Preface

Design patterns are reusable solutions to commonly occurring problems in software design. They are both exciting and a fascinating topic to explore in any programming language.

One reason for this is that they help us build upon the combined experience of many developers that came before us and ensure we structure our code in an optimized way, meeting the needs of problems we're attempting to solve.

Design patterns also provide us a common vocabulary to describe solutions. This can be significantly simpler than describing syntax and semantics when we're attempting to convey a way of structuring a solution in code form to others.

In this book we will explore applying both classical and modern design patterns to the JavaScript programming language.

Target Audience

This book is targeted at professional developers wishing to improve their knowledge of design patterns and how they can be applied to the JavaScript programming language.

Some of the concepts covered (closures, prototypal inheritance) will assume a level of basic prior knowledge and understanding. If you find yourself needing to read further about these topics, a list of suggested titles is provided for convenience.

If you would like to learn how to write beautiful, structured and organized code, I believe this is the book for you.

Acknowledgments

I will always be grateful for the talented technical reviewers who helped review and improve this book, including those from the community at large. The knowledge and enthusiasm they brought to the project was simply amazing. The official technical reviewers tweets and blogs are also a regular source of both ideas and inspiration and I wholeheartedly recommend checking them out.

• Nicholas Zakas (http://nczonline.net, @slicknet)
• Andrée Hansson (http://andreehansson.se, @peolanha)
• Luke Smith (http://lucassmith.name, @ls_n)
• Eric Ferraiuolo (http://ericf.me/, @ericf)
• Peter Michaux (http://michaux.ca, @petermichaux)
• Alex Sexton (http://alexsexton.com, @slexaxton)

I would also like to thank Rebecca Murphey (http://rebeccamurphey.com, @rmurphey) for providing the inspiration to write this book and more importantly, continue to make it both available on GitHub and via O'Reilly.

Finally, I would like to thank my wonderful wife Ellie, for all of her support while I was putting together this publication.

Credits

Whilst some of the patterns covered in this book were implemented based on personal experience, many of them have been previously identified by the JavaScript community. This work is as such the production of the combined experience of a number of developers. Similar to Stoyan Stefanov's logical approach to preventing interruption of the narrative with credits (in JavaScript Patterns), I have listed credits and suggested reading for any content covered in the references section.

If any articles or links have been missed in the list of references, please accept my heartfelt apologies. If you contact me I'll be sure to update them to include you on the list.

Reading

Whilst this book is targeted at both beginners and intermediate developers, a basic understanding of JavaScript fundamentals is assumed. Should you wish to learn more about the language, I am happy to recommend the following titles:

• JavaScript: The Definitive Guide by David Flanagan
• Eloquent JavaScript by Marijn Haverbeke
• JavaScript Patterns by Stoyan Stefanov
• Writing Maintainable JavaScript by Nicholas Zakas
• JavaScript: The Good Parts by Douglas Crockford

Table Of Contents

# Introduction

One of the most important aspects of writing maintainable code is being able to notice the recurring themes in that code and optimize them. This is an area where knowledge of design patterns can prove invaluable.

In the first part of this book, we will explore the history and importance of design patterns which can really be applied to any programming language. If you're already sold on or are familiar with this history, feel free to skip to the chapter "What is a Pattern?" to continue reading.

Design patterns can be traced back to the early work of an architect named Christopher Alexander. He would often write publications about his experience in solving design issues and how they related to buildings and towns. One day, it occurred to Alexander that when used time and time again, certain design constructs lead to a desired optimal effect.

In collaboration with Sara Ishikawa and Murray Silverstein, Alexander produced a pattern language that would help empower anyone wishing to design and build at any scale. This was published back in 1977 in a paper titled "A Pattern Language", which was later released as a complete hardcover book.

Some 30 years ago, software engineers began to incorporate the principles Alexander had written about into the first documentation about design patterns, which was to be a guide for novice developers looking to improve their coding skills. It's important to note that the concepts behind design patterns have actually been around in the programming industry since its inception, albeit in a less formalized form.

One of the first and arguably most iconic formal works published on design patterns in software engineering was a book in 1995 called Design Patterns: Elements Of Reusable Object-Oriented Software. This was written by Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides - a group that became known as the Gang of Four (or GoF for short).

The GoF's publication is considered quite instrumental to pushing the concept of design patterns further in our field as it describes a number of development techniques and pitfalls as well as providing twenty-three core Object-Oriented design patterns frequently used around the world today. We will be covering these patterns in more detail in the section "Categories of Design Patterns".

In this book, we will take a look at a number of popular JavaScript design patterns and explore why certain patterns may be more suitable for your projects than others. Remember that patterns can be applied not just to vanilla JavaScript (i.e standard JavaScript code), but also to abstracted libraries such as jQuery or dojo as well. Before we begin, let’s look at the exact definition of a "pattern" in software design.

What is a Pattern?

A pattern is a reusable solution that can be applied to commonly occurring problems in software design - in our case - in writing JavaScript web applications. Another way of looking at patterns are as templates for how we solve problems - ones which can be used in quite a few different situations.

So, why is it important to understand patterns and be familiar with them? Design patterns have three main benefits:

1. Patterns are proven solutions: They provide solid approaches to solving issues in software development using proven techniques that reflect the experience and insights the developers that helped define them bring to the pattern.
2. Patterns can be easily reused: A pattern usually reflects an out of the box solution that can be adapted to suit our own needs. This feature makes them quite robust.
3. Patterns can be expressive: When we look at a pattern there’s generally a set structure and vocabulary to the solution presented that can help express rather large solutions quite elegantly.

• Reusing patterns assists in preventing minor issues that can cause major problems in the application development process. What this means is when code is built on proven patterns, we can afford to spend less time worrying about the structure of our code and more time focusing on the quality of our overall solution. This is because patterns can encourage us to code in a more structured and organized fashion avoiding the need to refactor it for cleanliness purposes in the future.

• Patterns can provide generalized solutions which are documented in a fashion that doesn't require them to be tied to a specific problem. This generalized approach means that regardless of the application (and in many cases the programming language) we are working with, design patterns can be applied to improve the structure of our code.
• Certain patterns can actually decrease the overall file-size footprint of our code by avoiding repetition. By encouraging developers to look more closely at their solutions for areas where instant reductions in repetition can be made, e.g. reducing the number of functions performing similar processes in favor of a single generalized function, the overall size of our codebase can be decreased. This is also known as making code more DRY.
• Patterns add to a developers vocabulary, which makes communication faster.
• Patterns that are frequently used can be improved over time by harnessing the collective experiences other developers using those patterns contribute back to the design pattern community. In some cases this leads to the creation of entirely new design patterns whilst in others it can lead to the provision of improved guidelines on how specific patterns can be best used. This can ensure that pattern-based solutions continue to become more robust than ad-hoc solutions may be.