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Functional Programming in JavaScript Functional Programming in JavaScript LUIS ATENCIO MANNING SHELTER ISLAND For online information and ordering of this and other Manning books, please visit www.manning.com. The publisher offers discounts on this book when ordered in quantity. For more information, please contact Special Sales Department Manning Publications Co. 20 Baldwin Road PO Box 761 Shelter Island, NY 11964 Email: [email protected] ©2016 by Manning Publications Co. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by means electronic, mechanical, photocopying, or otherwise, without prior written permission of the publisher. Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in the book, and Manning Publications was aware of a trademark claim, the designations have been printed in initial caps or all caps. Recognizing the importance of preserving what has been written, it is Manning’s policy to have the books we publish printed on acid-free paper, and we exert our best efforts to that end. Recognizing also our responsibility to conserve the resources of our planet, Manning books are printed on paper that is at least 15 percent recycled and processed without the use of elemental chlorine. Manning Publications Co. Development editor: Marina Michaels 20 Baldwin Road Technical development editor: Dean Iverson PO Box 761 Review editor: Aleksandar Dragosavljevic Shelter Island, NY 11964 Project editor: Tiffany Taylor Copy editor: Tiffany Taylor Proofreader: Katie Tennant Technical proofreader: Daniel Lamb Typesetter: Dennis Dalinnik Cover designer: Leslie Haimes ISBN: 9781617292828 Printed in the United States of America 1 2 3 4 5 6 7 8 9 10 – EBM – 21 20 19 18 17 16 To my wonderful wife, Ana. Thank you for your unconditional support and for being the source of passion and inspiration in my life. brief contents PART 1 THINK FUNCTIONALLY ..................................................1 1 ■ Becoming functional 3 2 ■ Higher-order JavaScript 23 PART 2 GET FUNCTIONAL........................................................55 3 ■ Few data structures, many operations 57 4 ■ Toward modular, reusable code 84 5 ■ Design patterns against complexity 117 PART 3 ENHANCING YOUR FUNCTIONAL SKILLS......................151 6 ■ Bulletproofing your code 153 7 ■ Functional optimizations 180 8 ■ Managing asynchronous events and data 205 vii contents preface xv acknowledgments xvii about this book xix P 1 T .......................................1 ART HINK FUNCTIONALLY 1 Becoming functional 3 1.1 Can functional programming help? 5 1.2 What is functional programming? 5 Functional programming is declarative 7 ■ Pure functions and the problem with side effects 9 ■ Referential transparency and substitutability 13 ■ Preserving immutable data 15 1.3 Benefits of functional programming 16 Encouraging the decomposition of complex tasks 16 Processing data using fluent chains 18 ■ Reacting to the complexity of asynchronous applications 19 1.4 Summary 22 ix