JavaScript 设计模式指南

侯赛因·阿里夫撰写✏️

想象一下这样的情况：一群建筑师想要设计一座摩天大楼。在设计阶段，他们必须考虑很多因素，例如：

• 建筑风格——建筑应该是野兽派、极简主义还是其他风格？
• 底座的宽度——需要多大的尺寸才能防止大风天倒塌？
• 抵御自然灾害 - 根据该建筑物的位置需要采取哪些预防性结构措施来防止地震、洪水等造成的损坏？

需要考虑的因素有很多，但有一点是可以确定的：很可能已经有一份蓝图可以帮助建造这座摩天大楼。如果没有共同的设计或计划，这些架构师将不得不重新发明轮子，这可能会导致混乱和多重低效率。

类似地，在编程世界中，开发人员经常参考一组设计模式来帮助他们构建软件，同时遵循干净的代码原则。此外，这些模式无处不在，从而让程序员专注于交付新功能，而不是每次都重新发明轮子。

在本文中，您将了解一些常用的 JavaScript 设计模式，并且我们将一起构建小型 Node.js 项目来说明每种设计模式的用法。

软件工程中的设计模式是什么？

设计模式是预先制作的蓝图，开发人员可以对其进行定制以解决编码过程中的重复设计问题。要记住的一件重要的事情是，这些蓝图不是代码片段，而是应对即将到来的挑战的一般概念。

设计模式有很多好处：

• 经过尝试和测试 - 它们解决了软件设计中的无数问题。了解并应用代码中的模式很有用，因为这样做可以帮助您使用面向对象设计的原则解决各种问题
• 定义通用语言 — 设计模式帮助团队以有效的方式进行沟通。例如，队友可以说“我们应该使用工厂方法来解决这个问题”，每个人都会明白他的意思以及他们建议背后的动机

在本文中，我们将介绍三类设计模式：

• 创建型 — 用于创建对象
• 结构 - 组装这些对象以形成一个工作结构
• 行为 - 在这些对象之间分配职责

让我们看看这些设计模式的实际应用！

创意设计模式

顾名思义，创建模式包含各种帮助开发人员创建对象的方法。

工厂

工厂方法是一种创建对象的模式，可以更好地控制对象的创建。这种方法适用于我们希望将对象创建逻辑集中在一处的情况。

以下是一些示例代码，展示了此模式的实际效果：

//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();

让我们逐段分解这段代码：createCarCar

• 每辆汽车都有三个属性：公司、型号和尺寸。此外，我们还定义了一个 showDescription 函数，它将注销对象的属性。此外，请注意 createCar 方法演示了我们如何在内存中实例化对象时进行精细控制
• 后来，我们使用 createCar 实例来初始化一个名为challenger的对象
• 最后，在最后一行，我们在挑战者实例上调用了 showDescription

让我们测试一下！我们应该期望程序注销我们新创建的 Car 实例的详细信息：

建设者

构建器方法让我们可以使用逐步的对象构造来构建对象。因此，这种设计模式非常适合我们想要创建对象并仅应用必要功能的情况。因此，这提供了更大的灵活性。

这是使用构建器模式创建 Car 对象的代码块：

//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();

这是我们在上面的代码块中所做的事情：

• 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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