ES6中class的實現原理

一、在ES6以前實現類和繼承

　　實現類的代碼如下：

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

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

　　實現繼承的代碼如下：一般使用原型鏈繼承和call繼承混合的形式

function Person(name) {
    this.name = name;
}

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

function Student(name, skill) {
    Person.call(this, name);//繼承屬性
    this.skill = skill;
}

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

二、ES6使用class定義類

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

　　經過babel轉碼之后

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類的底層還是通過構造函數去創建的。

　　 通過ES6創建的類，是不允許你直接調用的。在ES5中，構造函數是可以直接運行的，比如Parent()。但是在ES6就不行。我們可以看到轉碼的構造函數中有_classCallCheck(this, Parent)語句,這句話是防止你通過構造函數直接運行的。你直接在ES6運行Parent(),這是不允許的,ES6中拋出Class constructor Parent cannot be invoked without 'new'錯誤。轉碼后的會拋出Cannot call a class as a function.能夠規范化類的使用方式。

　　轉碼中_createClass方法，它調用Object.defineProperty方法去給新創建的Parent添加各種屬性。defineProperties(Constructor.prototype, protoProps)是給原型添加屬性。如果你有靜態屬性，會直接添加到構造函數defineProperties(Constructor, staticProps)上。

三、ES6實現繼承

　　我們給Parent添加靜態屬性，原型屬性，內部屬性。

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'


//定義子類，繼承父類
class Child extends Parent {
    static width = 18
    constructor(name,age){
        super(name,age);
    }
    coding(){
        console.log("I can code JS");
    }
}

　　經過babel轉碼之后

"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';

//定義子類，繼承父類

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;

　　構造類的方法都沒變，只是添加了_inherits核心方法來實現繼承。具體步驟如下：

　　首先是判斷父類的類型，然后：

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

　　這段代碼翻譯下來就是

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

　　接下來就是subClass.__proto__ = superClass

　　_inherits核心思想就是下面兩句：　　

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

　　如下圖所示：

　　首先 subClass.prototype.__proto__ = superClass.prototype保證了子類的實例instanceof父類是true，子類的實例可以訪問到父類的屬性，包括內部屬性，以及原型屬性。

　　其次，subClass.__proto__ = superClass，保證了靜態屬性也能訪問到，也就是這個例子中的Child.height。