scenes, deeper in the codebase—is referred to in this book as the implementation. For refactoring, the most important distinction is between the implementation and the next layer. This second layer can be called the interface or API and describes the “public” functions and objects that one should expect to interact with if a codebase is used as a module. The third layer of conse‐ quence is sometimes called the client code or calling code. It refers to the code that is written to interact with the interface layer. This is the code that people using a mod‐ ule would write, as well as the testing code that we will write to test the interface layer.

Basics of architecture Throughout this book, we’re creating programs that start out very unstructured, and our main thrust (regardless of a paradigm like OOP or FP) is to make divisions between these three layers. This is what allows code to be testable and portable. If you’re mostly reli‐ ant on frameworks that provide their own organization, this pro‐ cess might be unfamiliar.


| Preface

Inputs (Nonlocal and Free Variables) Throughout the book (especially in Chapter 5), we distinguish between three types of inputs: • Explicit inputs (the parameters passed into a function) • Implicit inputs (the this that refers to the containing context object, function, or class) • Nonlocal inputs (the values used within a function or object that are defined elsewhere) There are two things of note here. First, local variables (or constants) created within the scope of a function are not considered “inputs” for the sake of diagramming or otherwise. Second, although the term nonlocal input is used as a precise term in this text, free variable is a more common name. However, it is a bit imprecise given that nonlocal inputs may be constants rather than variables. Similarly, some use the term bound variables to refer to what we call explicit inputs and, to some degree, implicit inputs as well.

Conventions Used in This Book The following typographical conventions are used in this book: Italic Indicates new terms, URLs, email addresses, filenames, and file extensions. Also used on occasion for emphasis and contrast. Constant width

Used for program listings, as well as within paragraphs to refer to program ele‐ ments such as variable or function names, databases, data types, environment variables, statements, and keywords. Constant width bold

Shows commands or other text that should be typed literally by the user. Constant width italic

Shows text that should be replaced with user-supplied values or by values deter‐ mined by context. This element signifies a tip or suggestion.




This element signifies a general note.

This element indicates a warning or caution.

Using Code Examples Supplemental material (code examples, exercises, etc.) is available for download at This book is here to help you get your job done. In general, if example code is offered with this book, you may use it in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a signifi‐ cant amount of example code from this book into your product’s documentation does require permission. We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: “Refactoring JavaScript by Evan Burchard (O’Reilly). Copyright 2017 O’Reilly Media, 978-1-491-96492-7.” If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at [email protected] Tools used within the code of this book can be found in the appendix, along with resources for further information on topics covered. For reference, the tools used in this book along with their versions are: • • • • • • • •


node 6.7.0 npm 3.10.3 wish 0.1.2 mocha 3.2.0 deep-equal 1.0.1 testdouble 1.10.0 tape 4.6.3 lodash 4.17.2



• • • •

assert 1.4.1 underscore 1.8.3 ramda 0.22.1 sanctuary 0.11.1

Later versions are unlikely to cause problems, but earlier ones might. At lower ver‐ sions, node in particular is known to not fully support the code in this book.

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Acknowledgments Thanks to my family for their support in making this book happen: Mom, Dad, Amy, Scott, Gretchen, Max, and Jade. Special thanks to the people who helped kick everything off: Zeke Templin, Steve Souders, Mary Treseler, Simon St. Laurent, and Tyler Ortman. And to those who gave technical inspiration and feedback: Jacob Barss-Bailey, Matt Blake, Charles Baakel, Stefano De Vuono, and Ryan Duchin. And the rest of the O’Reilly staff that helped along the way: Annalis Clint, Nena Cavi‐ ness, Michelle Gilliland, Rita Scordamalgia, Josh Garstka, Kristen Brown, Rebecca Demarest, Rachel Monaghan, Shiny Kalapurakkel, and especially my editors, Nan Barber and Ally MacDonald. And to my technical reviewers: Steve Suering, Shelley Powers, Chris Deely, Darrell Heath, and Jade Applegate. And to those whose work I found useful and inspirational: William F. Opdyke, Mar‐ tin Fowler, Kent Beck, John Brant, Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides, Douglas Crockford, Tony Hoare, Alexis Deveria, Addy Osmani, Rob‐ ert Nystrom, Brian Lonsdorf, Reginald Braithwaite, Miran Lipovaca, Kyle Simpson, Tom Stuart, Michael Fogus, David Chambers, Michael Hurley, Scott Sauyet, Yehuda Katz, Jay Fields, Shane Harvie, Russ Olsen, Joshua Kerievsky, James Halliday, TJ Hol‐ owaychuk, Justin Searls, Eric Elliot, Jake Archibald, Arnau Sanchez, Alex Chaffee, Eric Hodel, Sean Hussey, Brian Cardarella, Foy Savas, and Katrina Owen, and Bryan Liles. A special thanks to Dr. Axel Rauschmayer for his amazing work interpreting specs for us mere mortals, as well as providing the foreword to this book.

Psst...Hey, reader! I know it looks like just a big list of names, but the people in this section are all really awesome. The resources in the appendix are less important than this list. A lot of these people made that stuff. And searching their names will let you know about their new stuff, which is probably better than their old stuff. Look these people up.

And thanks in general to all the people at TC39 and MDN. xxii

| Preface

And to my dog for taking me on walks, even when I was right in the middle of something. Also, to you. Thanks for supporting my work. Hit me up if you need anything.





What Is Refactoring?

Refactoring is not changing code. Okay, yes, it is, but there’s more to it. Refactoring is a type of changing code, but has one major constraint that makes “changing code” an imprecise way to describe it: you don’t change the behavior of the code. Two immediate questions should come to mind: • How do you guarantee that behavior does not change? • What is the point of changing the code if the behavior doesn’t change? In the rest of the chapter, we will pursue the answers to these questions. We’re not going to go through the full history of JavaScript, as that’s covered extensively on the web already.

How Can You Guarantee Behavior Doesn’t Change? Unqualified, the answer to that question is that it is incredibly hard. Fortunately, many types of behavior are not our primary concern when refactoring. We’ll cover these next: • Details of implementation • Unspecified and untested behavior • Performance The shorter answer, for us, is using tests and version control. Another approach, supported by William Opdyke, whose thesis is the foundational work on refactoring, stresses using automated tools that are responsible for changing the code as well as guaranteeing safety before doing so. Professional coders might find that removing the human element limits the types of changes that can be made, as the 1

number of changes that can be guaranteed as “safe” is confined to the functionality of the tools. Writing tools to encompass the whole refactoring catalog proposed by Martin Fowler in his seminal book, Refactoring: Improving the Design of Existing Code (AddisonWesley), would prove extremely difficult. And in JavaScript, a dynamic, multipara‐ digmatic language with an ecosystem teeming with variants (see Chapter 2), these tools are bound to lag behind a refactorer’s imagination even more so. Fowler’s approach pulls away from automation, while at the same time stressing the “mechanics” of the refactoring: steps of altering code that minimize unsafe states. If we relied on an “Opdykian,” automated approach for this book, the tooling would hold us back significantly. And we’re straying from Fowler’s emphasis on mechanics (step-by-step processes) as well. The reason is that, as we move toward confidence in our code through a given refactoring, if it is backed up by tests, verifying the success of our changes should be straightforward. And when we fail to execute a refactoring properly, version control (we’ll be using Git) should give us an easy way to simply “roll back” to the state of the code beforehand.

Warning! Use version control! Any form of “changing code” carries significant risk to your code‐ base if you cannot easily revert it to a previous, safe version. If you don’t have versioned backups of the codebase that you plan on refactoring, put this book down and don’t pick it up again until you have your code under version control. If you’re not using version control already, you probably want to use Git, and you probably want to back your work up on GitHub.

Admittedly, the approach of this book might seem reactive and cavalier in compari‐ son to the earlier paths of automation and mechanics. However, the process—the “red” (failure state of a test), “green” (passing state of a test), “refactor” cycle, with an eye on rolling back quickly if things go wrong—employed in this book is based upon how quality-focused teams operate with tools that are popular among them. Perhaps later, automated refactoring will catch up with Fowler’s extensive catalog of refactor‐ ings, as well as all that are presented in this book, but I wouldn’t count on it happen‐ ing soon. Our goal here is to get JavaScript developers out of the muck. Although it is tempting to try to automate and mechanize that process, the most valuable parts of the work of the giants (Opdyke, Fowler, Johnson, et al.) whose shoulders this book humbly stands on is that they gave us a new mindset around making code better and doing it safely.



Chapter 1: What Is Refactoring?

Why Don’t We Care About Details of Implementation? Let’s say we have a simple function that multiplies numbers by 2: function byTwo(number){ return number * 2; }

We could instead write a function that accomplishes the same goal in a slightly differ‐ ent way: function byTwo(number){ return number