Mastering JavaScript Functional Programming

457 Pages·2020·6.616 MB·English

by  Federico Kereki

Mastering JavaScript
Functional Programming
Second Edition
Write clean, robust, and maintainable web and server code
using functional JavaScript
Federico Kereki
BIRMINGHAM - MUMBAI
Mastering JavaScript Functional
Programming
Second Edition
Copyright © 2020 Packt Publishing
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Commissioning Editor: Wilson D'souza
Acquisition Editor: Shweta Bairoliya
Content Development Editor: Aamir Ahmed
Senior Editor: Hayden Edwards
Technical Editor: Jane Dsouza
Copy Editor: Safis Editing
Project Coordinator: Manthan Patel
Proofreader: Safis Editing
Indexer: Manju Arasan
Production Designer: Joshua Misquitta
First published: November 2017
Second edition: January 2020
Production reference: 1240120
Published by Packt Publishing Ltd.
Livery Place
35 Livery Street
Birmingham
B3 2PB, UK.
ISBN 978-1-83921-306-9
www.packt.com
Writing a book involves many people, and even if I cannot mention and name all of them, there
are some who really deserve to be highlighted.
At Packt Publishing, I want to thank Larissa Pinto, Senior Acquisition Editor, for proposing
the theme for this book and helping me get started with it. Thanks must also go to Mohammed
Yusuf Imaratwale, Content Development Editor, and Ralph Rosario, Technical Editor, for their
help in giving shape to the book and making it clearer and better structured. I also want to send
my appreciation to the reviewers, Gerónimo García Sgritta and Steve Perkins, who went
through the initial draft, enhancing it with their comments.
There are some other people who deserve extra consideration. This book was written under
unusual circumstances, around 10,000 miles away from home! I had gone from Uruguay,
where I live, to work on a project in India, and that's where I wrote every single page of the text.
This would not have been possible if I hadn't had complete support from my family, who stayed
in Montevideo, but who were constantly nearby, thanks to the internet and modern
communications. In particular, I must single out my wife, Sylvia Tosar, not only for
supporting and aiding me both with the project and the book, but also for dealing with
everything, and the rest of the family on her own in Uruguay—this book wouldn't have been
possible otherwise, and she is the greatest reason the book could be written!
For the second edition
Revisiting and expanding a book for a second edition is a challenging, interesting task. I had
great support from Packt, and I must now thank Aamir Ahmed, Content Development Editor;
Jane D'souza, Technical Editor; and Crystian Bietti and (again, for double merit!) Steve
Perkins, reviewers—both of whom helped produce a much better text.
– Federico Kereki
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Contributors
About the author
Federico Kereki is an Uruguayan systems engineer, with a master's degree in education,
and more than 30 years of experience as a consultant, system developer, university
professor, and writer.
He is currently a subject matter expert at Globant, where he gets to use a good mixture of
development frameworks, programming tools, and operating systems, such as JavaScript,
Node.js, React and Redux, SOA, Containers, and PHP, with both Windows and Linux.
He has taught several computer science courses at Universidad de la República,
Universidad ORT Uruguay, and Universidad de la Empresa. He has also written texts for
these courses.
He has written several articles—on JavaScript, web development, and open source
topics—for magazines such as Linux Journal and LinuxPro Magazine in the United States,
Linux+ and Mundo Linux in Europe, and for websites such as Linux.com and IBM Developer
Works. He has also written booklets on computer security (Linux in the Time of
Malware and SSH: A Modern Lock for Your Server), a book on GWT programming (Essential
GWT: Building for the Web with Google Web Toolkit), and another one on JavaScript
development (Modern JavaScript Web Development Cookbook).
Federico has given talks on functional programming with JavaScript at public conferences
(such as JSCONF 2016 and Development Week Santiago 2019) and has used these
techniques to develop internet systems for businesses in Uruguay and abroad.
His current interests tend toward software quality and software engineering—with agile
methodologies topmost—while on the practical side, he works with diverse languages,
tools, and frameworks, and open source software (FLOSS) wherever possible!
He usually resides, works, and teaches in Uruguay, but this book was fully written while
on a project in India, and the revisions for the second edition were finished during a
sojourn in Mexico.
About the reviewers
Steve Perkins is the author of Hibernate Search by Example. He has been working with Java
and JavaScript since the late-1990's, with forays into Scala, Groovy, and Go. Steve lives in
Atlanta, GA, with his wife and two children, and is currently a software architect at Banyan
Hills Technologies, where he works on a platform for IoT device management and
analytics.
When he is not writing code or spending time with family, Steve plays guitar and loses
games at bridge and backgammon. You can visit his technical blog at steveperkins.com,
and follow him on Twitter at @stevedperkins.
Cristian "Pusher" Bietti is an entrepreneur who is proactive and has a creative attitude to
facing challenges in new technologies, with a great hunger to learn more! A senior
developer with more than 18 years of experience in software development and software
design and trained in a wide variety of technologies, he has participated in big banking
projects and small applications for mobile and social networks, including video games.
He has focused on frontend and user experience (UI/UX). He is a subject matter expert at
Globant, and he works in the Fintech industry as a technical leader and developer because
he loves coding.
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Table of Contents
Preface 1
Technical Requirements 7
Chapter 1: Becoming Functional - Several Questions 8
What is functional programming? 8
Theory versus practice 9
A different way of thinking 10
What FP is not 10
Why use FP? 11
What we need 11
What we get 12
Not all is gold 13
Is JavaScript functional? 13
JavaScript as a tool 14
Going functional with JavaScript 15
Key features of JavaScript 16
Functions as first-class objects 16
Recursion 17
Closures 18
Arrow functions 19
Spread 20
How do we work with JavaScript? 22
Using transpilers 23
Working online 25
Testing 26
Summary 27
Questions 27
Chapter 2: Thinking Functionally - A First Example 29
Our problem – doing something only once 29
Solution 1 – hoping for the best! 30
Solution 2 – using a global flag 31
Solution 3 – removing the handler 32
Solution 4 – changing the handler 33
Solution 5 – disabling the button 33
Solution 6 – redefining the handler 34
Solution 7 – using a local flag 34
A functional solution to our problem 35
A higher-order solution 36
Testing the solution manually 37
Table of Contents
Testing the solution automatically 39
Producing an even better solution 41
Summary 42
Questions 43
Chapter 3: Starting Out with Functions - A Core Concept 44
All about functions 44
Of lambdas and functions 45
Arrow functions – the modern way 48
Returning values 49
Handling the this value 49
Working with arguments 51
One argument or many? 53
Functions as objects 54
A React-Redux reducer 55
An unnecessary mistake 57
Working with methods 58
Using functions in FP ways 59
Injection – sorting it out 59
Callbacks, promises, and continuations 62
Continuation passing style 62
Polyfills 64
Detecting Ajax 64
Adding missing functions 66
Stubbing 67
Immediate invocation 68
Summary 71
Questions 71
Chapter 4: Behaving Properly - Pure Functions 73
Pure functions 73
Referential transparency 74
Side effects 76
Usual side effects 76
Global state 77
Inner state 78
Argument mutation 80
Troublesome functions 81
Advantages of pure functions 83
Order of execution 83
Memoization 84
Self-documentation 88
Testing 88
Impure functions 89
Avoiding impure functions 89
Avoiding the usage of state 89
Injecting impure functions 91
Is your function pure? 93
[ ii ]
Table of Contents
Testing – pure versus impure 94
Testing pure functions 95
Testing purified functions 96
Testing impure functions 99
Summary 102
Questions 102
Chapter 5: Programming Declaratively - A Better Style 104
Transformations 105
Reducing an array to a value 105
Summing an array 107
Calculating an average 108
Calculating several values at once 110
Folding left and right 111
Applying an operation – map 113
Extracting data from objects 115
Parsing numbers tacitly 116
Working with ranges 117
Emulating map() with reduce() 119
Dealing with arrays of arrays 120
Flattening an array 120
Mapping and flattening – flatMap() 123
Emulating flat() and flatMap() 125
More general looping 127
Logical higher-order functions 129
Filtering an array 130
A reduce() example 131
Emulating filter() with reduce() 132
Searching an array 133
A special search case 134
Emulating find() and findIndex() with reduce() 134
Higher-level predicates – some, every 135
Checking negatives – none 136
Working with async functions 137
Some strange behaviors 138
Async-ready looping 140
Looping over async calls 140
Mapping async calls 141
Filtering with async calls 142
Reducing async calls 143
Summary 144
Questions 144
Chapter 6: Producing Functions - Higher-Order Functions 147
Wrapping functions – keeping behavior 148
Logging 149
Logging in a functional way 149
Taking exceptions into account 151
[ iii ]