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and Design Patterns Pdf. Book Details. Book Name. JavaScript Patterns Build. Better Applications with. Coding and Design. Patterns. Edition. 1st Edition. curriculum of JavaScript development and design patterns. As a digital Did you know that Packt offers eBook versions of every book published, with PDF and. This book helps you answer that question with numerous JavaScript coding patterns and best practices. If you're an - Selection from JavaScript Patterns [ Book].

Module Pattern Variations Import mixins This variation of the pattern demonstrates how globals e. This effectively allows us to import them and locally alias them as we wish. This takes as its first argument a dot-separated string such as 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. Here, we see an example of how to define a namespace which can then be populated with a module containing both a private and public API. 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: Y. 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. 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. Disadvantages 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 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. 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.

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. Disadvantages 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.

What makes the Singleton is the global access to the instance generally through MySingleton. This is however possible in JavaScript.

In the GoF book, the applicability of the Singleton pattern is described as follows: There must be exactly one instance of a class, and it must be accessible to clients from a well-known access point. 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 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 called Design Patterns: 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.

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:.

Patterns are not an exact solution. 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:. 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".

Use a modern native browser feature such as querySelectorAll to select all of the elements with the class "foo". 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. In a real-world application however, 3. 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. 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.

Although we know that querySelectorAll attempts to solve this problem, compare the effort involved in using jQuery's Facade interfaces vs. There's no contest! Abstractions using patterns can offer real-world value. 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 which should create the "thing". This is a reason why patterns generally focus on addressing a visually identifiable structure i.

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.

One of the additional requirements for a pattern to be valid is that they display some recurring phenomenon.

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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:. 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:. 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. 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.

Your Answer

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.

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

Let us qualify the reason behind this. 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. The bigger challenges happen after an application has hit production and is ready to go into maintenance mode.

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. 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 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.

Creational design patterns focus on handling object creation mechanisms where objects are created in a manner suitable for the situation we're 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: 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.

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

ES introduced native support for classes to JavaScript, however, they are primarily syntactical sugar over JavaScript's existing prototype-based inheritance model. Keep in mind that there will be patterns in this table that reference the concept of "classes". In ES5, 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. For more ways to define "classes" using JavaScript, see Stoyan Stefanov's useful post on them. Let us now proceed to review the table. 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. 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. 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.

JavaScript

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. 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.

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.

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. As we will see a little later in the book, these methods can even be used for inheritance, as follows:. As we saw earlier, JavaScript doesn't support the concept of classes but it does support 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:. 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. Functions, like almost all objects in JavaScript, contain a "prototype" object.

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:. 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.

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

Pro 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. Names inside the object may be either strings or identifiers that are followed by a colon. Outside of an object, new members may be added to it using assignment as follows myModule. Where in the world is Paul Irish today?

Caching is: 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 was originally defined as a way to provide both private and public encapsulation for classes in conventional software engineering. 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 utilizes an immediately-invoked function expression IIFE - see the section on namespacing patterns for more on this where an object is returned. 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 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. 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. 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:. Looking at another example, below we can see a shopping basket implemented using 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 i. 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:.

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:.

This variation of the pattern demonstrates how globals e. This effectively allows us to import them and locally alias them as we wish. 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. Dojo Dojo provides a convenience method for working with objects called dojo. This takes as its first argument a dot-separated string such as 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. For more information on dojo. For those using Sencha's ExtJS, an example demonstrating how to correctly use the Module pattern with the framework can be found below. Here, we 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:.

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:.

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.

We've seen why the Constructor 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 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.

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

What makes the Singleton is the global access to the instance generally through MySingleton. This is however possible in JavaScript. There must be exactly one instance of a class, and it must be accessible to clients from a well-known access point.

When the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code.

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.

It is important to note the difference between a static instance of a class object and a Singleton: 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.

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:.

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 is a design pattern where 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.

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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.

Elements of Reusable Object-Oriented Software , is:. 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:.

Next, let's model the Subject and the ability to add, remove or notify observers on the observer list. We then define a skeleton for creating new Observers. The update functionality here will be overwritten later with custom behaviour. 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. In this example, we looked at how to implement and utilize the Observer pattern, covering the concepts of a Subject, Observer, ConcreteSubject and ConcreteObserver.

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. 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. 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.

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.

When you wish to change the visibility of a member, you need to modify the code wherever you have used this member because of the different nature of accessing public and private parts. Also, methods added to the object after their creation cannot access the private members of the object. Revealing Module Pattern This pattern is an improvement made to the module pattern as illustrated above. The main difference is that we write the entire object logic in the private scope of the module and then simply expose the parts we want to be public by returning an anonymous object.

We can also change the naming of private members when mapping private members to their corresponding public members. The differences between the revealing module pattern and the other variants of the module pattern are primarily in how public members are referenced. As a result, the revealing module pattern is much easier to use and modify; however, it may prove fragile in certain scenarios, like using RMP objects as prototypes in an inheritance chain.

The problematic situations are the following: If we have a private function which is referring to a public function, we cannot override the public function, as the private function will continue to refer to the private implementation of the function, thus introducing a bug into our system. If we have a public member pointing to a private variable, and try to override the public member from outside the module, the other functions would still refer to the private value of the variable, introducing a bug into our system.

Singleton Pattern The singleton pattern is used in scenarios when we need exactly one instance of a class. For example, we need to have an object which contains some configuration for something.

In these cases, it is not necessary to create a new object whenever the configuration object is required somewhere in the system. It is important to note that the access point for retrieving the singleton value needs to be only one and very well known. A downside to using this pattern is that it is rather difficult to test. Observer Pattern The observer pattern is a very useful tool when we have a scenario where we need to improve the communication between disparate parts of our system in an optimized way.

It promotes loose coupling between objects. There are various versions of this pattern, but in its most basic form, we have two main parts of the pattern. The first is a subject and the second is observers. A subject handles all of the operations regarding a certain topic that the observers subscribe to.

These operations subscribe an observer to a certain topic, unsubscribe an observer from a certain topic, and notify observers about a certain topic when an event is published. The publisher fires an event and simply executes the callback function sent for that event. You would have to nest the AJAX calls one within the other, possibly entering into a situation known as callback hell.

A downside to using this pattern is difficult testing of various parts of our system. There is no elegant way for us to know whether or not the subscribing parts of the system are behaving as expected. Mediator Pattern We will briefly cover a pattern which is also very useful when talking about decoupled systems.Here, other parts of the code are unable to directly read the value of our incrementCounter or resetCounter. 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.

Iterator Sequentially access the elements of a collection without knowing the inner workings of the collection.

Mixins assist in decreasing functional repetition and increasing function re-use in a system. 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.

As the majority of JavaScript developers currently using these patterns opt to utilize libraries such as Backbone. The Superhero constructor creates an object which descends from Person. You just define your class and instantiate it later on when you need it.

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. That said, if we're opting for this technique, we may be equally as interested in the Module pattern.

SOILA from Vermont
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