Functional Programming in JavaScript Table of Contents Functional Programming in JavaScript Credits About the Author About the Reviewers www.PacktPub.com Support files, eBooks, discount offers and more Why Subscribe? Free Access for Packt account holders Preface What this book covers What you need for this book Who this book is for Conventions Reader feedback Customer support Downloading the example code Errata Piracy Questions 1. The Powers of JavaScript’s Functional Side – a Demonstration Introduction The demonstration The application – an e-commerce website Imperative methods Functional programming Summary 2. Fundamentals of Functional Programming Functional programming languages What makes a language functional? Advantages Cleaner code Modularity Reusability Reduced coupling Mathematically correct Functional programming in a nonfunctional world Is JavaScript a functional programming language? Working with functions Self-invoking functions and closures Higher-order functions Pure functions Anonymous functions Method chains Recursion Divide and conquer Lazy evaluation The functional programmer’s toolkit Callbacks Array.prototype.map() Array.prototype.filter() Array.prototype.reduce() Honorable mentions Array.prototype.forEach Array.prototype.concat Array.prototype.reverse Array.prototype.sort Array.prototype.every and Array.prototype.some Summary 3. Setting Up the Functional Programming Environment Introduction Functional libraries for JavaScript Underscore.js Fantasy Land Bilby.js Lazy.js Bacon.js Honorable mentions Development and production environments Browsers Server-side JavaScript A functional use case in the server-side environment CLI Using functional libraries with other JavaScript modules Functional languages that compile into JavaScript Summary 4. Implementing Functional Programming Techniques in JavaScript Partial function application and currying Function manipulation Apply, call, and the this keyword Binding arguments Function factories Partial application Partial application from the left Partial application from the right Currying Function composition Compose Sequence – compose in reverse Compositions versus chains Programming with compose Mostly functional programming Handling events Functional reactive programming Reactivity Putting it all together Summary 5. Category Theory Category theory Category theory in a nutshell Type safety Object identities Functors Creating functors Arrays and functors Function compositions, revisited Monads Maybes Promises Lenses jQuery is a monad Implementing categories Summary 6. Advanced Topics and Pitfalls in JavaScript Recursion Tail recursion The Tail-call elimination Trampolining The Y-combinator Memoization Variable scope Scope resolutions Global scope Local scope Object properties Closures Gotchas Function declarations versus function expressions versus the function constructor Function declarations Function expressions The function constructor Unpredictable behavior Summary 7. Functional and Object-oriented Programming in JavaScript JavaScript – the multi-paradigm language JavaScript’s object-oriented implementation – using prototypes Inheritance JavaScript’s prototype chain Inheritance in JavaScript and the Object.create() method Mixing functional and object-oriented programming in JavaScript Functional inheritance Strategy Pattern Mixins Classical mixins Functional mixins Summary A. Common Functions for Functional Programming in JavaScript B. Glossary of Terms Index Functional Programming in JavaScript Functional Programming in JavaScript Copyright © 2015 Packt Publishing All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, without the prior written permission of the publisher, except in the case of brief quotations embedded in critical articles or reviews. Every effort has been made in the preparation of this book to ensure the accuracy of the information presented. However, the information contained in this book is sold without warranty, either express or implied. Neither the author nor Packt Publishing, and its dealers and distributors will be held liable for any damages caused or alleged to be caused directly or indirectly by this book. Packt Publishing has endeavored to provide trademark information about all of the companies and products mentioned in this book by the appropriate use of capitals. However, Packt Publishing cannot guarantee the accuracy of this information. First published: March 2015 Production reference: 1230315 Published by Packt Publishing Ltd. Livery Place 35 Livery Street Birmingham B3 2PB, UK ISBN 978-1-78439-822-4 www.packtpub.com Cover Image by Dan Mantyla Credits Author Dan Mantyla Reviewers Dom Derrien Joe Dorocak Peter Ehrlich Edward E. Griebel Jr. Commissioning Editor Julian Ursell Acquisition Editor Owen Roberts Content Development Editor Kirti Patil Technical Editor Abhishek R. Kotian Copy Editors Aditya Nair Aarti Saldanha Vikrant Phadkey Project Coordinator Nidhi Joshi Proofreaders Stephen Copestake Maria Gould Paul Hindle Indexer Tejal Daruwale Soni Production Coordinator Aparna Bhagat Cover Work Aparna Bhagat About the Author Dan Mantyla works as a web application developer for the University of Kansas. He enjoys contributing to open source web frameworks and wrenching on motorcycles. Dan is currently living in Lawrence, Kansas, USA—the birthplace of Python Django and home to Linux News Media. Dan has also clicked the cover image, which was taken outside his home in Lawrence, Kansas, USA, where the sunflower fields are in bloom for only one short week in September. About the Reviewers Dom Derrien is a full stack web developer who has recently been defining application environments with a focus on high availability and scalability. He’s been in the development field for more than 15 years and has worked for big and small companies and as an entrepreneur. He’s currently working for the game company Ubisoft, where he defines the next generation services platform for its successful AAA games. To extend the gamer experience on to the Web and on mobiles, he provides technical means that are transparent, efficient, and highly flexible. Having developed smart clients before the introduction of XHR, using a frameset tag to keep the context and a hidden frame of size=0 to dynamically exchange data with servers, he had a great pleasure of reviewing this book, which pushes the language to its limits. He hopes that it will help developers improve their programming skills. I want to thank my wife, Sophie, and our sons, Erwan and Goulven, with whom I enjoy a peaceful life in Montréal, Québec, Canada. Joe Dorocak, whose Internet moniker is Joe Codeswell, is a very experienced programmer. He enjoys creating readable code that implements project requirements efficiently and in a manner that can be easily understood. He considers writing code akin to writing poetry. Joe prides himself on the ability to communicate clearly and professionally. He considers his code to be communication, not only with the machine platforms on which it runs, but also with human programmers who might read it in the future. Joe has worked as an employee as well as in a contractual role for major brands such as IBM, HP, GTE/Sprint, and other top-shelf companies. He is presently consulting on web, mobile, and desktop applications, which are coded primarily, but not exclusively, in Python and JavaScript. For more details about him, please visit https://www.linkedin.com/in/joedorocak. Peter Ehrlich taught himself web programming in 2007, and now works on performance JavaScript and WebGL at Leap Motion, Inc. In his spare time, he enjoys dancing, rock climbing, and taking naps. Edward E. Griebel Jr. has been developing enterprise software for over 20 years in C, C++, and Java. He has a bachelor of science degree in computer engineering. 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Here, you can access, read and search across Packt’s entire library of books. Why Subscribe? Fully searchable across every book published by Packt Copy and paste, print and bookmark content On demand and accessible via web browser Free Access for Packt account holders If you have an account with Packt at www.PacktPub.com, you can use this to access PacktLib today and view nine entirely free books. Simply use your login credentials for immediate access. Preface Functional programming is a style that emphasizes and enables the writing of smarter code, which minimizes complexity and increases modularity. It’s a way of writing cleaner code through clever ways of mutating, combining, and using functions. JavaScript