JavaScript Design Patterns - Command Pattern

JavaScript Design Patterns - Command Pattern

JavaScript Design Patterns - Command Pattern. Command pattern is a data driven design pattern and falls under behavioral pattern category. What's the command pattern. Why the command pattern is important. How to implement the command pattern in JavaScript. When to use the command pattern. The idea of the command pattern is to create an abstraction between the operations an object can do, its commands, and the actual commands themselves

The command pattern is probably my favorite design pattern, because of all the fun things you can do with it. The idea of the command pattern is to create an abstraction between the operations an object can do, its commands, and the actual commands themselves. This makes it really easy to combine together or chain different commands without having to change the code. The program can dynamically chain and combine these actions. The best part is since each command is its own object you can easily implement and undo function for each command and make a set of undo-able actions.

ūü߆ Concepts Covered:

  • What the command pattern is
  • Why the command pattern is important
  • How to implement the command pattern in JavaScript
  • When to use the command pattern

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Best Design Patterns for writing JavaScript Web applications

Best Design Patterns for writing JavaScript Web applications

Design patterns in JavaScript are reusable solutions applied to commonly occurring problems in writing JavaScript web applications. In this article, I will explore best and most popular JavaScript design patterns.

**Introduction

It is quite appropriate to refer JavaScript design patterns as templates to provide solutions to problems but not quite to say that these patterns can replace the developers.

Object constructors are used to create specific types of objects ‚ÄĒ 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.

  1. Object Creation

2. Basic Constructors

function person(firstName,lastName){
  
  this.firstName = firstName;
  
  this.lastName = lastName;
  
  this.fullName = function(){
    return this.firstName + " " + this.lastName;
  }
  
}

var person1 = new person('Akash','Pal');
var person2 = new person('Black','Panther');
person1.fullName(); //"Akash Pal"
person2.fullName(); //"Black Panther"
person1 //{firstName: "Akash", lastName: "Pal", fullName: ƒ}
person2 //{firstName: "Black", lastName: "Panther", fullName: ƒ}

3. Constructor with prototypes.

In this article, I will explore seven best and most popular JavaScript design patterns, which of course most of them will fall under three categories namely; creation design patterns, structural design patterns and behavioral design patterns. A pattern is something like the following image; just to acquaint you into the context.

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.

var personModule = (function(){
  
  var firstName;
  var lastName;
  
  return{
    setName(f,l){
      firstName = f;
      lastName = l;
    },
    getName(){
      return firstName + " " + lastName;
    }
  }
  
})();

personModule.setName('akash','pal')
personModule.getName() //"akash pal"

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.

var personModule = (function(){
  
  var firstName;
  var lastName;
  
  function setName(f,l){
    firstName = f;
    lastName = l;
  }
  
  function getName(){
    return firstName + " " + lastName;
  }
  
  return{
    setName:setName,
    getName:getName
  }
  
})();

personModule.setName('akash','pal');
personModule.getName(); //"akash pal"

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.

var singleton = (function(){

  var instance;
  
  function init(){
    
    var name;
    
    this.setName = function(name){
       this.name = name;
    }
    
    this.getName = function(){
      return this.name;
    }
    
    return{
      setName:setName,
      getName:getName
    }
      
  }
  
  function getInstance(){
    
    if(!instance){
      instance = init();
    }
    
    return instance;
  }
    
  return{
    getInstance:getInstance
  }  
  
})();


var one = singleton.getInstance();
var two = singleton.getInstance();

//the two instance are same
one == two //true

one.setName('Akash');
two.getName(); //"Akash"

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.

  • Subject: maintains a list of observers, facilitates adding or removing observers
  • Observer: provides an update interface for objects that need to be notified of a Subject‚Äôs changes of state
  • ConcreteSubject: broadcasts notifications to observers on changes of state, stores the state of ConcreteObservers
  • ConcreteObserver: stores a reference to the ConcreteSubject, implements an update interface for the Observer to ensure state is consistent with the Subject‚Äôs
Publish/Subscribe Pattern

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.

A Mediator is an object that coordinates interactions (logic and behavior) between multiple objects. It makes decisions on when to call which objects, based on the actions (or inaction) of other objects and input.

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.

Real prototypical inheritance, as defined in the ECMAScript 5 standard, requires the use of Object.create

function person(firstName,lastName){
  
  this.firstName = firstName;
  
  this.lastName = lastName;
  
}

person.prototype.fullName = function(){
    return this.firstName + " " + this.lastName;
}

var person1 = new person('Akash','Pal');
var person2 = new person('Black','Panther');
person1 //{firstName: "Akash", lastName: "Pal"}
person2 //{firstName: "Black", lastName: "Panther"}
person1.fullName() //"Akash Pal"
person2.fullName() //"Black Panther"

The prototype pattern without directly using Object.create

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).

var name = {
  fName:'aaa',
  lName:'bbb',
  setName:function(fName,lName){
    this.fName = fName;
    this.lName = lName;
  },
  getName:function(){
     return this.fName + " " + this.lName;
  }
}

name.execute = function(key){
   var methodName = name[key];
   var functionParamsArray = [].splice.call(arguments,1);
   return methodName.apply(name,functionParamsArray);   
}

name.execute('setName','Akash','Pal');
console.log(name.execute('getName'));//Akash Pal

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 ‚Äúfacade‚ÄĚ or limited abstraction of these methods is presented to the public for use.

Facades can also be integrated with other patterns such as the Module 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 to 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.

function Bike(options){
  this.wheels = 2;
  this.color = options.color;
}

function Car(options){
  this.wheels = 4;
  this.color = options.color;
}

function VehicleFactory(){}

VehicleFactory.prototype.createVehicle = function(options){
		
    switch(options.type){
    	case 'Bike': 
          this.vehicleClass = Bike;
      break;
      case 'Car': 
          this.vehicleClass = Car;
      break;
      default: 
          this.vehicleClass = Bike;
    }
    
    return new this.vehicleClass(options);
}

var vehicleFactory = new VehicleFactory();

var bike = vehicleFactory.createVehicle({
	type:'Bike',
  color:'black'
});

console.log(bike); //Bike {wheels: 2, color: "black"}

var car = vehicleFactory.createVehicle({
	type:'Car',
  color:'white'
});

console.log(car); //Car {wheels: 4, color: "white"}

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.

function Person(firstName,lastName){
  this.firstName = firstName; 
  this.lastName = lastName;
}

Person.prototype.fullName = function(){
    return this.firstName + " " + this.lastName;
}

function Superhero(firstName,lastName,powers){
  //super call
	Person.call(this,firstName,lastName);
  this.powers = powers;
}

Superhero.prototype = new Object(Person.prototype);

Superhero.prototype.showPowers = function(){
		return this.powers;
}

var superhero1 = new Superhero('Iron','Man',['Flying suit','Jarvis']);
console.log(superhero1.fullName() + '-' + superhero1.showPowers()); //Iron Man-Flying suit,Jarvis

var superhero2 = new Superhero('Captain','America',['strength','endurance','healing']);
console.log(superhero2.fullName() + '-' + superhero2.showPowers()); //Captain America-strength,endurance,healing

This articles includes examples derived from the below publication including my own inference of the patterns with the intention of simplifying the understanding of the patterns.

Conclusion

It is beneficial for JavaScript developers to use design patterns. Some major advantages of using design patterns include project maintainability and also cuts off unnecessary work on the development cycle. Even though JavaScript design patterns can provide solutions to complex problems, needless to say, rapid development and productivity, it is improper to conclude that these design patterns can replace the developers.

Thanks for reading, Hope this tutorial will surely help and you! Please share if you liked it!

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Top 10 JavaScript Design Patterns Every Developer Should Know

Top 10 JavaScript Design Patterns Every Developer Should Know

In this article, you'll see top 10 JavaScript Design Patterns every Developer should know

1. The constructor Pattern

In classical object-oriented programming languages, a constructor is a special method used to initialize a newly created object once the memory has been allocated for it. In JavaScript almost everything is an object, we have most often interested in object constructors. Since object constructors are used to creating specific types of objects, for example, both preparing the object for use and accepting arguments a constructor can use to set the values of member properties and methods when the object is first created.

As we saw know JavaScript does not support the concept of classes, so inside a constructor, the keyword this references the new object that's being created revisiting object creation, a basic constructor may look as follows:

function Car(model, year, miles) {
 this.model = model;
 this.year = year;
 this.miles = miles;
}

// Usage:
var bmw = new Car('M4', '2019', '1000');

2. The module Pattern

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

There are several option for implementing modules. These include:

  • Object literal notation
  • The module Pattern
  • AMD modules
  • CommonJS module
  • ECMAScript Harmony modules

Object literals :

var newObject = {
 variableKey: variableValue,
 functionKey: function() {
   //…
 }
};

The modules Pattern:

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

var testModule = (function() {
 var counter = 0;
 return {
   incrementCounter: function() {
     return ++counter;
   },
   resetCounter: function() {
     counter = 0;
   }
 };
})();

// Usage:
testModule.incrementCounter();
testModule.resetCounter();

3. The Revealing Module Pattern

One thing that the revealing module can do is avoiding repeat the name of the main object when we want to call one public method from another or access public variables.

var myRevealingModule = (function() {
 var privateVariable = 'not okay',
   publicVariable = 'okay';
 function privateFun() {
   return privateVariable;
 }

 function publicSetName(strName) {
   privateVariable = strName;
 }

 function publicGetName() {
   privateFun();
 }

 return {
   setName: publicSetName,
   message: publicVariable,
   getName: publicGetName
 };
})();

//Usage:

myRevealingModule.setName('Marvin King');

4. The Singleton Pattern

The Singleton pattern is thus known because it restricts instantiation of a class to single object. Singletons differ from static classes as we can delay their initialization. generally because they require some information that may not be available during initialization time. For code that is unaware of a previous reference to them, they do not provide a method for easy retrieval. Let's have a look of the structure of singleton:

var singletonPattern = (function() {
 var instance;
 function init() {
   // Singleton
   function privateMethod() {
     console.log('privateMethod');
   }
   var privateVariable = 'this is private variable';
   var privateRandomNumber = Math.random();
   return {
     publicMethod: function() {
       console.log('publicMethod');
     },
     publicProperty: 'this is public property',
     getRandomNumber: function() {
       return privateRandomNumber;
     }
   };
 }

 return {
   // Get the singleton instance if one exists
   // or create if it doesn't
   getInstance: function() {
     if (!instance) {
       instance = init();
     }
     return instance;
   }
 };
})();

// Usage:
var single = singletonPattern.getInstance();

5. The Observer Pattern

The observer is a design pattern in which an object maintains a list of objects depending on it observers, automatically notifying them of any changes to state.

  • Subject
  • Maintains a list of observers, facilities adding or removing observers
  • Observer
  • Provides an update interface for object that need to be notified of a subject‚Äôs change of state
  • ConcreteSubject
  • Broadcasts notifications to Observers on changes of state, stores the state of ConcreteObservers
  • ConcreteObserver
  • Stores a reference to the ConcreteSubject, implements an update interface for the observer to ensure state is consistent with subjects
function ObserverList() {
 this.observerList = [];
}

ObserverList.prototype.Add = function(obj) {
 return this.observerList.push(obj);
};

ObserverList.prototype.Empty = function() {
 this.observerList = [];
};

ObserverList.prototype.Count = function() {
 return this.observerList.length;
};

ObserverList.prototype.Get = function(index) {
 if (index > -1 && index < this.observerList.length) {
   return this.observerList[index];
 }
};

//...

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

When we no longer wish for a particular observer the notified of changes by the subject it is registered with, the subject can remove it from the list of observers. In the future, I will talk more about the feature of how the observer can be used in JavaScript widely.

6. The Mediator Pattern

If it appears a system has too many direct relationships between components. it may be time to have a central point of the control that components communicate through instead. The mediator pattern promotes loose coupling by ensuring that instead of components referring to each other explicitly.

var mediator = (function() {
 var topics = {};
 var subscribe = function(topic, fn) {
   if (!topics[topic]) {
     topics[topic] = [];
   }
   topics[topic].push({ context: this, callback: fn });
   return this;
 };

 // publish/broadcast an event to the rest of the application
 var publish = function(topic) {
   var args;
   if (!topics[topic]) {
     return false;
   }
   args = Array.prototype.slice.call(arguments, 1);
   for (var i = 0, l = topics[topic].length; i < l; i++) {
     var subscription = topics[topic][i];
     subscription.callback.apply(subscription.content, args);
   }
   return this;
 };
 return {
   publish: publish,
   subscribe: subscribe,
   installTo: function(obj) {
     obj.subscribe = subscribe;
     obj.publish = publish;
   }
 };
})();

7. The Prototype Pattern

One of the benefits of using the Prototype pattern is that we've working with the prototype strengths JavaScript has to offer natively rather than attempting to imitate features of other languages. let's look at the pattern example.

var myCar = {
 name: 'bmw',
 drive: function() {
   console.log('I am driving!');
 },
 panic: function() {
   console.log('wait, how do you stop this thing?');
 }
};

//Usages:

var yourCar = Object.create(myCar);

console.log(yourCar.name); //'bmw'

8. The Factory Pattern

Factory can provide a generic interface for creating objects, where we can specify the type of factory object we wish to create. see the diagram below.

function Car(options) {
 this.doors = options.doors || 4;
 this.state = options.state || 'brand new';
 this.color = options.color || 'silver';
}
9. The Mixin Pattern

Mixins are classes that offer functionality that can be easily inherited by a sub-class or group of sub-classes for the purpose of the function reuse.

var Person = function(firstName, lastName) {
 this.firstName = firstName;
 this.lastName = lastName;
 this.gender = 'male';
};

var clark = new Person('Clark', 'kent');

var Superhero = function(firstName, lastName, powers) {
 Person.call(this.firstName, this.lastName);
 this.powers = powers;
};

SuperHero.prototype = Object.create(Person.prototype);
var superman = new Superhero('Clark', 'Kent', ['flight', 'heat-vision']);

console.log(superman); //output personal attributes as well as power

In this case, superhero is capable of overriding any inherited values with values specific to its object.

10. The Decorator Pattern

The Decorators are a structural design pattern that aim to promote code reuse. Similar to Mixins, they can be considered another viable alternative to object subclassing. Classically, Decorators offered that ability to add behavior to existing classes in a system dynamically. The idea was the decoration itself wasn't essential to the base functionality of the class. Let's checkout how decorator work in JavaScript

function MacBook() {
 this.cost = function() {
   return 997;
 };
 this.screenSize = function() {
   return 11.6;
 };
}

// Decorator 1

function Memory(macbook) {
 var v = macbook.cost();
 macbook.cost = function() {
   return v + 75;
 };
}

// Decorator 2

function Engraving(macbook) {
 var v = macbook.cost();
 macbook.cost = function() {
   return v + 200;
 };
}

// Decorator 3

function Insurance(macbook) {
 var v = macbook.cost();
 macbook.cost = function() {
   return v + 250;
 };
}

var mb = new MacBook();

Memory(mb);
Engraving(mb);
Insurance(mb);

mb.cost(); // 1522

All the patterns may not use to one project, and some projects may benefit from the decoupling benefits offered by the Observer pattern. That said, once we have a firm grasp-of design patterns and the specific problems they are best suited to. Thus, it becomes much easier to integrate into our application architecture.

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