JavaScript for impatient programmers (ES2021 edition) (Dr. Axel Rauschmayer) (z-library.sk, 1lib.sk, z-lib.sk)

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JavaScript for impatient programmers (ES2021 edition) Dr. Axel Rauschmayer 2021 “An exhaustive resource, yet cuts out the fluff that clutters many programming books – with explanations that are understandable and to the point, as promised by the title! The quizzes and exercises are a very useful feature to check and lock in your knowledge. And you can definitely tear through the book fairly quickly, to get up and running in JavaScript.” — Pam Selle, thewebivore.com “The best introductory book for modern JavaScript.” — Tejinder Singh, Senior Software Engineer, IBM “This is JavaScript. No filler. No frameworks. No third-party libraries. If you want to learn JavaScript, you need this book.” — Shelley Powers, Software Engineer/Writer

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Copyright © 2021 by Dr. Axel Rauschmayer Cover by Fran Caye All rights reserved. This book or any portion thereof may not be reproduced or used in any manner whatsoever without the express written permission of the publisher except for the use of brief quotations in a book review or scholarly journal. ISBN 978-1-09-121009-7 exploringjs.com

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Contents I Background 13 1 Before you buy the book 15 1.1 About the content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.2 Previewing and buying this book . . . . . . . . . . . . . . . . . . . . . . 16 1.3 About the author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 1.4 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2 FAQ: book and supplementary material 19 2.1 How to read this book . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.2 I own a digital version . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.3 I own the print version . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.4 Notations and conventions . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3 Why JavaScript? (bonus) 23 3.1 The cons of JavaScript . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.2 The pros of JavaScript . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.3 Pro and con of JavaScript: innovation . . . . . . . . . . . . . . . . . . . . 25 4 The nature of JavaScript (bonus) 27 4.1 JavaScript’s influences . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 4.2 The nature of JavaScript . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 4.3 Tips for getting started with JavaScript . . . . . . . . . . . . . . . . . . . 28 5 History and evolution of JavaScript 31 5.1 How JavaScript was created . . . . . . . . . . . . . . . . . . . . . . . . . 31 5.2 Standardizing JavaScript . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 5.3 Timeline of ECMAScript versions . . . . . . . . . . . . . . . . . . . . . . 32 5.4 Ecma Technical Committee 39 (TC39) . . . . . . . . . . . . . . . . . . . . 33 5.5 The TC39 process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 5.6 FAQ: TC39 process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 5.7 Evolving JavaScript: Don’t break the web . . . . . . . . . . . . . . . . . . 35 6 New JavaScript features 37 6.1 New in ECMAScript 2021 . . . . . . . . . . . . . . . . . . . . . . . . . . 37 6.2 New in ECMAScript 2020 . . . . . . . . . . . . . . . . . . . . . . . . . . 37 6.3 New in ECMAScript 2019 . . . . . . . . . . . . . . . . . . . . . . . . . . 38 6.4 New in ECMAScript 2018 . . . . . . . . . . . . . . . . . . . . . . . . . . 38 3

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4 CONTENTS 6.5 New in ECMAScript 2017 . . . . . . . . . . . . . . . . . . . . . . . . . . 38 6.6 New in ECMAScript 2016 . . . . . . . . . . . . . . . . . . . . . . . . . . 39 6.7 Source of this chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 7 FAQ: JavaScript 41 7.1 What are good references for JavaScript? . . . . . . . . . . . . . . . . . . 41 7.2 How do I find out what JavaScript features are supported where? . . . . 41 7.3 Where can I look up what features are planned for JavaScript? . . . . . . 42 7.4 Why does JavaScript fail silently so often? . . . . . . . . . . . . . . . . . 42 7.5 Why can’t we clean up JavaScript, by removing quirks and outdated fea- tures? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 7.6 How can I quickly try out a piece of JavaScript code? . . . . . . . . . . . 42 II First steps 43 8 Using JavaScript: the big picture 45 8.1 What are you learning in this book? . . . . . . . . . . . . . . . . . . . . . 45 8.2 The structure of browsers and Node.js . . . . . . . . . . . . . . . . . . . 45 8.3 JavaScript references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 8.4 Further reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 9 Syntax 47 9.1 An overview of JavaScript’s syntax . . . . . . . . . . . . . . . . . . . . . 48 9.2 (Advanced) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 9.3 Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 9.4 Statement vs. expression . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 9.5 Ambiguous syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 9.6 Semicolons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 9.7 Automatic semicolon insertion (ASI) . . . . . . . . . . . . . . . . . . . . 60 9.8 Semicolons: best practices . . . . . . . . . . . . . . . . . . . . . . . . . . 61 9.9 Strict mode vs. sloppy mode . . . . . . . . . . . . . . . . . . . . . . . . . 62 10 Consoles: interactive JavaScript command lines 65 10.1 Trying out JavaScript code . . . . . . . . . . . . . . . . . . . . . . . . . . 65 10.2 The console.* API: printing data and more . . . . . . . . . . . . . . . . 67 11 Assertion API 71 11.1 Assertions in software development . . . . . . . . . . . . . . . . . . . . . 71 11.2 How assertions are used in this book . . . . . . . . . . . . . . . . . . . . 71 11.3 Normal comparison vs. deep comparison . . . . . . . . . . . . . . . . . . 72 11.4 Quick reference: module assert . . . . . . . . . . . . . . . . . . . . . . . 73 12 Getting started with quizzes and exercises 77 12.1 Quizzes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 12.2 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 12.3 Unit tests in JavaScript . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

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CONTENTS 5 III Variables and values 83 13 Variables and assignment 85 13.1 let . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 13.2 const . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 13.3 Deciding between const and let . . . . . . . . . . . . . . . . . . . . . . 87 13.4 The scope of a variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 13.5 (Advanced) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 13.6 Terminology: static vs. dynamic . . . . . . . . . . . . . . . . . . . . . . . 89 13.7 Global variables and the global object . . . . . . . . . . . . . . . . . . . . 90 13.8 Declarations: scope and activation . . . . . . . . . . . . . . . . . . . . . . 92 13.9 Closures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 14 Values 99 14.1 What’s a type? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 14.2 JavaScript’s type hierarchy . . . . . . . . . . . . . . . . . . . . . . . . . . 100 14.3 The types of the language specification . . . . . . . . . . . . . . . . . . . 100 14.4 Primitive values vs. objects . . . . . . . . . . . . . . . . . . . . . . . . . . 101 14.5 The operators typeof and instanceof: what’s the type of a value? . . . . 103 14.6 Classes and constructor functions . . . . . . . . . . . . . . . . . . . . . . 105 14.7 Converting between types . . . . . . . . . . . . . . . . . . . . . . . . . . 106 15 Operators 109 15.1 Making sense of operators . . . . . . . . . . . . . . . . . . . . . . . . . . 109 15.2 The plus operator (+) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 15.3 Assignment operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 15.4 Equality: == vs. === . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 15.5 Ordering operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 15.6 Various other operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 IV Primitive values 117 16 The non-values undefined and null 119 16.1 undefined vs. null . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 16.2 Occurrences of undefined and null . . . . . . . . . . . . . . . . . . . . . 120 16.3 Checking for undefined or null . . . . . . . . . . . . . . . . . . . . . . . 121 16.4 The nullish coalescing operator (??) for default values [ES2020] . . . . . . 121 16.5 undefined and null don’t have properties . . . . . . . . . . . . . . . . . 124 16.6 The history of undefined and null . . . . . . . . . . . . . . . . . . . . . 125 17 Booleans 127 17.1 Converting to boolean . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 17.2 Falsy and truthy values . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 17.3 Truthiness-based existence checks . . . . . . . . . . . . . . . . . . . . . . 129 17.4 Conditional operator (? :) . . . . . . . . . . . . . . . . . . . . . . . . . . 131 17.5 Binary logical operators: And (x && y), Or (x || y) . . . . . . . . . . . . 132 17.6 Logical Not (!) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

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6 CONTENTS 18 Numbers 135 18.1 Numbers are used for both floating point numbers and integers . . . . . 136 18.2 Number literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 18.3 Arithmetic operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 18.4 Converting to number . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 18.5 Error values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 18.6 The precision of numbers: careful with decimal fractions . . . . . . . . . 144 18.7 (Advanced) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 18.8 Background: floating point precision . . . . . . . . . . . . . . . . . . . . 144 18.9 Integer numbers in JavaScript . . . . . . . . . . . . . . . . . . . . . . . . 146 18.10Bitwise operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 18.11Quick reference: numbers . . . . . . . . . . . . . . . . . . . . . . . . . . 151 19 Math 157 19.1 Data properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 19.2 Exponents, roots, logarithms . . . . . . . . . . . . . . . . . . . . . . . . . 158 19.3 Rounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 19.4 Trigonometric Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 19.5 Various other functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 19.6 Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 20 Bigints – arbitrary-precision integers [ES2020] (advanced) 165 20.1 Why bigints? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 20.2 Bigints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 20.3 Bigint literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 20.4 Reusing number operators for bigints (overloading) . . . . . . . . . . . . 168 20.5 The wrapper constructor BigInt . . . . . . . . . . . . . . . . . . . . . . . 172 20.6 Coercing bigints to other primitive types . . . . . . . . . . . . . . . . . . 174 20.7 TypedArrays and DataView operations for 64-bit values . . . . . . . . . . 174 20.8 Bigints and JSON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 20.9 FAQ: Bigints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 21 Unicode – a brief introduction (advanced) 177 21.1 Code points vs. code units . . . . . . . . . . . . . . . . . . . . . . . . . . 177 21.2 Encodings used in web development: UTF-16 and UTF-8 . . . . . . . . . 180 21.3 Grapheme clusters – the real characters . . . . . . . . . . . . . . . . . . . 180 22 Strings 183 22.1 Plain string literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 22.2 Accessing characters and code points . . . . . . . . . . . . . . . . . . . . 184 22.3 String concatenation via + . . . . . . . . . . . . . . . . . . . . . . . . . . 185 22.4 Converting to string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 22.5 Comparing strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 22.6 Atoms of text: Unicode characters, JavaScript characters, grapheme clusters188 22.7 Quick reference: Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 23 Using template literals and tagged templates 199 23.1 Disambiguation: “template” . . . . . . . . . . . . . . . . . . . . . . . . . 199 23.2 Template literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

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CONTENTS 7 23.3 Tagged templates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 23.4 Examples of tagged templates (as provided via libraries) . . . . . . . . . 203 23.5 Raw string literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 23.6 (Advanced) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 23.7 Multiline template literals and indentation . . . . . . . . . . . . . . . . . 204 23.8 Simple templating via template literals . . . . . . . . . . . . . . . . . . . 206 24 Symbols 209 24.1 Symbols are primitives that are also like objects . . . . . . . . . . . . . . 209 24.2 The descriptions of symbols . . . . . . . . . . . . . . . . . . . . . . . . . 210 24.3 Use cases for symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210 24.4 Publicly known symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 24.5 Converting symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 V Control flow and data flow 217 25 Control flow statements 219 25.1 Controlling loops: break and continue . . . . . . . . . . . . . . . . . . . 220 25.2 Conditions of control flow statements . . . . . . . . . . . . . . . . . . . . 221 25.3 if statements [ES1] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 25.4 switch statements [ES3] . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 25.5 while loops [ES1] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 25.6 do-while loops [ES3] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 25.7 for loops [ES1] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 25.8 for-of loops [ES6] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 25.9 for-await-of loops [ES2018] . . . . . . . . . . . . . . . . . . . . . . . . 229 25.10for-in loops (avoid) [ES1] . . . . . . . . . . . . . . . . . . . . . . . . . . 229 25.11Recomendations for looping . . . . . . . . . . . . . . . . . . . . . . . . . 229 26 Exception handling 231 26.1 Motivation: throwing and catching exceptions . . . . . . . . . . . . . . . 231 26.2 throw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 26.3 The try statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 26.4 Error classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 27 Callable values 237 27.1 Kinds of functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238 27.2 Ordinary functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238 27.3 Specialized functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 27.4 Summary: kinds of callable values . . . . . . . . . . . . . . . . . . . . . . 246 27.5 Returning values from functions and methods . . . . . . . . . . . . . . . 247 27.6 Parameter handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248 27.7 Methods of functions: .call(), .apply(), .bind() . . . . . . . . . . . . . 252 28 Evaluating code dynamically: eval(), new Function() (advanced) 255 28.1 eval() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 28.2 new Function() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 28.3 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

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8 CONTENTS VI Modularity 259 29 Modules 261 29.1 Overview: syntax of ECMAScript modules . . . . . . . . . . . . . . . . . 262 29.2 JavaScript source code formats . . . . . . . . . . . . . . . . . . . . . . . . 263 29.3 Before we had modules, we had scripts . . . . . . . . . . . . . . . . . . . 263 29.4 Module systems created prior to ES6 . . . . . . . . . . . . . . . . . . . . 264 29.5 ECMAScript modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 29.6 Named exports and imports . . . . . . . . . . . . . . . . . . . . . . . . . 267 29.7 Default exports and imports . . . . . . . . . . . . . . . . . . . . . . . . . 269 29.8 More details on exporting and importing . . . . . . . . . . . . . . . . . . 272 29.9 npm packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 29.10Naming modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 29.11Module specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 29.12Loading modules dynamically via import() [ES2020] . . . . . . . . . . . 278 29.13import.meta – metadata for the current module [ES2020] . . . . . . . . . 280 29.14Polyfills: emulating native web platform features (advanced) . . . . . . . 282 30 Single objects 283 30.1 What is an object? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 30.2 Objects as records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 30.3 Spreading into object literals (...) [ES2018] . . . . . . . . . . . . . . . . . 288 30.4 Methods and the special variable this . . . . . . . . . . . . . . . . . . . 290 30.5 Optional chaining for property accesses and method calls [ES2020] (ad- vanced) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295 30.6 Objects as dictionaries (advanced) . . . . . . . . . . . . . . . . . . . . . . 298 30.7 Standard methods (advanced) . . . . . . . . . . . . . . . . . . . . . . . . 306 30.8 Advanced topics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 31 Prototype chains and classes 311 31.1 Prototype chains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 31.2 Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 31.3 Private data for classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321 31.4 Subclassing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323 31.5 FAQ: objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331 VII Collections 333 32 Synchronous iteration 335 32.1 What is synchronous iteration about? . . . . . . . . . . . . . . . . . . . . 335 32.2 Core iteration constructs: iterables and iterators . . . . . . . . . . . . . . 336 32.3 Iterating manually . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 32.4 Iteration in practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338 32.5 Quick reference: synchronous iteration . . . . . . . . . . . . . . . . . . . 339 33 Arrays (Array) 341 33.1 The two roles of Arrays in JavaScript . . . . . . . . . . . . . . . . . . . . 342 33.2 Basic Array operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342

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CONTENTS 9 33.3 for-of and Arrays [ES6] . . . . . . . . . . . . . . . . . . . . . . . . . . . 346 33.4 Array-like objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346 33.5 Converting iterable and Array-like values to Arrays . . . . . . . . . . . . 347 33.6 Creating and filling Arrays with arbitrary lengths . . . . . . . . . . . . . 348 33.7 Multidimensional Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . 350 33.8 More Array features (advanced) . . . . . . . . . . . . . . . . . . . . . . . 350 33.9 Adding and removing elements (destructively and non-destructively) . . 353 33.10Methods: iteration and transformation (.find(), .map(), .filter(), etc.) 355 33.11.sort(): sorting Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . 363 33.12Quick reference: Array<T> . . . . . . . . . . . . . . . . . . . . . . . . . . 364 34 Typed Arrays: handling binary data (advanced) 373 34.1 The basics of the API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374 34.2 Element types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376 34.3 More information on Typed Arrays . . . . . . . . . . . . . . . . . . . . . 379 34.4 Quick references: indices vs. offsets . . . . . . . . . . . . . . . . . . . . . 382 34.5 Quick reference: ArrayBuffers . . . . . . . . . . . . . . . . . . . . . . . . 383 34.6 Quick reference: Typed Arrays . . . . . . . . . . . . . . . . . . . . . . . . 384 34.7 Quick reference: DataViews . . . . . . . . . . . . . . . . . . . . . . . . . 387 35 Maps (Map) 389 35.1 Using Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390 35.2 Example: Counting characters . . . . . . . . . . . . . . . . . . . . . . . . 393 35.3 A few more details about the keys of Maps (advanced) . . . . . . . . . . 393 35.4 Missing Map operations . . . . . . . . . . . . . . . . . . . . . . . . . . . 394 35.5 Quick reference: Map<K,V> . . . . . . . . . . . . . . . . . . . . . . . . . . 396 35.6 FAQ: Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399 36 WeakMaps (WeakMap) (advanced) 401 36.1 WeakMaps are black boxes . . . . . . . . . . . . . . . . . . . . . . . . . . 401 36.2 The keys of a WeakMap are weakly held . . . . . . . . . . . . . . . . . . . 402 36.3 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403 36.4 WeakMap API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404 37 Sets (Set) 405 37.1 Using Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406 37.2 Examples of using Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407 37.3 What Set elements are considered equal? . . . . . . . . . . . . . . . . . . 407 37.4 Missing Set operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408 37.5 Quick reference: Set<T> . . . . . . . . . . . . . . . . . . . . . . . . . . . 409 37.6 FAQ: Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411 38 WeakSets (WeakSet) (advanced) 413 38.1 Example: Marking objects as safe to use with a method . . . . . . . . . . 413 38.2 WeakSet API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414 39 Destructuring 415 39.1 A first taste of destructuring . . . . . . . . . . . . . . . . . . . . . . . . . 416 39.2 Constructing vs. extracting . . . . . . . . . . . . . . . . . . . . . . . . . . 416

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10 CONTENTS 39.3 Where can we destructure? . . . . . . . . . . . . . . . . . . . . . . . . . . 417 39.4 Object-destructuring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 39.5 Array-destructuring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 39.6 Examples of destructuring . . . . . . . . . . . . . . . . . . . . . . . . . . 420 39.7 What happens if a pattern part does not match anything? . . . . . . . . . 422 39.8 What values can’t be destructured? . . . . . . . . . . . . . . . . . . . . . 422 39.9 (Advanced) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423 39.10Default values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423 39.11Parameter definitions are similar to destructuring . . . . . . . . . . . . . 424 39.12Nested destructuring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424 40 Synchronous generators (advanced) 427 40.1 What are synchronous generators? . . . . . . . . . . . . . . . . . . . . . 427 40.2 Calling generators from generators (advanced) . . . . . . . . . . . . . . . 431 40.3 Background: external iteration vs. internal iteration . . . . . . . . . . . . 433 40.4 Use case for generators: reusing traversals . . . . . . . . . . . . . . . . . 434 40.5 Advanced features of generators . . . . . . . . . . . . . . . . . . . . . . . 435 VIII Asynchronicity 437 41 Asynchronous programming in JavaScript 439 41.1 A roadmap for asynchronous programming in JavaScript . . . . . . . . . 440 41.2 The call stack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442 41.3 The event loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443 41.4 How to avoid blocking the JavaScript process . . . . . . . . . . . . . . . 444 41.5 Patterns for delivering asynchronous results . . . . . . . . . . . . . . . . 446 41.6 Asynchronous code: the downsides . . . . . . . . . . . . . . . . . . . . . 449 41.7 Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450 42 Promises for asynchronous programming [ES6] 451 42.1 The basics of using Promises . . . . . . . . . . . . . . . . . . . . . . . . . 452 42.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460 42.3 Error handling: don’t mix rejections and exceptions . . . . . . . . . . . . 464 42.4 Promise-based functions start synchronously, settle asynchronously . . . 465 42.5 Promise combinator functions: working with Arrays of Promises . . . . . 466 42.6 Concurrency and Promise.all() (advanced) . . . . . . . . . . . . . . . . 478 42.7 Tips for chaining Promises . . . . . . . . . . . . . . . . . . . . . . . . . . 480 42.8 Quick reference: Promise combinator functions . . . . . . . . . . . . . . 482 43 Async functions 485 43.1 Async functions: the basics . . . . . . . . . . . . . . . . . . . . . . . . . . 485 43.2 Returning from async functions . . . . . . . . . . . . . . . . . . . . . . . 487 43.3 await: working with Promises . . . . . . . . . . . . . . . . . . . . . . . . 489 43.4 (Advanced) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 43.5 Immediately invoked async arrow functions . . . . . . . . . . . . . . . . 491 43.6 Concurrency and await . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492 43.7 Tips for using async functions . . . . . . . . . . . . . . . . . . . . . . . . 493

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CONTENTS 11 44 Asynchronous iteration 495 44.1 Basic asynchronous iteration . . . . . . . . . . . . . . . . . . . . . . . . . 495 44.2 Asynchronous generators . . . . . . . . . . . . . . . . . . . . . . . . . . 498 44.3 Async iteration over Node.js streams . . . . . . . . . . . . . . . . . . . . 502 IX More standard library 505 45 Regular expressions (RegExp) 507 45.1 Creating regular expressions . . . . . . . . . . . . . . . . . . . . . . . . . 508 45.2 Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509 45.3 Flags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513 45.4 Properties of regular expression objects . . . . . . . . . . . . . . . . . . . 517 45.5 Methods for working with regular expressions . . . . . . . . . . . . . . . 518 45.6 The flags /g and /y, and the property .lastIndex (advanced) . . . . . . . 525 45.7 Techniques for working with regular expressions . . . . . . . . . . . . . . 535 46 Dates (Date) 537 46.1 Best practice: avoid the built-in Date . . . . . . . . . . . . . . . . . . . . 537 46.2 Time standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 538 46.3 Background: date time formats (ISO) . . . . . . . . . . . . . . . . . . . . 539 46.4 Time values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 541 46.5 Creating Dates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 542 46.6 Getters and setters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543 46.7 Converting Dates to strings . . . . . . . . . . . . . . . . . . . . . . . . . 543 47 Creating and parsing JSON (JSON) 545 47.1 The discovery and standardization of JSON . . . . . . . . . . . . . . . . 546 47.2 JSON syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 546 47.3 Using the JSON API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 547 47.4 Customizing stringification and parsing (advanced) . . . . . . . . . . . . 549 47.5 FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553 X Miscellaneous topics 555 48 Next steps: overview of web development (bonus) 557 48.1 Tips against feeling overwhelmed . . . . . . . . . . . . . . . . . . . . . . 557 48.2 Things worth learning for web development . . . . . . . . . . . . . . . . 558 48.3 Example: tool-based JavaScript workflow . . . . . . . . . . . . . . . . . . 560 48.4 An overview of JavaScript tools . . . . . . . . . . . . . . . . . . . . . . . 562 48.5 Tools not related to JavaScript . . . . . . . . . . . . . . . . . . . . . . . . 564 XI Appendices 567 49 Index 569

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12 CONTENTS

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Part I Background 13

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Chapter 1 Before you buy the book Contents 1.1 About the content . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 1.1.1 What’s in this book? . . . . . . . . . . . . . . . . . . . . . . . 15 1.1.2 What is not covered by this book? . . . . . . . . . . . . . . . . 16 1.1.3 Isn’t this book too long for impatient people? . . . . . . . . . . 16 1.2 Previewing and buying this book . . . . . . . . . . . . . . . . . . . 16 1.2.1 How can I preview the book, the exercises, and the quizzes? . 16 1.2.2 How can I buy a digital version of this book? . . . . . . . . . . 16 1.2.3 How can I buy the print version of this book? . . . . . . . . . 16 1.3 About the author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 1.4 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1.1 About the content 1.1.1 What’s in this book? This bookmakes JavaScript less challenging to learn for newcomers by offering amodern view that is as consistent as possible. Highlights: • Get started quickly by initially focusing on modern features. • Test-driven exercises and quizzes available for most chapters. • Covers all essential features of JavaScript, up to and including ES2021. • Optional advanced sections let you dig deeper. No prior knowledge of JavaScript is required, but you should know how to program. 15

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16 1 Before you buy the book 1.1.2 What is not covered by this book? • Some advanced language features are not explained, but references to appropri- ate material are provided – for example, to my other JavaScript books at Explor- ingJS.com, which are free to read online. • This book deliberately focuses on the language. Browser-only features, etc. are not described. 1.1.3 Isn’t this book too long for impatient people? There are several ways in which you can read this book. One of them involves skipping much of the content in order to get started quickly. For details, see §2.1.1 “In which order should I read the content in this book?”. 1.2 Previewing and buying this book 1.2.1 How can I preview the book, the exercises, and the quizzes? Go to the homepage of this book: • All essential chapters of this book are free to read online. • The first half of the test-driven exercises can be downloaded. • The first half of the quizzes can be tried online. 1.2.2 How can I buy a digital version of this book? There are two digital versions of JavaScript for impatient programmers: • Ebooks: PDF, EPUB, MOBI, HTML (all without DRM) • Ebooks plus exercises and quizzes The home page of this book describes how you can buy them. 1.2.3 How can I buy the print version of this book? The print version of JavaScript for impatient programmers is available on Amazon. 1.3 About the author Dr. Axel Rauschmayer specializes in JavaScript and web development. He has been de- veloping web applications since 1995. In 1999, he was technical manager at a German internet startup that later expanded internationally. In 2006, he held his first talk on Ajax. In 2010, he received a PhD in Informatics from the University of Munich. Since 2011, he has been blogging about web development at 2ality.com and has written several books on JavaScript. He has held trainings and talks for companies such as eBay, Bank of America, and O’Reilly Media. He lives in Munich, Germany.

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1.4 Acknowledgements 17 1.4 Acknowledgements • Cover by Fran Caye • Parts of this book were edited by Adaobi Obi Tulton. • Thanks for answering questions, discussing language topics, etc.: – Allen Wirfs-Brock (@awbjs) – Benedikt Meurer (@bmeurer) – Brian Terlson (@bterlson) – Daniel Ehrenberg (@littledan) – Jordan Harband (@ljharb) – Maggie Johnson-Pint (@maggiepint) – Mathias Bynens (@mathias) – Myles Borins (@MylesBorins) – Rob Palmer (@robpalmer2) – Šime Vidas (@simevidas) – And many others • Thanks for reviewing: – Johannes Weber (@jowe) [Generated: 2021-02-05 10:30]

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18 1 Before you buy the book