ES6中class的实现原理

Posted 2022-10-28 gg-qq

一、在ES6以前实现类和继承

　　实现类的代码如下：

function Person(name, age) 
    this.name = name;
    this.age = age;


Person.prototype.speakSomething = function () 
    console.log("I can speek chinese");
;

　　实现继承的代码如下：一般使用原型链继承和call继承混合的形式

function Person(name) 
    this.name = name;


Person.prototype.showName = function () 
    return `名字是：$this.name`;
;

function Student(name, skill) 
    Person.call(this, name);//继承属性
    this.skill = skill;


Student.prototype = new Person();//继承方法

二、ES6使用class定义类

class Parent 
    constructor(name,age)
        this.name = name;
        this.age = age;
    
    speakSomething()
        console.log("I can speek chinese");
    

　　经过babel转码之后

function _classCallCheck(instance, Constructor) 
    if (!(instance instanceof Constructor)) 
        throw new TypeError("Cannot call a class as a function");
    


var Parent = function () 
    function Parent(name, age) 
        _classCallCheck(this, Parent);

        this.name = name;
        this.age = age;
    

    _createClass(Parent, [
        key: "speakSomething",
        value: function speakSomething() 
            console.log("I can speek chinese");
        
    ]);

    return Parent;
();

　　可以看到ES6类的底层还是通过构造函数去创建的。

　　 通过ES6创建的类，是不允许你直接调用的。在ES5中，构造函数是可以直接运行的，比如Parent()。但是在ES6就不行。我们可以看到转码的构造函数中有_classCallCheck(this, Parent)语句,这句话是防止你通过构造函数直接运行的。你直接在ES6运行Parent(),这是不允许的,ES6中抛出Class constructor Parent cannot be invoked without ‘new‘错误。转码后的会抛出Cannot call a class as a function.能够规范化类的使用方式。

　　转码中_createClass方法，它调用Object.defineProperty方法去给新创建的Parent添加各种属性。defineProperties(Constructor.prototype, protoProps)是给原型添加属性。如果你有静态属性，会直接添加到构造函数defineProperties(Constructor, staticProps)上。

三、ES6实现继承

　　我们给Parent添加静态属性，原型属性，内部属性。

class Parent 
    static height = 12
    constructor(name,age)
        this.name = name;
        this.age = age;
    
    speakSomething()
        console.log("I can speek chinese");
    

Parent.prototype.color = ‘yellow‘


//定义子类，继承父类
class Child extends Parent 
    static width = 18
    constructor(name,age)
        super(name,age);
    
    coding()
        console.log("I can code JS");
    

　　经过babel转码之后

"use strict";

var _createClass = function () 
    function defineProperties(target, props) 
        for (var i = 0; i < props.length; i++) 
            var descriptor = props[i];
            descriptor.enumerable = descriptor.enumerable || false;
            descriptor.configurable = true;
            if ("value" in descriptor) descriptor.writable = true;
            Object.defineProperty(target, descriptor.key, descriptor);
        
    

    return function (Constructor, protoProps, staticProps) 
        if (protoProps) defineProperties(Constructor.prototype, protoProps);
        if (staticProps) defineProperties(Constructor, staticProps);
        return Constructor;
    ;
();

function _possibleConstructorReturn(self, call) 
    if (!self) 
        throw new ReferenceError("this hasn‘t been initialised - super() hasn‘t been called");
    
    return call && (typeof call === "object" || typeof call === "function") ? call : self;


function _inherits(subClass, superClass) 
    if (typeof superClass !== "function" && superClass !== null) 
        throw new TypeError("Super expression must either be null or a function, not " + typeof superClass);
    
    subClass.prototype = Object.create(superClass && superClass.prototype, 
        constructor: 
            value: subClass,
            enumerable: false,
            writable: true,
            configurable: true
        
    );
    if (superClass) Object.setPrototypeOf ? Object.setPrototypeOf(subClass, superClass) : subClass.__proto__ = superClass;


function _classCallCheck(instance, Constructor) 
    if (!(instance instanceof Constructor)) 
        throw new TypeError("Cannot call a class as a function");
    


var Parent = function () 
    function Parent(name, age) 
        _classCallCheck(this, Parent);

        this.name = name;
        this.age = age;
    

    _createClass(Parent, [
        key: "speakSomething",
        value: function speakSomething() 
            console.log("I can speek chinese");
        
    ]);

    return Parent;
();

Parent.height = 12;

Parent.prototype.color = ‘yellow‘;

//定义子类，继承父类

var Child = function (_Parent) 
    _inherits(Child, _Parent);

    function Child(name, age) 
        _classCallCheck(this, Child);

        return _possibleConstructorReturn(this, (Child.__proto__ || Object.getPrototypeOf(Child)).call(this, name, age));
    

    _createClass(Child, [
        key: "coding",
        value: function coding() 
            console.log("I can code JS");
        
    ]);

    return Child;
(Parent); 

Child.width = 18;

　　构造类的方法都没变，只是添加了_inherits核心方法来实现继承。具体步骤如下：

　　首先是判断父类的类型，然后：

subClass.prototype = Object.create(superClass && superClass.prototype, 
        constructor: 
            value: subClass,
            enumerable: false,
            writable: true,
            configurable: true
        
    );

　　这段代码翻译下来就是

function F()
F.prototype = superClass.prototype
subClass.prototype = new F()
subClass.prototype.constructor = subClass

　　接下来就是subClass.__proto__ = superClass

　　_inherits核心思想就是下面两句：　　

subClass.prototype.__proto__ = superClass.prototype
subClass.__proto__ = superClass

　　如下图所示：

　　首先 subClass.prototype.__proto__ = superClass.prototype保证了子类的实例instanceof父类是true，子类的实例可以访问到父类的属性，包括内部属性，以及原型属性。

　　其次，subClass.__proto__ = superClass，保证了静态属性也能访问到，也就是这个例子中的Child.height。