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Learning JavaScript Design Patterns

Cover of Learning JavaScript Design Patterns by Addy Osmani Published by O'Reilly Media, Inc.
  1. Learning JavaScript Design Patterns
  2. SPECIAL OFFER: Upgrade this ebook with O’Reilly
  3. Preface
    1. Target Audience
    2. Credits
    3. Reading
    4. Conventions Used in This Book
    5. Using Code Examples
    6. Safari® Books Online
    7. How to Contact Us
    8. Acknowledgments
  4. 1. Introduction
  5. 2. What Is a Pattern?
    1. We Already Use Patterns Every Day
  6. 3. “Pattern”-ity Testing, Proto-Patterns, and the Rule of Three
  7. 4. The Structure of a Design Pattern
  8. 5. Writing Design Patterns
  9. 6. Anti-Patterns
  10. 7. Categories of Design Patterns
    1. Creational Design Patterns
    2. Structural Design Patterns
    3. Behavioral Design Patterns
  11. 8. Design Pattern Categorization
    1. A Brief Note on Classes
  12. 9. JavaScript Design Patterns
    1. The Constructor Pattern
      1. Object Creation
      2. Basic Constructors
      3. Constructors with Prototypes
    2. The Module Pattern
      1. Object Literals
      2. The Module Pattern
      3. Module Pattern Variations
    3. The Revealing Module Pattern
      1. Advantages
      2. Disadvantages
    4. The Singleton Pattern
    5. The Observer Pattern
      1. Differences Between the Observer and Publish/Subscribe Pattern
      2. Advantages
      3. Disadvantages
      4. Publish/Subscribe Implementations
    6. The Mediator Pattern
      1. Basic Implementation
      2. Advanced Implementation
      3. Example
      4. Advantages and Disadvantages
      5. Mediator Versus Observer
      6. Mediator Versus Facade
    7. The Prototype Pattern
    8. The Command Pattern
    9. The Facade Pattern
      1. Notes on Abstraction
    10. The Factory Pattern
      1. When to Use the Factory Pattern
      2. When Not to Use the Factory Pattern
      3. Abstract Factories
    11. The Mixin Pattern
    12. Subclassing
    13. Mixins
      1. Advantages and Disadvantages
    14. The Decorator Pattern
    15. Pseudoclassical Decorators
      1. Interfaces
      2. Abstract Decorators
    16. Decorators with jQuery
    17. Advantages and Disadvantages
    18. Flyweight
      1. Using Flyweights
      2. Flyweights and Sharing Data
      3. Implementing Classical Flyweights
      4. Converting Code to Use the Flyweight Pattern
      5. A Basic Factory
      6. Managing the Extrinsic States
      7. The Flyweight Pattern and the DOM
  13. 10. JavaScript MV* Patterns
    1. MVC
      1. Smalltalk-80 MVC
    2. MVC for JavaScript Developers
      1. Models
      2. Views
      3. Controllers
      4. Controllers in Another Library (Spine.js) Versus Backbone.js
    3. What Does MVC Give Us?
    4. Smalltalk-80 MVC in JavaScript
      1. Delving Deeper
      2. Summary
    5. MVP
      1. Models, Views, and Presenters
      2. MVP or MVC?
      3. MVC, MVP, and Backbone.js
    6. MVVM
      1. History
      2. Model
      3. View
      4. ViewModel
      5. Recap: The View and the ViewModel
      6. Recap: The ViewModel and the Model
    7. Pros and Cons
      1. Advantages
      2. Disadvantages
    8. MVVM with Looser Data Bindings
    9. MVC Versus MVP Versus MVVM
    10. Backbone.js Versus KnockoutJS
  14. 11. Modern Modular JavaScript Design Patterns
    1. A Note on Script Loaders
    2. AMD
      1. Getting Started with Modules
      2. AMD Modules with Dojo
      3. AMD Module Design Patterns (Dojo)
      4. AMD Modules with jQuery
      5. AMD Conclusions
    3. CommonJS
      1. Getting Started
      2. Consuming Multiple Dependencies
      3. Loaders and Frameworks that Support CommonJS
      4. Is CommonJS Suitable for the Browser?
      5. Related Reading
    4. AMD and CommonJS: Competing, but Equally Valid Standards
      1. UMD: AMD and CommonJS-Compatible Modules for Plug-ins
    5. ES Harmony
      1. Modules with Imports and Exports
      2. Modules Loaded from Remote Sources
      3. Module Loader API
      4. CommonJS-like Modules for the Server
      5. Classes with Constructors, Getters, and Setters
      6. ES Harmony Conclusions
      7. Related Reading
    6. Conclusions
  15. 12. Design Patterns in jQuery
    1. The Composite Pattern
    2. The Adapter Pattern
    3. The Facade Pattern
    4. The Observer Pattern
    5. The Iterator Pattern
    6. Lazy Initialization
    7. The Proxy Pattern
    8. The Builder Pattern
  16. 13. jQuery Plug-in Design Patterns
    1. Patterns
    2. A Lightweight Start Pattern
      1. Further Reading
    3. Complete Widget Factory Pattern
      1. Further Reading
    4. Nested Namespacing Plug-in Pattern
      1. Further Reading
    5. Custom Events Plug-in Pattern (with the Widget Factory)
      1. Further Reading
    6. Prototypal Inheritance with the DOM-to-Object Bridge Pattern
      1. Further Reading
    7. jQuery UI Widget Factory Bridge Pattern
      1. Further Reading
    8. jQuery Mobile Widgets with the Widget Factory
    9. RequireJS and the jQuery UI Widget Factory
      1. Usage
      2. Further Reading
    10. Globally and Per-Call Overridable Options (Best Options Pattern)
      1. Further Reading
    11. A Highly Configurable and Mutable Plug-in Pattern
      1. Further Reading
    12. What Makes a Good Plug-in Beyond Patterns?
      1. Quality
      2. Code Style
      3. Compatibility
      4. Reliability
      5. Performance
      6. Documentation
      7. Likelihood of maintenance
    13. Conclusions
    14. Namespacing Patterns
    15. Namespacing Fundamentals
      1. Single Global Variables
      2. Prefix Namespacing
      3. Object Literal Notation
      4. Nested Namespacing
      5. Immediately Invoked Function Expressions (IIFE)s
      6. Namespace Injection
    16. Advanced Namespacing Patterns
      1. Automating Nested Namespacing
      2. Dependency Declaration Pattern
      3. Deep Object Extension
      4. Recommendation
  17. 14. Conclusions
  18. A. References
  19. Index
  20. About the Author
  21. Colophon
  22. SPECIAL OFFER: Upgrade this ebook with O’Reilly
  23. Copyright
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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 the use of 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 (Figure 9-9).

Factory pattern

Figure 9-9. Factory pattern

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:

// Types.js - Constructors used behind the scenes

// A constructor for defining new cars
function Car( options ) {

  // some defaults
  this.doors = options.doors || 4;
  this.state = options.state || "brand new";
  this.color = options.color || "silver";

}

// A constructor for defining new trucks
function Truck( options){

  this.state = options.state || "used";
  this.wheelSize = options.wheelSize || "large";
  this.color = options.color || "blue";
}


// FactoryExample.js

// Define a skeleton vehicle factory
function VehicleFactory() {}

// Define the prototypes and utilities for this factory

// Our default vehicleClass is Car
VehicleFactory.prototype.vehicleClass = Car;

// Our Factory method for creating new Vehicle instances
VehicleFactory.prototype.createVehicle = function ( options ) {

  if( options.vehicleType === "car" ){
    this.vehicleClass = Car;
  }else{
    this.vehicleClass = Truck;
  }

  return new this.vehicleClass( options );

};

// Create an instance of our factory that makes cars
var carFactory = new VehicleFactory();
var car = carFactory.createVehicle( { 
            vehicleType: "car", 
            color: "yellow", 
            doors: 6 } );

// Test to confirm our car was created using the vehicleClass/prototype Car

// Outputs: true
console.log( car instanceof Car ); 

// Outputs: Car object of color "yellow", doors: 6 in a "brand new" state
console.log( car );

In Approach 1, we modify a VehicleFactory instance to use the Truck class:

var movingTruck = carFactory.createVehicle( { 
                      vehicleType: "truck", 
                      state: "like new", 
                      color: "red", 
                      wheelSize: "small" } );

// Test to confirm our truck was created with the vehicleClass/prototype Truck

// Outputs: true
console.log( movingTruck instanceof Truck );

// Outputs: Truck object of color "red", a "like new" state 
// and a "small" wheelSize
console.log( movingTruck );

In Approach 2, we subclass VehicleFactory to create a factory class that builds Trucks:

function TruckFactory () {}
TruckFactory.prototype = new VehicleFactory();
TruckFactory.prototype.vehicleClass = Truck;

var truckFactory = new TruckFactory();
var myBigTruck = truckFactory.createVehicle( { 
                    state: "omg..so bad.", 
                    color: "pink", 
                    wheelSize: "so big" } );

// Confirms that myBigTruck was created with the prototype Truck
// Outputs: true
console.log( myBigTruck instanceof Truck );

// Outputs: Truck object with the color "pink", wheelSize "so big" 
// and state "omg. so bad"
console.log( myBigTruck );

When to Use the Factory Pattern

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

  • When our object or component setup involves a high level of complexity.

  • When we need to easily generate different instances of objects depending on the environment we are in.

  • When we’re working with many small objects or components that share the same properties.

  • When composing objects with instances of other objects that need only satisfy an API contract (a.k.a., duck typing) to work. This is useful for decoupling.

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 pattern 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 that 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).

  var AbstractVehicleFactory = (function () {

    // Storage for our vehicle types
    var types = {};

    return {
        getVehicle: function ( type, customizations ) {
            var Vehicle = types[type];

            return (Vehicle) ? return new Vehicle(customizations) : null;
        },

        registerVehicle: function ( type, Vehicle ) {
            var proto = Vehicle.prototype;

            // only register classes that fulfill the vehicle contract
            if ( proto.drive && proto.breakDown ) {
                types[type] = Vehicle;
            }

            return AbstractVehicleFactory;
        }
    };
})();


// Usage: 

AbstractVehicleFactory.registerVehicle( "car", Car );
AbstractVehicleFactory.registerVehicle( "truck", Truck );

// Instantiate a new car based on the abstract vehicle type
var car = AbstractVehicleFactory.getVehicle( "car" , { 
            color: "lime green", 
            state: "like new" } );

// Instantiate a new truck in a similar manner
var truck = AbstractVehicleFactory.getVehicle( "truck" , { 
            wheelSize: "medium", 
            color: "neon yellow" } );

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