How to use JS classes, constructors, and factory functions

This time I will bring you how to use JS class, constructor , and factory function. What are the precautions when using JS class, constructor, and factory function? The following is a practical case, let’s take a look.

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.

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 based on prototype creation, and all support the implementation of private variables in the constructor function. 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 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

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

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

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

2. 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 do something during the construction process, it will be a very troublesome thing.

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

Because of the detailed handling 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 that upgrading the class function to a factory function will not have any impact, but in JavaScript, it does have an impact.

If you start to write a constructor or class, 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 started with a class and finally changed 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 a constructor and a factory function is instanceof operator. Many people use instanceof to ensure the correctness of their code. But to be honest, this is a big problem, 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 fail, 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 above situation where it was rewritten as a factory function. At this time instanceof will also have problems.

All in all, instanceof is another big change in constructor and factory function calls.

Benefits of using classes

• A convenient, self-contained keyword

• that is the only authoritative way to implement a class in JavaScript.

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

Disadvantages of using classes

All the bad things about constructors, plus:

Creating a problematic class using the extends keyword is a big temptation for users.
Hierarchical inheritance of classes can cause many well-known problems, including fragile base class (the base class will be destroyed due to inheritance), gorilla banana problem (objects mixed with complex contexts), duplication by necessity (classes need to be modified from time to time when inheritance is diversified), etc.

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 down the wrong path. It also won’t get you stuck in a deep inheritance chain. There are many ways you can simulate inheritance

1. Return any object using any prototype

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

Factory functions can also instantiate objects through execution contexts, benefiting from object pooling and a more flexible inheritance model.

2. No worries about complex refactoring

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

3. No new

You don't use the new keyword to create a new object, you can master this process 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

• There is no automatic processing of 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.

in conclusion

In my opinion, class may be a convenient keyword, but it cannot hide that it will lead unsuspecting users into the inheritance pit. 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 have no access to, so you cannot turn a blind eye to the impact of modified functions.

Factory PatternThe great thing is that it is not only more powerful and flexible, but also encourages the entire team to make the API simpler, safer, and lighter.

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the php Chinese website!

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