About This Book

A good understanding of ECMAScript 6 features is key for all JavaScript developers going forward. The language features introduced in ECMAScript 6 represent the foundation upon which JavaScript applications will be built for the foreseeable future. That’s where this book comes in. My hope is that you’ll read this book to learn about ECMAScript 6 features so that you’ll be ready to start using them as soon as you need to.

Browser and Node.js Compatibility

Many JavaScript environments, such as web browsers and Node.js, are actively working on implementing ECMAScript 6. This book doesn’t attempt to address the inconsistencies between implementations and instead focuses on what the specification defines as the correct behavior. As such, it’s possible that your JavaScript environment may not conform to the behavior described in this book.

Who This Book is For

This book is intended as a guide for those who are already familiar with JavaScript and ECMAScript 5. While a deep understanding of the language isn’t necessary to use this book, it will help you understand the differences between ECMAScript 5 and 6. In particular, this book is aimed at intermediate-to-advanced JavaScript developers programming for a browser or Node.js environment who want to learn about the latest developments in the language.

This book is not for beginners who have never written JavaScript. You will need to have a good basic understanding of the language to make use of this book.

Overview

Each of this book’s thirteen chapters covers a different aspect of ECMAScript 6. Many chapters start by discussing problems that ECMAScript 6 changes were made to solve, to give you a broader context for those changes, and all chapters include code examples to help you learn new syntax and concepts.

Chapter 1: How Block Bindings Work talks about let and const, the block-level replacement for var.

Chapter 2: Strings and Regular Expressions covers additional functionality for string manipulation and inspection as well as the introduction of template strings.

Chapter 3: Functions in ECMAScript 6 discusses the various changes to functions. This includes the arrow function form, default parameters, rest parameters, and more.

Chapter 4: Expanded Object Functionality explains the changes to how objects are created, modified, and used. Topics include changes to object literal syntax, and new reflection methods.

Chapter 5: Destructuring for Easier Data Access introduces object and array destructuring, which allow you to decompose objects and arrays using a concise syntax.

Chapter 6: Symbols and Symbol Properties introduces the concept of symbols, a new way to define properties. Symbols are a new primitive type that can be used to obscure (but not hide) object properties and methods.

Chapter 7: Sets and Maps details the new collection types of Set, WeakSet, Map, and WeakMap. These types expand on the usefulness of arrays by adding semantics, de-duping, and memory management designed specifically for JavaScript.

Chapter 8: Iterators and Generators discusses the addition of iterators and generators to the language. These features allow you to work with collections of data in powerful ways that were not possible in previous versions of JavaScript.

Chapter 9: Introducing JavaScript Classes introduces the first formal concept of classes in JavaScript. Often a point of confusion for those coming from other languages, the addition of class syntax in JavaScript makes the language more approachable to others and more concise for enthusiasts.

Chapter 10: Improved Array Capabilities details the changes to native arrays and the interesting new ways they can be used in JavaScript.

Chapter 11: Promises and Asynchronous Programming introduces promises as a new part of the language. Promises were a grassroots effort that eventually took off and gained in popularity due to extensive library support. ECMAScript 6 formalizes promises and makes them available by default.

Chapter 12: Proxies and the Reflection API introduces the formalized reflection API for JavaScript and the new proxy object that allows you to intercept every operation performed on an object. Proxies give developers unprecedented control over objects and, as such, unlimited possibilities for defining new interaction patterns.

Chapter 13: Encapsulating Code with Modules details the official module format for JavaScript. The intent is that these modules can replace the numerous ad-hoc module definition formats that have appeared over the years.

Appendix A: Smaller ECMAScript 6 Changes covers other changes implemented in ECMAScript 6 that you’ll use less frequently or that didn’t quite fit into the broader major topics covered in each chapter.

Appendix B: Understanding ECMAScript 7 (2016) describes the two additions to the standard that were implemented for ECMAScript 7, which didn’t impact JavaScript nearly as much as ECMAScript 6.

Conventions Used

The following typographical conventions are used in this book:

• Italics introduces new terms
• Constant width indicates a piece of code or filename

Additionally, longer code examples are contained in constant width code blocks such as:

function doSomething() {
    // empty
}

Within a code block, comments to the right of a console.log() statement indicate the output you’ll see in the browser or Node.js console when the code is executed, for example:

console.log("Hi");      // "Hi"

If a line of code in a code block throws an error, this is also indicated to the right of the code:

doSomething();          // error!

Help and Support

You can file issues, suggest changes, and open pull requests against this book by visiting: https://github.com/nzakas/understandinges6

If you have questions as you read this book, please send a message to my mailing list: http://groups.google.com/group/zakasbooks.