Guide des modèles de conception JavaScript

Écrit par Hussain Arif✏️

Imaginez une situation dans laquelle un groupe d'architectes souhaite concevoir un gratte-ciel. Lors de la phase de conception, ils devraient prendre en compte une multitude de facteurs, par exemple :

• Le style architectural : le bâtiment doit-il être brutaliste, minimaliste ou autre ?
• La largeur de la base — quelle dimension est nécessaire pour éviter l'effondrement par temps venteux ?
• Protection contre les catastrophes naturelles — quelles mesures structurelles préventives doivent être mises en place en fonction de l'emplacement de ce bâtiment pour éviter les dommages causés par les tremblements de terre, les inondations, etc. ?

Il peut y avoir de nombreux facteurs à prendre en compte, mais une chose peut être sûre : il existe très probablement déjà un plan pour aider à construire ce gratte-ciel. Sans une conception ou un plan commun, ces architectes devraient réinventer la roue, ce qui peut entraîner de la confusion et de multiples inefficacités.

De même, dans le monde de la programmation, les développeurs se réfèrent souvent à un ensemble de modèles de conception pour les aider à créer des logiciels tout en suivant les principes d'un code propre. De plus, ces modèles sont omniprésents, permettant ainsi aux programmeurs de se concentrer sur la création de nouvelles fonctionnalités au lieu de réinventer la roue à chaque fois.

Dans cet article, vous découvrirez quelques modèles de conception JavaScript couramment utilisés et, ensemble, nous créerons de petits projets Node.js pour illustrer l'utilisation de chaque modèle de conception.

Que sont les modèles de conception en génie logiciel ?

Les modèles de conception sont des plans prédéfinis que les développeurs peuvent adapter pour résoudre des problèmes de conception répétitifs lors du codage. Une chose cruciale à retenir est que ces plans ne sont pas des extraits de code mais plutôt des concepts généraux pour aborder les défis à venir.

Les modèles de conception présentent de nombreux avantages :

• Essayés et testés — ils résolvent d'innombrables problèmes de conception de logiciels. Connaître et appliquer des modèles dans le code est utile car cela peut vous aider à résoudre toutes sortes de problèmes en utilisant les principes de la conception orientée objet
• Définir un langage commun — les modèles de conception aident les équipes à communiquer de manière efficace. Par exemple, un coéquipier peut dire : « Nous devrions simplement utiliser la méthode d'usine pour résoudre ce problème », et tout le monde comprendra ce qu'il veut dire et le motif de sa suggestion

Dans cet article, nous couvrirons trois catégories de modèles de conception :

• Créationnel — Utilisé pour créer des objets
• Structurel — Assembler ces objets pour former une structure de travail
• Comportemental — Attribuer des responsabilités entre ces objets

Voyons ces modèles de conception en action !

Modèles de conception créatifs

Comme leur nom l'indique, les modèles de création comprennent diverses méthodes pour aider les développeurs à créer des objets.

Usine

La méthode factory est un modèle de création d'objets qui permet plus de contrôle sur la création d'objets. Cette méthode convient aux cas où nous souhaitons conserver la logique de création d’objets centralisée en un seul endroit.

Voici un exemple de code qui présente ce modèle en action :

//file name: factory-pattern.js
//use the factory JavaScript design pattern:
//Step 1: Create an interface for our object. In this case, we want to create a car
const createCar = ({ company, model, size }) => ({
//the properties of the car:
  company,
  model,
  size,
  //a function that prints out the car's properties:
  showDescription() {
    console.log(
      "The all new ",
      model,
      " is built by ",
      company,
      " and has an engine capacity of ",
      size,
      " CC "
    );
  },
});
//Use the 'createCar' interface to create a car
const challenger = createCar({
  company: "Dodge",
  model: "Challenger",
  size: 6162,
});
//print out this object's traits:
challenger.showDescription();

Décomposons ce code morceau par morceau :createCarCar

• Chaque voiture a trois propriétés : entreprise, modèle et taille. De plus, nous avons également défini une fonction showDescription, qui déconnectera les propriétés de l'objet. De plus, notez que la méthode createCar montre comment nous pouvons avoir un contrôle granulaire lorsqu'il s'agit d'instancier des objets en mémoire
• Plus tard, nous avons utilisé notre instance createCar pour initialiser un objet appelé challenger
• Enfin, dans la dernière ligne, nous avons invoqué la showDescription sur notre instance challenger

Testons-le ! Nous devrions nous attendre à ce que le programme déconnecte les détails de notre instance Car nouvellement créée :

Constructeur

La méthode builder nous permet de construire des objets en utilisant une construction d'objet étape par étape. En conséquence, ce modèle de conception est idéal pour les situations dans lesquelles nous souhaitons créer un objet et appliquer uniquement les fonctions nécessaires. En conséquence, cela permet une plus grande flexibilité.

Voici un bloc de code qui utilise le modèle builder pour créer un objet Car :

//builder-pattern.js
//Step 1: Create a class reperesentation for our toy car:
class Car {
  constructor({ model, company, size }) {
    this.model = model;
    this.company = company;
    this.size = size;
  }
}
//Use the 'builder' pattern to extend this class and add functions
//note that we have seperated these functions in their entities.
//this means that we have not defined these functions in the 'Car' definition.
Car.prototype.showDescription = function () {
  console.log(
    this.model +
      " is made by " +
      this.company +
      " and has an engine capacity of " +
      this.size +
      " CC "
  );
};
Car.prototype.reduceSize = function () {
  const size = this.size - 2; //function to reduce the engine size of the car.
  this.size = size;
};
const challenger = new Car({
  company: "Dodge",
  model: "Challenger",
  size: 6162,
});
//finally, print out the properties of the car before and after reducing the size:
challenger.showDescription();
console.log('reducing size...');
//reduce size of car twice:
challenger.reduceSize();
challenger.reduceSize();
challenger.showDescription();

Voici ce que nous faisons dans le bloc de code ci-dessus :

• As a first step, we created a Car class which will help us instantiate objects. Notice that earlier in the factory pattern, we used a createCar function, but here we are using classes. This is because classes in JavaScript let developers construct objects in pieces. Or, in simpler words, to implement the JavaScript builder design pattern, we have to opt for the object-oriented paradigm
• Afterwards, we used the prototype object to extend the Car class. Here, we created two functions — showDescription and reduceSize
• Later on, we then created our Car instance, named it challenger, and then logged out its information
• Finally, we invoked the reduceSize method on this object to decrement its size, and then we printed its properties once more

The expected output should be the properties of the challenger object before and after we reduced its size by four units:   This confirms that our builder pattern implementation in JavaScript was successful!

Structural design patterns

Structural design patterns focus on how different components of our program work together.

Adapter

The adapter method allows objects with conflicting interfaces to work together. A great use case for this pattern is when we want to adapt old code to a new codebase without introducing breaking changes:

//adapter-pattern.js
//create an array with two fields: 
//'name' of a band and the number of 'sold' albums
const groupsWithSoldAlbums = [
  {
    name: "Twice",
    sold: 23,
  },
  { name: "Blackpink", sold: 23 },
  { name: "Aespa", sold: 40 },
  { name: "NewJeans", sold: 45 },
];
console.log("Before:");
console.log(groupsWithSoldAlbums);
//now we want to add this object to the 'groupsWithSoldAlbums' 
//problem: Our array can't accept the 'revenue' field
// we want to change this field to 'sold'
var illit = { name: "Illit", revenue: 300 };
//Solution: Create an 'adapter' to make both of these interfaces..
//..work with each other
const COST_PER_ALBUM = 30;
const convertToAlbumsSold = (group) => {
  //make a copy of the object and change its properties
  const tempGroup = { name: group.name, sold: 0 };
  tempGroup.sold = parseInt(group.revenue / COST_PER_ALBUM);
  //return this copy:
  return tempGroup;
};
//use our adapter to make a compatible copy of the 'illit' object:
illit = convertToAlbumsSold(illit);
//now that our interfaces are compatible, we can add this object to the array
groupsWithSoldAlbums.push(illit);
console.log("After:");
console.log(groupsWithSoldAlbums);

Here’s what’s happening in this snippet:

• First, we created an array of objects called groupsWithSoldAlbums. Each object will have a name and sold property
• We then made an illit object which had two properties — name and revenue. Here, we want to append this to the groupsWithSoldAlbums array. This might be an issue, since the array doesn’t accept a revenue property
• To mitigate this problem, use the adapter method. The convertToAlbumsSold function will adjust the illit object so that it can be added to our array

When this code is run, we expect our illit object to be part of the groupsWithSoldAlbums list:

Decorator

This design pattern lets you add new methods and properties to objects after creation. This is useful when we want to extend the capabilities of a component during runtime.

If you come from a React background, this is similar to using Higher Order Components. Here is a block of code that demonstrates the use of the JavaScript decorator design pattern:

//file name: decorator-pattern.js
//Step 1: Create an interface
class MusicArtist {
  constructor({ name, members }) {
    this.name = name;
    this.members = members;
  }
  displayMembers() {
    console.log(
      "Group name",
      this.name,
      " has",
      this.members.length,
      " members:"
    );
    this.members.map((item) => console.log(item));
  }
}
//Step 2: Create another interface that extends the functionality of MusicArtist
class PerformingArtist extends MusicArtist {
  constructor({ name, members, eventName, songName }) {
    super({ name, members });
    this.eventName = eventName;
    this.songName = songName;
  }
  perform() {
    console.log(
      this.name +
        " is now performing at " +
        this.eventName +
        " They will play their hit song " +
        this.songName
    );
  }
}
//create an instance of PerformingArtist and print out its properties:
const akmu = new PerformingArtist({
  name: "Akmu",
  members: ["Suhyun", "Chanhyuk"],
  eventName: "MNET",
  songName: "Hero",
});
akmu.displayMembers();
akmu.perform();

Let's explain what's happening here:

• In the first step, we created a MusicArtist class which has two properties: name and members. It also has a displayMembers method, which will print out the name and the members of the current music band
• Later on, we extended MusicArtist and created a child class called PerformingArtist. In addition to the properties of MusicArtist, the new class will have two more properties: eventName and songName. Furthermore, PerformingArtist also has a perform function, which will print out the name and the songName properties to the console
• Afterwards, we created a PerformingArtist instance and named it akmu
• Finally, we logged out the details of akmu and invoked the perform function

The output of the code should confirm that we successfully added new capabilities to our music band via the PerformingArtist class:

Behavioral design patterns

This category focuses on how different components in a program communicate with each other.

Chain of Responsibility

The Chain of Responsibility design pattern allows for passing requests through a chain of components. When the program receives a request, components in the chain either handle it or pass it on until the program finds a suitable handler.

Here’s an illustration that explains this design pattern: The bucket, or request, is passed down the chain of components until a capable component is found. When a suitable component is found, it will process the request. Source: Refactoring Guru.[/caption] The best use for this pattern is a chain of Express middleware functions, where a function would either process an incoming request or pass it to the next function via the next() method:

//Real-world situation: Event management of a concert
//implement COR JavaScript design pattern:
//Step 1: Create a class that will process a request
class Leader {
  constructor(responsibility, name) {
    this.responsibility = responsibility;
    this.name = name;
  }
  //the 'setNext' function will pass the request to the next component in the chain.
  setNext(handler) {
    this.nextHandler = handler;
    return handler;
  }
  handle(responsibility) {
  //switch to the next handler and throw an error message:
    if (this.nextHandler) {
      console.log(this.name + " cannot handle operation: " + responsibility);
      return this.nextHandler.handle(responsibility);
    }
    return false;
  }
}
//create two components to handle certain requests of a concert
//first component: Handle the lighting of the concert:
class LightsEngineerLead extends Leader {
  constructor(name) {
    super("Light management", name);
  }
  handle(responsibility) {
  //if 'LightsEngineerLead' gets the responsibility(request) to handle lights,
  //then they will handle it
    if (responsibility == "Lights") {
      console.log("The lights are now being handled by ", this.name);
      return;
    }
    //otherwise, pass it to the next component.
    return super.handle(responsibility);
  }
}

//second component: Handle the sound management of the event:
class SoundEngineerLead extends Leader {
  constructor(name) {
    super("Sound management", name);
  }
  handle(responsibility) {
  //if 'SoundEngineerLead' gets the responsibility to handle sounds,
  // they will handle it
    if (responsibility == "Sound") {
      console.log("The sound stage is now being handled by ", this.name);
      return;
    }
    //otherwise, forward this request down the chain:
    return super.handle(responsibility);
  }
}
//create two instances to handle the lighting and sounds of an event:
const minji = new LightsEngineerLead("Minji");
const danielle = new SoundEngineerLead("Danielle");
//set 'danielle' to be the next handler component in the chain.
minji.setNext(danielle);
//ask Minji to handle the Sound and Lights:
//since Minji can't handle Sound Management, 
// we expect this request to be forwarded 
minji.handle("Sound");
//Minji can handle Lights, so we expect it to be processed
minji.handle("Lights");

In the above code, we’ve modeled a situation at a music concert. Here, we want different people to handle different responsibilities. If a person cannot handle a certain task, it’s delegated to the next person in the list.

Initially, we declared a Leader base class with two properties:

• responsibility — the kind of task the leader can handle
• name — the name of the handler

Additionally, each Leader will have two functions:

• setNext: As the name suggests, this function will add a Leader to the responsibility chain
• handle: The function will check if the current Leader can process a certain responsibility; otherwise, it will forward that responsibility to the next person via the setNext method

Next, we created two child classes called LightsEngineerLead (responsible for lighting), and SoundEngineerLead (handles audio). Later on, we initialized two objects — minji and danielle. We used the setNext function to set danielle as the next handler in the responsibility chain.

Lastly, we asked minji to handle Sound and Lights.

When the code is run, we expect minji to attempt at processing our Sound and Light responsibilities. Since minji is not an audio engineer, it should hand over Sound to a capable handler. In this case, it is danielle:

Strategy

The strategy method lets you define a collection of algorithms and swap between them during runtime. This pattern is useful for navigation apps. These apps can leverage this pattern to switch between routes for different user types (cycling, driving, or running):

This code block demonstrates the strategy design pattern in JavaScript code:

//situation: Build a calculator app that executes an operation between 2 numbers.
//depending on the user input, change between division and modulus operations

class CalculationStrategy {
  performExecution(a, b) {}
}
//create an algorithm for division
class DivisionStrategy extends CalculationStrategy {
  performExecution(a, b) {
    return a / b;
  }
}
//create another algorithm for performing modulus
class ModuloStrategy extends CalculationStrategy {
  performExecution(a, b) {
    return a % b;
  }
}
//this class will help the program switch between our algorithms:
class StrategyManager {
  setStrategy(strategy) {
    this.strategy = strategy;
  }
  executeStrategy(a, b) {
    return this.strategy.performExecution(a, b);
  }
}

const moduloOperation = new ModuloStrategy();
const divisionOp = new DivisionStrategy();
const strategyManager = new StrategyManager();
//use the division algorithm to divide two numbers:
strategyManager.setStrategy(divisionOp);
var result = strategyManager.executeStrategy(20, 4);
console.log("Result is: ", result);
//switch to the modulus strategy to perform modulus:
strategyManager.setStrategy(moduloOperation);
result = strategyManager.executeStrategy(20, 4);
console.log("Result of modulo is ", result);

Here’s what we did in the above block:

• First we created a base CalculationStrategy abstract class which will process two numbers — a and b
• We then defined two child classes — DivisionStrategy and ModuloStrategy. These two classes consist of division and modulo algorithms and return the output
• Next, we declared a StrategyManager class which will let the program alternate between different algorithms
• In the end, we used our DivisionStrategy and ModuloStrategy algorithms to process two numbers and return its output. To switch between these strategies, the strategyManager instance was used

When we execute this program, the expected output is strategyManager first using DivisionStrategy to divide two numbers and then switching to ModuloStrategy to return the modulo of those inputs:

Conclusion

In this article, we learned about what design patterns are, and why they are useful in the software development industry. Furthermore, we also learned about different categories of JavaScript design patterns and implemented them in code.

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