Introduction xxxi
Chapter 1: The New Toys In ES2015–ES2020, and Beyond 1
Definitions, Who’s Who, and Terminology 2
What are the “New Toys”? 4
How Do New Toys Get Created? 6
Keeping Up with the New Toys 9
Using Today’s Toys in Yesterday’s Environments, and Tomorrow’s Toys Today 10
Review 15
Chapter 2: Block-Scoped Declarations: Let and Const 17
An Introduction to let and const 18
True Block Scope 18
Repeated Declarations are an Error 19
Hoisting and the Temporal Dead Zone 20
A New Kind of Global 22
const: Constants for JavaScript 24
Block Scope in Loops 26
Old Habits to New 36
Chapter 3: New Function Features 39
Arrow Functions and Lexical this, super, etc. 40
Default Parameter Values 45
“Rest” Parameters 50
Trailing Commas in Parameter Lists and Function Calls 52
The Function name Property 53
Function Declarations in Blocks 55
Function Declarations in Blocks: Standard Semantics 57
Old Habits to New 60
Chapter 4: Classes 65
What is a Class? 66
Introducing the New class Syntax 66
Comparing with the Older Syntax 75
Creating Subclasses 77
Leaving Off Object.prototype 97
new.target 98
class Declarations vs. class Expressions 101
More to Come 103
Old Habits to New 104
Chapter 5: New Object Features 105
Computed Property Names 106
Shorthand Properties 107
Getting and Setting an Object’s Prototype 107
Method Syntax, and super Outside Classes 109
Symbol 112
New Object Functions 120
Symbol.toPrimitive 123
Property Order 125
Property Spread Syntax 127
Old Habits to New 128
Chapter 6: Iterables, Iterators, For-Of, Iterable Spread, Generators 131
Iterators, Iterables, the for-of Loop, and Iterable Spread Syntax 131
Generator Functions 146
Old Habits to New 163
Chapter 7: Destructuring 165
Overview 165
Basic Object Destructuring 166
Basic Array (and Iterable) Destructuring 169
Defaults 170
Rest Syntax in Destructuring Patterns 172
Using Different Names 173
Computed Property Names 174
Nested Destructuring 174
Parameter Destructuring 175
Destructuring in Loops 178
Old Habits to New 179
Chapter 8: Promises 181
Why Promises? 182
Promise Fundamentals 182
Using an Existing Promise 186
Adding Handlers to Already Settled Promises 201
Creating Promises 202
Other Promise Utility Methods 207
Promise Patterns 210
Handle Errors or Return the Promise 210
Promises in Series 211
Promises in Parallel 213
Promise Subclasses 218
Old Habits to New 219
Chapter 9: Asynchronous Functions, Iterators, and Generators 221
async Functions 222
async Iterators, Iterables, and Generators 232
for-await-of 238
Old Habits to New 238
Chapter 10: Templates, Tag Functions, and New String Features 241
Template Literals 241
Improved Unicode Support 250
Iteration 255
New String Methods 256
Updates to the match, split, search, and replace Methods 259
Old Habits to New 260
Chapter 11: New Array Features, Typed Arrays 263
New Array Methods 264
Iteration, Spread, Destructuring 276
Stable Array Sort 276
Typed Arrays 277
Old Habits to New 292
Chapter 12: Maps and Sets 293
Maps 293
Sets 300
WeakMaps 304
WeakSets 314
Old Habits to New 316
Chapter 13: Modules 319
Introduction to Modules 319
Module Fundamentals 320
Renaming Exports 331
Re-Exporting Exports from Another Module 332
Renaming Imports 333
Importing a Module’s Namespace Object 333
Exporting Another Module’s Namespace Object 334
Importing a Module Just for Side Effects 335
Import and Export Entries 335
Imports are Live and Read-Only 338
Module Instances are Realm-Specific 340
How Modules are Loaded 341
Import/Export Syntax Review 348
Dynamic Import 350
Tree Shaking 357
Bundling 359
Import Metadata 360
Worker Modules 360
Old Habits to New 362
Chapter 14: Reflection—Reflect and Proxy 365
Reflect 365
Proxy 371
Old Habits to New 395
Chapter 15: Regular Expression Updates 397
The Flags Property 398
New Flags 398
Named Capture Groups 400
Lookbehind Assertions 405
Unicode Features 408
Old Habits to New 413
Chapter 16: Shared Memory 417
Introduction 417
Here There Be Dragons! 418
Browser Support 418
Shared Memory Basics 420
Memory is Shared, Not Objects 426
Race Conditions, Out-of-Order Stores, Stale Values, Tearing, and More 427
The Atomics Object 429
Shared Memory Example 434
Here There Be Dragons! (Again) 455
Old Habits to New 460
Chapter 17: Miscellany 461
BigInt 462
New Integer Literals 465
New Math Methods 467
Exponentiation Operator (**) 468
Date.prototype.toString Change 470
Function.prototype.toString Change 471
Number Additions 471
Symbol.isConcatSpreadable 474
Various Syntax Tweaks 475
Various Standard Library / Global Additions 479
Annex B: Browser-Only Features 482
Tail Call Optimization 488
Old Habits to New 491
Chapter 18: Upcoming Class Features 493
Public and Private Class Fields, Methods, and Accessors 493
Old Habits to New 514
Chapter 19: A Look Ahead . . .517
Top-Level await 518
WeakRefs and Cleanup Callbacks 525
RegExp Match Indices 533
String.prototype.replaceAll 535
Atomics asyncWait 535
Various Syntax Tweaks 536
Legacy Deprecated RegExp Features 537
Thank You for Reading! 538
Appendix: Fantastic Features and Where to Find Them 539
Index 557
About the author
T.J. Crowder is a software engineer with 30 years of experience, including over 15 years of professional work in JavaScript. He runs Farsight Software, a software contracting and product company. He's often found helping people on Stack Overflow, where he's a top 10 all-time contributor and the top JavaScript contributor. When not working or writing, he…no, sorry, he's always working or writing – or spending time with his wonderful, supportive wife and fantastic son.
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