design-pattern-categorization.md

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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

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Note: ES2015 introduced native support for classes to JavaScript, however, they are primarily syntactical sugar over JavaScript's existing prototype-based inheritance model. We will not be covering ES2015 classes in this book, but MDN has an excellent introduction to them.

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. this can be used to help define new properties and methods for the object as follows:

// A car "class"
function Car(model) {
  this.model = model;
  this.color = 'silver';
  this.year = '2012';

  this.getInfo = function () {
    return this.model + ' ' + this.year;
  };
}

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

var myCar = new Car('ford');

myCar.year = '2010';

console.log(myCar.getInfo());

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

Let us now proceed to review the table.

Creational

Creational	Based on the concept of creating an object.
Class
Factory Method	This makes an instance of several derived classes based on interfaced data or events.
Object
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

Structural	Based on the idea of building blocks of objects.
Class
Adapter	Match interfaces of different classes therefore classes can work together despite incompatible interfaces.
Object
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

Behavioral	Based on the way objects play and work together.
Class
Interpreter	A way to include language elements in an application to match the grammar of the intended language.
Template Method	Creates the shell of an algorithm in a method, then defer the exact steps to a subclass.
Object
Chain of Responsibility	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.