Learning JavaScript Design Patterns

A book by Addy Osmani

Volume 1.5.2

(original online at: http://addyosmani.com/resources/essentialjsdesignpatterns/book/)

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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.


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.

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.


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.


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:


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.

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 have to offer.


We already use patterns everyday

To understand how useful patterns can be, let's review a very simple element selection problem that the jQuery library solves for us.

Imagine that we have a script where for each DOM element found on a page with class "foo" we wish to increment a counter. What's the most efficient way to query for this collection of elements? Well, there are a few different ways this problem could be tackled:

  1. Select all of the elements in the page and then store references to them. Next, filter this collection and use regular expressions (or another means) to only store those with the class "foo".
  2. Use a modern native browser feature such as querySelectorAll() to select all of the elements with the class "foo".
  3. Use a native feature such as getElementsByClassName() to similarly get back the desired collection.

So, which of these options is the fastest? It's actually option 3. by a factor of 8-10 times the alternatives. In a real-world application however, 3. will not work in versions of Internet Explorer below 9 and thus it's necessary to use 1. where both 2. and 3. aren't supported.

Developers using jQuery don't have to worry about this problem however, as it's luckily abstracted away for us using the Facade pattern. As we'll review in more detail later, this pattern provides a simple set of abstracted interfaces (e.g $el.css(), $el.animate()) to several more complex underlying bodies of code. As we've seen, this means less time having to be concerned about implementation level details.

Behind the scenes, the library simply opts for the most optimal approach to selecting elements depending on what our current browser supports and we just consume the abstraction layer.

We're probably all also familiar with jQuery's $("selector"). This is significantly more easy to use for selecting HTML elements on a page versus having to manually handle opt for getElementById(), getElementsByClassName(), getElementByTagName and so on.

Although we know that querySelectorAll() attempts to solve this problem, compare the effort involved in using jQuery's Facade interfaces vs. selecting the most optimal selection paths ourselves. There's no contest! Abstractions using patterns can offer real-world value.

We'll be looking at this and more design patterns later on in the book.


"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 wary 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 it is the process should create the "thing". This is a reason why patterns generally focus on addressing a visually identifiable structure i.e we 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, it's not irregular to come across the term "proto-pattern". 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 its 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 or snippets 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:

We would be forgiven for thinking that a proto-pattern which fails to meet guidelines isn't worth learning from, however, this is far from the truth. Many proto-patterns are actually quite good. I’m not saying that all proto-patterns are worth looking at, but there are quite a few useful ones in the wild that could assist us with future projects. Use best judgment with the above list in mind and you’ll be fine in your selection process.

One of the additional requirements for a pattern to be valid is that they display some recurring phenomenon. This is often something that can be qualified in at least three key areas, referred to as the rule of three. To show recurrence using this rule, one must demonstrate:

  1. Fitness of purpose - how is the pattern considered successful?
  2. Usefulness - why is the pattern considered successful?
  3. Applicability - is the design worthy of being a pattern because it has wider applicability? If so, this needs to be explained. When reviewing or defining a pattern, it is important to keep the above in mind.


# The Structure Of A Design Pattern

You may be curious about how a pattern author might approach outlining structure, implementation and purpose of a new pattern.  A pattern is initially presented in the form of a rule that establishes a relationship between:

With this in mind, let’s now take a look at a summary of the component elements for a design pattern. A design pattern should have a:

Design patterns are quite a powerful approach to getting all of the developers in an organization or team on the same page when creating or maintaining solutions. If considering working on a pattern of your own, remember that although they may have a heavy initial cost in the planning and write-up phases, the value returned from that investment can be quite worth it. Always research thoroughly before working on new patterns however, as you may find it more beneficial to use or build on top of existing proven patterns than starting afresh.


# Writing Design Patterns

Although this book is aimed at those new to design patterns, a fundamental understanding of how a design pattern is written can offer a number of useful benefits. For starters, we can gain a deeper appreciation for the reasoning behind why a pattern is needed. We can also learn how to tell if a pattern (or proto-pattern) is up to scratch when reviewing it for our own needs.

Writing good patterns is a challenging task. Patterns not only need to (ideally) provide a substantial quantity of reference material for end-users, but they also need to be able to defend why they are necessary.

Having read the previous section on what a pattern is, we may think that this in itself is enough to help us identify patterns we see in the wild. This is actually not completely true. It's not always clear if a piece of code we're looking at is following a set pattern or just accidentally happens to appear like it does.

When we're looking at a body of code we think may be using a pattern, we should consider writing down some of the aspects of the code that we believe falls under a particular existing pattern or set of patterns.

In many cases of pattern-analysis we can find that we're just looking at code that follows good principles and design practices that could happen to overlap with the rules for a pattern by accident. Remember - solutions in which neither interactions nor defined rules appear are not patterns.

If interested in venturing down the path of writing your own design patterns I recommend learning from others who have already been through the process and done it well. Spend time absorbing the information from a number of different design pattern descriptions and take in what’s meaningful to you.

Explore structure and semantics - this can be done by examining the interactions and context of the patterns you are interested in so you can identify the principles that assist in organizing those patterns together in useful configurations.

Once we've exposed ourselves to a wealth of information on pattern literature, we may wish to begin writing our pattern using an existing format and see if we can brainstorm new ideas for improving it or integrating our ideas in there.

An example of a developer that did this is in recent years is Christian Heilmann, who took the existing Module pattern and made some fundamentally useful changes to it to create the Revealing Module pattern (this is one of the patterns covered later in this book).

The following are tips I would suggest if interesting in creating a new design pattern:

Pattern writing is a careful balance between creating a design that is general, specific and above all, useful. Try to ensure that if writing a pattern you cover the widest possible areas of application and you should be fine.  I hope that this brief introduction to writing patterns has given you some insights that will assist your learning process for the next sections of this book.



If we consider that a pattern represents a best practice, an anti-pattern represents a lesson that has been learned. The term anti-patterns was coined in 1995 by Andrew Koenig in the November C++ Report that year, inspired by the GoF's book Design Patterns. In Koenig’s report, there are two notions of anti-patterns that are presented. Anti-Patterns:

On this topic, Alexander writes about the difficulties in achieving a good balance between good design structure and good context:

“These notes are about the process of design; the process of inventing physical things which display a new physical order, organization, form, in response to function.…every design problem begins with an effort to achieve fitness between two entities: the form in question and its context. The form is the solution to the problem; the context defines the problem”.

While it’s quite important to be aware of design patterns, it can be equally important to understand anti-patterns. Let us qualify the reason behind this. When creating an application, a project’s life-cycle begins with construction however once you’ve got the initial release done, it needs to be maintained. The quality of a final solution will either be good or bad, depending on the level of skill and time the team have invested in it. Here good and bad are considered in context - a ‘perfect’ design may qualify as an anti-pattern if applied in the wrong context.

The bigger challenges happen after an application has hit production and is ready to go into maintenance mode. A developer working on such a system who hasn’t worked on the application before may introduce a bad design into the project by accident. If said bad practices are created as anti-patterns, they allow developers a means to recognize these in advance so that they can avoid common mistakes that can occur - this is parallel to the way in which design patterns provide us with a way to recognize common techniques that are useful.

To summarize, an anti-pattern is a bad design that is worthy of documenting. Examples of anti-patterns in JavaScript are the following:

Knowledge of anti-patterns is critical for success. Once we are able to recognize such anti-patterns, we're able to refactor our code to negate them so that the overall quality of our solutions improves instantly.


# Categories Of Design Pattern


A glossary from the well-known design book, Domain-Driven Terms, rightly states that:

“A design pattern names, abstracts, and identifies the key aspects of a common design structure that make it useful for creating a reusable object-oriented design. The design pattern identifies the participating classes and their instances, their roles and collaborations, and the distribution of responsibilities.

Each design pattern focuses on a particular object-oriented design problem or issue. It describes when it applies, whether or not it can be applied in view of other design constraints, and the consequences and trade-offs of its use. Since we must eventually implement our designs, a design pattern also provides sample ... code to illustrate an implementation.

Although design patterns describe object-oriented designs, they are based on practical solutions that have been implemented in mainstream object-oriented programming languages ....”

Design patterns can be broken down into a number of different categories. In this section we’ll review three of these categories and briefly mention a few examples of the patterns that fall into these categories before exploring specific ones in more detail.


Creational Design Patterns

Creational design patterns focus on handling object creation mechanisms where objects are created in a manner suitable for the situation we're are working in. The basic approach to object creation might otherwise lead to added complexity in a project whilst these patterns aim to solve this problem by controlling the creation process.

Some of the patterns that fall under this category are: Constructor, Factory, Abstract, Prototype, Singleton and Builder.

Structural Design Patterns

Structural patterns are concerned with object composition and typically identify simple ways to realize relationships between different objects. They help ensure that when one part of a system changes, the entire structure of the system doesn't need to do the same. They also assist in recasting parts of the system which don't fit a particular purpose into those that do.

Patterns that fall under this category include: Decorator, Facade, Flyweight, Adapter and Proxy.

Behavioral Design Patterns

Behavioral patterns focus on improving or streamlining the communication between disparate objects in a system.

Some behavioral patterns include: Iterator, Mediator, Observer and Visitor.


# Design Pattern Categorization

In my early experiences of learning about design patterns, I personally found the following table a very useful reminder of what a number of patterns has to offer - it covers the 23 Design Patterns mentioned by the GoF. The original table was summarized by Elyse Nielsen back in 2004 and I've modified it where necessary to suit our discussion in this section of the book.

I recommend using this table as reference, but do remember that there are a number of additional patterns that are not mentioned here but will be discussed later in the book.

A brief note on classes

Keep in mind that there will be patterns in this table that reference the concept of "classes". JavaScript is a class-less language, however classes can be simulated using functions.

The most common approach to achieving this is by defining a JavaScript function where we then create an object using the new keyword. this can be used to help define new properties and methods for the object as follows:

01// A car "class"
02function Car( model ) {
04  this.model = model;
05  this.color = "silver";
06  this.year  = "2012";
08  this.getInfo = function () {
09    return this.model + " " + this.year;
10  };

We can then instantiate the object using the Car constructor we defined above like this:

1var myCar = new Car("ford");
3myCar.year = "2010";
5console.log( myCar.getInfo() );

For more ways to define "classes" using JavaScript, see Stoyan Stefanov's useful post on them:


JavaScript is a very flexible object-oriented language when it comes to syntax. In this article you can find three ways of defining and instantiating an object. Even if you have already picked your favorite way of doing it, it helps to know some alternatives in order to read other people's code.

It's important to note that there are no classes in JavaScript. Functions can be used to somewhat simulate classes, but in general JavaScript is a class-less language. Everything is an object. And when it comes to inheritance, objects inherit from objects, not classes from classes as in the "class"-ical languages.

1. Using a function

This is probably one of the most common ways. You define a normal JavaScript function and then create an object by using the new keyword. To define properties and methods for an object created using function(), you use the this keyword, as seen in the following example.

function Apple (type) {
    this.type = type;
    this.color = "red";
    this.getInfo = getAppleInfo;
// anti-pattern! keep reading...
function getAppleInfo() {
    return this.color + ' ' + this.type + ' apple';

To instantiate an object using the Apple constructor function, set some properties and call methods you can do the following:

var apple = new Apple('macintosh');
apple.color = "reddish";

1.1. Methods defined internally

In the example above you see that the method getInfo() of the Apple "class" was defined in a separate function getAppleInfo(). While this works fine, it has one drawback – you may end up defining a lot of these functions and they are all in the "global namespece". This means you may have naming conflicts if you (or another library you are using) decide to create another function with the same name. The way to prevent pollution of the global namespace, you can define your methods within the constructor function, like this:

function Apple (type) {
    this.type = type;
    this.color = "red";
    this.getInfo = function() {
        return this.color + ' ' + this.type + ' apple';

Using this syntax changes nothing in the way you instantiate the object and use its properties and methods.

1.2. Methods added to the prototype

A drawback of 1.1. is that the method getInfo() is recreated every time you create a new object. Sometimes that may be what you want, but it's rare. A more inexpensive way is to add getInfo() to the prototype of the constructor function.

function Apple (type) {
    this.type = type;
    this.color = "red";
Apple.prototype.getInfo = function() {
    return this.color + ' ' + this.type + ' apple';

Again, you can use the new objects exactly the same way as in 1. and 1.1.

2. Using object literals

Literals are shorter way to define objects and arrays in JavaScript. To create an empty object using you can do:
var o = {};
instead of the "normal" way:
var o = new Object();
For arrays you can do:
var a = [];
instead of:
var a = new Array();
So you can skip the class-like stuff and create an instance (object) immediately. Here's the same functionality as described in the previous examples, but using object literal syntax this time:

var apple = {
    type: "macintosh",
    color: "red",
    getInfo: function () {
        return this.color + ' ' + this.type + ' apple';

In this case you don't need to (and cannot) create an instance of the class, it already exists. So you simply start using this instance.

apple.color = "reddish";

Such an object is also sometimes called singleton. It "classical" languages such as Java, singleton means that you can have only one single instance of this class at any time, you cannot create more objects of the same class. In JavaScript (no classes, remember?) this concept makes no sense anymore since all objects are singletons to begin with.

3. Singleton using a function

Again with the singleton, eh? :)

The third way presented in this article is a combination of the other two you already saw. You can use a function to define a singleton object. Here's the syntax:

var apple = new function() {
    this.type = "macintosh";
    this.color = "red";
    this.getInfo = function () {
        return this.color + ' ' + this.type + ' apple';

So you see that this is very similar to 1.1. discussed above, but the way to use the object is exactly like in 2.

apple.color = "reddish";

new function(){...} does two things at the same time: define a function (an anonymous constructor function) and invoke it with new. It might look a bit confusing if you're not used to it and it's not too common, but hey, it's an option, when you really want a constructor function that you'll use only once and there's no sense of giving it a name.


You saw three (plus one) ways of creating objects in JavaScript. Remember that (despite the article's title) there's no such thing as a class in JavaScript. Looking forward to start coding using the new knowledge? Happy JavaScript-ing!

Tell your friends about this post: Facebook, Twitter, Google+

Let us now proceed to review the table.

  Creational   Based on the concept of creating an object.
      Factory Method This makes an instance of several derived classes based on interfaced data or events.
      Abstract Factory Creates an instance of several families of classes without detailing concrete classes.
      Builder Separates object construction from its representation, always creates the same type of object.
      Prototype A fully initialized instance used for copying or cloning.
      Singleton A class with only a single instance with global access points.
  Structural   Based on the idea of building blocks of objects
      Adapter Match interfaces of different classes therefore classes can work together despite incompatible interfaces
      Adapter Match interfaces of different classes therefore classes can work together despite incompatible interfaces
      Bridge Separates an object's interface from its implementation so the two can vary independently
      Composite A structure of simple and composite objects which makes the total object more than just the sum of its parts.
      Decorator Dynamically add alternate processing to objects.
      Facade A single class that hides the complexity of an entire subsystem.
      Flyweight A fine-grained instance used for efficient sharing of information that is contained elsewhere.
      Proxy A place holder object representing the true object
  Behavioral   Based on the way objects play and work together.
      Interpreter A way to include language elements in an application to match the grammar of the intended language.
Creates the shell of an algorithm in a method, then defer the exact steps to a subclass.
      Chain of
A way of passing a request between a chain of objects to find the object that can handle the request.
      Command Encapsulate a command request as an object to enable, logging and/or queuing of requests, and provides error-handling for unhandled requests.
      Iterator Sequentially access the elements of a collection without knowing the inner workings of the collection.
      Mediator Defines simplified communication between classes to prevent a group of classes from referring explicitly to each other.
      Memento Capture an object's internal state to be able to restore it later.
      Observer A way of notifying change to a number of classes to ensure consistency between the classes.
      State Alter an object's behavior when its state changes
      Strategy Encapsulates an algorithm inside a class separating the selection from the implementation
      Visitor Adds a new operation to a class without changing the class



# JavaScript Design Patterns


In this section, we will explore JavaScript implementations of a number of both classic and modern design patterns.

Developers commonly wonder whether there is an ideal pattern or set of patterns they should be using in their workflow. There isn't a true single answer to this question; each script and web application we work on is likely to have its own individual needs and we need to think about where we feel a pattern can offer real value to an implementation.

For example, some projects may benefit from the decoupling benefits offered by the Observer pattern (which reduces how dependent parts of an application are on one another) whilst others may simply be too small for decoupling to be a concern at all.

That said, once we have a firm grasp of design patterns and the specific problems they are best suited to, it becomes much easier to integrate them into our application architectures.


The patterns we will be exploring in this section are the:


# The Constructor Pattern

In classical object-oriented programming languages, a constructor is a special method used to initialize a newly created object once memory has been allocated for it. In JavaScript, as almost everything is an object, we're most often interested in object constructors.

Object constructors are used to create specific types of objects - both preparing the object for use and accepting arguments which a constructor can use to set the values of member properties and methods when the object is first created.

Object Creation

The three common ways to create new objects in JavaScript are as follows:

1// Each of the following options will create a new empty object:
3var newObject = {};
5// or
6var newObject = Object.create( null );
8// or
9var newObject = new Object();

Where the "Object" constructor in the final example creates an object wrapper for a specific value, or where no value is passed, it will create an empty object and return it.

There are then four ways in which keys and values can then be assigned to an object:

01// ECMAScript 3 compatible approaches
03// 1. Dot syntax
05// Set properties
06newObject.someKey = "Hello World";
08// Get properties
09var key = newObject.someKey;
13// 2. Square bracket syntax
15// Set properties
16newObject["someKey"] = "Hello World";
18// Get properties
19var key = newObject["someKey"];
23// ECMAScript 5 only compatible approaches
24// For more information see: http://kangax.github.com/es5-compat-table/
26// 3. Object.defineProperty
28// Set properties
29Object.defineProperty( newObject, "someKey", {
30    value: "for more control of the property's behavior",
31    writable: true,
32    enumerable: true,
33    configurable: true
36// If the above feels a little difficult to read, a short-hand could
37// be written as follows:
39var defineProp = function ( obj, key, value ){
40  config.value = value;
41  Object.defineProperty( obj, key, config );
44// To use, we then create a new empty "person" object
45var person = Object.create( null );
47// Populate the object with properties
48defineProp( person, "car""Delorean" );
49defineProp( person, "dateOfBirth", "1981" );
50defineProp( person, "hasBeard", false );
53// 4. Object.defineProperties
55// Set properties
56Object.defineProperties( newObject, {
58  "someKey": { 
59    value: "Hello World"
60    writable: true 
61  },
63  "anotherKey": { 
64    value: "Foo bar"
65    writable: false 
70// Getting properties for 3. and 4. can be done using any of the
71// options in 1. and 2.

As we will see a little later in the book, these methods can even be used for inheritance, as follows:

01// Usage:
03// Create a race car driver that inherits from the person object
04var driver = Object.create( person );
06// Set some properties for the driver
07defineProp(driver, "topSpeed", "100mph");
09// Get an inherited property (1981)
10console.log( driver.dateOfBirth );
12// Get the property we set (100mph)
13console.log( driver.topSpeed );

Basic Constructors

As we saw earlier, JavaScript didn't support the concept of classes until the incorporation of ECMAScript 6 recommendations but it has supported special constructor functions that work with objects. By simply prefixing a call to a constructor function with the keyword "new", we can tell JavaScript we would like the function to behave like a constructor and instantiate a new object with the members defined by that function.

Inside a constructor, the keyword this references the new object that's being created. Revisiting object creation, a basic constructor may look as follows:

01function Car( model, year, miles ) {
03  this.model = model;
04  this.year = year;
05  this.miles = miles;
07  this.toString = function () {
08    return this.model + " has done " + this.miles + " miles";
09  };
12// Usage:
14// We can create new instances of the car
15var civic = new Car( "Honda Civic", 2009, 20000 );
16var mondeo = new Car( "Ford Mondeo", 2010, 5000 );
18// and then open our browser console to view the
19// output of the toString() method being called on
20// these objects
21console.log( civic.toString() );
22console.log( mondeo.toString() );

The above is a simple version of the constructor pattern but it does suffer from some problems. One is that it makes inheritance difficult and the other is that functions such as toString() are redefined for each of the new objects created using the Car constructor. This isn't very optimal as the function should ideally be shared between all of the instances of the Car type.

Thankfully as there are a number of both ES3 and ES5-compatible alternatives to constructing objects, it's trivial to work around this limitation.


Constructors With Prototypes

Functions in JavaScript have a property called a prototype. When we call a JavaScript constructor to create an object, all the properties of the constructor's prototype are then made available to the new object. In this fashion, multiple Car objects can be created which access the same prototype. We can thus extend the original example as follows:

01function Car( model, year, miles ) {
03  this.model = model;
04  this.year = year;
05  this.miles = miles;
10// Note here that we are using Object.prototype.newMethod rather than
11// Object.prototype so as to avoid redefining the prototype object
12Car.prototype.toString = function () {
13  return this.model + " has done " + this.miles + " miles";
16// Usage:
18var civic = new Car( "Honda Civic", 2009, 20000 );
19var mondeo = new Car( "Ford Mondeo", 2010, 5000 );
21console.log( civic.toString() );
22console.log( mondeo.toString() );

Above, a single instance of toString() will now be shared between all of the Car objects.


# The Module Pattern


Modules are an integral piece of any robust application's architecture and typically help in keeping the units of code for a project both cleanly separated and organized.

In JavaScript, there are several options for implementing modules. These include:

We will be exploring the latter three of these options later on in the book in the section Modern Modular JavaScript Design Patterns.

The Module pattern is based in part on object literals and so it makes sense to refresh our knowledge of them first.

Object Literals

In object literal notation, an object is described as a set of comma-separated name/value pairs enclosed in curly braces ({}). Names inside the object may be either strings or identifiers that are followed by a colon. There should be no comma used after the final name/value pair in the object as this may result in errors.

1var myObjectLiteral = {
3    variableKey: variableValue,
5    functionKey: function () {
6      // ...
7    };

Object literals don't require instantiation using the new operator but shouldn't be used at the start of a statement as the opening { may be interpreted as the beginning of a block. Outside of an object, new members may be added to it using assignment as follows myModule.property = "someValue";

Below we can see a more complete example of a module defined using object literal notation:

01var myModule = {
03  myProperty: "someValue",
05  // object literals can contain properties and methods.
06  // e.g we can define a further object for module configuration:
07  myConfig: {
08    useCaching: true,
09    language: "en"
10  },
12  // a very basic method
13  myMethod: function () {
14    console.log( "Where in the world is Paul Irish today?" );
15  },
17  // output a value based on the current configuration
18  myMethod2: function () {
19    console.log( "Caching is:" + ( this.myConfig.useCaching ) ? "enabled" : "disabled" );
20  },
22  // override the current configuration
23  myMethod3: function( newConfig ) {
25    if ( typeof newConfig === "object" ) {
26      this.myConfig = newConfig;
27      console.log( this.myConfig.language );
28    }
29  }
32// Outputs: Where in the world is Paul Irish today?
35// Outputs: enabled
38// Outputs: fr
40  language: "fr",
41  useCaching: false

Using object literals can assist in encapsulating and organizing your code and Rebecca Murphey has previously written about this topic in depth should you wish to read into object literals further.

That said, if we're opting for this technique, we may be equally as interested in the Module pattern. It still uses object literals but only as the return value from a scoping function.

The Module Pattern

The Module pattern was originally defined as a way to provide both private and public encapsulation for classes in conventional software engineering.

In JavaScript, the Module pattern is used to further emulate the concept of classes in such a way that we're able to include both public/private methods and variables inside a single object, thus shielding particular parts from the global scope. What this results in is a reduction in the likelihood of our function names conflicting with other functions defined in additional scripts on the page.


The Module pattern encapsulates "privacy", state and organization using closures. It provides a way of wrapping a mix of public and private methods and variables, protecting pieces from leaking into the global scope and accidentally colliding with another developer's interface. With this pattern, only a public API is returned, keeping everything else within the closure private.

This gives us a clean solution for shielding logic doing the heavy lifting whilst only exposing an interface we wish other parts of our application to use. The pattern is quite similar to an immediately-invoked functional expression (IIFE - see the section on namespacing patterns for more on this) except that an object is returned rather than a function.

It should be noted that there isn't really an explicitly true sense of "privacy" inside JavaScript because unlike some traditional languages, it doesn't have access modifiers. Variables can't technically be declared as being public nor private and so we use function scope to simulate this concept. Within the Module pattern, variables or methods declared are only available inside the module itself thanks to closure. Variables or methods defined within the returning object however are available to everyone.


From a historical perspective, the Module pattern was originally developed by a number of people including Richard Cornford in 2003. It was later popularized by Douglas Crockford in his lectures. Another piece of trivia is that if you've ever played with Yahoo's YUI library, some of its features may appear quite familiar and the reason for this is that the Module pattern was a strong influence for YUI when creating their components.


Let's begin looking at an implementation of the Module pattern by creating a module which is self-contained.

01var testModule = (function () {
03  var counter = 0;
05  return {
07    incrementCounter: function () {
08      return counter++;
09    },
11    resetCounter: function () {
12      console.log( "counter value prior to reset: " + counter );
13      counter = 0;
14    }
15  };
19// Usage:
21// Increment our counter
24// Check the counter value and reset
25// Outputs: 1

Here, other parts of the code are unable to directly read the value of our incrementCounter() or resetCounter(). The counter variable is actually fully shielded from our global scope so it acts just like a private variable would - its existence is limited to within the module's closure so that the only code able to access its scope are our two functions. Our methods are effectively namespaced so in the test section of our code, we need to prefix any calls with the name of the module (e.g. "testModule").

When working with the Module pattern, we may find it useful to define a simple template that we use for getting started with it. Here's one that covers namespacing, public and private variables:

01var myNamespace = (function () {
03  var myPrivateVar, myPrivateMethod;
05  // A private counter variable
06  myPrivateVar = 0;
08  // A private function which logs any arguments
09  myPrivateMethod = function( foo ) {
10      console.log( foo );
11  };
13  return {
15    // A public variable
16    myPublicVar: "foo",
18    // A public function utilizing privates
19    myPublicFunction: function( bar ) {
21      // Increment our private counter
22      myPrivateVar++;
24      // Call our private method using bar
25      myPrivateMethod( bar );
27    }
28  };

Looking at another example, below we can see a shopping basket implemented using the this pattern. The module itself is completely self-contained in a global variable called basketModule. The basket array in the module is kept private and so other parts of our application are unable to directly read it. It only exists with the module's closure and so the only methods able to access it are those with access to its scope (ie. addItem(), getItem() etc).

01var basketModule = (function () {
03  // privates
05  var basket = [];
07  function doSomethingPrivate() {
08    //...
09  }
11  function doSomethingElsePrivate() {
12    //...
13  }
15  // Return an object exposed to the public
16  return {
18    // Add items to our basket
19    addItem: function( values ) {
20      basket.push(values);
21    },
23    // Get the count of items in the basket
24    getItemCount: function () {
25      return basket.length;
26    },
28    // Public alias to a  private function
29    doSomething: doSomethingPrivate,
31    // Get the total value of items in the basket
32    getTotal: function () {
34      var q = this.getItemCount(),
35          p = 0;
37      while (q--) {
38        p += basket[q].price;
39      }
41      return p;
42    }
43  };

Inside the module, you may have noticed that we return an object. This gets automatically assigned to basketModule so that we can interact with it as follows:

01// basketModule returns an object with a public API we can use
04  item: "bread",
05  price: 0.5
09  item: "butter",
10  price: 0.3
13// Outputs: 2
14console.log( basketModule.getItemCount() );
16// Outputs: 0.8
17console.log( basketModule.getTotal() );
19// However, the following will not work:
21// Outputs: undefined
22// This is because the basket itself is not exposed as a part of our
23// the public API
24console.log( basketModule.basket );
26// This also won't work as it only exists within the scope of our
27// basketModule closure, but not the returned public object
28console.log( basket );

The methods above are effectively namespaced inside basketModule.

Notice how the scoping function in the above basket module is wrapped around all of our functions, which we then call and immediately store the return value of. This has a number of advantages including:


Module Pattern Variations

Import mixins

This variation of the pattern demonstrates how globals (e.g jQuery, Underscore) can be passed in as arguments to our module's anonymous function. This effectively allows us to import them and locally alias them as we wish.

01// Global module
02var myModule = (function ( jQ, _ ) {
04    function privateMethod1(){
05        jQ(".container").html("test");
06    }
08    function privateMethod2(){
09      console.log( _.min([10, 5, 100, 2, 1000]) );
10    }
12    return{
13        publicMethod: function(){
14            privateMethod1();               
15        }           
16    };
18// Pull in jQuery and Underscore
19}( jQuery, _ ));

This next variation allows us to declare globals without consuming them and could similarly support the concept of global imports seen in the last example.

01// Global module
02var myModule = (function () {
04    // Module object
05  var module = {},
06    privateVariable = "Hello World";
08  function privateMethod() {
09    // ...
10  }
12  module.publicProperty = "Foobar";
13  module.publicMethod = function () {
14    console.log( privateVariable );
15  };
17  return module;

Toolkit And Framework-specific Module Pattern Implementations


Dojo provides a convenience method for working with objects called dojo.setObject(). This takes as its first argument a dot-separated string such as myObj.parent.child which refers to a property called "child" within an object "parent" defined inside "myObj". Using setObject() allows us to set the value of children, creating any of the intermediate objects in the rest of the path passed if they don't already exist.

For example, if we wanted to declare basket.core as an object of the store namespace, this could be achieved as follows using the traditional way:

01var store = window.store || {};
03if ( !store["basket"] ) {
04  store.basket = {};
07if ( !store.basket["core"] ) {
08  store.basket.core = {};
11store.basket.core = {
12  // ...rest of our logic

Or as follows using Dojo 1.7 (AMD-compatible version) and above:

01require(["dojo/_base/customStore"], function( store ){
03  // using dojo.setObject()
04  store.setObject( "basket.core", (function() {
06      var basket = [];
08      function privateMethod() {
09          console.log(basket);
10      }
12      return {
13          publicMethod: function(){
14                  privateMethod();
15          }
16      };
18  }()));

For more information on dojo.setObject(), see the official documentation.


For those using Sencha's ExtJS, you're in for some luck as the official documentation incorporates examples that do demonstrate how to correctly use the Module pattern with the framework.

Below we can see an example of how to define a namespace which can then be populated with a module containing both a private and public API. With the exception of some semantic differences, it's quite close to how the Module pattern is implemented in vanilla JavaScript:

01// create namespace
04// create application
05myNameSpace.app = function () {
07  // do NOT access DOM from here; elements don't exist yet
08  // private variables
10  var btn1,
11      privVar1 = 11;
13  // private functions
14  var btn1Handler = function ( button, event ) {
15      console.log( "privVar1=" + privVar1 );
16      console.log( "this.btn1Text=" + this.btn1Text );
17    };
19  // public space
20  return {
21    // public properties, e.g. strings to translate
22    btn1Text: "Button 1",
24    // public methods
25    init: function () {
27      if ( Ext.Ext2 ) {
29        btn1 = new Ext.Button({
30          renderTo: "btn1-ct",
31          text: this.btn1Text,
32          handler: btn1Handler
33        });
35      } else {
37        btn1 = new Ext.Button( "btn1-ct", {
38          text: this.btn1Text,
39          handler: btn1Handler
40        });
42      }
43    }
44  };



Similarly, we can also implement the Module pattern when building applications using YUI3. The following example is heavily based on the original YUI Module pattern implementation by Eric Miraglia, but again, isn't vastly different from the vanilla JavaScript version:

01Y.namespace( "store.basket" ) = (function () {
03    var myPrivateVar, myPrivateMethod;
05    // private variables:
06    myPrivateVar = "I can be accessed only within Y.store.basket.";
08    // private method:
09    myPrivateMethod = function () {
10        Y.log( "I can be accessed only from within YAHOO.store.basket" );
11    }
13    return {
14        myPublicProperty: "I'm a public property.",
16        myPublicMethod: function () {
17            Y.log( "I'm a public method." );
19            // Within basket, I can access "private" vars and methods:
20            Y.log( myPrivateVar );
21            Y.log( myPrivateMethod() );
23            // The native scope of myPublicMethod is store so we can
24            // access public members using "this":
25            Y.log( this.myPublicProperty );
26        }
27    };


There are a number of ways in which jQuery code unspecific to plugins can be wrapped inside the Module pattern. Ben Cherry previously suggested an implementation where a function wrapper is used around module definitions in the event of there being a number of commonalities between modules.

In the following example, a library function is defined which declares a new library and automatically binds up the init function to document.ready when new libraries (ie. modules) are created.

01function library( module ) {
03  $( function() {
04    if ( module.init ) {
05      module.init();
06    }
07  });
09  return module;
12var myLibrary = library(function () {
14  return {
15    init: function () {
16      // module implementation
17    }
18  };


We've seen why the Singleton pattern can be useful, but why is the Module pattern a good choice? For starters, it's a lot cleaner for developers coming from an object-oriented background than the idea of true encapsulation, at least from a JavaScript perspective.

Secondly, it supports private data - so, in the Module pattern, public parts of our code are able to touch the private parts, however the outside world is unable to touch the class's private parts (no laughing! Oh, and thanks to David Engfer for the joke).


The disadvantages of the Module pattern are that as we access both public and private members differently, when we wish to change visibility, we actually have to make changes to each place the member was used.

We also can't access private members in methods that are added to the object at a later point. That said, in many cases the Module pattern is still quite useful and when used correctly, certainly has the potential to improve the structure of our application.

Other disadvantages include the inability to create automated unit tests for private members and additional complexity when bugs require hot fixes. It's simply not possible to patch privates. Instead, one must override all public methods which interact with the buggy privates. Developers can't easily extend privates either, so it's worth remembering privates are not as flexible as they may initially appear.

For further reading on the Module pattern, see Ben Cherry's excellent in-depth article on it.


# The Revealing Module Pattern

Now that we're a little more familiar with the module pattern, let’s take a look at a slightly improved version - Christian Heilmann’s Revealing Module pattern.

The Revealing Module pattern came about as Heilmann was frustrated with the fact that he had to repeat the name of the main object when we wanted to call one public method from another or access public variables.  He also disliked the Module pattern’s requirement for having to switch to object literal notation for the things he wished to make public.

The result of his efforts was an updated pattern where we would simply define all of our functions and variables in the private scope and return an anonymous object with pointers to the private functionality we wished to reveal as public.

An example of how to use the Revealing Module pattern can be found below:

01var myRevealingModule = function () {
03        var privateVar = "Ben Cherry",
04            publicVar  = "Hey there!";
06        function privateFunction() {
07            console.log( "Name:" + privateVar );
08        }
10        function publicSetName( strName ) {
11            privateVar = strName;
12        }
14        function publicGetName() {
15            privateFunction();
16        }
19        // Reveal public pointers to 
20        // private functions and properties
22        return {
23            setName: publicSetName,
24            greeting: publicVar,
25            getName: publicGetName
26        };
28    }();
30myRevealingModule.setName( "Paul Kinlan" );

The pattern can also be used to reveal private functions and properties with a more specific naming scheme if we would prefer:

01var myRevealingModule = function () {
03        var privateCounter = 0;
05        function privateFunction() {
06            privateCounter++;
07        }
09        function publicFunction() {
10            publicIncrement();
11        }
13        function publicIncrement() {
14            privateFunction();
15        }
17        function publicGetCount(){
18          return privateCounter;
19        }
21        // Reveal public pointers to
22        // private functions and properties       
24       return {
25            start: publicFunction,
26            increment: publicIncrement,
27            count: publicGetCount
28        };
30    }();


This pattern allows the syntax of our scripts to be more consistent. It also makes it more clear at the end of the module which of our functions and variables may be accessed publicly which eases readability.


A disadvantage of this pattern is that if a private function refers to a public function, that public function can't be overridden if a patch is necessary. This is because the private function will continue to refer to the private implementation and the pattern doesn't apply to public members, only to functions.

Public object members which refer to private variables are also subject to the no-patch rule notes above.

As a result of this, modules created with the Revealing Module pattern may be more fragile than those created with the original Module pattern, so care should be taken during usage.


# The Singleton Pattern

The Singleton pattern is thus known because it restricts instantiation of a class to a single object. Classically, the Singleton pattern can be implemented by creating a class with a method that creates a new instance of the class if one doesn't exist. In the event of an instance already existing, it simply returns a reference to that object.

Singletons differ from static classes (or objects) as we can delay their initialization, generally because they require some information that may not be available during initialization time. They don't provide a way for code that is unaware of a previous reference to them to easily retrieve them. This is because it is neither the object or "class" that's returned by a Singleton, it's a structure. Think of how closured variables aren't actually closures - the function scope that provides the closure is the closure.

In JavaScript, Singletons serve as a shared resource namespace which isolate implementation code from the global namespace so as to provide a single point of access for functions.

We can implement a Singleton as follows:

01var mySingleton = (function () {
03  // Instance stores a reference to the Singleton
04  var instance;
06  function init() {
08    // Singleton
10    // Private methods and variables
11    function privateMethod(){
12        console.log( "I am private" );
13    }
15    var privateVariable = "Im also private";
17    var privateRandomNumber = Math.random();
19    return {
21      // Public methods and variables
22      publicMethod: function () {
23        console.log( "The public can see me!" );
24      },
26      publicProperty: "I am also public",
28      getRandomNumber: function() {
29        return privateRandomNumber;
30      }
32    };
34  };
36  return {
38    // Get the Singleton instance if one exists
39    // or create one if it doesn't
40    getInstance: function () {
42      if ( !instance ) {
43        instance = init();
44      }
46      return instance;
47    }
49  };
53var myBadSingleton = (function () {
55  // Instance stores a reference to the Singleton
56  var instance;
58  function init() {
60    // Singleton
62    var privateRandomNumber = Math.random();
64    return {
66      getRandomNumber: function() {
67        return privateRandomNumber;
68      }
70    };
72  };
74  return {
76    // Always create a new Singleton instance
77    getInstance: function () {
79      instance = init();
81      return instance;
82    }
84  };
89// Usage:
91var singleA = mySingleton.getInstance();
92var singleB = mySingleton.getInstance();
93console.log( singleA.getRandomNumber() === singleB.getRandomNumber() ); // true
95var badSingleA = myBadSingleton.getInstance();
96var badSingleB = myBadSingleton.getInstance();
97console.log( badSingleA.getRandomNumber() !== badSingleB.getRandomNumber() ); // true

What makes the Singleton is the global access to the instance (generally through MySingleton.getInstance()) as we don't (at least in static languages) call new MySingleton() directly. This is however possible in JavaScript.

In the GoF book, the applicability of the Singleton pattern is described as follows:

The second of these points refers to a case where we might need code such as:

01mySingleton.getInstance = function(){
02  if ( this._instance == null ) {
03    if ( isFoo() ) {
04       this._instance = new FooSingleton();
05    } else {
06       this._instance = new BasicSingleton();
07    }
08  }
09  return this._instance;


Here, getInstance becomes a little like a Factory method and we don't need to update each point in our code accessing it. FooSingleton above would be a subclass of BasicSingleton and implement the same interface.

Why is deferring execution considered important for a Singleton?:

In C++ it serves to isolate from the unpredictability of the order of dynamic initialization, returning control to the programmer.

It is important to note the difference between a static instance of a class (object) and a Singleton: whilst a Singleton can be implemented as a static instance, it can also be constructed lazily, without the need for resources nor memory until this is actually needed.

If we have a static object that can be initialized directly, we need to ensure the code is always executed in the same order (e.g in case objCar needs objWheel during its initialization) and this doesn't scale when you have a large number of source files.

Both Singletons and static objects are useful but they shouldn't be overused - the same way in which we shouldn't overuse other patterns.

In practice, the Singleton pattern is useful when exactly one object is needed to coordinate others across a system. Here is one example with the pattern being used in this context:

01var SingletonTester = (function () {
03  // options: an object containing configuration options for the singleton
04  // e.g var options = { name: "test", pointX: 5}; 
05  function Singleton( options )  {
07    // set options to the options supplied
08    // or an empty object if none are provided
09    options = options || {};
11    // set some properties for our singleton
12    this.name = "SingletonTester";
14    this.pointX = options.pointX || 6;
16    this.pointY = options.pointY || 10; 
18  }
20  // our instance holder 
21  var instance;
23  // an emulation of static variables and methods
24  var _static  = {  
26    name:  "SingletonTester",
28    // Method for getting an instance. It returns
29    // a singleton instance of a singleton object
30    getInstance:  function( options ) {   
31      if( instance  ===  undefined )  {    
32        instance = new Singleton( options );   
33      }   
35      return  instance; 
38  }; 
40  return  _static;
44var singletonTest  =  SingletonTester.getInstance({
45  pointX:  5
48// Log the output of pointX just to verify it is correct
49// Outputs: 5
50console.log( singletonTest.pointX ); 

Whilst the Singleton has valid uses, often when we find ourselves needing it in JavaScript it's a sign that we may need to re-evaluate our design.

They're often an indication that modules in a system are either tightly coupled or that logic is overly spread across multiple parts of a codebase. Singletons can be more difficult to test due to issues ranging from hidden dependencies, the difficulty in creating multiple instances, difficulty in stubbing dependencies and so on.

Miller Medeiros has previously recommended this excellent article on the Singleton and its various issues for further reading as well as the comments to this article, discussing how Singletons can increase tight coupling. I'm happy to second these recommendations as both pieces raise many important points about this pattern that are also worth noting.


# The Observer Pattern

The Observer is a design pattern where an an object (known as a subject) maintains a list of objects depending on it (observers), automatically notifying them of any changes to state.

When a subject needs to notify observers about something interesting happening, it broadcasts a notification to the observers (which can include specific data related to the topic of the notification).

When we no longer wish for a particular observer to be notified of changes by the subject they are registered with, the subject can remove them from the list of observers.

It's often useful to refer back to published definitions of design patterns that are language agnostic to get a broader sense of their usage and advantages over time. The definition of the Observer pattern provided in the GoF book, Design Patterns: Elements of Reusable Object-Oriented Software, is:

"One or more observers are interested in the state of a subject and register their interest with the subject by attaching themselves. When something changes in our subject that the observer may be interested in, a notify message is sent which calls the update method in each observer. When the observer is no longer interested in the subject's state, they can simply detach themselves."

We can now expand on what we've learned to implement the Observer pattern with the following components:

First, let's model the list of dependent Observers a subject may have:

01function ObserverList(){
02  this.observerList = [];
05ObserverList.prototype.Add = function( obj ){
06  return this.observerList.push( obj );
09ObserverList.prototype.Empty = function(){
10  this.observerList = [];
13ObserverList.prototype.Count = function(){
14  return this.observerList.length;
18ObserverList.prototype.Get = function( index ){
19  if( index > -1 && index < this.observerList.length ){
20    return this.observerList[ index ];
21  }
24ObserverList.prototype.Insert = function( obj, index ){
25  var pointer = -1;
27  if( index === 0 ){
28    this.observerList.unshift( obj );
29    pointer = index;
30  }else if( index === this.observerList.length ){
31    this.observerList.push( obj );
32    pointer = index;
33  }
35  return pointer;
38ObserverList.prototype.IndexOf = function( obj, startIndex ){
39  var i = startIndex, pointer = -1;
41  while( i < this.observerList.length ){
42    if( this.observerList[i] === obj ){
43      pointer = i;
44    }
45    i++;
46  }
48  return pointer;
52ObserverList.prototype.RemoveAt = function( index ){
53  if( index === 0 ){
54    this.observerList.shift();
55  }else if( index === this.observerList.length -1 ){
56    this.observerList.pop();
57  }
61// Extend an object with an extension
62function extend( extension, obj ){
63  for ( var key in extension ){
64    obj[key] = extension[key];
65  }

Next, let's model the Subject and the ability to add, remove or notify observers on the observer list.

01function Subject(){
02  this.observers = new ObserverList();
05Subject.prototype.AddObserver = function( observer ){
06  this.observers.Add( observer );
09Subject.prototype.RemoveObserver = function( observer ){
10  this.observers.RemoveAt( this.observers.IndexOf( observer, 0 ) );
13Subject.prototype.Notify = function( context ){
14  var observerCount = this.observers.Count();
15  for(var i=0; i < observerCount; i++){
16    this.observers.Get(i).Update( context );
17  }

We then define a skeleton for creating new Observers. The Update functionality here will be overwritten later with custom behaviour.

1// The Observer
2function Observer(){
3  this.Update = function(){
4    // ...
5  };

In our sample application using the above Observer components, we now define:

We then define ConcreteSubject and ConcreteObserver handlers for both adding new observers to the page and implementing the updating interface. See below for inline comments on what these components do in the context of our example.


1<button id="addNewObserver">Add New Observer checkbox</button>
2<input id="mainCheckbox" type="checkbox"/>
3<div id="observersContainer"></div>

Sample script:

01// References to our DOM elements
03var controlCheckbox = document.getElementById( "mainCheckbox" ),
04  addBtn = document.getElementById( "addNewObserver" ),
05  container = document.getElementById( "observersContainer" );
08// Concrete Subject
10// Extend the controlling checkbox with the Subject class
11extend( new Subject(), controlCheckbox );
13// Clicking the checkbox will trigger notifications to its observers
14controlCheckbox["onclick"] = new Function( "controlCheckbox.Notify(controlCheckbox.checked)" );
17addBtn["onclick"] = AddNewObserver;
19// Concrete Observer
21function AddNewObserver(){
23  // Create a new checkbox to be added
24  var check  = document.createElement( "input" );
25  check.type = "checkbox";
27  // Extend the checkbox with the Observer class
28  extend( new Observer(), check );
30  // Override with custom update behaviour
31  check.Update = function( value ){
32    this.checked = value;
33  };
35  // Add the new observer to our list of observers
36  // for our main subject
37  controlCheckbox.AddObserver( check );
39  // Append the item to the container
40  container.appendChild( check );

In this example, we looked at how to implement and utilize the Observer pattern, covering the concepts of a Subject, Observer, ConcreteSubject and ConcreteObserver.

Differences Between The Observer And Publish/Subscribe Pattern

Whilst the Observer pattern is useful to be aware of, quite often in the JavaScript world, we'll find it commonly implemented using a variation known as the Publish/Subscribe pattern. Whilst very similar, there are differences between these patterns worth noting.

The Observer pattern requires that the observer (or object) wishing to receive topic notifications must subscribe this interest to the object firing the event (the subject).

The Publish/Subscribe pattern however uses a topic/event channel which sits between the objects wishing to receive notifications (subscribers) and the object firing the event (the publisher). This event system allows code to define application specific events which can pass custom arguments containing values needed by the subscriber. The idea here is to avoid dependencies between the subscriber and publisher.

This differs from the Observer pattern as it allows any subscriber implementing an appropriate event handler to register for and receive topic notifications broadcast by the publisher.

Here is an example of how one might use the Publish/Subscribe if provided with a functional implementation powering publish(),subscribe() and unsubscribe() behind the scenes:

01// A very simple new mail handler
03// A count of the number of messages received
04var mailCounter = 0;
06// Initialize subscribers that will listen out for a topic
07// with the name "inbox/newMessage".
09// Render a preview of new messages
10var subscriber1 = subscribe( "inbox/newMessage", function( topic, data ) {
12  // Log the topic for debugging purposes
13  console.log( "A new message was received: ", topic );
15  // Use the data that was passed from our subject
16  // to display a message preview to the user
17  $( ".messageSender" ).html( data.sender );
18  $( ".messagePreview" ).html( data.body );
22// Here's another subscriber using the same data to perform
23// a different task.
25// Update the counter displaying the number of new
26// messages received via the publisher
28var subscriber2 = subscribe( "inbox/newMessage", function( topic, data ) {
30  $('.newMessageCounter').html( mailCounter++ );
34publish( "inbox/newMessage", [{
35  sender:"hello@google.com",
36  body: "Hey there! How are you doing today?"
39// We could then at a later point unsubscribe our subscribers
40// from receiving any new topic notifications as follows:
41// unsubscribe( subscriber1,  );
42// unsubscribe( subscriber2 );

The general idea here is the promotion of loose coupling. Rather than single objects calling on the methods of other objects directly, they instead subscribe to a specific task or activity of another object and are notified when it occurs.


The Observer and Publish/Subscribe patterns encourage us to think hard about the relationships between different parts of our application. They also help us identify what layers containing direct relationships which could instead be replaced with sets of subjects and observers. This effectively could be used to break down an application into smaller, more loosely coupled blocks to improve code management and potentials for re-use.

Further motivation behind using the Observer pattern is where we need to maintain consistency between related objects without making classes tightly coupled. For example, when an object needs to be able to notify other objects without making assumptions regarding those objects.

Dynamic relationships can exist between observers and subjects when using either pattern. This provides a great deal of flexibility which may not be as easy to implement when disparate parts of our application are tightly coupled.

Whilst it may not always be the best solution to every problem, these patterns remain one of the best tools for designing decoupled systems and should be considered an important tool in any JavaScript developer's utility belt.


Consequently, some of the issues with these patterns actually stem from their main benefits. In Publish/Subscribe, by decoupling publishers from subscribers, it can sometimes become difficult to obtain guarantees that particular parts of our applications are functioning as we may expect.

For example, publishers may make an assumption that one or more subscribers are listening to them. Say that we're using such an assumption to log or output errors regarding some application process. If the subscriber performing the logging crashes (or for some reason fails to function), the publisher won't have a way of seeing this due to the decoupled nature of the system.

Another draw-back of the pattern is that subscribers are quite ignorant to the existence of each other and are blind to the cost of switching publishers. Due to the dynamic relationship between subscribers and publishers, the update dependency can be difficult to track.

Publish/Subscribe Implementations

Publish/Subscribe fits in very well in JavaScript ecosystems, largely because at the core, ECMAScript implementations are event driven. This is particularly true in browser environments as the DOM uses events as its main interaction API for scripting.

That said, neither ECMAScript nor DOM provide core objects or methods for creating custom events systems in implementation code (with the exception of perhaps the DOM3 CustomEvent, which is bound to the DOM and is thus not generically useful).

Luckily, popular JavaScript libraries such as dojo, jQuery (custom events) and YUI already have utilities that can assist in easily implementing a Publish/Subscribe system with very little effort. Below we can see some examples of this:

01// Publish
03// jQuery: $(obj).trigger("channel", [arg1, arg2, arg3]);
04$( el ).trigger( "/login", [{username:"test", userData:"test"}] );
06// Dojo: dojo.publish("channel", [arg1, arg2, arg3] );
07dojo.publish( "/login", [{username:"test", userData:"test"}] );
09// YUI: el.publish("channel", [arg1, arg2, arg3]);
10el.publish( "/login", {username:"test", userData:"test"} );
13// Subscribe
15// jQuery: $(obj).on( "channel", [data], fn );
16$( el ).on( "/login", function( event ){...} );
18// Dojo: dojo.subscribe( "channel", fn);
19var handle = dojo.subscribe( "/login", function(data){..} );
21// YUI: el.on("channel", handler);
22el.on( "/login", function( data ){...} );
25// Unsubscribe
27// jQuery: $(obj).off( "channel" );
28$( el ).off( "/login" );
30// Dojo: dojo.unsubscribe( handle );
31dojo.unsubscribe( handle );
33// YUI: el.detach("channel");
34el.detach( "/login" );

For those wishing to use the Publish/Subscribe pattern with vanilla JavaScript (or another library) AmplifyJS includes a clean, library-agnostic implementation that can be used with any library or toolkit. Radio.js (http://radio.uxder.com/), PubSubJS (https://github.com/mroderick/PubSubJS) or Pure JS PubSub by Peter Higgins (https://github.com/phiggins42/bloody-jquery-plugins/blob/55e41df9bf08f42378bb08b93efcb28555b61aeb/pubsub.js) are also similar alternatives worth checking out.

jQuery developers in particular have quite a few other options and can opt to use one of the many well-developed implementations ranging from Peter Higgins's jQuery plugin to Ben Alman's (optimized) Pub/Sub jQuery gist on GitHub. Links to just a few of these can be found below.

So that we are able to get an appreciation for how many of the vanilla JavaScript implementations of the Observer pattern might work, let's take a walk through of a minimalist version of Publish/Subscribe I released on GitHub under a project called pubsubz. This demonstrates the core concepts of subscribe, publish as well as the concept of unsubscribing.

I've opted to base our examples on this code as it sticks closely to both the method signatures and approach of implementation I would expect to see in a JavaScript version of the classic Observer pattern.

A Publish/Subscribe Implementation

01var pubsub = {};
03(function(q) {
05    var topics = {},
06        subUid = -1;
08    // Publish or broadcast events of interest
09    // with a specific topic name and arguments
10    // such as the data to pass along
11    q.publish = function( topic, args ) {
13        if ( !topics[topic] ) {
14            return false;
15        }
17        var subscribers = topics[topic],
18            len = subscribers ? subscribers.length : 0;
20        while (len--) {
21            subscribers[len].func( topic, args );
22        }
24        return this;
25    };
27    // Subscribe to events of interest
28    // with a specific topic name and a
29    // callback function, to be executed
30    // when the topic/event is observed
31    q.subscribe = function( topic, func ) {
33        if (!topics[topic]) {
34            topics[topic] = [];
35        }
37        var token = ( ++subUid ).toString();
38        topics[topic].push({
39            token: token,
40            func: func
41        });
42        return token;
43    };
45    // Unsubscribe from a specific
46    // topic, based on a tokenized reference
47    // to the subscription
48    q.unsubscribe = function( token ) {
49        for ( var m in topics ) {
50            if ( topics[m] ) {
51                for ( var i = 0, j = topics[m].length; i < j; i++ ) {
52                    if ( topics[m][i].token === token) {
53                        topics[m].splice( i, 1 );
54                        return token;
55                    }
56                }
57            }
58        }
59        return this;
60    };
61}( pubsub ));

Example: Using Our Implementation

We can now use the implementation to publish and subscribe to events of interest as follows:

01// Another simple message handler
03// A simple message logger that logs any topics and data received through our
04// subscriber
05var messageLogger = function ( topics, data ) {
06    console.log( "Logging: " + topics + ": " + data );
09// Subscribers listen for topics they have subscribed to and
10// invoke a callback function (e.g messageLogger) once a new
11// notification is broadcast on that topic
12var subscription = pubsub.subscribe( "inbox/newMessage", messageLogger );
14// Publishers are in charge of publishing topics or notifications of
15// interest to the application. e.g:
17pubsub.publish( "inbox/newMessage", "hello world!" );
19// or
20pubsub.publish( "inbox/newMessage", ["test", "a", "b", "c"] );
22// or
23pubsub.publish( "inbox/newMessage", {
24  sender: "hello@google.com",
25  body: "Hey again!"
28// We cab also unsubscribe if we no longer wish for our subscribers
29// to be notified
30// pubsub.unsubscribe( subscription );
32// Once unsubscribed, this for example won't result in our
33// messageLogger being executed as the subscriber is
34// no longer listening
35pubsub.publish( "inbox/newMessage", "Hello! are you still there?" );

Example: User-Interface Notifications

Next, let's imagine we have a web application responsible for displaying real-time stock information.

The application might have a grid for displaying the stock stats and a counter for displaying the last point of update. When the data model changes, the application will need to update the grid and counter. In this scenario, our subject (which will be publishing topics/notifications) is the data model and our subscribers are the grid and counter.

When our subscribers receive notification that the model itself has changed, they can update themselves accordingly.

In our implementation, our subscriber will listen to the topic "newDataAvailable" to find out if new stock information is available. If a new notification is published to this topic, it will trigger gridUpdate to add a new row to our grid containing this information. It will also update a last updated counter to log the last time data was added

01// Return the current local time to be used in our UI later
02getCurrentTime = function (){
04   var date = new Date(),
05         m = date.getMonth() + 1,
06         d = date.getDate(),
07         y = date.getFullYear(),
08         t = date.toLocaleTimeString().toLowerCase();
10        return (m + "/" + d + "/" + y + " " + t);
13// Add a new row of data to our fictional grid component
14function addGridRow( data ) {
16   // ui.grid.addRow( data );
17   console.log( "updated grid component with:" + data );
21// Update our fictional grid to show the time it was last
22// updated
23function updateCounter( data ) {
25   // ui.grid.updateLastChanged( getCurrentTime() );  
26   console.log( "data last updated at: " + getCurrentTime() + " with " + data);
30// Update the grid using the data passed to our subscribers
31gridUpdate = function( topic, data ){
33  if ( data !== "undefined" ) {
34     addGridRow( data );
35     updateCounter( data );
36   }
40// Create a subscription to the newDataAvailable topic
41var subscriber = pubsub.subscribe( "newDataAvailable", gridUpdate );
43// The following represents updates to our data layer. This could be
44// powered by ajax requests which broadcast that new data is available
45// to the rest of the application.
47// Publish changes to the gridUpdated topic representing new entries
48pubsub.publish( "newDataAvailable", {
49  summary: "Apple made $5 billion",
50  identifier: "APPL",
51  stockPrice: 570.91
54pubsub.publish( "newDataAvailable", {
55  summary: "Microsoft made $20 million",
56  identifier: "MSFT",
57  stockPrice: 30.85

Example: Decoupling applications using Ben Alman's Pub/Sub implementation

In the following movie ratings example, we'll be using Ben Alman's jQuery implementation of Publish/Subscribe to demonstrate how we can decouple a user interface. Notice how submitting a rating only has the effect of publishing the fact that new user and rating data is available.

It's left up to the subscribers to those topics to then delegate what happens with that data. In our case we're pushing that new data into existing arrays and then rendering them using the Underscore library's .template() method for templating.

01<script id="userTemplate" type="text/html">
02   <li><%= name %></li>
06<script id="ratingsTemplate" type="text/html">
07   <li><strong><%= title %></strong> was rated <%= rating %>/5</li>
11<div id="container">
13   <div class="sampleForm">
14       <p>
15           <label for="twitter_handle">Twitter handle:</label>
16           <input type="text" id="twitter_handle" />
17       </p>
18       <p>
19           <label for="movie_seen">Name a movie you've seen this year:</label>
20           <input type="text" id="movie_seen" />
21       </p>
22       <p>
24           <label for="movie_rating">Rate the movie you saw:</label>
25           <select id="movie_rating">
26                 <option value="1">1</option>
27                  <option value="2">2</option>
28                  <option value="3">3</option>
29                  <option value="4">4</option>
30                  <option value="5" selected>5</option>
32          </select>
33        </p>
34        <p>
36            <button id="add">Submit rating</button>
37        </p>
38    </div>
42    <div class="summaryTable">
43        <div id="users"><h3>Recent users</h3></div>
44        <div id="ratings"><h3>Recent movies rated</h3></div>
45    </div>
47 </div>
01;(function( $ ) {
03  // Pre-compile templates and "cache" them using closure
04  var
05    userTemplate = _.template($( "#userTemplate" ).html()),
06    ratingsTemplate = _.template($( "#ratingsTemplate" ).html());
08  // Subscribe to the new user topic, which adds a user
09  // to a list of users who have submitted reviews
10  $.subscribe( "/new/user", function( e, data ){
12    if( data ){
14      $('#users').append( userTemplate( data ));
16    }
18  });
20  // Subscribe to the new rating topic. This is composed of a title and
21  // rating. New ratings are appended to a running list of added user
22  // ratings.
23  $.subscribe( "/new/rating", function( e, data ){
25    var compiledTemplate;
27    if( data ){
29      $( "#ratings" ).append( ratingsTemplate( data );
31    }
33  });
35  // Handler for adding a new user
36  $("#add").on("click", function( e ) {
38    e.preventDefault();
40    var strUser = $("#twitter_handle").val(),
41       strMovie = $("#movie_seen").val(),
42       strRating = $("#movie_rating").val();
44    // Inform the application a new user is available
45    $.publish( "/new/user",  { name: strUser } );
47    // Inform the app a new rating is available
48    $.publish( "/new/rating",  { title: strMovie, rating: strRating} );
50    });
52})( jQuery );

Example: Decoupling an Ajax-based jQuery application

In our final example, we're going to take a practical look at how decoupling our code using Pub/Sub early on in the development process can save us some potentially painful refactoring later on.

Quite often in Ajax-heavy applications, once we've received a response to a request we want to achieve more than just one unique action. One could simply add all of their post-request logic into a success callback, but there are drawbacks to this approach.

Highly coupled applications sometimes increase the effort required to reuse functionality due to the increased inter-function/code dependency. What this means is that although keeping our post-request logic hardcoded in a callback might be fine if we're just trying to grab a result set once, it's not as appropriate when we want to make further Ajax-calls to the same data source (and different end-behavior) without rewriting parts of the code multiple times. Rather than having to go back through each layer that calls the same data-source and generalizing them later on, we can use pub/sub from the start and save time.

Using Observers, we can also easily separate application-wide notifications regarding different events down to whatever level of granularity we're comfortable with - something which can be less elegantly done using other patterns.

Notice how in our sample below, one topic notification is made when a user indicates they want to make a search query and another is made when the request returns and actual data is available for consumption. It's left up to the subscribers to then decide how to use knowledge of these events (or the data returned). The benefits of this are that, if we wanted, we could have 10 different subscribers utilizing the data returned in different ways but as far as the Ajax-layer is concerned, it doesn't care. Its sole duty is to request and return data then pass it on to whoever wants to use it. This separation of concerns can make the overall design of our code a little cleaner.

01<form id="flickrSearch">
03   <input type="text" name="tag" id="query"/>
05   <input type="submit" name="submit" value="submit"/>
11<div id="lastQuery"></div>
13<div id="searchResults"></div>
17<script id="resultTemplate" type="text/html">
18    <% _.each(items, function( item ){  %>
19            <li><p><img src="<%= item.media.m %>"/></p></li>
20    <% });%>
01;(function( $ ) {
03   // Pre-compile template and "cache" it using closure
04   var resultTemplate = _.template($( "#resultTemplate" ).html());
06   // Subscribe to the new search tags topic
07   $.subscribe( "/search/tags" , function( tags ) {
08       $( "#searchResults" )
09                .html("<p>Searched for:<strong>" + tags + "</strong></p>");
10   });
12   // Subscribe to the new results topic
13   $.subscribe( "/search/resultSet" , function( results ){
15       $( "#searchResults" ).append(resultTemplate( results ));
17   });
19   // Submit a search query and publish tags on the /search/tags topic
20   $( "#flickrSearch" ).submit( function( e ) {
22       e.preventDefault();
23       var tags = $(this).find( "#query").val();
25       if ( !tags ){
26        return;
27       }
29       $.publish( "/search/tags" , [ $.trim(tags) ]);
31   });
34   // Subscribe to new tags being published and perform
35   // a search query using them. Once data has returned
36   // publish this data for the rest of the application
37   // to consume
39   $.subscribe("/search/tags", function( tags ) {
42              tags: tags,
43              tagmode: "any",
44              format: "json"
45            },
47          function( data ){
49              if( !data.items.length ) {
50                return;
51              }
53              $.publish( "/search/resultSet" , data.items  );
54       });
56   });

The Observer pattern is useful for decoupling a number of different scenarios in application design and if you haven't been using it, I recommend picking up one of the pre-written implementations mentioned today and just giving it a try out. It's one of the easier design patterns to get started with but also one of the most powerful.


# The Mediator Pattern

The dictionary refers to a mediator as a neutral party that assists in negotiations and conflict resolution. In our world, a mediator is a behavioral design pattern that allows us to expose a unified interface through which the different parts of a system may communicate.

If it appears a system has too many direct relationships between components, it may be time to have a central point of control that components communicate through instead. The Mediator promotes loose coupling by ensuring that instead of components referring to each other explicitly, their interaction is handled through this central point. This can help us decouple systems and improve the potential for component reusability.

A real-world analogy could be a typical airport traffic control system. A tower (Mediator) handles what planes can take off and land because all communications (notifications being listened out for or broadcast) are done from the planes to the control tower, rather than from plane-to-plane. A centralized controller is key to the success of this system and that's really the role a Mediator plays in software design.

In implementation terms, the Mediator pattern is essentially a shared subject in the Observer pattern. This might assume that a direction Publish/Subscribe relationship between objects or modules in such systems is sacrificed in order to maintain a central point of contact.

It may also be considered supplemental - perhaps used for application-level notifications such as a communication between different subsystems that are themselves complex and may desire internal component decoupling through Publish/Subscribe relationships.

Another analogy would be DOM event bubbling and event delegation. If all subscriptions in a system are made against the document rather than individual nodes, the document effectively serves as a Mediator. Instead of binding to the events of the individual nodes, a higher level object is given the responsibility of notifying subscribers about interaction events.

Basic Implementation

A simple implementation of the Mediator pattern can be found below, exposing both publish() and subscribe() methods for use:

01var mediator = (function(){
03    // Storage for topics that can be broadcast or listened to
04    var topics = {};
06    // Subscribe to a topic, supply a callback to be executed
07    // when that topic is broadcast to
08    var subscribe = function( topic, fn ){
10        if ( !topics[topic] ){
11          topics[topic] = [];
12        }
14        topics[topic].push( { context: this, callback: fn } );
16        return this;
17    };
19    // Publish/broadcast an event to the rest of the application
20    var publish = function( topic ){
22        var args;
24        if ( !topics[topic] ){
25          return false;
26        }
28        args = Array.prototype.slice.call( arguments, 1 );
29        for ( var i = 0, l = topics[topic].length; i < l; i++ ) {
31            var subscription = topics[topic][i];
32            subscription.callback.apply( subscription.context, args );
33        }
34        return this;
35    };
37    return {
38        publish: publish,
39        subscribe: subscribe,
40        installTo: function( obj ){
41            obj.subscribe = subscribe;
42            obj.publish = publish;
43        }
44    };

Advanced Implementation

For those interested in a more advanced implementation, read on for a walk-through of my trimmed down version of Jack Lawson's excellent Mediator.js. Amongst other improvements, this version supports topic namespaces, subscriber removal and a much more robust Publish/Subscribe system for our Mediator. If however, you wish to skip this walk-through, you can go directly to the next example to continue reading.

Thanks to Jack for his excellent code comments which assisted with this section.

To start, let's implement the notion of Subscriber, which we can consider an instance of a Mediators topic registration.

By generating object instances, we can easily update Subscribers later without the need to unregister and re-register them. Subscribers can be written as constructors that take a function fn to be called, an options object and a context.

01// Pass in a context to attach our Mediator to.
02// By default this will be the window object
03(function( root ){
05  function guidGenerator() { /*..*/}
07  // Our Subscriber constructor
08  function Subscriber( fn, options, context ){
10    if ( !(this instanceof Subscriber) ) {
12      return new Subscriber( fn, context, options );
14    }else{
16      // guidGenerator() is a function that generates
17      // GUIDs for instances of our Mediators Subscribers so
18      // we can easily reference them later on. We're going
19      // to skip its implementation for brevity
21      this.id = guidGenerator();
22      this.fn = fn;
23      this.options = options;
24      this.context = context;
25      this.topic = null;
27    }
28  }

Topics in our Mediator hold a list of callbacks and sub-topics that are fired when Mediator.Publish is called on our Mediator instance. It also contains methods for manipulating lists of data.

01// Let's model the Topic.
02// JavaScript lets us use a Function object as a
03// conjunction of a prototype for use with the new
04// object and a constructor function to be invoked.
05function Topic( namespace ){
07  if ( !(this instanceof Topic) ) {
08    return new Topic( namespace );
09  }else{
11    this.namespace = namespace || "";
12    this._callbacks = [];
13    this._topics = [];
14    this.stopped = false;
16  }
20// Define the prototype for our topic, including ways to
21// add new subscribers or retrieve existing ones.
22Topic.prototype = {
24  // Add a new subscriber
25  AddSubscriber: function( fn, options, context ){
27    var callback = new Subscriber( fn, options, context );
29    this._callbacks.push( callback );
31    callback.topic = this;
33    return callback;
34  },

Our topic instance is passed along as an argument to the Mediator callback. Further callback propagation can then be called using a handy utility method called StopPropagation():

1StopPropagation: function(){
2  this.stopped = true;

We can also make it easy to retrieve existing Subscribers when given a GUID identifier:

01GetSubscriber: function( identifier ){
03  for(var x = 0, y = this._callbacks.length; x < y; x++ ){
04    if( this._callbacks[x].id == identifier || this._callbacks[x].fn == identifier ){
05      return this._callbacks[x];
06    }
07  }
09  for( var z in this._topics ){
10    if( this._topics.hasOwnProperty( z ) ){
11      var sub = this._topics[z].GetSubscriber( identifier );
12      if( sub !== undefined ){
13        return sub;
14      }
15    }
16  }

Next, in case we need them, we can offer easy ways to add new topics, check for existing topics or retrieve topics as well:

01AddTopic: function( topic ){
02  this._topics[topic] = new Topic( (this.namespace ? this.namespace + ":" : "") + topic );
05HasTopic: function( topic ){
06  return this._topics.hasOwnProperty( topic );
09ReturnTopic: function( topic ){
10  return this._topics[topic];

We can also explicitly remove Subscribers if we feel they are no longer necessary. The following will recursively remove a Subscriber through its sub-topics:

01RemoveSubscriber: function( identifier ){
03  if( !identifier ){
04    this._callbacks = [];
06    for( var z in this._topics ){
07      if( this._topics.hasOwnProperty(z) ){
08        this._topics[z].RemoveSubscriber( identifier );
09      }
10    }
11  }
13  for( var y = 0, x = this._callbacks.length; y < x; y++ ) {
14    if( this._callbacks[y].fn == identifier || this._callbacks[y].id == identifier ){
15      this._callbacks[y].topic = null;
16      this._callbacks.splice( y,1 );
17      x--; y--;
18    }
19  }

Next we include the ability to Publish arbitrary arguments to Subscribers recursively through sub-topics.

01  Publish: function( data ){
03    for( var y = 0, x = this._callbacks.length; y < x; y++ ) {
05        var callback = this._callbacks[y], l;
06          callback.fn.apply( callback.context, data );
08      l = this._callbacks.length;
10      if( l < x ){
11        y--;
12        x = l;
13      }
14    }
16    for( var x in this._topics ){
17      if( !this.stopped ){
18        if( this._topics.hasOwnProperty( x ) ){
19          this._topics[x].Publish( data );
20        }
21      }
22    }
24    this.stopped = false;
25  }

Here we expose the Mediator instance we will primarily be interacting with. It is through here that events are registered and removed from topics.

1function Mediator() {
3  if ( !(this instanceof Mediator) ) {
4    return new Mediator();
5  }else{
6    this._topics = new Topic( "" );
7  }

For more advanced use-cases, we can get our Mediator supporting namespaces for topics such as inbox:messages:new:read.GetTopic below returns topic instances based on a namespace.

01Mediator.prototype = {
03  GetTopic: function( namespace ){
04    var topic = this._topics,
05        namespaceHierarchy = namespace.split( ":" );
07    if( namespace === "" ){
08      return topic;
09    }
11    if( namespaceHierarchy.length > 0 ){
12      for( var i = 0, j = namespaceHierarchy.length; i < j; i++ ){
14        if( !topic.HasTopic( namespaceHierarchy[i]) ){
15          topic.AddTopic( namespaceHierarchy[i] );
16        }
18        topic = topic.ReturnTopic( namespaceHierarchy[i] );
19      }
20    }
22    return topic;
23  },

In this section we define a Mediator.Subscribe method, which accepts a topic namespace, a function fn to be executed, options and once again a context to call the function in to Subscribe. This creates a topic if one doesn't exist.

1Subscribe: function( topiclName, fn, options, context ){
2  var options = options || {},
3      context = context || {},
4      topic = this.GetTopic( topicName ),
5      sub = topic.AddSubscriber( fn, options, context );
7  return sub;

Following on from this, we can also define further utilities for accessing specific subscribers or removing them from topics recursively.

01// Returns a subscriber for a given subscriber id / named function and topic namespace
03GetSubscriber: function( identifier, topic ){
04  return this.GetTopic( topic || "" ).GetSubscriber( identifier );
07// Remove a subscriber from a given topic namespace recursively based on
08// a provided subscriber id or named function.
10Remove: function( topicName, identifier ){
11  this.GetTopic( topicName ).RemoveSubscriber( identifier );

Our primary Publish method allows us to arbitrarily publish data to a chosen topic namespace and can be seen below.

Topics are called recursively downwards. For example, a post to inbox:messages will post to inbox:messages:new and inbox:messages:new:read. It is used as follows: Mediator.Publish( "inbox:messages:new", [args] );

1  Publish: function( topicName ){
2    var args = Array.prototype.slice.call( arguments, 1),
3        topic = this.GetTopic( topicName );
5    args.push( topic );
7    this.GetTopic( topicName ).Publish( args );
8  }

Finally we can easily expose our Mediator for attachment to the object passed in to root:

1  root.Mediator = Mediator;
2  Mediator.Topic = Topic;
3  Mediator.Subscriber = Subscriber;
5// Remember we can pass anything in here. I've passed in <code>window</code> to
6// attach the Mediator to, but we can just as easily attach it to another
7// object if desired.
8})( window );


Using either of the implementations from above (both the simple option and the more advanced one), we can then put together a simple chat logging system as follows:


02<form id="chatForm">
03    <label for="fromBox">Your Name:</label>
04    <input id="fromBox" type="text"/>
05    <br />
06    <label for="toBox">Send to:</label>
07    <input id="toBox" type="text"/>
08    <br />
09    <label for="chatBox">Message:</label>
10    <input id="chatBox" type="text"/>
11    <button type="submit">Chat</button>
14<div id="chatResult"></div>


01$( "#chatForm" ).on( "submit", function(e) {
02    e.preventDefault();
04    // Collect the details of the chat from our UI
05    var text = $( "#chatBox" ).val(),
06        from = $( "#fromBox" ).val(),
07        to = $( "#toBox" ).val();
09    // Publish data from the chat to the newMessage topic
10    mediator.publish( "newMessage" , { message: text, from: from, to: to } );
13// Append new messages as they come through
14function displayChat( data ) {
15    var date = new Date(),
16        msg = data.from + " said \"" + data.message + "\" to " + data.to;
18    $( "#chatResult" )
19        .prepend("<p>" + msg + " (" + date.toLocaleTimeString() + ")</p>");
22// Log messages
23function logChat( data ) {
24    if ( window.console ) {
25        console.log( data );
26    }
31// Subscribe to new chat messages being submitted
32// via the mediator
33mediator.subscribe( "newMessage", displayChat );
34mediator.subscribe( "newMessage", logChat );
37// The following will however only work with the more advanced implementation:
39function amITalkingToMyself( data ) {
40    return data.from === data.to;
43function iAmClearlyCrazy( data ) {
44    $( "#chatResult" ).prepend("<p>" + data.from + " is talking to himself.</p>");
47 mediator.Subscribe( amITalkingToMyself, iAmClearlyCrazy );

# Advantages & Disadvantages

The largest benefit of the Mediator pattern is that it reduces the communication channels needed between objects or components in a system from many to many to just many to one. Adding new publishers and subscribers is relatively easy due to the level of decoupling present.

Perhaps the biggest downside of using the pattern is that it can introduce a single point of failure. Placing a Mediator between modules can also cause a performance hit as they are always communicating indirectly. Because of the nature of loose coupling, it's difficult to establish how a system might react by only looking at the broadcasts.

That said, it's useful to remind ourselves that decoupled systems have a number of other benefits - if our modules communicated with each other directly, changes to modules (e.g another module throwing an exception) could easily have a domino effect on the rest of our application. This problem is less of a concern with decoupled systems.

At the end of the day, tight coupling causes all kinds of headaches and this is just another alternative solution, but one which can work very well if implemented correctly.

# Mediator Vs. Observer

Developers often wonder what the differences are between the Mediator pattern and the Observer pattern. Admittedly, there is a bit of overlap, but let's refer back to the GoF for an explanation:

"In the Observer pattern, there is no single object that encapsulates a constraint. Instead, the Observer and the Subject must cooperate to maintain the constraint. Communication patterns are determined by the way observers and subjects are interconnected: a single subject usually has many observers, and sometimes the observer of one subject is a subject of another observer."

Both Mediators and Observers promote loose coupling, however, the Mediator achieves this by having objects communicate strictly through the Mediator. The Observer pattern creates observable objects which publish events of interest occur to objects which are subscribed to them.

# Mediator Vs. Facade

We will be covering the Facade pattern shortly, but for reference purposes some developers may also wonder whether there are similarities between the Mediator and Facade patterns. They do both abstract the functionality of existing modules, but there are some subtle differences.

The Mediator centralizes communication between modules where it's explicitly referenced by these modules. In a sense this is multidirectional. The Facade however just defines a simpler interface to a module or system but doesn't add any additional functionality. Other modules in the system aren't directly aware of the concept of a facade and could be considered unidirectional.



# The Prototype Pattern

The GoF refer to the prototype pattern as one which creates objects based on a template of an existing object through cloning.

We can think of the prototype pattern as being based on prototypal inheritance where we create objects which act as prototypes for other objects. The prototype object itself is effectively used as a blueprint for each object the constructor creates. If the prototype of the constructor function used contains a property called name for example (as per the code sample lower down), then each object created by that same constructor will also have this same property.

Reviewing the definitions for this pattern in existing (non-JavaScript) literature, we may find references to classes once again. The reality is that prototypal inheritance avoids using classes altogether. There isn't a "definition" object nor a core object in theory. we're simply creating copies of existing functional objects.

One of the benefits of using the prototype pattern is that we're working with the prototypal strengths JavaScript has to offer natively rather than attempting to imitate features of other languages. With other design patterns, this isn't always the case.

Not only is the pattern an easy way to implement inheritance, but it can also come with a performance boost as well: when defining a function in an object, they're all created by reference (so all child objects point to the same function) instead of creating their own individual copies.

For those interested, real prototypal inheritance, as defined in the ECMAScript 5 standard, requires the use of Object.create (which we previously looked at earlier in this section). To remind ourselves, Object.create creates an object which has a specified prototype and optionally contains specified properties as well (e.g Object.create( prototype, optionalDescriptorObjects )).

We can see this demonstrated in the example below:

01var myCar = {
03  name: "Ford Escort",
05  drive: function () {
06    console.log( "Weeee. I'm driving!" );
07  },
09  panic: function () {
10    console.log( "Wait. How do you stop this thing?" );
11  }
15// Use Object.create to instantiate a new car
16var yourCar = Object.create( myCar );
18// Now we can see that one is a prototype of the other
19console.log( yourCar.name );

Object.create also allows us to easily implement advanced concepts such as differential inheritance where objects are able to directly inherit from other objects. We saw earlier that Object.create allows us to initialise object properties using the second supplied argument. For example:

01var vehicle = {
02  getModel: function () {
03    console.log( "The model of this vehicle is.." + this.model );
04  }
07var car = Object.create(vehicle, {
09  "id": {
10    value: MY_GLOBAL.nextId(),
11    // writable:false, configurable:false by default
12    enumerable: true
13  },
15  "model": {
16    value: "Ford",
17    enumerable: true
18  }

Here the properties can be initialized on the second argument of Object.create using an object literal with a syntax similar to that used by the Object.defineProperties and Object.defineProperty methods that we looked at previously.

It is worth noting that prototypal relationships can cause trouble when enumerating properties of objects and (as Crockford recommends) wrapping the contents of the loop in a hasOwnProperty() check.

If we wish to implement the prototype pattern without directly using Object.create, we can simulate the pattern as per the above example as follows:


01var vehiclePrototype = {
03  init: function ( carModel ) {
04    this.model = carModel;
05  },
07  getModel: function () {
08    console.log( "The model of this vehicle is.." + this.model);
09  }
13function vehicle( model ) {
15  function F() {};
16  F.prototype = vehiclePrototype;
18  var f = new F();
20  f.init( model );
21  return f;
25var car = vehicle( "Ford Escort" );

Note: This alternative does not allow the user to define read-only properties in the same manner (as the vehiclePrototype may be altered if not careful).

A final alternative implementation of the Prototype pattern could be the following:

1var beget = (function () {
3    function F() {}
5    return function ( proto ) {
6        F.prototype = proto;
7        return new F();
8    };

One could reference this method from the vehicle function. Note, however that vehicle here is emulating a constructor, since the prototype pattern does not include any notion of initialization beyond linking an object to a prototype.


# The Command Pattern

The Command pattern aims to encapsulate method invocation, requests or operations into a single object and gives us the ability to both parameterize and pass method calls around that can be executed at our discretion. In addition, it enables us to decouple objects invoking the action from the objects which implement them, giving us a greater degree of overall flexibility in swapping out concrete classes (objects).

Concrete classes are best explained in terms of class-based programming languages and are related to the idea of abstract classes. An abstract class defines an interface, but doesn't necessarily provide implementations for all of its member functions. It acts as a base class from which others are derived. A derived class which implements the missing functionality is called a concrete class.

The general idea behind the Command pattern is that it provides us a means to separate the responsibilities of issuing commands from anything executing commands, delegating this responsibility to different objects instead.

Implementation wise, simple command objects bind together both an action and the object wishing to invoke the action. They consistently include an execution operation (such as run() or execute()). All Command objects with the same interface can easily be swapped as needed and this is considered one of the larger benefits of the pattern.

To demonstrate the Command pattern we're going to create a simple car purchasing service.

03  var CarManager = {
05      // request information
06      requestInfo: function( model, id ){
07        return "The information for " + model + " with ID " + id + " is foobar";
08      },
10      // purchase the car
11      buyVehicle: function( model, id ){
12        return "You have successfully purchased Item " + id + ", a " + model;
13      },
15      // arrange a viewing
16      arrangeViewing: function( model, id ){
17        return "You have successfully booked a viewing of " + model + " ( " + id + " ) ";
18      }
20    };

Taking a look at the above code, it would be trivial to invoke our CarManager methods by directly accessing the object. We would all be forgiven for thinking there is nothing wrong with this - technically, it's completely valid JavaScript. There are however scenarios where this may be disadvantageous.

For example, imagine if the core API behind the CarManager changed. This would require all objects directly accessing these methods within our application to also be modified. This could be viewed as a layer of coupling which effectively goes against the OOP methodology of loosely coupling objects as much as possible. Instead, we could solve this problem by abstracting the API away further.

Let's now expand on our CarManager so that our application of the Command pattern results in the following: accept any named methods that can be performed on the CarManager object, passing along any data that might be used such as the Car model and ID.

Here is what we would like to be able to achieve:

1CarManager.execute( "buyVehicle", "Ford Escort", "453543" );

As per this structure we should now add a definition for the "CarManager.execute" method as follows:

1CarManager.execute = function ( name ) {
2    return CarManager[name] && CarManager[name].apply( CarManager, [].slice.call(arguments, 1) );

Our final sample calls would thus look as follows:

1CarManager.execute( "arrangeViewing", "Ferrari", "14523" );
2CarManager.execute( "requestInfo", "Ford Mondeo", "54323" );
3CarManager.execute( "requestInfo", "Ford Escort", "34232" );
4CarManager.execute( "buyVehicle", "Ford Escort", "34232" );



# The Facade Pattern

When we put up a facade, we present an outward appearance to the world which may conceal a very different reality. This was the inspiration for the name behind the next pattern we're going to review - the Facade pattern. This pattern provides a convenient higher-level interface to a larger body of code, hiding its true underlying complexity. Think of it as simplifying the API being presented to other developers, something which almost always improves usability.

Facades are a structural pattern which can often be seen in JavaScript libraries like jQuery where, although an implementation may support methods with a wide range of behaviors, only a "facade" or limited abstraction of these methods is presented to the public for use.

This allows us to interact with the Facade directly rather than the subsystem behind the scenes. Whenever we use jQuery's $(el).css() or $(el).animate() methods, we're actually using a Facade - the simpler public interface that avoid us having to manually call the many internal methods in jQuery core required to get some behavior working. This also avoids the need to manually interact with DOM APIs and maintain state variables.

The jQuery core methods should be considered intermediate abstractions. The more immediate burden to developers is the DOM API and facades are what make the jQuery library so easy to use.

To build on what we've learned, the Facade pattern both simplifies the interface of a class and it also decouples the class from the code that utilizes it. This gives us the ability to indirectly interact with subsystems in a way that can sometimes be less prone to error than accessing the subsystem directly. A Facade's advantages include ease of use and often a small size-footprint in implementing the pattern.

Let’s take a look at the pattern in action. This is an unoptimized code example, but here we're utilizing a Facade to simplify an interface for listening to events cross-browser. We do this by creating a common method that can be used in one’s code which does the task of checking for the existence of features so that it can provide a safe and cross-browser compatible solution.

01var addMyEvent = function( el,ev,fn ){
03   if( el.addEventListener ){
04            el.addEventListener( ev,fn, false );
05      }else if(el.attachEvent){
06            el.attachEvent( "on" + ev, fn );
07      } else{
08           el["on" + ev] = fn;
09    }

In a similar manner, we're all familiar with jQuery's $(document).ready(..). Internally, this is actually being powered by a method called bindReady(), which is doing this:

01bindReady: function() {
02    ...
03    if ( document.addEventListener ) {
04      // Use the handy event callback
05      document.addEventListener( "DOMContentLoaded", DOMContentLoaded, false );
07      // A fallback to window.onload, that will always work
08      window.addEventListener( "load", jQuery.ready, false );
10    // If IE event model is used
11    } else if ( document.attachEvent ) {
13      document.attachEvent( "onreadystatechange", DOMContentLoaded );
15      // A fallback to window.onload, that will always work
16      window.attachEvent( "onload", jQuery.ready );
17               ...

This is another example of a Facade, where the rest of the world simply uses the limited interface exposed by $(document).ready(..) and the more complex implementation powering it is kept hidden from sight.

Facades don't just have to be used on their own, however. They can also be integrated with other patterns such as the Module pattern. As we can see below, our instance of the module patterns contains a number of methods which have been privately defined. A Facade is then used to supply a much simpler API to accessing these methods:

01var module = (function() {
03    var _private = {
04        i:5,
05        get : function() {
06            console.log( "current value:" + this.i);
07        },
08        set : function( val ) {
09            this.i = val;
10        },
11        run : function() {
12            console.log( "running" );
13        },
14        jump: function(){
15            console.log( "jumping" );
16        }
17    };
19    return {
21        facade : function( args ) {
22            _private.set(args.val);
23            _private.get();
24            if ( args.run ) {
25                _private.run();
26            }
27        }
28    };
32// Outputs: "current value: 10" and "running"
33module.facade( {run: true, val:10} );

In this example, calling module.facade() will actually trigger a set of private behavior within the module, but again, the user isn't concerned with this. we've made it much easier for them to consume a feature without needing to worry about implementation-level details.

Notes on abstraction

Facades generally have few disadvantages, but one concern worth noting is performance. Namely, one must determine whether there is an implicit cost to the abstraction a Facade offers to our implementation and if so, whether this cost is justifiable. Going back to the jQuery library, most of us are aware that both getElementById("identifier") and $("#identifier") can be used to query an element on a page by its ID.

Did you know however that getElementById() on its own is significantly faster by a high order of magnitude? Take a look at this jsPerf test to see results on a per-browser level: http://jsperf.com/getelementbyid-vs-jquery-id. Now of course, we have to keep in mind that jQuery (and Sizzle - its selector engine) are doing a lot more behind the scenes to optimize our query (and that a jQuery object, not just a DOM node is returned).

The challenge with this particular Facade is that in order to provide an elegant selector function capable of accepting and parsing multiple types of queries, there is an implicit cost of abstraction. The user isn't required to access jQuery.getById("identifier") or jQuery.getbyClass("identifier") and so on. That said, the trade-off in performance has been tested in practice over the years and given the success of jQuery, a simple Facade actually worked out very well for the team.

When using the pattern, try to be aware of any performance costs involved and make a call on whether they are worth the level of abstraction offered.


# The Factory Pattern

The Factory pattern is another creational pattern concerned with the notion of creating objects. Where it differs from the other patterns in its category is that it doesn't explicitly require us use a constructor. Instead, a Factory can provide a generic interface for creating objects, where we can specify the type of factory object we wish to be created.

Imagine that we have a UI factory where we are asked to create a type of UI component. Rather than creating this component directly using the new operator or via another creational constructor, we ask a Factory object for a new component instead. We inform the Factory what type of object is required (e.g "Button", "Panel") and it instantiates this, returning it to us for use.

This is particularly useful if the object creation process is relatively complex, e.g. if it strongly depends on dynamic factors or application configuration.

Examples of this pattern can be found in UI libraries such as ExtJS where the methods for creating objects or components may be further subclassed.

The following is an example that builds upon our previous snippets using the Constructor pattern logic to define cars. It demonstrates how a Vehicle Factory may be implemented using the Factory pattern:

01// Types.js - Constructors used behind the scenes
03// A constructor for defining new cars
04function Car( options ) {
06  // some defaults
07  this.doors = options.doors || 4;
08  this.state = options.state || "brand new";
09  this.color = options.color || "silver";
13// A constructor for defining new trucks
14function Truck( options){
16  this.state = options.state || "used";
17  this.wheelSize = options.wheelSize || "large";
18  this.color = options.color || "blue";
22// FactoryExample.js
24// Define a skeleton vehicle factory
25function VehicleFactory() {}
27// Define the prototypes and utilities for this factory
29// Our default vehicleClass is Car
30VehicleFactory.prototype.vehicleClass = Car;
32// Our Factory method for creating new Vehicle instances
33VehicleFactory.prototype.createVehicle = function ( options ) {
35  if( options.vehicleType === "car" ){
36    this.vehicleClass = Car;
37  }else{
38    this.vehicleClass = Truck;
39  }
41  return new this.vehicleClass( options );
45// Create an instance of our factory that makes cars
46var carFactory = new VehicleFactory();
47var car = carFactory.createVehicle( {
48            vehicleType: "car",
49            color: "yellow",
50            doors: 6 } );
52// Test to confirm our car was created using the vehicleClass/prototype Car
54// Outputs: true
55console.log( car instanceof Car );
57// Outputs: Car object of color "yellow", doors: 6 in a "brand new" state
58console.log( car );

Approach #1: Modify a VehicleFactory instance to use the Truck class

01var movingTruck = carFactory.createVehicle( {
02                      vehicleType: "truck",
03                      state: "like new",
04                      color: "red",
05                      wheelSize: "small" } );
07// Test to confirm our truck was created with the vehicleClass/prototype Truck
09// Outputs: true
10console.log( movingTruck instanceof Truck );
12// Outputs: Truck object of color "red", a "like new" state
13// and a "small" wheelSize
14console.log( movingTruck );

Approach #2: Subclass VehicleFactory to create a factory class that builds Trucks

01function TruckFactory () {}
02TruckFactory.prototype = new VehicleFactory();
03TruckFactory.prototype.vehicleClass = Truck;
05var truckFactory = new TruckFactory();
06var myBigTruck = truckFactory.createVehicle( {
07                    state: "omg..so bad.",
08                    color: "pink",
09                    wheelSize: "so big" } );
11// Confirms that myBigTruck was created with the prototype Truck
12// Outputs: true
13console.log( myBigTruck instanceof Truck );
15// Outputs: Truck object with the color "pink", wheelSize "so big"
16// and state "omg. so bad"
17console.log( myBigTruck );


When To Use The Factory Pattern

The Factory pattern can be especially useful when applied to the following situations:


When Not To Use The Factory Pattern

When applied to the wrong type of problem, this pattern can introduce an unnecessarily great deal of complexity to an application. Unless providing an interface for object creation is a design goal for the library or framework we are writing, I would suggest sticking to explicit constructors to avoid the unnecessary overhead.

Due to the fact that the process of object creation is effectively abstracted behind an interface, this can also introduce problems with unit testing depending on just how complex this process might be.

Abstract Factories

It is also useful to be aware of the Abstract Factory pattern, which aims to encapsulate a group of individual factories with a common goal. It separates the details of implementation of a set of objects from their general usage.

An Abstract Factory should be used where a system must be independent from the way the objects it creates are generated or it needs to work with multiple types of objects.

An example which is both simple and easier to understand is a vehicle factory, which defines ways to get or register vehicles types. The abstract factory can be named AbstractVehicleFactory. The Abstract factory will allow the definition of types of vehicle like "car" or "truck" and concrete factories will implement only classes that fulfill the vehicle contract (e.g Vehicle.prototype.drive and Vehicle.prototype.breakDown).

01  var AbstractVehicleFactory = (function () {
03    // Storage for our vehicle types
04    var types = {};
06    return {
07        getVehicle: function ( type, customizations ) {
08            var Vehicle = types[type];
10            return (Vehicle ? new Vehicle(customizations) : null);
11        },
13        registerVehicle: function ( type, Vehicle ) {
14            var proto = Vehicle.prototype;
16            // only register classes that fulfill the vehicle contract
17            if ( proto.drive && proto.breakDown ) {
18                types[type] = Vehicle;
19            }
21            return AbstractVehicleFactory;
22        }
23    };
27// Usage:
29AbstractVehicleFactory.registerVehicle( "car", Car );
30AbstractVehicleFactory.registerVehicle( "truck", Truck );
32// Instantiate a new car based on the abstract vehicle type
33var car = AbstractVehicleFactory.getVehicle( "car" , {
34            color: "lime green",
35            state: "like new" } );
37// Instantiate a new truck in a similar manner
38var truck = AbstractVehicleFactory.getVehicle( "truck" , {
39            wheelSize: "medium",
40            color: "neon yellow" } );

# The Mixin Pattern

In traditional programming languages such as C++ and Lisp, Mixins are classes which offer functionality that can be easily inherited by a sub-class or group of sub-classes for the purpose of function re-use.


For developers unfamiliar with sub-classing, we will go through a brief beginners primer on them before diving into Mixins and Decorators further.

Sub-classing is a term that refers to inheriting properties for a new object from a base or superclass object. In traditional object-oriented programming, a class B is able to extend another class A. Here we consider A a superclass and B a subclass of A. As such, all instances of B inherit the methods from A. B is however still able to define its own methods, including those that override methods originally defined by A.

Should B need to invoke a method in A that has been overridden, we refer to this as method chaining. Should B need to invoke the constructor A (the superclass), we call this constructor chaining.

In order to demonstrate sub-classing, we first need a base object that can have new instances of itself created. let's model this around the concept of a person.

1var Person =  function( firstName , lastName ){
3  this.firstName = firstName;
4  this.lastName =  lastName;
5  this.gender = "male";

Next, we'll want to specify a new class (object) that's a subclass of the existing Person object. Let us imagine we want to add distinct properties to distinguish a Person from a Superhero whilst inheriting the properties of the Person "superclass". As superheroes share many common traits with normal people (e.g. name, gender), this should hopefully illustrate how sub-classing works adequately.

01// a new instance of Person can then easily be created as follows:
02var clark = new Person( "Clark" , "Kent" );
04// Define a subclass constructor for for "Superhero":
05var Superhero = function( firstName, lastName , powers ){
07    // Invoke the superclass constructor on the new object
08    // then use .call() to invoke the constructor as a method of
09    // the object to be initialized.
11    Person.call( this, firstName, lastName );
13    // Finally, store their powers, a new array of traits not found in a normal "Person"
14    this.powers = powers;
17SuperHero.prototype = Object.create( Person.prototype );
18var superman = new Superhero( "Clark" ,"Kent" , ["flight","heat-vision"] );
19console.log( superman );
21// Outputs Person attributes as well as powers

The Superhero constructor creates an object which descends from Person. Objects of this type have attributes of the objects that are above it in the chain and if we had set default values in the Person object, Superhero is capable of overriding any inherited values with values specific to it's object.


In JavaScript, we can look at inheriting from Mixins as a means of collecting functionality through extension. Each new object we define has a prototype from which it can inherit further properties. Prototypes can inherit from other object prototypes but, even more importantly, can define properties for any number of object instances. We can leverage this fact to promote function re-use.

Mixins allow objects to borrow (or inherit) functionality from them with a minimal amount of complexity. As the pattern works well with JavaScripts object prototypes, it gives us a fairly flexible way to share functionality from not just one Mixin, but effectively many through multiple inheritance.

They can be viewed as objects with attributes and methods that can be easily shared across a number of other object prototypes. Imagine that we define a Mixin containing utility functions in a standard object literal as follows:

01var myMixins = {
03  moveUp: function(){
04    console.log( "move up" );
05  },
07  moveDown: function(){
08    console.log( "move down" );
09  },
11  stop: function(){
12    console.log( "stop! in the name of love!" );
13  }

We can then easily extend the prototype of existing constructor functions to include this behavior using a helper such as the Underscore.js _.extend() method:

01// A skeleton carAnimator constructor
02function carAnimator(){
03  this.moveLeft = function(){
04    console.log( "move left" );
05  };
08// A skeleton personAnimator constructor
09function personAnimator(){
10  this.moveRandomly = function(){ /*..*/ };
13// Extend both constructors with our Mixin
14_.extend( carAnimator.prototype, myMixins );
15_.extend( personAnimator.prototype, myMixins );
17// Create a new instance of carAnimator
18var myAnimator = new carAnimator();
23// Outputs:
24// move left
25// move down
26// stop! in the name of love!

As we can see, this allows us to easily "mix" in common behaviour into object constructors fairly trivially.

In the next example, we have two constructors: a Car and a Mixin. What we're going to do is augment (another way of saying extend) the Car so that it can inherit specific methods defined in the Mixin, namely driveForward() and driveBackward(). This time we won't be using Underscore.js.

Instead, this example will demonstrate how to augment a constructor to include functionality without the need to duplicate this process for every constructor function we may have.

01// Define a simple Car constructor
02var Car = function ( settings ) {
04        this.model = settings.model || "no model provided";
05        this.color = settings.color || "no colour provided";
07    };
09// Mixin
10var Mixin = function () {};
12Mixin.prototype = {
14    driveForward: function () {
15        console.log( "drive forward" );
16    },
18    driveBackward: function () {
19        console.log( "drive backward" );
20    },
22    driveSideways: function () {
23        console.log( "drive sideways" );
24    }
29// Extend an existing object with a method from another
30function augment( receivingClass, givingClass ) {
32    // only provide certain methods
33    if ( arguments[2] ) {
34        for ( var i = 2, len = arguments.length; i < len; i++ ) {
35            receivingClass.prototype[arguments[i]] = givingClass.prototype[arguments[i]];
36        }
37    }
38    // provide all methods
39    else {
40        for ( var methodName in givingClass.prototype ) {
42            // check to make sure the receiving class doesn't
43            // have a method of the same name as the one currently
44            // being processed
45            if ( !Object.hasOwnProperty(receivingClass.prototype, methodName) ) {
46                receivingClass.prototype[methodName] = givingClass.prototype[methodName];
47            }
49            // Alternatively:
50            // if ( !receivingClass.prototype[methodName] ) {
51            //  receivingClass.prototype[methodName] = givingClass.prototype[methodName];
52            // }
53        }
54    }
58// Augment the Car constructor to include "driveForward" and "driveBackward"
59augment( Car, Mixin, "driveForward", "driveBackward" );
61// Create a new Car
62var myCar = new Car({
63    model: "Ford Escort",
64    color: "blue"
67// Test to make sure we now have access to the methods
71// Outputs:
72// drive forward
73// drive backward
75// We can also augment Car to include all functions from our mixin
76// by not explicitly listing a selection of them
77augment( Car, Mixin );
79var mySportsCar = new Car({
80    model: "Porsche",
81    color: "red"
86// Outputs:
87// drive sideways

Advantages & Disadvantages

Mixins assist in decreasing functional repetition and increasing function re-use in a system. Where an application is likely to require shared behaviour across object instances, we can easily avoid any duplication by maintaining this shared functionality in a Mixin and thus focusing on implementing only the functionality in our system which is truly distinct.

That said, the downsides to Mixins are a little more debatable. Some developers feel that injecting functionality into an object prototype is a bad idea as it leads to both prototype pollution and a level of uncertainly regarding the origin of our functions. In large systems this may well be the case.

I would argue that strong documentation can assist in minimizing the amount of confusion regarding the source of mixed in functions, but as with every pattern, if care is taken during implementation we should be okay.


The Decorator Pattern

Decorators are a structural design pattern that aim to promote code re-use. Similar to Mixins, they can be considered another viable alternative to object sub-classing.

Classically, Decorators offered the ability to add behaviour to existing classes in a system dynamically. The idea was that the decoration itself wasn't essential to the base functionality of the class, otherwise it would be baked into the superclass itself.

They can be used to modify existing systems where we wish to add additional features to objects without the need to heavily modify the underlying code using them. A common reason why developers use them is their applications may contain features requiring a large quantity of distinct types of object. Imagine having to define hundreds of different object constructors for say, a JavaScript game.

The object constructors could represent distinct player types, each with differing capabilities. A Lord of the Rings game could require constructors for Hobbit, Elf, Orc, Wizard, Mountain Giant, Stone Giant and so on, but there could easily be hundreds of these. If we then factored in capabilities, imagine having to create sub-classes for each combination of capability type e.g HobbitWithRing,HobbitWithSword, HobbitWithRingAndSword and so on.This isn't very practical and certainly isn't manageable when we factor in a growing number of different abilities.

The Decorator pattern isn't heavily tied to how objects are created but instead focuses on the problem of extending their functionality. Rather than just relying on prototypal inheritance, we work with a single base object and progressively add decorator objects which provide the additional capabilities. The idea is that rather than sub-classing, we add (decorate) properties or methods to a base object so it's a little more streamlined.

Adding new attributes to objects in JavaScript is a very straight-forward process so with this in mind, a very simplistic decorator may be implemented as follows:

Example 1: Decorating Constructors With New Functionality

01// A vehicle constructor
02function vehicle( vehicleType ){
04    // some sane defaults
05    this.vehicleType = vehicleType || "car";
06    this.model = "default";
07    this.license = "00000-000";
11// Test instance for a basic vehicle
12var testInstance = new vehicle( "car" );
13console.log( testInstance );
15// Outputs:
16// vehicle: car, model:default, license: 00000-000
18// Lets create a new instance of vehicle, to be decorated
19var truck = new vehicle( "truck" );
21// New functionality we're decorating vehicle with
22truck.setModel = function( modelName ){
23    this.model = modelName;
26truck.setColor = function( color ){
27    this.color = color;
30// Test the value setters and value assignment works correctly
31truck.setModel( "CAT" );
32truck.setColor( "blue" );
34console.log( truck );
36// Outputs:
37// vehicle:truck, model:CAT, color: blue
39// Demonstrate "vehicle" is still unaltered
40var secondInstance = new vehicle( "car" );
41console.log( secondInstance );
43// Outputs:
44// vehicle: car, model:default, license: 00000-000

This type of simplistic implementation is functional, but it doesn't really demonstrate all of the strengths Decorators have to offer. For this, we're first going to go through my variation of the Coffee example from an excellent book called Head First Design Patterns by Freeman, Sierra and Bates, which is modeled around a Macbook purchase.

Example 2: Decorating Objects With Multiple Decorators

01// The constructor to decorate
02function MacBook() {
04  this.cost = function () { return 997; };
05  this.screenSize = function () { return 11.6; };
09// Decorator 1
10function Memory( macbook ) {
12  var v = macbook.cost();
13  macbook.cost = function() {
14    return v + 75;
15  };
19// Decorator 2
20function Engraving( macbook ){
22  var v = macbook.cost();
23  macbook.cost = function(){
24    return  v + 200;
25  };
29// Decorator 3
30function Insurance( macbook ){
32  var v = macbook.cost();
33  macbook.cost = function(){
34     return  v + 250;
35  };
39var mb = new MacBook();
40Memory( mb );
41Engraving( mb );
42Insurance( mb );
44// Outputs: 1522
45console.log( mb.cost() );
47// Outputs: 11.6
48console.log( mb.screenSize() );

In the above example, our Decorators are overriding the MacBook() super-class objects .cost() function to return the current price of the Macbook plus the cost of the upgrade being specified.

It's considered a decoration as the original Macbook objects constructor methods which are not overridden (e.g. screenSize()) as well as any other properties which we may define as a part of the Macbook remain unchanged and intact.

There isn't really a defined interface in the above example and we're shifting away the responsibility of ensuring an object meets an interface when moving from the creator to the receiver.

Pseudo-classical Decorators

We're now going to examine a variation of the Decorator first presented in a JavaScript form in Pro JavaScript Design Patterns (PJDP) by Dustin Diaz and Ross Harmes.

Unlike some of the examples from earlier, Diaz and Harmes stick more closely to how decorators are implemented in other programming languages (such as Java or C++) using the concept of an "interface", which we will define in more detail shortly.

Note: This particular variation of the Decorator pattern is provided for reference purposes. If finding it overly complex, I recommend opting for one of the simpler implementations covered earlier.


PJDP describes the Decorator as a pattern that is used to transparently wrap objects inside other objects of the same interface. An interface is a way of defining the methods an object should have, however, it doesn't actually directly specify how those methods should be implemented.

They can also indicate what parameters the methods take, but this is considered optional.

So, why would we use an interface in JavaScript? The idea is that they're self-documenting and promote reusability. In theory, interfaces also make code more stable by ensuring changes to them must also be made to the objects implementing them.

Below is an example of an implementation of interfaces in JavaScript using duck-typing - an approach that helps determine whether an object is an instance of constructor/object based on the methods it implements.

01// Create interfaces using a pre-defined Interface
02// constructor that accepts an interface name and
03// skeleton methods to expose.
05// In our reminder example summary() and placeOrder()
06// represent functionality the interface should
07// support
08var reminder = new Interface( "List", ["summary", "placeOrder"] );
10var properties = {
11  name: "Remember to buy the milk",
12  date: "05/06/2016",
13  actions:{
14    summary: function (){
15      return "Remember to buy the milk, we are almost out!";
16   },
17    placeOrder: function (){
18      return "Ordering milk from your local grocery store";
19    }
20  }
23// Now create a constructor implementing the above properties
24// and methods
26function Todo( config ){
28  // State the methods we expect to be supported
29  // as well as the Interface instance being checked
30  // against
32  Interface.ensureImplements( config.actions, reminder );
34  this.name = config.name;
35  this.methods = config.actions;
39// Create a new instance of our Todo constructor
41var todoItem = Todo( properties );
43// Finally test to make sure these function correctly
45console.log( todoItem.methods.summary() );
46console.log( todoItem.methods.placeOrder() );
48// Outputs:
49// Remember to buy the milk, we are almost out!
50// Ordering milk from your local grocery store

In the above, Interface.ensureImplements provides strict functionality checking and code for both this and the Interface constructor can be found here.

The biggest problem with interfaces is that, as there isn't built-in support for them in JavaScript, there is a danger of us attempting to emulate a feature of another language that may not be an ideal fit. Lightweight interfaces can be used without a great performance cost however and we will next look at Abstract Decorators using this same concept.

Abstract Decorators

To demonstrate the structure of this version of the Decorator pattern, we're going to imagine we have a superclass that models a Macbook once again and a store that allows us to "decorate" our Macbook with a number of enhancements for an additional fee.

Enhancements can include upgrades to 4GB or 8GB Ram, engraving, Parallels or a case. Now if we were to model this using an individual sub-class for each combination of enhancement options, it might look something like this:

01var Macbook = function(){
02        //...
05var  MacbookWith4GBRam =  function(){},
06     MacbookWith8GBRam = function(){},
07     MacbookWith4GBRamAndEngraving = function(){},
08     MacbookWith8GBRamAndEngraving = function(){},
09     MacbookWith8GBRamAndParallels = function(){},
10     MacbookWith4GBRamAndParallels = function(){},
11     MacbookWith8GBRamAndParallelsAndCase = function(){},
12     MacbookWith4GBRamAndParallelsAndCase = function(){},
13     MacbookWith8GBRamAndParallelsAndCaseAndInsurance = function(){},
14     MacbookWith4GBRamAndParallelsAndCaseAndInsurance = function(){};

and so on.

This would be an impractical solution as a new subclass would be required for every possible combination of enhancements that are available. As we would prefer to keep things simple without maintaining a large set of subclasses, let's look at how decorators may be used to solve this problem better.

Rather than requiring all of the combinations we saw earlier, we should simply have to create five new decorator classes. Methods that are called on these enhancement classes would be passed on to our Macbook class.

In our next example, decorators transparently wrap around their components and can interestingly be interchanged astray use the same interface.

Here's the interface we're going to define for the Macbook:

01var Macbook = new Interface( "Macbook",
02  ["addEngraving",
03  "addParallels",
04  "add4GBRam",
05  "add8GBRam",
06  "addCase"]);
08// A Macbook Pro might thus be represented as follows:
09var MacbookPro = function(){
10    // implements Macbook
13MacbookPro.prototype = {
14    addEngraving: function(){
15    },
16    addParallels: function(){
17    },
18    add4GBRam: function(){
19    },
20    add8GBRam:function(){
21    },
22    addCase: function(){
23    },
24    getPrice: function(){
25      // Base price
26      return 900.00;        
27    }

To make it easier for us to add as many more options as needed later on, an Abstract Decorator class is defined with default methods required to implement the Macbook interface, which the rest of the options will sub-class. Abstract Decorators ensure that we can decorate a base class independently with as many decorators as needed in different combinations (remember the example earlier?) without needing to derive a class for every possible combination.

01// Macbook decorator abstract decorator class
03var MacbookDecorator = function( macbook ){
05    Interface.ensureImplements( macbook, Macbook );
06    this.macbook = macbook; 
10MacbookDecorator.prototype = {
11    addEngraving: function(){
12        return this.macbook.addEngraving();
13    },
14    addParallels: function(){
15        return this.macbook.addParallels();
16    },
17    add4GBRam: function(){
18        return this.macbook.add4GBRam();
19    },
20    add8GBRam:function(){
21        return this.macbook.add8GBRam();
22    },
23    addCase: function(){
24        return this.macbook.addCase();
25    },
26    getPrice: function(){
27        return this.macbook.getPrice();
28    }       

What's happening in the above sample is that the Macbook Decorator is taking an object to use as the component. It's using the Macbook interface we defined earlier and for each method is just calling the same method on the component. We can now create our option classes just by using the Macbook Decorator - simply call the superclass constructor and any methods can be overridden as per necessary.

01var CaseDecorator = function( macbook ){
03   // call the superclass's constructor next
04   this.superclass.constructor( macbook );  
08// Let's now extend the superclass
09extend( CaseDecorator, MacbookDecorator );
11CaseDecorator.prototype.addCase = function(){
12    return this.macbook.addCase() + "Adding case to macbook";  
15CaseDecorator.prototype.getPrice = function(){
16    return this.macbook.getPrice() + 45.00; 

As we can see, most of this is relatively straight-forward to implement. What we're doing is overriding the addCase() and getPrice() methods that need to be decorated and we're achieving this by first executing the component's method and then adding to it.

As there's been quite a lot of information presented in this section so far, let's try to bring it all together in a single example that will hopefully highlight what we have learned.

01// Instantiation of the macbook
02var myMacbookPro = new MacbookPro(); 
04// Outputs: 900.00
05console.log( myMacbookPro.getPrice() );
07// Decorate the macbook
08myMacbookPro = new CaseDecorator( myMacbookPro );
10// This will return 945.00
11console.log( myMacbookPro.getPrice() );

As decorators are able to modify objects dynamically, they're a perfect pattern for changing existing systems. Occasionally, it's just simpler to create decorators around an object versus the trouble of maintaining individual sub-classes for each object type. This makes maintaining applications that may require a large number of sub-classed objects significantly more straight-forward.

Decorators With jQuery

As with other patterns we've covered, there are also examples of the Decorator pattern that can be implemented with jQuery. jQuery.extend() allows us to extend (or merge) two or more objects (and their properties) together into a single object either at run-time or dynamically at a later point.

In this scenario, a target object can be decorated with new functionality without necessarily breaking or overriding existing methods in the source/superclass object (although this can be done).

In the following example, we define three objects: defaults, options and settings. The aim of the task is to decorate the defaults object with additional functionality found in optionssettings. We must:

(a) Leave "defaults" in an untouched state where we don't lose the ability to access the properties or functions found in it a later point (b) Gain the ability to use the decorated properties and functions found in "options"

01var decoratorApp = decoratorApp || {};
03// define the objects we're going to use
04decoratorApp = {
06    defaults: {
07        validate: false,
08        limit: 5,
09        name: "foo",
10        welcome: function () {
11            console.log( "welcome!" );
12        }
13    },
15    options: {
16        validate: true,
17        name: "bar",
18        helloWorld: function () {
19            console.log( "hello world" );
20        }
21    },
23    settings: {},
25    printObj: function ( obj ) {
26        var arr = [],
27            next;
28        $.each( obj, function ( key, val ) {
29            next = key + ": ";
30            next += $.isPlainObject(val) ? printObj( val ) : val;
31            arr.push( next );
32        } );
34        return "{ " + arr.join(", ") + " }";
35    }
39// merge defaults and options, without modifying defaults explicitly
40decoratorApp.settings = $.extend({}, decoratorApp.defaults, decoratorApp.options);
42// what we have done here is decorated defaults in a way that provides
43// access to the properties and functionality it has to offer (as well as
44// that of the decorator "options"). defaults itself is left unchanged
47    .append( decoratorApp.printObj(decoratorApp.settings) + 
48          + decoratorApp.printObj(decoratorApp.options) +
49          + decoratorApp.printObj(decoratorApp.defaults));
51// settings -- { validate: true, limit: 5, name: bar, welcome: function (){ console.log( "welcome!" ); },
52// helloWorld: function (){ console.log("hello!"); } }
53// options -- { validate: true, name: bar, helloWorld: function (){ console.log("hello!"); } }
54// defaults -- { validate: false, limit: 5, name: foo, welcome: function (){ console.log("welcome!"); } }

Advantages & Disadvantages

Developers enjoy using this pattern as it can be used transparently and is also fairly flexible - as we've seen, objects can be wrapped or "decorated" with new behavior and then continue to be used without needing to worry about the base object being modified. In a broader context, this pattern also avoids us needing to rely on large numbers of subclasses to get the same benefits.

There are however drawbacks that we should be aware of when implementing the pattern. If poorly managed, it can significantly complicate our application architecture as it introduces many small, but similar objects into our namespace. The concern here is that in addition to becoming hard to manage, other developers unfamiliar with the pattern may have a hard time grasping why it's being used.

Sufficient commenting or pattern research should assist with the latter, however as long as we keep a handle on how widespread we use the decorator in our applications we should be fine on both counts.



# Flyweight


The Flyweight pattern is a classical structural solution for optimizing code that is repetitive, slow and inefficiently shares data. It aims to minimize the use of memory in an application by sharing as much data as possible with related objects (e.g application configuration, state and so on).

The pattern was first conceived by Paul Calder and Mark Linton in 1990 and was named after the boxing weight class that includes fighters weighing less than 112lb. The name Flyweight itself is derived from this weight classification as it refers to the small weight (memory footprint) the pattern aims to help us achieve.

In practice, Flyweight data sharing can involve taking several similar objects or data constructs used by a number of objects and placing this data into a single external object. We can pass through this object through to those depending on this data, rather than storing identical data across each one.

Using Flyweights

There are two ways in which the Flyweight pattern can be applied. The first is at the data-layer, where we deal with the concept of sharing data between large quantities of similar objects stored in memory.

The second is at the DOM-layer where the Flyweight can be used as a central event-manager to avoid attaching event handlers to every child element in a parent container we wish to have some similar behavior.

As the data-layer is where the flyweight pattern is most used traditionally, we'll take a look at this first.

Flyweights and sharing data

For this application, there are a few more concepts around the classical Flyweight pattern that we need to be aware of. In the Flyweight pattern there's a concept of two states - intrinsic and extrinsic. Intrinsic information may be required by internal methods in our objects which they absolutely cannot function without. Extrinsic information can however be removed and stored externally.

Objects with the same intrinsic data can be replaced with a single shared object, created by a factory method. This allows us to reduce the overall quantity of implicit data being stored quite significantly.

The benefit of this is that we're able to keep an eye on objects that have already been instantiated so that new copies are only ever created should the intrinsic state differ from the object we already have.

We use a manager to handle the extrinsic states. How this is implemented can vary, but one approach to this to have the manager object contain a central database of the extrinsic states and the flyweight objects which they belong to.

Implementing Classical Flyweights

As the Flyweight pattern hasn't been heavily used in JavaScript in recent years, many of the implementations we might use for inspiration come form the Java and C++ worlds.

Our first look at Flyweights in code is my JavaScript implementation of the Java sample of the Flyweight pattern from Wikipedia (http://en.wikipedia.org/wiki/Flyweight_pattern).

We will be making use of three types of Flyweight components in this implementation, which are listed below:

These correspond to the following definitions in our implementation:

Duck punching "implements"

Duck punching allows us to extend the capabilities of a language or solution without necessarily needing to modify the runtime source. As this next solution requires the use of a Java keyword (implements) for implementing interfaces and isn't found in JavaScript natively, let's first duck punch it.

Function.prototype.implementsFor works on an object constructor and will accept a parent class (function) or object and either inherit from this using normal inheritance (for functions) or virtual inheritance (for objects).

01// Simulate pure virtual inheritance/"implement" keyword for JS
02Function.prototype.implementsFor = function( parentClassOrObject ){
03    if ( parentClassOrObject.constructor === Function )
04    {
05        // Normal Inheritance
06        this.prototype = new parentClassOrObject();
07        this.prototype.constructor = this;
08        this.prototype.parent = parentClassOrObject.prototype;
09    }
10    else
11    {
12        // Pure Virtual Inheritance
13        this.prototype = parentClassOrObject;
14        this.prototype.constructor = this;
15        this.prototype.parent = parentClassOrObject;
16    }
17    return this;

We can use this to patch the lack of an implements keyword by having a function inherit an interface explicitly. Below, CoffeeFlavor implements the CoffeeOrder interface and must contain its interface methods in order for us to assign the functionality powering these implementations to an object.

001// Flyweight object
002var CoffeeOrder = {
004  // Interfaces
005  serveCoffee:function(context){},
006    getFlavor:function(){}
011// ConcreteFlyweight object that creates ConcreteFlyweight
012// Implements CoffeeOrder
013function CoffeeFlavor( newFlavor ){
015    var flavor = newFlavor;
017    // If an interface has been defined for a feature
018    // implement the feature
019    if( typeof this.getFlavor === "function" ){
020      this.getFlavor = function() {
021          return flavor;
022      };
023    }
025    if( typeof this.serveCoffee === "function" ){
026      this.serveCoffee = function( context ) {
027        console.log("Serving Coffee flavor "
028          + flavor
029          + " to table number "
030          + context.getTable());
031    };     
032    }
037// Implement interface for CoffeeOrder
038CoffeeFlavor.implementsFor( CoffeeOrder );
041// Handle table numbers for a coffee order
042function CoffeeOrderContext( tableNumber ) {
043   return{
044      getTable: function() {
045         return tableNumber;
046     }
047   };
051function CoffeeFlavorFactory() {
052    var flavors = {},
053    length = 0;
055    return {
056        getCoffeeFlavor: function (flavorName) {
058            var flavor = flavors[flavorName];
059            if (flavor === undefined) {
060                flavor = new CoffeeFlavor(flavorName);
061                flavors[flavorName] = flavor;
062                length++;
063            }
064            return flavor;
065        },
067        getTotalCoffeeFlavorsMade: function () {
068            return length;
069        }
070    };
073// Sample usage:
074// testFlyweight()
076function testFlyweight(){
079  // The flavors ordered.
080  var flavors = new CoffeeFlavor(),
082  // The tables for the orders.
083    tables = new CoffeeOrderContext(),
085  // Number of orders made
086    ordersMade = 0,
088  // The CoffeeFlavorFactory instance
089    flavorFactory;
091  function takeOrders( flavorIn, table) {
092     flavors[ordersMade] = flavorFactory.getCoffeeFlavor( flavorIn );
093     tables[ordersMade++] = new CoffeeOrderContext( table );
094  }
096   flavorFactory = new CoffeeFlavorFactory();
098   takeOrders("Cappuccino", 2);
099   takeOrders("Cappuccino", 2);
100   takeOrders("Frappe", 1);
101   takeOrders("Frappe", 1);
102   takeOrders("Xpresso", 1);
103   takeOrders("Frappe", 897);
104   takeOrders("Cappuccino", 97);
105   takeOrders("Cappuccino", 97);
106   takeOrders("Frappe", 3);
107   takeOrders("Xpresso", 3);
108   takeOrders("Cappuccino", 3);
109   takeOrders("Xpresso", 96);
110   takeOrders("Frappe", 552);
111   takeOrders("Cappuccino", 121);
112   takeOrders("Xpresso", 121);
114   for (var i = 0; i < ordersMade; ++i) {
115       flavors[i].serveCoffee(tables[i]);
116   }
117   console.log(" ");
118   console.log("total CoffeeFlavor objects made: " +  flavorFactory.getTotalCoffeeFlavorsMade());

Converting code to use the Flyweight pattern

Next, let's continue our look at Flyweights by implementing a system to manage all of the books in a library. The important meta-data for each book could probably be broken down as follows:

We'll also require the following properties to keep track of which member has checked out a particular book, the date they've checked it out on as well as the expected date of return.

Each book would thus be represented as follows, prior to any optimization using the Flyweight pattern:

01var Book = function( id, title, author, genre, pageCount,publisherID, ISBN, checkoutDate, checkoutMember, dueReturnDate,availability ){
03   this.id = id;
04   this.title = title;
05   this.author = author;
06   this.genre = genre;
07   this.pageCount = pageCount;
08   this.publisherID = publisherID;
09   this.ISBN = ISBN;
10   this.checkoutDate = checkoutDate;
11   this.checkoutMember = checkoutMember;
12   this.dueReturnDate = dueReturnDate;
13   this.availability = availability;
17Book.prototype = {
19  getTitle: function () {
20     return this.title;
21  },
23  getAuthor: function () {
24     return this.author;
25  },
27  getISBN: function (){
28     return this.ISBN;
29  },
31  // For brevity, other getters are not shown
32  updateCheckoutStatus: function( bookID, newStatus, checkoutDate , checkoutMember, newReturnDate ){
34     this.id  = bookID;
35     this.availability = newStatus;
36     this.checkoutDate = checkoutDate;
37     this.checkoutMember = checkoutMember;
38     this.dueReturnDate = newReturnDate;
40  },
42  extendCheckoutPeriod: function( bookID, newReturnDate ){
44      this.id =  bookID;
45      this.dueReturnDate = newReturnDate;
47  },
49  isPastDue: function(bookID){
51     var currentDate = new Date();
52     return currentDate.getTime() > Date.parse( this.dueReturnDate );
54   }

This probably works fine initially for small collections of books, however as the library expands to include a larger inventory with multiple versions and copies of each book available, we may find the management system running slower and slower over time. Using thousands of book objects may overwhelm the available memory, but we can optimize our system using the Flyweight pattern to improve this.

We can now separate our data into intrinsic and extrinsic states as follows: data relevant to the book object (title, author etc) is intrinsic whilst the checkout data (checkoutMember, dueReturnDate etc) is considered extrinsic. Effectively this means that only one Book object is required for each combination of book properties. it's still a considerable quantity of objects, but significantly fewer than we had previously.

The following single instance of our book meta-data combinations will be shared among all of the copies of a book with a particular title.

01// Flyweight optimized version
02var Book = function ( title, author, genre, pageCount, publisherID, ISBN ) {
04    this.title = title;
05    this.author = author;
06    this.genre = genre;
07    this.pageCount = pageCount;
08    this.publisherID = publisherID;
09    this.ISBN = ISBN;

As we can see, the extrinsic states have been removed. Everything to do with library check-outs will be moved to a manager and as the object data is now segmented, a factory can be used for instantiation.

A Basic Factory

Let's now define a very basic factory. What we're going to have it do is perform a check to see if a book with a particular title has been previously created inside the system; if it has, we'll return it - if not, a new book will be created and stored so that it can be accessed later. This makes sure that we only create a single copy of each unique intrinsic piece of data:

01// Book Factory singleton
02var BookFactory = (function () {
03  var existingBooks = {}, existingBook;
05  return {
06    createBook: function ( title, author, genre, pageCount, publisherID, ISBN ) {
08      // Find out if a particular book meta-data combination has been created before
09      // !! or (bang bang) forces a boolean to be returned
10      existingBook = existingBooks[ISBN];
11      if ( !!existingBook ) {
12        return existingBook;
13      } else {
15        // if not, let's create a new instance of the book and store it
16        var book = new Book( title, author, genre, pageCount, publisherID, ISBN );
17        existingBooks[ISBN] = book;
18        return book;
20      }
21    }
22  };

Managing the extrinsic states

Next, we need to store the states that were removed from the Book objects somewhere - luckily a manager (which we'll be defining as a Singleton) can be used to encapsulate them. Combinations of a Book object and the library member that's checked them out will be called Book records. Our manager will be storing both and will also include checkout related logic we stripped out during our flyweight optimization of the Book class.

01// BookRecordManager singleton
02var BookRecordManager = (function () {
04  var bookRecordDatabase = {};
06  return {
07    // add a new book into the library system
08    addBookRecord: function ( id, title, author, genre, pageCount, publisherID, ISBN, checkoutDate, checkoutMember, dueReturnDate, availability ) {
10      var book = bookFactory.createBook( title, author, genre, pageCount, publisherID, ISBN );
12      bookRecordDatabase[id] = {
13        checkoutMember: checkoutMember,
14        checkoutDate: checkoutDate,
15        dueReturnDate: dueReturnDate,
16        availability: availability,
17        book: book
18      };
19    },
20    updateCheckoutStatus: function ( bookID, newStatus, checkoutDate, checkoutMember, newReturnDate ) {
22      var record = bookRecordDatabase[bookID];
23      record.availability = newStatus;
24      record.checkoutDate = checkoutDate;
25      record.checkoutMember = checkoutMember;
26      record.dueReturnDate = newReturnDate;
27    },
29    extendCheckoutPeriod: function ( bookID, newReturnDate ) {
30      bookRecordDatabase[bookID].dueReturnDate = newReturnDate;
31    },
33    isPastDue: function ( bookID ) {
34      var currentDate = new Date();
35      return currentDate.getTime() > Date.parse( bookRecordDatabase[bookID].dueReturnDate );
36    }
37  };

The result of these changes is that all of the data that's been extracted from the Book class is now being stored in an attribute of the BookManager singleton (BookDatabase) - something considerably more efficient than the large number of objects we were previously using. Methods related to book checkouts are also now based here as they deal with data that's extrinsic rather than intrinsic.

This process does add a little complexity to our final solution, however it's a small concern when compared to the performance issues that have been tackled.Data wise, if we have 30 copies of the same book, we are now only storing it once. Also, every function takes up memory. With the flyweight pattern these functions exist in one place (on the manager) and not on every object, thus saving on memory use.

The Flyweight pattern and the DOM

The DOM (Document Object Model) supports two approaches that allow objects to detect events - either top down (event capture) or bottom up (event bubbling).

In event capture, the event is first captured by the outer-most element and propagated to the inner-most element. In event bubbling, the event is captured and given to the inner-most element and then propagated to the outer-elements.

One of the best metaphors for describing Flyweights in this context was written by Gary Chisholm and it goes a little like this:

Try to think of the flyweight in terms of a pond. A fish opens its mouth (the event), bubbles rise to the surface (the bubbling) a fly sitting on the top flies away when the bubble reaches the surface (the action). In this example we can easily transpose the fish opening its mouth to a button being clicked, the bubbles as the bubbling effect and the fly flying away to some function being run

Bubbling was introduced to handle situations where a single event (e.g a click) may be handled by multiple event handlers defined at different levels of the DOM hierarchy. Where this happens, event bubbling executes event handlers defined for specific elements at the lowest level possible. From there on, the event bubbles up to containing elements before going to those even higher up.

Flyweights can be used to tweak the event bubbling process further, as we will see shortly.

Example 1: Centralized event handling

For our first practical example, imagine we have a number of similar elements in a document with similar behavior executed when a user-action (e.g click, mouse-over) is performed against them.

Normally what we do when constructing our own accordion component, menu or other list-based widget is bind a click event to each link element in the parent container (e.g $('ul li a').on(..). Instead of binding the click to multiple elements, we can easily attach a Flyweight to the top of our container which can listen for events coming from below. These can then be handled using logic that is as simple or complex as required.

As the types of components mentioned often have the same repeating markup for each section (e.g. each section of an accordion), there's a good chance the behavior of each element that may be clicked is going to be quite similar and relative to similar classes nearby. We'll use this information to construct a very basic accordion using the Flyweight below.

A stateManager namespace is used here to encapsulate our flyweight logic whilst jQuery is used to bind the initial click to a container div. In order to ensure that no other logic on the page is attaching similar handles to the container, an unbind event is first applied.

Now to establish exactly what child element in the container is clicked, we make use of a target check which provides a reference to the element that was clicked, regardless of its parent. We then use this information to handle the click event without actually needing to bind the event to specific children when our page loads.

01<div id="container">
02   <div class="toggle" href="#">More Info (Address)
03       <span class="info">
04           This is more information
05       </span></div>
06   <div class="toggle" href="#">Even More Info (Map)
07       <span class="info">
09       </span>
10   </div>
01var stateManager = {
03  fly: function () {
05    var self = this;
07    $( "#container" ).unbind().on( "click" , function ( e ) {
08      var target = $( e.originalTarget || e.srcElement );
09        if ( target.is( "div.toggle") ) {
10          self.handleClick( target );
11        }
12    });
13  },
15  handleClick: function ( elem ) {
16    elem.find( "span" ).toggle( "slow" );
17  }

The benefit here is that we're converting many independent actions into a shared ones (potentially saving on memory).

Example 2: Using the Flyweight for performance optimization

In our second example, we'll reference some further performance gains that can be achieved using Flyweights with jQuery.

James Padolsey previously wrote an article called 76 bytes for faster jQuery where he reminded us that each time jQuery fires off a callback, regardless of type (filter, each, event handler), we're able to access the function's context (the DOM element related to it) via the this keyword.

Unfortunately, many of us have become used to the idea of wrapping this in $() or jQuery(), which means that a new instance of jQuery is unnecessarily constructed every time, rather than simply doing this:

01$("div").on( "click", function () {
02  console.log( "You clicked: " + $( this ).attr( "id" ));
05// we should avoid using the DOM element to create a
06// jQuery object (with the overhead that comes with it)
07// and just use the DOM element itself like this:
09$( "div" ).on( "click", function () {
10  console.log( "You clicked:"  + this.id );

James had wanted to use jQuery's jQuery.text in the following context, however he disagreed with the notion that a new jQuery object had to be created on each iteration:

$( "a" ).map( function () { 
  return $( this ).text(); 

Now with respect to redundant wrapping, where possible with jQuery's utility methods, it's better to use jQuery.methodName (e.g jQuery.text) as opposed to jQuery.fn.methodName (e.g jQuery.fn.text) where methodName represents a utility such as each() or text. This avoids the need to call a further level of abstraction or construct a new jQuery object each time our function is called as as jQuery.methodName is what the library itself uses at a lower-level to power jQuery.fn.methodName.

Because however not all of jQuery's methods have corresponding single-node functions, Padolsey devised the idea of a jQuery.single utility.

The idea here is that a single jQuery object is created and used for each call to jQuery.single (effectively meaning only one jQuery object is ever created). The implementation for this can be found below and as we're consolidating data for multiple possible objects into a more central singular structure, it is technically also a Flyweight.

01jQuery.single = (function( o ){
03   var collection = jQuery([1]);
04   return function( element ) {
06       // Give collection the element:
07       collection[0] = element;
09        // Return the collection:
10       return collection;
12   };

An example of this in action with chaining is:

1$( "div" ).on( "click", function () {
3   var html = jQuery.single( this ).next().html();
4   console.log( html );

Note: Although we may believe that simply caching our jQuery code may offer just as equivalent performance gains, Padolsey claims that $.single() is still worth using and can perform better. That's not to say don't apply any caching at all, just be mindful that this approach can assist. For further details about $.single, I recommend reading Padolsey's full post:

# JavaScript MV* Patterns

In this section, we're going to review three very important architectural patterns - MVC (Model-View-Controller), MVP (Model-View-Presenter) and MVVM (Model-View-ViewModel). In the past, these patterns have been heavily used for structuring desktop and server-side applications but it's only been in recent years that come to being applied to JavaScript.

As the majority of JavaScript developers currently using these patterns opt to utilize libraries such as Backbone.js for implementing an MVC/MV*-like structure, we will compare how modern solutions such as it differ in their interpretation of MVC compared to classical takes on these patterns.

Let us first now cover the basics.


MVC is an architectural design pattern that encourages improved application organization through a separation of concerns. It enforces the isolation of business data (Models) from user interfaces (Views), with a third component (Controllers) traditionally managing logic and user-input. The pattern was originally designed by Trygve Reenskaug during his time working on Smalltalk-80 (1979) where it was initially called Model-View-Controller-Editor. MVC went on to be described in depth in 1995's “Design Patterns: Elements of Reusable Object-Oriented Software” (The "GoF" book), which played a role in popularizing its use.

Smalltalk-80 MVC

It's important to understand what the original MVC pattern was aiming to solve as it's mutated quite heavily since the days of its origin. Back in the 70's, graphical user-interfaces were few and far between and a concept known as Separated Presentation began to be used as a means to make a clear division between domain objects which modeled concepts in the real world (e.g a photo, a person) and the presentation objects which were rendered to the user's screen.

The Smalltalk-80 implementation of MVC took this concept further and had an objective of separating out the application logic from the user interface. The idea was that decoupling these parts of the application would also allow the reuse of models for other interfaces in the application. There are some interesting points worth noting about Smalltalk-80's MVC architecture:

Developers are sometimes surprised when they learn that the Observer pattern (nowadays commonly implemented as the Publish/Subscribe variation) was included as a part of MVC's architecture many decades ago. In Smalltalk-80's MVC, the View observes the Model. As mentioned in the bullet point above, anytime the Model changes, the Views react. A simple example of this is an application backed by stock market data - in order for the application to be useful, any change to the data in our Models should result in the View being refreshed instantly.

Martin Fowler has done an excellent job of writing about the origins of MVC over the years and if interested in some further historical information about Smalltalk-80's MVC, I recommend reading his work.

MVC For JavaScript Developers

We've reviewed the 70's, but let us now return to the here and now. In modern times, the MVC pattern has been applied to a diverse range of programming languages including of most relevance to us: JavaScript. JavaScript now has a number of frameworks boasting support for MVC (or variations on it, which we refer to as the MV* family), allowing developers to easily add structure to their applications without great effort.

These frameworks include the likes of Backbone, Ember.js and AngularJS. Given the importance of avoiding "spaghetti" code, a term which describes code that is very difficult to read or maintain due to its lack of structure, it's imperative that the modern JavaScript developer understand what this pattern provides. This allows us to effectively appreciate what these frameworks enable us to do differently.

We know that MVC is composed of three core components:


Models manage the data for an application. They are concerned with neither the user-interface nor presentation layers but instead represent unique forms of data that an application may require. When a model changes (e.g when it is updated), it will typically notify its observers (e.g views, a concept we will cover shortly) that a change has occurred so that they may react accordingly.

To understand models further, let us imagine we have a JavaScript photo gallery application. In a photo gallery, the concept of a photo would merit its own model as it represents a unique kind of domain-specific data. Such a model may contain related attributes such as a caption, image source and additional meta-data. A specific photo would be stored in an instance of a model and a model may also be reusable. Below we can see an example of a very simplistic model implemented using Backbone.

01var Photo = Backbone.Model.extend({
03    // Default attributes for the photo
04    defaults: {
05      src: "placeholder.jpg",
06      caption: "A default image",
07      viewed: false
08    },
10    // Ensure that each photo created has an `src`.
11    initialize: function() {
12       this.set( { "src": this.defaults.src} );
13    }

The built-in capabilities of models vary across frameworks, however it is quite common for them to support validation of attributes, where attributes represent the properties of the model, such as a model identifier. When using models in real-world applications we generally also desire model persistence. Persistence allows us to edit and update models with the knowledge that its most recent state will be saved in either: memory, in a user's localStorage data-store or synchronized with a database.

In addition, a model may also have multiple views observing it. If say, our photo model contained meta-data such as its location (longitude and latitude), friends that were present in the photo (a list of identifiers) and a list of tags, a developer may decide to provide a single view to display each of these three facets.

It is not uncommon for modern MVC/MV* frameworks to provide a means to group models together (e.g. in Backbone, these groups are referred to as "collections"). Managing models in groups allows us to write application logic based on notifications from the group should any model it contains be changed. This avoids the need to manually observe individual model instances.

A sample grouping of models into a simplified Backbone collection can be seen below.

01var PhotoGallery = Backbone.Collection.extend({
03    // Reference to this collection's model.
04    model: Photo,
06    // Filter down the list of all photos
07    // that have been viewed
08    viewed: function() {
09        return this.filter(function( photo ){
10           return photo.get( "viewed" );
11        });
12    },
14    // Filter down the list to only photos that
15    // have not yet been viewed
16    unviewed: function() {
17      return this.without.apply( this, this.viewed() );
18    }

Older texts on MVC may also contain reference to a notion of models managing application state.In JavaScript applications state has a different connotation, typically referring to the current "state" i.e view or sub-view (with specific data) on a users screen at a fixed point. State is a topic which is regularly discussed when looking at Single-page applications, where the concept of state needs to be simulated.

So to summarize, models are primarily concerned with business data.


Views are a visual representation of models that present a filtered view of their current state. Whilst Smalltalk views are about painting and maintaining a bitmap, JavaScript views are about building and maintaining a DOM element.

A view typically observes a model and is notified when the model changes, allowing the view to update itself accordingly. Design pattern literature commonly refers to views as "dumb" given that their knowledge of models and controllers in an application is limited.

Users are able to interact with views and this includes the ability to read and edit (i.e get or set the attribute values in) models. As the view is the presentation layer, we generally present the ability to edit and update in a user-friendly fashion. For example, in the former photo gallery application we discussed earlier, model editing could be facilitated through an "edit' view where a user who has selected a specific photo could edit its meta-data.

The actual task of updating the model falls to controllers (which we will be covering shortly).

Let's explore views a little further using a vanilla JavaScript sample implementation. Below we can see a function that creates a single Photo view, consuming both a model instance and a controller instance.

We define a render() utility within our view which is responsible for rendering the contents of the photoModel using a JavaScript templating engine (Underscore templating) and updating the contents of our view, referenced by photoEl.

The photoModel then adds our render() callback as one of its subscribers so that through the Observer pattern we can trigger the view to update when the model changes.

One may wonder where user-interaction comes into play here. When users click on any elements within the view, it's not the view's responsibility to know what to do next. It relies on a controller to make this decision for it. In our sample implementation, this is achieved by adding an event listener to photoEl which will delegate handling the click behavior back to the controller, passing the model information along with it in case it's needed.

The benefit of this architecture is that each component plays its own separate role in making the application function as needed.

01var buildPhotoView = function ( photoModel, photoController ) {
03  var base = document.createElement( "div" ),
04      photoEl = document.createElement( "div" );
06  base.appendChild(photoEl);
08  var render = function () {
09          // We use a templating library such as Underscore
10          // templating which generates the HTML for our
11          // photo entry
12          photoEl.innerHTML = _.template( "#photoTemplate" , {
13              src: photoModel.getSrc()
14          });
15      };
17  photoModel.addSubscriber( render );
19  photoEl.addEventListener( "click", function () {
20    photoController.handleEvent( "click", photoModel );
21  });
23  var show = function () {
24    photoEl.style.display = "";
25  };
27  var hide = function () {
28    photoEl.style.display = "none";
29  };
31  return {
32    showView: show,
33    hideView: hide
34  };


In the context of JavaScript frameworks that support MVC/MV*, it is worth briefly discussing JavaScript templating and its relationship to views as we briefly touched upon it in the last section.

It has long been considered (and proven) a performance bad practice to manually create large blocks of HTML markup in-memory through string concatenation. Developers doing so have fallen prey to inperformantly iterating through their data, wrapping it in nested divs and using outdated techniques such as document.write to inject the "template" into the DOM. As this typically means keeping scripted markup inline with our standard markup, it can quickly become both difficult to read and more importantly, maintain such disasters, especially when building non-trivially sized applications.

JavaScript templating solutions (such as Handlebars.js and Mustache) are often used to define templates for views as markup (either stored externally or within script tags with a custom type - e.g text/template) containing template variables. Variables may be delimitated using a variable syntax (e.g {{name}}) and frameworks are typically smart enough to accept data in a JSON form (of which model instances can be converted to) such that we only need be concerned with maintaining clean models and clean templates. Most of the grunt work to do with population is taken care of by the framework itself. This has a large number of benefits, particularly when opting to store templates externally as this can give way to templates being dynamically loaded on an as-needed basis when it comes to building larger applications.

Below we can see two examples of HTML templates. One implemented using the popular Handlebars.js framework and another using Underscore's templates.


1<li class="photo">
2  <h2>{{caption}}</h2>
3  <img class="source" src="{{src}}"/>
4  <div class="meta-data">
5    {{metadata}}
6  </div>

Underscore.js Microtemplates:

1<li class="photo">
2  <h2><%= caption %></h2>
3  <img class="source" src="<%= src %>"/>
4  <div class="meta-data">
5    <%= metadata %>
6  </div>

Note that templates are not themselves views. Developers coming from a Struts Model 2 architecture may feel like a template *is a view, but it isn't. A view is an object which observes a model and keeps the visual representation up-to-date. A template *might* be a declarative way to specify part or even all of a view object so that it can be generated from the template specification.

It is also worth noting that in classical web development, navigating between independent views required the use of a page refresh. In Single-page JavaScript applications however, once data is fetched from a server via Ajax, it can simply be dynamically rendered in a new view within the same page without any such refresh being necessary.

The role of navigation thus falls to a "router", which assists in managing application state (e.g allowing users to bookmark a particular view they have navigated to). As routers are, however, neither a part of MVC nor present in every MVC-like framework, I will not be going into them in greater detail in this section.

To summarize, views are a visual representation of our application data.


Controllers are an intermediary between models and views which are classically responsible for updating the model when the user manipulates the view.

In our photo gallery application, a controller would be responsible for handling changes the user made to the edit view for a particular photo, updating a specific photo model when a user has finished editing.

Remember that the controllers fulfill one role in MVC: the facilitation of the Strategy pattern for the view. In the Strategy pattern regard, the view delegates to the controller at the view's discretion. So, that's how the strategy pattern works. The view could delegate handling user events to the controller when the view sees fit. The view *could* delegate handling model change events to the controller if the view sees fit, but this is not the traditional role of the controller.

In terms of where most JavaScript MVC frameworks detract from what is conventionally considered "MVC" however, it is with controllers. The reasons for this vary, but in my honest opinion, it is that framework authors initially look at the server-side interpretation of MVC, realize that it doesn't translate 1:1 on the client-side and re-interpret the C in MVC to mean something they feel makes more sense. The issue with this however is that it is subjective, increases the complexity in both understanding the classical MVC pattern and of course the role of controllers in modern frameworks.

As an example, let's briefly review the architecture of the popular architectural framework Backbone.js. Backbone contains models and views (somewhat similar to what we reviewed earlier), however it doesn't actually have true controllers. Its views and routers act a little similar to a controller, but neither are actually controllers on their own.

In this respect, contrary to what might be mentioned in the official documentation or in blog posts, Backbone is neither a truly MVC/MVP nor MVVM framework. It's in fact better to consider it a member of the MV* family which approaches architecture in its own way. There is of course nothing wrong with this, but it is important to distinguish between classical MVC and MV* should we begin relying on advice from classical literature on the former to help with the latter.

Controllers in another library (Spine.js) vs Backbone.js


We now know that controllers are traditionally responsible for updating the model when the user updates the view. It's interesting to note that the most popular JavaScript MVC/MV* framework at the time of writing (Backbone) does not have it's own explicit concept of controllers.

It can thus be useful for us to review the controller from another MVC framework to appreciate the difference in implementations and further demonstrate how nontraditionally frameworks approach the role of the controller. For this, let's take a look at a sample controller from Spine.js:

In this example, we're going to have a controller called PhotosController which will be in charge of individual photos in the application. It will ensure that when the view updates (e.g a user edited the photo meta-data) the corresponding model does too.

Note: We won't be delving heavily into Spine.js at all, but will just take a ten-foot view of what its controllers can do:

01// Controllers in Spine are created by inheriting from Spine.Controller
03var PhotosController = Spine.Controller.sub({ 
05  init: function () {
06    this.item.bind( "update" , this.proxy( this.render ));
07    this.item.bind( "destroy", this.proxy( this.remove ));
08  },
10  render: function () {
11    // Handle templating
12    this.replace( $( "#photoTemplate" ).tmpl( this.item ) );
13    return this;
14  },
16  remove: function () {
17    this.el.remove();
18    this.release();
19  }

In Spine, controllers are considered the glue for an application, adding and responding to DOM events, rendering templates and ensuring that views and models are kept in sync (which makes sense in the context of what we know to be a controller).

What we're doing in the above example is setting up listeners in the update and destroy events using render() and remove(). When a photo entry gets updated, we re-render the view to reflect the changes to the meta-data. Similarly, if the photo gets deleted from the gallery, we remove it from the view. In the render() function, we're using Underscore micro-templating (via _.template()) to render a JavaScript template with the ID #photoTemplate. This simply returns a compiled HTML string used to populate the contents of photoEl.

What this provides us with is a very lightweight, simple way to manage changes between the model and the view.


Later on in this section we're going to revisit the differences between Backbone and traditional MVC, but for now let's focus on controllers.

In Backbone, one shares the responsibility of a controller with both the Backbone.View and Backbone.Router. Some time ago Backbone did once come with its own Backbone.Controller, but as the naming for this component didn't make sense for the context in which it was being used, it was later renamed to Router.

Routers handle a little more of the controller responsibility as it's possible to bind the events there for models and have our view respond to DOM events and rendering. As Tim Branyen (another Bocoup-based Backbone contributor) has also previously pointed out, it's possible to get away with not needing Backbone.Router at all for this, so a way to think about it using the Router paradigm is probably:

01var PhotoRouter = Backbone.Router.extend({
02  routes: { "photos/:id": "route" },
04  route: function( id ) {
05    var item = photoCollection.get( id );
06    var view = new PhotoView( { model: item } );
08    $('.content').html( view.render().el );
09  }

To summarize, the takeaway from this section is that controllers manage the logic and coordination between models and views in an application.

What does MVC give us?

This separation of concerns in MVC facilitates simpler modularization of an application's functionality and enables:

Smalltalk-80 MVC In JavaScript

Although the majority of modern-day JavaScript frameworks attempt to evolve the MVC paradigm to better fit the differing needs of web application development, there is one framework which attempts to adhere to the pure form of the pattern found in Smalltalk-80. Maria.js (https://github.com/petermichaux/maria) by Peter Michaux offers an implementation which is faithful to MVCs origins - Models are models, Views are views and Controllers are nothing but controllers. Whilst some developers might feel an MV* framework should address more concerns, this is a useful reference to be aware of in case you would like a JavaScript implementation of the original MVC.

Delving deeper

At this point in the book, we should have a basic understanding of what the MVC pattern provides, but there's still some fascinating information about it worth noting.

The GoF do not refer to MVC as a design pattern, but rather consider it a set of classes to build a user interface. In their view, it's actually a variation of three classical design patterns: the Observer, Strategy and Composite patterns. Depending on how MVC has been implemented in a framework, it may also use the Factory and Template patterns. The GoF book mentions these patterns as useful extras when working with MVC.

As we have discussed, models represent application data whilst views are what the user is presented on screen. As such, MVC relies on the Observer pattern for some of its core communication (something that surprisingly isn't covered in many articles about the MVC pattern). When a model is changed it notifies its observers (Views) that something has been updated - this is perhaps the most important relationship in MVC. The observer nature of this relationship is also what facilitates multiple views being attached to the same model.

For developers interested in knowing more about the decoupled nature of MVC (once again, depending on the implementation), one of the goals of the pattern is to help define one-to-many relationships between a topic (data object) and its observers. When a topic changes, its observers are updated. Views and controllers have a slightly different relationship. Controllers facilitate views to respond to different user input and are an example of the Strategy pattern.


Having reviewed the classical MVC pattern, we should now understand how it allows us to cleanly separate concerns in an application. We should also now appreciate how JavaScript MVC frameworks may differ in their interpretation of the MVC pattern, which although quite open to variation, still shares some of the fundamental concepts the original pattern has to offer.

When reviewing a new JavaScript MVC/MV* framework, remember - it can be useful to step back and review how it's opted to approach architecture (specifically, how it supports implementing models, views, controllers or other alternatives) as this can better help us grok how the framework expects to be used.


Model-view-presenter (MVP) is a derivative of the MVC design pattern which focuses on improving presentation logic. It originated at a company named Taligent in the early 1990s while they were working on a model for a C++ CommonPoint environment. Whilst both MVC and MVP target the separation of concerns across multiple components, there are some fundamental differences between them.

For the purposes of this summary we will focus on the version of MVP most suitable for web-based architectures.

Models, Views & Presenters

The P in MVP stands for presenter. It's a component which contains the user-interface business logic for the view. Unlike MVC, invocations from the view are delegated to the presenter, which are decoupled from the view and instead talk to it through an interface. This allows for all kinds of useful things such as being able to mock views in unit tests.

The most common implementation of MVP is one which uses a Passive View (a view which is for all intents and purposes "dumb"), containing little to no logic. If MVC and MVP are different it is because the C and P do different things. In MVP, the P observes models and updates views when models change. The P effectively binds models to views, a responsibility which was previously held by controllers in MVC.

Solicited by a view, presenters perform any work to do with user requests and pass data back to them. In this respect, they retrieve data, manipulate it and determine how the data should be displayed in the view. In some implementations, the presenter also interacts with a service layer to persist data (models). Models may trigger events but it's the presenters role to subscribe to them so that it can update the view. In this passive architecture, we have no concept of direct data binding. Views expose setters which presenters can use to set data.

The benefit of this change from MVC is that it increases the testability of our application and provides a more clean separation between the view and the model. This isn't however without its costs as the lack of data binding support in the pattern can often mean having to take care of this task separately.

Although a common implementation of a Passive View is for the view to implement an interface, there are variations on it, including the use of events which can decouple the View from the Presenter a little more. As we don't have the interface construct in JavaScript, we're using more a protocol than an explicit interface here. It's technically still an API and it's probably fair for us to refer to it as an interface from that perspective.

There is also a Supervising Controller variation of MVP, which is closer to the MVC and MVVM patterns as it provides data-binding from the Model directly from the View. Key-value observing (KVO) plugins (such as Derick Bailey's Backbone.ModelBinding plugin) tend to bring Backbone out of the Passive View and more into the Supervising Controller or MVVM variations.


MVP is generally used most often in enterprise-level applications where it's necessary to reuse as much presentation logic as possible. Applications with very complex views and a great deal of user interaction may find that MVC doesn't quite fit the bill here as solving this problem may mean heavily relying on multiple controllers. In MVP, all of this complex logic can be encapsulated in a presenter, which can simplify maintenance greatly.

As MVP views are defined through an interface and the interface is technically the only point of contact between the system and the view (other than a presenter), this pattern also allows developers to write presentation logic without needing to wait for designers to produce layouts and graphics for the application.

Depending on the implementation, MVP may be easier to automatically unit test than MVC. The reason often cited for this is that the presenter can be used as a complete mock of the user-interface and so it can be unit tested independent of other components. In my experience this really depends on the languages we are implementing MVP in (there's quite a difference between opting for MVP for a JavaScript project over one for say, ASP.net).

At the end of the day, the underlying concerns we may have with MVC will likely hold true for MVP given that the differences between them are mainly semantic. As long as we are cleanly separating concerns into models, views and controllers (or presenters) we should be achieving most of the same benefits regardless of the variation we opt for.

MVC, MVP and Backbone.js

There are very few, if any architectural JavaScript frameworks that claim to implement the MVC or MVP patterns in their classical form as many JavaScript developers don't view MVC and MVP as being mutually exclusive (we are actually more likely to see MVP strictly implemented when looking at web frameworks such as ASP.net or GWT). This is because it's possible to have additional presenter/view logic in our application and yet still consider it a flavor of MVC.

Backbone contributor Irene Ros (of Boston-based Bocoup) subscribes to this way of thinking as when she separates views out into their own distinct components, she needs something to actually assemble them for her. This could either be a controller route (such as a Backbone.Router, covered later in the book) or a callback in response to data being fetched.

That said, some developers do however feel that Backbone.js better fits the description of MVP than it does MVC. Their view is that:

A response to this could be that the view can also just be a View (as per MVC) because Backbone is flexible enough to let it be used for multiple purposes. The V in MVC and the P in MVP can both be accomplished by Backbone.View because they're able to achieve two purposes: both rendering atomic components and assembling those components rendered by other views.

We've also seen that in Backbone the responsibility of a controller is shared with both the Backbone.View and Backbone.Router and in the following example we can actually see that aspects of that are certainly true.

Our Backbone PhotoView uses the Observer pattern to "subscribe" to changes to a View's model in the line this.model.bind("change",...). It also handles templating in the render() method, but unlike some other implementations, user interaction is also handled in the View (see events).

01var PhotoView = Backbone.View.extend({
03    //... is a list tag.
04    tagName:  "li",
06    // Pass the contents of the photo template through a templating
07    // function, cache it for a single photo
08    template: _.template( $("#photo-template").html() ),
10    // The DOM events specific to an item.
11    events: {
12      "click img" : "toggleViewed"
13    },
15    // The PhotoView listens for changes to
16    // its model, re-rendering. Since tHere's
17    // a one-to-one correspondence between a
18    // **Photo** and a **PhotoView** in this
19    // app, we set a direct reference on the model for convenience.
21    initialize: function() {
22      this.model.on( "change", this.render, this );
23      this.model.on( "destroy", this.remove, this );
24    },
26    // Re-render the photo entry
27    render: function() {
28      $( this.el ).html( this.template(this.model.toJSON() ));
29      return this;
30    },
32    // Toggle the `"viewed"` state of the model.
33    toggleViewed: function() {
34      this.model.viewed();
35    }

Another (quite different) opinion is that Backbone more closely resembles Smalltalk-80 MVC, which we went through earlier.

As regular Backbone blogger Derick Bailey has previously put it, it's ultimately best not to force Backbone to fit any specific design patterns. Design patterns should be considered flexible guides to how applications may be structured and in this respect, Backbone fits neither MVC nor MVP. Instead, it borrows some of the best concepts from multiple architectural patterns and creates a flexible framework that just works well.

It is however worth understanding where and why these concepts originated, so I hope that my explanations of MVC and MVP have been of help. Call it the Backbone way, MV* or whatever helps reference its flavor of application architecture. Most structural JavaScript frameworks will adopt their own take on classical patterns, either intentionally or by accident, but the important thing is that they help us develop applications which are organized, clean and can be easily maintained.



MVVM (Model View ViewModel) is an architectural pattern based on MVC and MVP, which attempts to more clearly separate the development of user-interfaces (UI) from that of the business logic and behavior in an application. To this end, many implementations of this pattern make use of declarative data bindings to allow a separation of work on Views from other layers.

This facilitates UI and development work occurring almost simultaneously within the same codebase. UI developers write bindings to the ViewModel within their document markup (HTML), where the Model and ViewModel are maintained by developers working on the logic for the application.


MVVM (by name) was originally defined by Microsoft for use with Windows Presentation Foundation (WPF) and Silverlight, having been officially announced in 2005 by John Grossman in a blog post about Avalon (the codename for WPF). It also found some popularity in the Adobe Flex community as an alternative to simply using MVC.

Prior to Microsoft adopting the MVVM name, there was however a movement in the community to go from MVP to MVPM: Model View PresentationModel. Martin Fowler wrote an article on PresentationModels back in 2004 for those interested in reading more about it. The idea of a PresentationModel had been around much longer than this article, however it was considered the big break in the idea and greatly helped popularize it.

There was quite a lot of uproar in the "alt.net" circles after Microsoft announced MVVM as an alternative to MVPM. Many claimed the company's dominance in the GUI world was giving them the opportunity to take over the community as a whole, renaming existing concepts as they pleased for marketing purposes. A progressive crowd recognized that whilst MVVM and MVPM were effectively the same idea, they came in slightly different packages.

In recent years, MVVM has been implemented in JavaScript in the form of structural frameworks such as KnockoutJS, Kendo MVVM and Knockback.js, with an overall positive response from the community.

Let’s now review the three components that compose MVVM.


As with other members of the MV* family, the Model in MVVM represents domain-specific data or information that our application will be working with. A typical example of domain-specific data might be a user account (e.g name, avatar, e-mail) or a music track (e.g title, year, album).

Models hold information, but typically don’t handle behavior. They don’t format information or influence how data appears in the browser as this isn’t their responsibility. Instead, formatting of data is handled by the View, whilst behavior is considered business logic that should be encapsulated in another layer that interacts with the Model - the ViewModel.

The only exception to this rule tends to be validation and it’s considered acceptable for Models to validate data being used to define or update existing models (e.g does an e-mail address being input meet the requirements of a particular regular expression?).

In KnockoutJS, Models fall under the above definition, but often make Ajax calls to a server-side service to both read and write Model data.

If we were constructing a simple Todo application, a KnockoutJS Model representing a single Todo item could look as follows:

1var Todo = function ( content, done ) {
2    this.content = ko.observable(content);
3    this.done = ko.observable(done);
4    this.editing = ko.observable(false);

Note: One may notice in the above snippet that we are calling the method observable() on the KnockoutJS namespace ko. In KnockoutJS, observables are special JavaScript objects that can notify subscribers about changes and automatically detect dependencies. This allows us to synchronize Models and ViewModels when the value of a Model attribute is modified.


As with MVC, the View is the only part of the application that users actually interact with. They are an interactive UI that represent the state of a ViewModel. In this sense, the view is considered active rather than passive, but this is also true for views in MVC and MVP. In MVC, MVP and MVVM a view can also be passive, but what does this mean?

A passive View only outputs a display and does not accept any user input.

Such a view may also have no real knowledge of the models in our application and could be manipulated by a presenter. MVVM’s active View contains the data-bindings, events and behaviors which requires an understanding of the ViewModel. Although these behaviors can be mapped to properties, the View is still responsible for handling events from the ViewModel.

It’s important to remember the View isn’t responsible here for handling state - it keeps this in sync with the ViewModel.

A KnockoutJS View is simply a HTML document with declarative bindings to link it to the ViewModel. KnockoutJS Views display information from the ViewModel, pass commands to it (e.g a user clicking on an element) and update as the state of the ViewModel changes. Templates generating markup using data from the ViewModel can however also be used for this purpose.

To give a brief initial example, we can look to the JavaScript MVVM framework KnockoutJS for how it allows the definition of a ViewModel and its related bindings in markup:


1var aViewModel = {
2    contactName: ko.observable("John")


1<p><input id="source" data-bind="value: contactName, valueUpdate: 'keyup'" /></p>
2<div data-bind="visible: contactName().length > 10">
3    You have a really long name!
5<p>Contact name: <strong data-bind="text: contactName"></strong></p>

Our input text-box (source) obtains it's initial value from contactName, automatically updating this value whenever contactName changes. As the data binding is two-way, typing into the text-box will update contactName accordingly so the values are always in sync.

Although implementation specific to KnockoutJS, the <div> containing the "You have a really long name!" text also contains simple validation (once again in the form of data bindings). If the input exceeds 10 characters, it will display, otherwise it will remain hidden.

Moving on to a more advanced example, we can return to our Todo application. A trimmed down KnockoutJS View for this, including all the necessary data-bindings may look as follows.

01<div id="todoapp">
02    <header>
03        <h1>Todos</h1>
04        <input id="new-todo" type="text" data-bind="value: current, valueUpdate: 'afterkeydown', enterKey: add"
05               placeholder="What needs to be done?"/>
06    </header>
07    <section id="main" data-bind="block: todos().length">
09        <input id="toggle-all" type="checkbox" data-bind="checked: allCompleted">
10        <label for="toggle-all">Mark all as complete</label>
12        <ul id="todo-list" data-bind="foreach: todos">
14           <!-- item -->
15            <li data-bind="css: { done: done, editing: editing }">
16                <div class="view" data-bind="event: { dblclick: $root.editItem }">
17                    <input class="toggle" type="checkbox" data-bind="checked: done">
18                    <label data-bind="text: content"></label>
19                    <a class="destroy" href="#" data-bind="click: $root.remove"></a>
20                </div>
21                <input class="edit' type="text"
22                       data-bind="value: content, valueUpdate: 'afterkeydown', enterKey: $root.stopEditing, selectAndFocus: editing, event: { blur: $root.stopEditing }"/>
23            </li>
25        </ul>
27    </section>

Note that the basic layout of the mark-up is relatively straight-forward, containing an input textbox (new-todo) for adding new items, togglers for marking items as complete and a list (todo-list) with a template for a Todo item in the form of an li.

The data bindings in the above markup can be broken down as follows:



The ViewModel can be considered a specialized Controller that acts as a data converter. It changes Model information into View information, passing commands from the View to the Model.

For example, let us imagine that we have a model containing a date attribute in unix format (e.g 1333832407). Rather than our models being aware of a user's view of the date (e.g 04/07/2012 @ 5:00pm), where it would be necessary to convert the attribute to its display format, our model simply holds the raw format of the data. Our View contains the formatted date and our ViewModel acts as a middle-man between the two.

In this sense, the ViewModel might be looked upon as more of a Model than a View but it does handle most of the View's display logic. The ViewModel may also expose methods for helping to maintain the View's state, update the model based on the action's on a View and trigger events on the View.

In summary, the ViewModel sits behind our UI layer. It exposes data needed by a View (from a Model) and can be viewed as the source our Views go to for both data and actions.

KnockoutJS interprets the ViewModel as the representation of data and operations that can be performed on a UI. This isn't the UI itself nor the data model that persists, but rather a layer that can also hold the yet to be saved data a user is working with. Knockout's ViewModels are implemented JavaScript objects with no knowledge of HTML markup. This abstract approach to their implementation allows them to stay simple, meaning more complex behavior can be more easily managed on-top as needed.

A partial KnockoutJS ViewModel for our Todo application could thus look as follows:

01// our main ViewModel
02    var ViewModel = function ( todos ) {
03        var self = this;
05    // map array of passed in todos to an observableArray of Todo objects
06    self.todos = ko.observableArray(
07    ko.utils.arrayMap( todos, function ( todo ) {
08        return new Todo( todo.content, todo.done );
09    }));
11    // store the new todo value being entered
12    self.current = ko.observable();
14    // add a new todo, when enter key is pressed
15    self.add = function ( data, event ) {
16        var newTodo, current = self.current().trim();
17        if ( current ) {
18            newTodo = new Todo( current );
19            self.todos.push( newTodo );
20            self.current("");
21        }
22    };
24    // remove a single todo
25    self.remove = function ( todo ) {
26        self.todos.remove( todo );
27    };
29    // remove all completed todos
30    self.removeCompleted = function () {
31        self.todos.remove(function (todo) {
32            return todo.done();
33        });
34    };
36    // writeable computed observable to handle marking all complete/incomplete
37    self.allCompleted = ko.computed({
39        // always return true/false based on the done flag of all todos
40        read:function () {
41            return !self.remainingCount();
42        },
44        // set all todos to the written value (true/false)
45        write:function ( newValue ) {
46            ko.utils.arrayForEach( self.todos(), function ( todo ) {
47                //set even if value is the same, as subscribers are not notified in that case
48                todo.done( newValue );
49            });
50        }
51    });
53    // edit an item
54    self.editItem = function( item ) {
55        item.editing( true );
56    };
57 ..

Above we are basically providing the methods needed to add, edit or remove items as well as the logic to mark all remaining items as having been completed Note: The only real difference worth noting from previous examples in our ViewModel are observable arrays. In KnockoutJS, if we wish to detect and respond to changes on a single object, we would use observables. If however we wish to detect and respond to changes of a collection of things, we can use an observableArray instead. A simpler example of how to use observables arrays may look as follows:

1// Define an initially an empty array
2var myObservableArray = ko.observableArray();
4// Add a value to the array and notify our observers
5myObservableArray.push( 'A new todo item' );

Note: The complete Knockout.js Todo application we reviewed above can be grabbed from TodoMVC if interested.

Recap: The View and the ViewModel

Views and ViewModels communicate using data-bindings and events. As we saw in our initial ViewModel example, the ViewModel doesn’t just expose Model attributes but also access to other methods and features such as validation.

Our Views handle their own user-interface events, mapping them to the ViewModel as necessary. Models and attributes on the ViewModel are synchronized and updated via two-way data-binding.

Triggers (data-triggers) also allow us to further react to changes in the state of our Model attributes.

Recap: The ViewModel and the Model

Whilst it may appear the ViewModel is completely responsible for the Model in MVVM, there are some subtleties with this relationship worth noting. The ViewModel can expose a Model or Model attributes for the purposes of data-binding and can also contain interfaces for fetching and manipulating properties exposed in the view.

Pros and Cons

We now hopefully have a better appreciation for what MVVM is and how it works. Let’s now review the advantages and disadvantages of employing the pattern:



MVVM With Looser Data-Bindings

It’s not uncommon for JavaScript developers from an MVC or MVP background to review MVVM and complain about its true separation of concerns. Namely, the quantity of inline data-bindings maintained in the HTML markup of a View.

I must admit that when I first reviewed implementations of MVVM (e.g KnockoutJS, Knockback), I was surprised that any developer would want to return to the days of old where we mixed logic (JavaScript) with our markup and found it quickly unmaintainable. The reality however is that MVVM does this for a number of good reasons (which we’ve covered), including facilitating designers to more easily bind to logic from their markup.

For the purists among us, you’ll be happy to know that we can now also greatly reduce how reliant we are on data-bindings thanks to a feature known as custom binding providers, introduced in KnockoutJS 1.3 and available in all versions since.

KnockoutJS by default has a data-binding provider which searches for any elements with data-bind attributes on them such as in the below example.

1<input id="new-todo" type="text" data-bind="value: current, valueUpdate: 'afterkeydown', enterKey: add" placeholder="What needs to be done?"/>

When the provider locates an element with this attribute, it parses it and turns it into a binding object using the current data context. This is the way KnockoutJS has always worked, allowing us to declaratively add bindings to elements which KnockoutJS binds to the data at that layer.

Once we start building Views that are no longer trivial, we may end up with a large number of elements and attributes whose bindings in markup can become difficult to manage. With custom binding providers however, this is no longer a problem.

A binding provider is primarily interested in two things:

Binding providers implement two functions:

A skeleton binding provider might thus look as follows:

1var ourBindingProvider = {
2  nodeHasBindings: function( node ) {
3      // returns true/false
4  },
6  getBindings: function( node, bindingContext ) {
7      // returns a binding object
8  }

Before we get to fleshing out this provider, let’s briefly discuss logic in data-bind attributes.

If when using Knockout’s MVVM we find yourself dissatisfied with the idea of application logic being overly tied into your View, we can change this. We could implement something a little like CSS classes to assign bindings by name to elements. Ryan Niemeyer (of knockmeout.net) has previously suggested using data-class for this to avoid confusing presentation classes with data classes, so let’s get our nodeHasBindings function supporting this:

1// does an element have any bindings?
2function nodeHasBindings( node ) {
3    return node.getAttribute ? node.getAttribute("data-class") : false;

Next, we need a sensible getBindings() function. As we’re sticking with the idea of CSS classes, why not also consider supporting space-separated classes to allow us to share binding specs between different elements?

Let’s first review what our bindings will look like. We create an object to hold them where our property names need to match the keys we wish to use in our data-classes.

Note: There isn’t a great deal of work required to convert a KnockoutJS application from using traditional data-bindings over to unobtrusive bindings with custom binding providers. We simply pull our all of our data-bind attributes, replace them with data-class attributes and place our bindings in a binding object as per below:

01var viewModel = new ViewModel( todos || [] ),
02    bindings = {
04        newTodo:  {
05            value: viewModel.current,
06            valueUpdate: "afterkeydown",
07            enterKey: viewModel.add
08        },
10        taskTooltip : {
11            visible: viewModel.showTooltip
12        },
13        checkAllContainer : {
14            visible: viewModel.todos().length
15        },
16        checkAll: {
17            checked: viewModel.allCompleted
18        },
20        todos: {
21            foreach: viewModel.todos
22        },
23        todoListItem: function() {
24            return {
25                css: {
26                    editing: this.editing
27                }
28            };
29        },
30        todoListItemWrapper: function() {
31            return {
32                css: {
33                    done: this.done
34                }
35            };
36        },
37        todoCheckBox: function() {
38            return {
39                checked: this.done
40            };
41        },
42        todoContent: function() {
43            return {
44                text: this.content,
45                event: {
46                    dblclick: this.edit
47                }
48            };
49        },
50        todoDestroy: function() {
51            return {
52                click: viewModel.remove
53            };
54        },       
56        todoEdit: function() {
57            return {
58                value: this.content,
59                valueUpdate: "afterkeydown",
60                enterKey: this.stopEditing,
61                event: {
62                    blur: this.stopEditing
63                }
64            };
65        },
67        todoCount: {
68            visible: viewModel.remainingCount
69        },
70        remainingCount: {
71            text: viewModel.remainingCount
72        },
73        remainingCountWord: function() {
74            return {
75                text: viewModel.getLabel(viewModel.remainingCount)
76            };
77        },
78        todoClear: {
79            visible: viewModel.completedCount
80        },
81        todoClearAll: {
82            click: viewModel.removeCompleted
83        },
84        completedCount: {
85            text: viewModel.completedCount
86        },
87        completedCountWord: function() {
88            return {
89                text: viewModel.getLabel(viewModel.completedCount)
90            };
91        },
92        todoInstructions: {
93            visible: viewModel.todos().length
94        }
95    };
97    ....

There are however two lines missing from the above snippet - we still need our getBindings function, which will loop through each of the keys in our data-class attributes and build up the resulting object from each of them. If we detect that the binding object is a function, we call it with our current data using the context this. Our complete custom binding provider would look as follows:

01    // We can now create a bindingProvider that uses
02    // something different than data-bind attributes
03    ko.customBindingProvider = function( bindingObject ) {
04        this.bindingObject = bindingObject;
06        // determine if an element has any bindings
07        this.nodeHasBindings = function( node ) {
08            return node.getAttribute ? node.getAttribute( "data-class" ) : false;
09        };
10      };
12    // return the bindings given a node and the bindingContext
13    this.getBindings = function( node, bindingContext ) {
15        var result = {},
16            classes = node.getAttribute( "data-class" );
18        if ( classes ) {
19            classes = classes.split( "" ); 
21            //evaluate each class, build a single object to return
22            for ( var i = 0, j = classes.length; i < j; i++ ) {
24               var bindingAccessor = this.bindingObject[classes[i]];
25               if ( bindingAccessor ) {
26                   var binding = typeof bindingAccessor === "function" ? bindingAccessor.call(bindingContext.$data) : bindingAccessor;
27                   ko.utils.extend(result, binding);              
28               }  
30            }
31        }
33        return result;
34    };

Thus, the final few lines of our bindings object can be defined as follows:

1// set ko's current bindingProvider equal to our new binding provider
2ko.bindingProvider.instance = new ko.customBindingProvider( bindings );  
4// bind a new instance of our ViewModel to the page
5ko.applyBindings( viewModel );

What we’re doing here is effectively defining constructor for our binding handler which accepts an object (bindings) which we use to lookup our bindings. We could then re-write the markup for our application View using data-classes as follows:

01<div id="create-todo">
02                <input id="new-todo" data-class="newTodo" placeholder="What needs to be done?" />
03                <span class="ui-tooltip-top" data-class="taskTooltip" style="display: none;">Press Enter to save this task</span>
04            </div>
05            <div id="todos">
06                <div data-class="checkAllContainer" >
07                    <input id="check-all" class="check" type="checkbox" data-class="checkAll" />
08                    <label for="check-all">Mark all as complete</label>
09                </div>
10                <ul id="todo-list" data-class="todos" >
11                    <li data-class="todoListItem" >
12                        <div class="todo" data-class="todoListItemWrapper" >
13                            <div class="display">
14                                <input class="check" type="checkbox" data-class="todoCheckBox" />
15                                <div class="todo-content" data-class="todoContent" style="cursor: pointer;"></div>
16                                <span class="todo-destroy" data-class="todoDestroy"></span>
17                            </div>
18                            <div class="edit'>
19                                <input class="todo-input" data-class="todoEdit'/>
20                            </div>
21                        </div>
22                    </li>
23                </ul>
24            </div>

Neil Kerkin has put together a complete TodoMVC demo app using the above, which can be accessed and played around with here.

Whilst it may look like quite a lot of work in the explanation above, now that we have a generic getBindings method written, it’s a lot more trivial to simply re-use it and use data-classes rather than strict data-bindings for writing our KnockoutJS applications instead. The net result is hopefully cleaner markup with our data bindings being shifted from the View to a bindings object instead.


Both MVP and MVVM are derivatives of MVC. The key difference between it and its derivatives is the dependency each layer has on other layers as well as how tightly bound they are to each other.

In MVC, the View sits on top of our architecture with the controller beside it. Models sit below the controller and so our Views know about our controllers and controllers know about Models. Here, our Views have direct access to Models. Exposing the complete Model to the View however may have security and performance costs, depending on the complexity of our application. MVVM attempts to avoid these issues.

In MVP, the role of the controller is replaced with a Presenter. Presenters sit at the same level as views, listening to events from both the View and model and mediating the actions between them. Unlike MVVM, there isn’t a mechanism for binding Views to ViewModels, so we instead rely on each View implementing an interface allowing the Presenter to interact with the View.

MVVM consequently allows us to create View-specific subsets of a Model which can contain state and logic information, avoiding the need to expose the entire Model to a View. Unlike MVP’s Presenter, a ViewModel is not required to reference a View. The View can bind to properties on the ViewModel which in turn expose data contained in Models to the View. As we’ve mentioned, the abstraction of the View means there is less logic required in the code behind it.

One of the downsides to this however is that a level of interpretation is needed between the ViewModel and the View and this can have performance costs. The complexity of this interpretation can also vary - it can be as simple as copying data or as complex as manipulating them to a form we would like the View to see. MVC doesn’t have this problem as the whole Model is readily available and such manipulation can be avoided.

Backbone.js Vs. KnockoutJS

Understanding the subtle differences between MVC, MVP and MVVM are important but developers ultimately will ask whether they should consider using KnockoutJS over Backbone based in what we’ve learned. The following notes may be of help here:

To conclude, I personally find KnockoutJS more suitable for smaller applications whilst Backbone’s feature set really shines when building anything non-trivial. That said, many developers have used both frameworks to write applications of varying complexity and I recommend trying out both at a smaller scale before making a decision on which might work best for your project.

For further reading about MVVM or Knockout, I recommend the following articles:

# Modern Modular JavaScript Design Patterns

The Importance Of Decoupling Applications

In the world of scalable JavaScript, when we say an application is modular, we often mean it's composed of a set of highly decoupled, distinct pieces of functionality stored in modules. Loose coupling facilitates easier maintainability of apps by removing dependencies where possible. When this is implemented efficiently, it's quite easy to see how changes to one part of a system may affect another.

Unlike some more traditional programming languages however, the current iteration of JavaScript (ECMA-262) doesn't provide developers with the means to import such modules of code in a clean, organized manner. It's one of the concerns with specifications that haven't required great thought until more recent years where the need for more organized JavaScript applications became apparent.

Instead, developers at present are left to fall back on variations of the module or object literal patterns, which we covered earlier in the book. With many of these, module scripts are strung together in the DOM with namespaces being described by a single global object where it's still possible to incur naming collisions in our architecture. There's also no clean way to handle dependency management without some manual effort or third party tools.

Whilst native solutions to these problems will be arriving in ES Harmony (likely to be the next version of JavaScript), the good news is that writing modular JavaScript has never been easier and we can start doing it today.

In this section, we're going to look at three formats for writing modular JavaScript: AMD, CommonJS and proposals for the next version of JavaScript, Harmony.

A Note On Script Loaders

It's difficult to discuss AMD and CommonJS modules without talking about the elephant in the room - script loaders. At the time of writing this book, script loading is a means to a goal, that goal being modular JavaScript that can be used in applications today - for this, use of a compatible script loader is unfortunately necessary. In order to get the most out of this section, I recommend gaining a basic understanding of how popular script loading tools work so the explanations of module formats make sense in context.

There are a number of great loaders for handling module loading in the AMD and CommonJS formats, but my personal preferences are RequireJS and curl.js. Complete tutorials on these tools are outside the scope of this book, but I can recommend reading John Hann's article about curl.js and James Burke's RequireJS API documentation for more.

From a production perspective, the use of optimization tools (like the RequireJS optimizer) to concatenate scripts is recommended for deployment when working with such modules. Interestingly, with the Almond AMD shim, RequireJS doesn't need to be rolled in the deployed site and what one might consider a script loader can be easily shifted outside of development.

That said, James Burke would probably say that being able to dynamically load scripts after page load still has its use cases and RequireJS can assist with this too. With these notes in mind, let's get started.


A Format For Writing Modular JavaScript In The Browser

The overall goal for the AMD (Asynchronous Module Definition) format is to provide a solution for modular JavaScript that developers can use today. It was born out of Dojo's real world experience using XHR+eval and proponents of this format wanted to avoid any future solutions suffering from the weaknesses of those in the past.

The AMD module format itself is a proposal for defining modules where both the module and dependencies can be asynchronously loaded. It has a number of distinct advantages including being both asynchronous and highly flexible by nature which removes the tight coupling one might commonly find between code and module identity. Many developers enjoy using it and one could consider it a reliable stepping stone towards the module system proposed for ES Harmony.

AMD began as a draft specification for a module format on the CommonJS list but as it wasn't able to reach full consensus, further development of the format moved to the amdjs group.

Today it's embraced by projects including Dojo, MooTools, Firebug and even jQuery . Although the term CommonJS AMD format has been seen in the wild on occasion, it's best to refer to it as just AMD or Async Module support as not all participants on the CommonJS list wished to pursue it.

Note: There was a time when the proposal was referred to as Modules Transport/C, however as the spec wasn't geared towards transporting existing CommonJS modules, but rather - for defining modules - it made more sense to opt for the AMD naming convention.

Getting Started With Modules

The first two concepts worth noting about AMD are the idea of a define method for facilitating module definition and a require method for handling dependency loading. define is used to define named or unnamed modules based using the following signature:

2    module_id /*optional*/,
3    [dependencies] /*optional*/,
4    definition function /*function for instantiating the module or object*/

As we can tell by the inline comments, the module_id is an optional argument which is typically only required when non-AMD concatenation tools are being used (there may be some other edge cases where it's useful too). When this argument is left out, we refer to the module as anonymous.

When working with anonymous modules, the idea of a module's identity is DRY, making it trivial to avoid duplication of filenames and code. Because the code is more portable, it can be easily moved to other locations (or around the file-system) without needing to alter the code itself or change its module ID. Consider the module_id similar to the concept of folder paths.

Note: Developers can run this same code on multiple environments just by using an AMD optimizer that works with a CommonJS environment such as r.js.

Back to the define signature, the dependencies argument represents an array of dependencies which are required by the module we are defining and the third argument ("definition function" or "factory function") is a function that's executed to instantiate our module. A bare bone module could be defined as follows:

Understanding AMD: define()

01// A module_id (myModule) is used here for demonstration purposes only
02define( "myModule",
04    ["foo", "bar"],
06    // module definition function
07    // dependencies (foo and bar) are mapped to function parameters
08    function ( foo, bar ) {
09        // return a value that defines the module export
10        // (i.e the functionality we want to expose for consumption)
12        // create your module here
13        var myModule = {
14            doStuff:function () {
15                console.log( "Yay! Stuff" );
16            }
17        };
19    return myModule;
22// An alternative version could be..
23define( "myModule",
25    ["math", "graph"],
27    function ( math, graph ) {
29        // Note that this is a slightly different pattern
30        // With AMD, it's possible to define modules in a few
31        // different ways due to it's flexibility with
32        // certain aspects of the syntax
33        return {
34            plot: function( x, y ){
35                return graph.drawPie( math.randomGrid( x, y ) );
36            }
37        };

require on the other hand is typically used to load code in a top-level JavaScript file or within a module should we wish to dynamically fetch dependencies. An example of its usage is:

Understanding AMD: require()

1// Consider "foo" and "bar" are two external modules
2// In this example, the "exports" from the two modules
3// loaded are passed as function arguments to the
4// callback (foo and bar) so that they can similarly be accessed
6require(["foo", "bar"], function ( foo, bar ) {
7        // rest of your code here
8        foo.doSomething();

Dynamically-loaded Dependencies

01define(function ( require ) {
02    var isReady = false, foobar;
04    // note the inline require within our module definition
05    require(["foo", "bar"], function ( foo, bar ) {
06        isReady = true;
07        foobar = foo() + bar();
08    });
10    // we can still return a module
11    return {
12        isReady: isReady,
13        foobar: foobar
14    };

Understanding AMD: plugins

The following is an example of defining an AMD-compatible plugin:

01// With AMD, it's possible to load in assets of almost any kind
02// including text-files and HTML. This enables us to have template
03// dependencies which can be used to skin components either on
04// page-load or dynamically.
06define( ["./templates", "text!./template.md","css!./template.css" ],
08    function( templates, template ){
09        console.log( templates );
10        // do something with our templates here
11    }

Note: Although css! is included for loading CSS dependencies in the above example, it's important to remember that this approach has some caveats such as it not being fully possible to establish when the CSS is fully loaded. Depending on how we approach our build process, it may also result in CSS being included as a dependency in the optimized file, so use CSS as a loaded dependency in such cases with caution. If interested in doing the above, we can also explore @VIISON's RequireJS CSS plugin further here: https://github.com/VIISON/RequireCSS

Loading AMD Modules Using RequireJS

3    function( myModule ){
4        // start the main module which in-turn
5        // loads other modules
6        var module = new myModule();
7        module.doStuff();

This example could simply be looked at as requirejs(["app/myModule"], function(){}) which indicates the loader's top level globals are being used. This is how to kick off top-level loading of modules with different AMD loaders however with a define() function, if it's passed a local require all require([]) examples apply to both types of loader (curl.js and RequireJS).

Loading AMD Modules Using curl.js

3    function( myModule ){
4        // start the main module which in-turn
5        // loads other modules
6        var module = new myModule();
7        module.doStuff();

Modules With Deferred Dependencies

01// This could be compatible with jQuery's Deferred implementation,
02// futures.js (slightly different syntax) or any one of a number
03// of other implementations
05define(["lib/Deferred"], function( Deferred ){
06    var defer = new Deferred();
08    require(["lib/templates/?index.html","lib/data/?stats"],
09        function( template, data ){
10            defer.resolve( { template: template, data:data } );
11        }
12    );
13    return defer.promise();


AMD Modules With Dojo

Defining AMD-compatible modules using Dojo is fairly straight-forward. As per above, define any module dependencies in an array as the first argument and provide a callback (factory) which will execute the module once the dependencies have been loaded. e.g:

1define(["dijit/Tooltip"], function( Tooltip ){
3    //Our dijit tooltip is now available for local use
4    new Tooltip(...);

Note the anonymous nature of the module, which can now be both consumed by a Dojo asynchronous loader, RequireJS or the standard dojo.require() module loader.

There are some interesting gotchas with module referencing that are useful to know here. Although the AMD-advocated way of referencing modules declares them in the dependency list with a set of matching arguments, this isn't supported by the older Dojo 1.6 build system - it really only works for AMD-compliant loaders. e.g:

1define(["dojo/cookie", "dijit/Tooltip"], function( cookie, Tooltip ){
3    var cookieValue = cookie( "cookieName" );
4    new Tooltip(...);

This has many advantages over nested namespacing as modules no longer need to directly reference complete namespaces every time - all we require is the "dojo/cookie" path in dependencies, which once aliased to an argument, can be referenced by that variable. This removes the need to repeatedly type out "dojo." in our applications.

The final gotcha to be aware of is that if we wish to continue using the older Dojo build system or wish to migrate older modules to this newer AMD-style, the following more verbose version enables easier migration. Notice that dojo and dijit and referenced as dependencies too:

1define(["dojo", "dijit', "dojo/cookie", "dijit/Tooltip"], function( dojo, dijit ){
2    var cookieValue = dojo.cookie( "cookieName" );
3    new dijit.Tooltip(...);

AMD Module Design Patterns (Dojo)

As we've seen in previous sections, design patterns can be highly effective in improving how we approach structuring solutions to common development problems. John Hann has given some excellent presentations about AMD module design patterns covering the Singleton, Decorator, Mediator and others and I highly recommend checking out his slides if we get a chance.

A selection of AMD design patterns can be found below.

Decorator pattern:

01// mylib/UpdatableObservable: a Decorator for dojo/store/Observable
02define(["dojo", "dojo/store/Observable"], function ( dojo, Observable ) {
03    return function UpdatableObservable ( store ) {
05        var observable = dojo.isFunction( store.notify ) ? store :
06                new Observable(store);
08        observable.updated = function( object ) {
09            dojo.when( object, function ( itemOrArray) {
10                dojo.forEach( [].concat(itemOrArray), this.notify, this );
11            });
12        };
14        return observable;
15    };
19// Decorator consumer
20// a consumer for mylib/UpdatableObservable
22define(["mylib/UpdatableObservable"], function ( makeUpdatable ) {
23    var observable,
24        updatable,
25        someItem;
27    // make the observable store updatable
28    updatable = makeUpdatable( observable ); // `new` is optional!
30    // we can then call .updated() later on if we wish to pass
31    // on data that has changed
32    //updatable.updated( updatedItem );

Adapter pattern

01// "mylib/Array" adapts `each` function to mimic jQuerys:
02define(["dojo/_base/lang", "dojo/_base/array"], function ( lang, array ) {
03    return lang.delegate( array, {
04        each: function ( arr, lambda ) {
05            array.forEach( arr, function ( item, i ) {
06                lambda.call( item, i, item ); // like jQuery's each
07            });
08        }
09    });
12// Adapter consumer
13// "myapp/my-module":
14define(["mylib/Array"], function ( array ) {
15    array.each( ["uno", "dos", "tres"], function ( i, esp ) {
16        // here, `this` == item
17    });

AMD Modules With jQuery

Unlike Dojo, jQuery really only comes with one file, however given the plugin-based nature of the library, we can demonstrate how straight-forward it is to define an AMD module that uses it below.

03    function( $, colorPlugin, _ ){
04        // Here we've passed in jQuery, the color plugin and Underscore
05        // None of these will be accessible in the global scope, but we
06        // can easily reference them below.
08        // Pseudo-randomize an array of colors, selecting the first
09        // item in the shuffled array
10        var shuffleColor = _.first( _.shuffle( "#666","#333","#111"] ) );
12        // Animate the background-color of any elements with the class
13        // "item" on the page using the shuf