Preface

Web Workers is a powerful feature of HTML5 that hasn’t received very much attention. It provides an API that allows you to run JavaScript in a separate thread that doesn’t interfere with the user interface of your web application. This JavaScript runs in parallel with the main renderer and any of your user interface scripts on it. This allows long and “processing-heavy” tasks to be executed without making the page unresponsive.

Like threads in other technologies, Web Workers are relatively heavyweight. You don’t want to use them in large numbers, as each one consumes significant system resources. Web Workers are expected to handle long tasks that rely on constrained resources (e.g., CPU, network bandwidth, etc.). They have a high startup cost and a high instance of memory cost.

Because it is a new, evolving standard, different browsers implement the Web Workers specification in different ways. Although some aspects of the implementation are stabilizing, I suspect that features like access to IndexedDB will be available soon in most modern browsers. I hope that with this book and the adoption of modern browsers we will see more usage of this powerful API.

Note

All the examples in this book were tested on Chrome (15+) and Firefox (7+). Web Workers also work in mobile Safari 5+, and will be especially useful because the new iPhone 4GS has a multi-core processor.

How This Book Is Organized

Before you can do much with Web Workers, it helps to know what they are and what they can do well. Next, you’ll learn how to confirm that your browser supports this feature with a simple “Web Worker Hello World” example. The next three chapters cover the different kinds of Web Workers (dedicated, shared, and inline), showing how to use each one of them and when it will be best to choose one over the other. After that, you’ll explore best practices for debugging your Web Workers. Finally, because Web Workers are also used outside of a browser, you’ll see how to apply them within the server-side Node environment.

Who This Book Is For

You should have a solid intermediate to advanced understanding of JavaScript before tackling the tools used in this book. In particular, you need to understand event handling and callbacks.

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

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Caution

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Using Code Examples

This book is here to help you get your job done. In general, you may use the code in this book 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: “Web Workers by Ido Green (O’Reilly). Copyright 2012 Ido Green, 9781449322137.”

If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at permissions@oreilly.com.

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