Effective JavaScript

David Herman is a principal researcher at Mozilla Research. He holds a BA in computer science from Grinnell College, and an MS and PhD in computer science from Northeastern University. David serves on Ecma TC39, the committee responsible for the standardization of JavaScript.

“Living up to the expectation of an Effective Software Development Series programming book, Effective JavaScript by Dave Herman is a must-read for anyone who wants to do serious JavaScript programming. The book provides detailed explanations of the inner workings of JavaScript, which helps readers take better advantage of the language.”—Erik Arvidsson, senior software engineer  “Before reading Effective JavaScript, I thought it would be just another book on how to write better JavaScript. But this book delivers that and so much more—it gives you a deep understanding of the language. And this is crucial. Without that understanding you’ll know absolutely nothing whatever about the language itself. You’ll only know how other programmers write their code.  “Read this book if you want to become a really good JavaScript developer. I, for one, wish I had it when I first started writing JavaScript.”—Anton Kovalyov, developer of JSHint  “If you’re looking for a book that gives you formal but highly readable insights into the JavaScript language, look no further. Intermediate JavaScript developers will find a treasure trove of knowledge inside, and even highly skilled JavaScripters are almost guaranteed to learn a thing or ten. For experienced practitioners of other languages looking to dive headfirst into JavaScript, this book is a mustread for quickly getting up to speed. No matter what your background, though, author Dave Herman does a fantastic job of exploring JavaScript—its beautiful parts, its warts, and everything in between.”—Rebecca Murphey, senior JavaScript developer, Bocoup  “Effective JavaScript is essential reading for anyone who understands that JavaScript is no mere toy and wants to fully grasp the power it has to offer. Dave Herman brings users a deep, studied, and practical understanding of the language, guiding them through example after example to help them come to the same conclusions he has. This is not a book for those looking for shortcuts; rather, it is hard-won experience distilled into a guided tour. It’s one of the few books on JavaScript that I’ll recommend without hesitation.”—Alex Russell, TC39 member, software engineer, Google  “Rarely does anyone have the opportunity to study alongside a master in their craft. This book is just that—the JavaScript equivalent of a time-traveling philosopher visiting fifth century BC to study with Plato.”—Rick Waldron, JavaScript evangelist, Bocoup

• Foreword xiiiPreface xvAcknowledgments xviiAbout the Author xixChapter 1: Accustoming Yourself to JavaScript 1Item 1: Know Which JavaScript You Are Using 1Item 2: Understand JavaScript’s Floating-Point Numbers 7Item 3: Beware of Implicit Coercions 9Item 4: Prefer Primitives to Object Wrappers 15Item 5: Avoid using == with Mixed Types 16Item 6: Learn the Limits of Semicolon Insertion 19Item 7: Think of Strings As Sequences of 16-Bit Code Units 25Chapter 2: Variable Scope 31Item 8: Minimize Use of the Global Object 31Item 9: Always Declare Local Variables 34Item 10: Avoid with 35Item 11: Get Comfortable with Closures 39Item 12: Understand Variable Hoisting 42Item 13: Use Immediately Invoked Function Expressions to Create Local Scopes 44Item 14: Beware of Unportable Scoping of Named Function Expressions 47Item 15: Beware of Unportable Scoping of Block-Local Function Declarations 50Item 16: Avoid Creating Local Variables with eval 52Item 17: Prefer Indirect eval to Direct eval 54Chapter 3: Working with Functions 57Item 18: Understand the Difference between Function, Method, and Constructor Calls 57Item 19: Get Comfortable Using Higher-Order Functions 60Item 20: Use call to Call Methods with a Custom Receiver 63Item 21: Use apply to Call Functions with Different Numbers of Arguments 65Item 22: Use arguments to Create Variadic Functions 67Item 23: Never Modify the arguments Object 68Item 24: Use a Variable to Save a Reference to arguments 70Item 25: Use bind to Extract Methods with a Fixed Receiver 72Item 26: Use bind to Curry Functions 74Item 27: Prefer Closures to Strings for Encapsulating Code 75Item 28: Avoid Relying on the toString Method of Functions 77Item 29: Avoid Nonstandard Stack Inspection Properties 79Chapter 4: Objects and Prototypes 83Item 30: Understand the Difference between prototype, getPrototypeOf, and__proto__ 83Item 31: Prefer Object.getPrototypeOf to __proto__ 87Item 32: Never Modify __proto__ 88Item 33: Make Your Constructors new-Agnostic 89Item 34: Store Methods on Prototypes 92Item 35: Use Closures to Store Private Data 94Item 36: Store Instance State Only on Instance Objects 95Item 37: Recognize the Implicit Binding of this 98Item 38: Call Superclass Constructors from Subclass Constructors 101Item 39: Never Reuse Superclass Property Names 105Item 40: Avoid Inheriting from Standard Classes 106Item 41: Treat Prototypes As an Implementation Detail 109Item 42: Avoid Reckless Monkey-Patching 110Chapter 5: Arrays and Dictionaries 113Item 43: Build Lightweight Dictionaries from Direct Instances of Object 113Item 44: Use null Prototypes to Prevent Prototype Pollution 116Item 45: Use hasOwnProperty to Protect Against Prototype Pollution 118Item 46: Prefer Arrays to Dictionaries for Ordered Collections 123Item 47: Never Add Enumerable Properties to Object.prototype 125Item 48: Avoid Modifying an Object during Enumeration 127Item 49: Prefer for Loops to for...in Loops for Array Iteration 132Item 50: Prefer Iteration Methods to Loops 133Item 51: Reuse Generic Array Methods on Array-Like Objects 138Item 52: Prefer Array Literals to the Array Constructor 140Chapter 6: Library and API Design 143Item 53: Maintain Consistent Conventions 143Item 54: Treat undefined As “No Value” 144Item 55: Accept Options Objects for Keyword Arguments 149Item 56: Avoid Unnecessary State 153Item 57: Use Structural Typing for Flexible Interfaces 156Item 58: Distinguish between Array and Array-Like 160Item 59: Avoid Excessive Coercion 164Item 60: Support Method Chaining 167Chapter 7: Concurrency 171Item 61: Don’t Block the Event Queue on I/O 172Item 62: Use Nested or Named Callbacks for Asynchronous Sequencing 175Item 63: Be Aware of Dropped Errors 179Item 64: Use Recursion for Asynchronous Loops 183Item 65: Don’t Block the Event Queue on Computation 186Item 66: Use a Counter to Perform Concurrent Operations 190Item 67: Never Call Asynchronous Callbacks Synchronously 194Item 68: Use Promises for Cleaner Asynchronous Logic 197Index 201