JavaScript with Promises

Preface

Asynchronous JavaScript is everywhere. AJAX, WebRTC, and Node.js are a few examples of where asynchronous APIs are found. Although it is easy to write a quick function to handle the result of one HTTP request, it is also easy to get lost in an unpredictable sea of callbacks as a codebase grows and more people contribute. That’s where a good approach for handling asynchronous code comes in and many developers are choosing to use Promises in their approach.

This is the book I needed when originally choosing an asynchronous strategy, and it is the result of my experience using promises in JavaScript applications. It explains their use and inner workings while exposing difficulties and missteps. Promises are made up of only a few concepts with a small API. But in the same way that JavaScript’s small number of simple constructs are used to create elegant and powerful solutions, I am surprised and pleased at the number of ways Promises can be used to effectively manage asynchronous code.

Intended Audience

This book is for intermediate and advanced JavaScript developers who want to write asynchronous code. These developers may be comfortable with JavaScript for traditional web APIs but are moving to environments such as Node.js, Google Chrome packaged apps, or building desktop applications with JavaScript. Developers who write browser-based code and want to use frameworks such as Angular or newer browser technologies such as Service Workers or WebRTC will also benefit. Even people who are already experienced with Promises may still enjoy reading the code and discovering additional ideas for their own work.

A Word on Style

This is not a book about JavaScript syntax dos and don’ts. All the examples are intended to be clear and casual; however, this style may conflict with some recommended practices. Those choices are independent of the ideas presented here and you are free to choose as you see fit when approaching these concepts in your code.

Conventions Used in This Book

The following typographical conventions are used in this book:

Italic

Indicates new terms, URLs, email addresses, filenames, and file extensions.

Constant width

Used for program listings, as well as within paragraphs to refer to program elements such as variable or function names, databases, data types, environment variables, statements, and keywords.

Constant width bold

Shows commands or other text that should be typed literally by the user.

Constant width italic

Shows text that should be replaced with user-supplied values or by values determined by context.

Tip

This element signifies a tip or suggestion.

Note

This element signifies a general note.

Warning

This element indicates a warning or caution.

Using Code Examples

Supplemental material (code examples, exercises, etc.) is available for download at https://github.com/dxparker/promises-book-examples.

This book is here to help you get your job done. In general, if example code is offered with this book, you may use it in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a significant amount of example code from this book into your product’s documentation does require permission.

We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN. For example: “JavaScript with Promises by Daniel Parker (O’Reilly). Copyright 2015 Daniel Parker, 978-1-449-37321-4.”

If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at [e-mail].

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Acknowledgments

Thank you to Kris Kowal, Domenic Denicola, and Petka Antonov for their ongoing contributions to JavaScript Promises and for their feedback during the writing of this book. Thanks also to Cody Lindley for his valuable feedback.

Thank you to the wonderful people at O’Reilly whose expertise, support, and patience made the publication of this book possible, especially Simon St.Laurent for his role in getting the book started, Brian MacDonald and Amy Jollymore for their guidance, and Colleen Lobner and Lindsy Gamble for sweating the details.

One of the best things about the programming culture is constantly learning from other people or alongside them in a collaborative effort. I am grateful for having some exceptional colleagues over the years, including Jerry Raschke, Nathan Price, Hank Beasley, Gregory Long, and Johnathan Hebert.

This book is dedicated to my loving wife Sarah. You are amazing!