ES6类以及继承的实现原理

儿独

ES6中通过class关键字,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

"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 _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.我觉得这样的规范挺好的,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上defineProperties(Constructor, staticProps)。但是貌似并没有用到,下面可以证明.

这两个流程走下来,其实就创建了一个类。

上面讲的是创建一个类的过程,那ES6如何实现继承的呢?还是上面的例子,这次我们给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");
    }
}

var c = new Child("job",30);
c.coding()

转码之后的代码变成了这样

"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;


var c = new Child("job", 30);
c.coding();

我们可以看到,构造类的方法都没变,只是添加了_inherits核心方法来实现继承,下面我们就看下这个方法做了什么?

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subClass.__proto__ = superClass
_inherits核心思想就是下面两句

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

一图胜千言

那为什么这样一倒腾,它就实现了继承了呢?
首先 subClass.prototype.__proto__ = superClass.prototype保证了c instanceof Parent是true,Child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Child.height也能访问到,也就是静态方法。

subClass.__proto__ = superClass不是很好理解,可以通过下面的方式理解

function A(){}
var a = new A()
a.__proto__ = A.prototype

a是一个实例,A.prototype是构造方法的原型。通过这种方式,那么a就可以访问A.prototype上面的方法。

那把 subClass类比成 a,superClass类比成A.prototype,那是不是subClass可以直接访问 superClass的静态属性,静态方法了。

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