An easy introduction to object-oriented JavaScript

This chapter assumes that everyone has read the author's previous article"JavaScriptEasy Introduction to Object-OrientedAbstraction"

Why encapsulation?

Encapsulation is to hide the internal properties and methods of an object. External code can only access the object through a specific interface. This is also part of the idea of interface-oriented programming.

Encapsulation is a very important part of object-oriented programming. Let’s take a look at what the code without encapsulation looks like:

1 function Dog(){2 this.hairColor = '白色';//string3 this.breed = '贵宾';//string4 this.age = 2;//number5 }6 var dog = new Dog();7 console.log(dog.breed);//log: '贵宾'

There seems to be no problem, but what if thebreedattribute name is changed? For example, ifthis.type = ‘VIP’,, then all codes using theDogclass must be changed.

If you write the code for the class and the code for using the class, and there are not many places to use this class, it doesn’t matter if you write it this way.

But if this class is used in many places, or other people also use your class during collaborative development, then doing so will make the code difficult to maintain. The correct approach is:

1 function Dog(){ 2 this.hairColor = '白色';//string 3 this.age = 2;//number 4 this._breed = '贵宾';//string 5 } 6 Dog.prototype.getBreed = function(){ 7 return this._breed; 8 } 9 Dog.prototype.setBreed = function(val){10 this._breed = val;11 }12 var dog = new Dog();13 console.log(dog.getBreed());//log: '贵宾'14 dog.setBreed('土狗');

　getBreed()is the interface. If the internal properties change, for example,breedis replaced bytype, then you only need to change the code ingetBreed(), and you can monitor all operations to obtain this attribute.

Therefore, encapsulation has many benefits:

1. As long as the interface does not change, the internal implementation can be changed arbitrarily; 2. It is very convenient for users to use. It doesn’t matter how it is implemented internally;

3. Reduce the coupling between codes;

4. Meet the collaborative development of large applications and multiple people;

Usegetter/setterto encapsulate private properties

getter/setter, as followsdemo, this chapter does not talk about the principles, only the use. You can check the information yourself for the principles:

1 function Dog(){ 2 this.hairColor = '白色';//string 3 this.age = 2;//number 4 this._breed = '贵宾';//string 5 Object.defineProperty(this, 'breed', {//传入this和属性名 6 get : function () { 7 console.log('监听到了有人调用这个get breed') 8 return this._breed; 9 },10 set : function (val) {11 this._breed = val;12 /*13 如果不设置setter的话默认这个属性是不可设置的14 但有点让人诟病的是，浏览器并不会报错15 所以即使你想让breed是只读的，你也应该设置一个setter让其抛出错误：16 throw 'attribute "breed" is read only!';17 */18 }19 });20 }21 var dog = new Dog();22 console.log(dog.breed);23 /*log：24 '监听到了有人调用这个get breed接口'25 '贵宾'26 */27 dog.breed = '土狗';28 console.log(dog.breed);29 /*log：30 '监听到了有人调用这个get breed接口'31 '土狗'32 */

getBreed(),getter/setterGenerally used inreadonlyattributes and some more important interfaces, as well as reconstructing attribute operations that do not encapsulate interfaces.You can also use closures to encapsulate private properties, which is the safest, but will cause additional memory overhead, so the author doesn't like to use it very much. You can learn about it yourself.

/Private conceptIn the first two sections, we briefly understood encapsulation, but these are definitely not enough Used, let's first understand the following concepts:

: Properties that can only be accessed and modified within the class, and external access is not allowed.

: A method that can only be called internally within the class, and is prohibited from being called externally.

: Attributes that can be obtained and modified outside the class. Theoretically, all attributes of a class should be private attributes and can only be accessed through encapsulated interfaces. However, for some smaller classes or classes that are used less frequently, you do not need to encapsulate the interface if you find it more convenient.

: Methods that can be called externally. Methods that implement interfaces such asgetBreed()are public methods, as well as behavioral methods exposed to the outside world.

: The properties and methods of the class itself. There is no need to distinguish between public and private. All static properties and static methods must be private and must be accessed through the encapsulated interface. This is why the author in the previous chapter usedgetInstanceNumber()to accessDog.instanceNumberProperty.

：

1 function Dog(){ 2 /*公有属性*/ 3 this.hairColor = null;//string 4 this.age = null;//number 5 /*私有属性，人们共同约定私有属性、私有方法前面加上_以便区分*/ 6 this._breed = null;//string 7 this._init(); 8 /*属性的初始化最好放一个私有方法里，构造函数最好只用来声明类的属性和调用方法*/ 9 Dog.instanceNumber++;10 }11 /*静态属性*/12 Dog.instanceNumber = 0;13 /*私有方法，只能类的内部调用*/14 Dog.prototype._init = function(){15 this.hairColor = '白色';16 this.age = 2;17 this._breed = '贵宾';18 }19 /*公有方法：获取属性的接口方法*/20 Dog.prototype.getBreed = function(){21 console.log('监听到了有人调用这个getBreed()接口')22 return this._breed;23 }24 /*公有方法：设置属性的接口方法*/25 Dog.prototype.setBreed = function(breed){26 this._breed = breed;27 return this;28 /*这是一个小技巧，可以链式调用方法，只要公有方法没有返回值都建议返回this*/29 }30 /*公有方法：对外暴露的行为方法*/31 Dog.prototype.gnawBone = function() {32 console.log('这是本狗最幸福的时候');33 return this;34 }35 /*公有方法：对外暴露的静态属性获取方法*/36 Dog.prototype.getInstanceNumber = function() {37 return Dog.instanceNumber;//也可以this.constructor.instanceNumber38 }39 var dog = new Dog();40 console.log(dog.getBreed());41 /*log:42 '监听到了有人调用这个getBreed()接口'43 '贵宾'44 */45 /*链式调用，由于getBreed()不是返回this,所以getBreed()后面就不可以链式调用了*/46 var dogBreed = dog.setBreed('土狗').gnawBone().getBreed();47 /*log：48 '这是本狗最幸福的时候'49 '监听到了有人调用这个getBreed()接口'50 */51 console.log(dogBreed);//log: '土狗'52 console.log(dog);

（Novices don’t want to watch this ES6 and TypeScrpt implementation part):

1 class Dog{ 2 constructor(){ 3 this.hairColor = null;//string 4 this.age = null;//number 5 this._breed = null;//string 6 this._init(); 7 Dog.instanceNumber++; 8 } 9 _init(){10 this.hairColor = '白色';11 this.age = 2;12 this._breed = '贵宾';13 }14 get breed(){15 /*其实就是通过getter实现的，只是ES6写起来更简洁*/16 console.log('监听到了有人调用这个get breed接口');17 return this._breed;18 }19 set breed(breed){20 /*跟ES5一样，如果不设置的话默认breed无法被修改，而且不会报错*/21 console.log('监听到了有人调用这个set breed接口');22 this._breed = breed;23 return this;24 }25 gnawBone() {26 console.log('这是本狗最幸福的时候');27 return this;28 }29 getInstanceNumber() {30 return Dog.instanceNumber;31 }32 }33 Dog.instanceNumber = 0;34 var dog = new Dog();35 console.log(dog.breed);36 /*log:37 '监听到了有人调用这个get breed接口'38 '贵宾'39 */40 dog.breed = '土狗';//log:'监听到了有人调用这个set breed接口'41 console.log(dog.breed);42 /*log:43 '监听到了有人调用这个get breed接口'44 '土狗'45 */

　　ES5、ES6中虽然我们把私有属性和方法用“_”放在名字前面以区分，但外部还是可以访问到属性和方法的。

TypeScrpt中就比较规范了，可以声明私有属性，私有方法，并且外部是无法访问私有属性、私有方法的：

1 class Dog{ 2 public hairColor: string; 3 readonly age: number;//可声明只读属性 4 private _breed: string;//虽然声明了private，但还是建议属性名加_以区分 5 static instanceNumber: number = 0;//静态属性 6 constructor(){ 7 this._init(); 8 Dog.instanceNumber++; 9 }10 private _init(){11 this.hairColor = '白色';12 this.age = 2;13 this._breed = '贵宾';14 }15 get breed(){16 console.log('监听到了有人调用这个get breed接口');17 return this._breed;18 }19 set breed(breed){20 console.log('监听到了有人调用这个set breed接口');21 this._breed = breed;22 }23 public gnawBone() {24 console.log('这是本狗最幸福的时候');25 return this;26 }27 public getInstanceNumber() {28 return Dog.instanceNumber;29 }30 }31 let dog = new Dog();32 console.log(dog.breed);33 /*log:34 '监听到了有人调用这个get breed接口'35 '贵宾'36 */37 dog.breed = '土狗';//log:'监听到了有人调用这个set breed接口'38 console.log(dog.breed);39 /*log:40 '监听到了有人调用这个get breed接口'41 '土狗'42 */43 console.log(dog._breed);//报错，无法通过编译44 dog._init();//报错，无法通过编译

注意事项：

　　1、暴露给别人的类，多个类组合成一个类时，所有属性一定都要封装起来；

　　2、如果你来不及封装属性，可以后期用getter/setter弥补；

　　3、每个公有方法，最好注释一下含义；

　　4、在重要的类前面最好用注释描述所有的公有方法；

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