Detailed explanation of class, constructor, and factory functions in Javascript

In the ES6 era, our methods of creating objects have increased. We can choose different methods to create them in different scenarios. There are currently three main ways to build objects, the class keyword, constructor, and factory function. They are all means of creating objects, but they are different. During daily development, you also need to choose based on these differences. This article mainly introduces the detailed explanation of classes, constructors, and factory functions in Javascript. Friends who need it can refer to it. I hope it can help everyone.

First let’s take a look at what these three methods look like

// class 关键字，ES6新特性
class ClassCar {
 drive () {
  console.log('Vroom!');
 }
}
const car1 = new ClassCar();
console.log(car1.drive());
// 构造函数
function ConstructorCar () {}
ConstructorCar.prototype.drive = function () {
 console.log('Vroom!');
};
const car2 = new ConstructorCar();
console.log(car2.drive());
// 工厂函数
const proto = {
 drive () {
  console.log('Vroom!');
 }
};
function factoryCar () {
 return Object.create(proto);
}
const car3 = factoryCar();
console.log(car3.drive());

These methods are all based on prototype creation, and all support construction. Implementation of private variables in functions. In other words, these functions have most of the same characteristics and are even equivalent in many scenarios.

In Javascript, every function can return a new object. When it is not a constructor or class, it is called a factory function.

ES6 classes are actually syntactic sugar for constructors (at least this is how they are implemented at this stage), so everything discussed next applies to constructors and ES6 classes:

class Foo {}
console.log(typeof Foo); // function

Benefits of constructors and ES6 classes

• Most books will teach you how to Use classes and constructors

• ' this ' points to the new object.

• Some people like the readability of the new keyword

• There may be some small differences in details, but if If there are no problems during the development process, don’t worry too much.
  

Disadvantages of constructors and ES6 classes

1. You need the new keyword

In ES6, both constructors and classes need to have the new keyword.

function Foo() {
 if (!(this instanceof Foo)) { return new Foo(); }
}

In ES6, if you try to call a class function without the new keyword, a task will be thrown. If you want one without the new keyword, you can only use a factory function to wrap it.

2. The details during the instantiation process are exposed to the external API

All calls are closely related to the implementation of the constructor. If you need to make some changes during the construction process yourself Hands and feet, that is a very troublesome thing.

3. The constructor does not comply with the Open / Closed rule

Because of the detailed processing of the new keyword, the constructor violates the Open / Closed rule: the API should be open for expansion and avoid modification.

I once questioned that classes and factory functions are so similar, upgrading the class function to a factory function will not have any impact, but in JavaScript, it does have an impact.

If you start writing constructors or classes, but as you continue, you find that you need the flexibility of the factory function. At this time, you cannot simply change the function and walk away.

Unfortunately, you are a JavaScript programmer, and transforming a constructor into a factory function is a major operation:

// 原来的实现：
// class Car {
//  drive () {
//   console.log('Vroom!');
//  }
// }
// const AutoMaker = { Car };
// 工厂函数改变的实现：
const AutoMaker = {
 Car (bundle) {
  return Object.create(this.bundle[bundle]);
 },
 bundle: {
  premium: {
   drive () {
    console.log('Vrooom!');
   },
   getOptions: function () {
    return ['leather', 'wood', 'pearl'];
   }
  }
 }
};
// 期望中的用法是：
const newCar = AutoMaker.Car('premium');
newCar.drive(); // 'Vrooom!'
// 但是因为他是一个库
// 许多地方依然这样用:
const oldCar = new AutoMaker.Car();
// 如此就会导致:
// TypeError: Cannot read property 'undefined' of
// undefined at new AutoMaker.Car

In the above example, we start from Start with a class, and finally change it into a factory function that can create objects based on a specific prototype. Such a function can be widely used in interface abstraction and special needs customization.

4. Use constructors to give instanceof an opportunity

The difference between constructors and factory functions is the instanceof operator. Many people use instanceof to ensure the correctness of their code. But to be honest, this is very problematic, and it is recommended to avoid the use of instanceof.

instanceof will lie.

// instanceof 是一个原型链检查
// 不是一个类型检查
// 这意味着这个检查是取决于执行上下文的,
// 当原型被动态的重新关联,
// 你就会得到这样令人费解的情况
function foo() {}
const bar = { a: 'a'};
foo.prototype = bar;
// bar是一个foo的实例吗，显示不是
console.log(bar instanceof foo); // false
// 上面我们看到了，他的确不是一个foo实例
// baz 显然也不是一个foo的实例，对吧?
const baz = Object.create(bar);
// ...不对.
console.log(baz instanceof foo); // true. oops.

instanceof does not check like other strongly typed languages, it just checks the object on the prototype chain.

In some execution contexts, it will become invalid, such as when you change Constructor.prototype.

Another example is that you start with a constructor or class, and then you expand it into another object, just like the case where it was rewritten as a factory function above. At this time instanceof will also have problems.

In short, instanceof is another big change in constructor and factory function calls.

Benefits of using classes

• A convenient, self-contained keyword
  

• The only authoritative way to implement classes in JavaScript.
  

• # It is a good experience for other developers who have experience in class language development.

Disadvantages of using classes

All the disadvantages of constructors, plus:

Use the extends keyword to create a problematic class, for users It's a big temptation.
Hierarchical inheritance of classes will cause many well-known problems, including fragmented base class (the base class will be destroyed due to inheritance), gorilla banana problem (objects mixed with complex contexts), duplication by necessity (classes are inherited diversified need to be modified from time to time) and so on.

Although the other two methods may also get you into these problems, when using the extend keyword, the environment will lead you down this path. In other words, it leads you toward writing code with inflexible relationships, rather than more reusable code.

Benefits of using factory functions

Factory functions are more flexible than classes and constructors, and will not lead people to errors path of. It also won’t get you stuck in a deep inheritance chain. You can use many methods to simulate inheritance

1. Return any object with any prototype

For example, you can create different instances of a media player through the same implementation Instances can be created for different media formats, using different APIs, or an event library can be for DOM events or ws events.

The factory function can also instantiate objects through the execution context, which can benefit from the object pool and a more flexible inheritance model.

2. No worries about complex refactoring

You will never have the need to convert a factory function into a constructor, so refactoring is not necessary.

3. Without new

You don’t need the new keyword to create a new object. You can master this process by yourself.

4. Standard this behavior

This is the this you are familiar with, and you can use it to get the parent object. For example, in player.create(), this points to player, and other this can also be bound through call and apply.

5. No trouble with instanceof

6. Some people like the readability and intuitiveness of writing directly without new.

Disadvantages of factory functions

• does not automatically handle prototypes, and factory function prototypes will not affect the prototype chain.

• this does not automatically point to the new object in the factory function.

• There may be some small differences in details, but if there are no problems during the development process, don’t worry too much.

Conclusion

In my opinion, class may be a convenient keyword, but it cannot hide it Will lead unsuspecting users into the pit of inheritance. Another risk is the possibility that in the future you want to use factory functions, and you have to make very big changes.

If you are working in a relatively large team, if you want to modify a public API, you may interfere with code that you cannot access, so you cannot turn a blind eye to the impact of modified functions.

One of the great things about the factory pattern is that it is not only more powerful and flexible, but also encourages the entire team to make the API simpler, safer, and lighter.

Related recommendations:

Detailed introduction to this and return in JavaScript constructor

How to use factory pattern and constructor in JavaScript to create objects?

Detailed examples of the difference between javascript function literals and Function() constructors

How to use factory mode, constructor mode, and prototype mode in javascript Detailed explanation of creating object instances

Detailed explanation of expressions and constructors in basic javascript tutorial

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