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Essential JavaScript Design Patterns For

Beginners

Authored By Addy Osmani

Creative Commons Attribution-NonCommercial-ShareAlike 3.0 unported license. You

are free to remix, tweak, and build upon this work non-commercially, as long as you

credit Addy Osmani (the copyright holder) and license your new creations under the

identical terms. Any of the above conditions can be waived if you get permission from

the copyright holder. For any reuse or distribution, you must make clear to others the

license terms of this work. The best way to do this is with a link to the license.

Foreword

I would like to thank Rebecca Murphey for inspiring me to open-source this mini-book

and release it for free download and distribution - making knowledge both open and

easily available is something we should all strive for where possible. I would also like to

extend my thanks to the very talented Alex Sexton who was kind enough to be the

technical reviewer for this publication. I hope that it helps you learn more about design

patterns and the usefulness of their application to JavaScript code.

Introduction

At the beginning of this book I will be focusing on a discussion about the importance

and history of design patterns in 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

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

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of design patterns can prove invaluable.

Design patterns can be traced back to the early work of a civil engineer 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 Sarah Ishikawra 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'.

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 more than likely 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, 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.

Contents

What is a Pattern?

'Pattern'-ity Testing, Proto-Patterns & The Rule Of Three

The Structure Of A Design Pattern

Writing Design Patterns

Anti-Patterns

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significantly easier to maintain.

You may ask why it’s important to understand patterns and be familiar with them.

Design patterns have three main benefits:

Patterns are proven solutions: They provide solid approaches to solving

issues in software development using proven solutions that reflect the

experience and insights the developers that helped define and improve them

bring to the pattern.

Patterns can be easily re-used: A pattern usually reflects an out of the box

solution that can be adapted to suit your own needs. This feature makes them

quite robust.

Patterns can be expressive: When you 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.

Patterns are not an exact solution. It’s important that we remember the role of a

pattern is merely to provide us with a solution scheme. Patterns don’t solve all design

problems nor do they replace good software designers, however, they do support

them. Next we’ll take a look at some of the other advantages patterns can offer us.

Reusing patterns assists in preventing minor issues that can cause major problems

in the application development process. What this means is that when your code

relies more on proven patterns, you can afford to spend less time worrying about your

code architecture and more time focusing on the quality of your overall solution. This

is because patterns can encourage you to code in a more structured and organized

fashion so the need to refactor it for cleanliness purposes in the future can be

significantly decreased.

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) you are working with, design patterns can be applied to improve the

structure of your code.

Certain patterns can actually decrease the overall file-size footprint of your 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 your codebase can be decreased.

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.

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'Pattern'-ity Testing, Proto-Patterns &

The Rule Of Three

Remember that not every algorithm, best practice or solution represents what might

be considered a complete pattern. There may be a few key ingredients here that are

missing and the pattern community is generally weary of something claiming to be

one unless it has been heavily vetted. Even if something is presented to us which

*appears* to meet the criteria for a pattern, it should not be considered one until it has

undergone suitable periods of scrutiny and testing by others.

Looking back upon the work by Alexander once more, he claims that a pattern should

both be a process and a ‘thing’. This definition is obtuse on purpose as he follows

by saying that the process should create the ‘thing’. This is a reason why patterns

generally focus on addressing a visually identifiable structure i.e you should be able to

visually depict (or draw) a picture representing the structure that placing the pattern

into practice results in.

In studying design patterns, you may come across the term ‘proto-pattern’ quite

frequently. What is this? Well, a pattern that has not yet been known to pass the

‘pattern’-ity tests is usually referred to as a proto-pattern. Proto-patterns may result

from the work of someone that has established a particular solution that is worthy of

sharing with the community, but may not have yet had the opportunity to have been

vetted heavily due to it’s very young age.

Alternatively, the individual(s) sharing the pattern may not have the time or interest of

going through the ‘pattern’-ity process and might release a short description of

their proto-pattern instead. Brief descriptions of this type of pattern are known as

patlets.

The work involved in fully documenting a qualified pattern can be quite daunting.

Looking back at some of the earliest work in the field of design patterns, a pattern may

be considered ‘good’ if it does the following:

Solves a particular problem - patterns are not supposed to just capture principles or

strategies. They need to capture solutions. This is one of the most essential ingredients

for a good pattern.

The solution to this problem cannot be obvious - you can often find that problem-

solving techniques attempt to derive from well-known first principles. The best design

patterns usually provide solutions to problems indirectly - this is considered a

necessary approach for the most challenging problems related to design.

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