ES5和ES6的继承对比

ES5的继承实现，这里以最佳实践：寄生组合式继承方式来实现。（为什么是最佳实践，前面有随笔讲过了，可以参考）

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

Super.prototype.sayName = function() {
   console.log(this.name)
}

function Sub(name, age) {
  Super.call(this, name);
  this.age = age;
}

Sub.prototype.sayAge = function() {
    console.log(this.age)
}

Sub.prototype = Object.create(Super.prototype, {
    constructor: {
        value: Sub,
        writable: true,
        configurable: true
    }
});

这里的Object.create可以替换成Object.setPrototypeOf，好处是不用再手动绑定constructor的指向。

这是ES5继承，再看下ES6的继承，同样实现上面的效果

class Super {
  constructor(name) {
    this.name = name;
  }
  sayName() {
    console.log(this.name);
  }
}

class Sub extends Super {
  constructor(name, age) {
    super(name);
    this.age = age;
  }
  sayAge() {
    console.log(this.age);
  }
}

很明显，ES6的继承要更简洁点，那区别是什么？

我们看一下babel解析后的ES6的继承：

"use strict";

function _typeof(obj) {
    if (typeof Symbol === "function" && typeof Symbol.iterator === "symbol") {
        _typeof = function _typeof(obj) {
            return typeof obj;
        };
    } else {
        _typeof = function _typeof(obj) {
            return obj && typeof Symbol === "function" && obj.constructor === Symbol && obj !== Symbol.prototype ? "symbol" : typeof obj;
        };
    } return _typeof(obj);
}

function _possibleConstructorReturn(self, call) {
    if (call && (_typeof(call) === "object" || typeof call === "function")) {
        return call;
    } return _assertThisInitialized(self);
}

function _assertThisInitialized(self) {
    if (self === void 0) {
        throw new ReferenceError("this hasn't been initialised - super() hasn't been called");
    } return self;
}

function _getPrototypeOf(o) {
    _getPrototypeOf = Object.setPrototypeOf ? Object.getPrototypeOf : function _getPrototypeOf(o) {
        return o.__proto__ || Object.getPrototypeOf(o);
    }; return _getPrototypeOf(o);
}

function _inherits(subClass, superClass) {
    if (typeof superClass !== "function" && superClass !== null) {
        throw new TypeError("Super expression must either be null or a function");
    }
    subClass.prototype = Object.create(superClass && superClass.prototype, {
        constructor: {
            value: subClass,
            writable: true,
            configurable: true
        }
    });
    if (superClass) _setPrototypeOf(subClass, superClass);
}

function _setPrototypeOf(o, p) {
    _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) {
        o.__proto__ = p;
        return o;
    };
    return _setPrototypeOf(o, p);
}

function _instanceof(left, right) {
    if (right != null && typeof Symbol !== "undefined" && right[Symbol.hasInstance]) {
        return !!right[Symbol.hasInstance](left);
    } else {
        return left instanceof right;
    }
}

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

function _createClass(Constructor, protoProps, staticProps) {
    if (protoProps) _defineProperties(Constructor.prototype, protoProps);
    if (staticProps) _defineProperties(Constructor, staticProps);
    return Constructor;
}

var Super =
    /*#__PURE__*/
    function () {
        function Super(name) {
            _classCallCheck(this, Super);

            this.name = name;
        }

        _createClass(Super, [{
            key: "sayName",
            value: function sayName() {
                console.log(this.name);
            }
        }]);

        return Super;
    }();

var Sub =
    /*#__PURE__*/
    function (_Super) {
        _inherits(Sub, _Super);

        function Sub(name, age) {
            var _this;

            _classCallCheck(this, Sub);

            _this = _possibleConstructorReturn(this, _getPrototypeOf(Sub).call(this, name));
            _this.age = age;
            return _this;
        }

        _createClass(Sub, [{
            key: "sayAge",
            value: function sayAge() {
                console.log(this.age);
            }
        }]);

        return Sub;
    }(Super);

代码有点多，但很多都是做了一些类型检查或者是polify的兼容，关键的方法是两个地方：

1，_inherits （这里等同于关键字extends的作用）

function _inherits(subClass, superClass) {
    if (typeof superClass !== "function" && superClass !== null) {
        throw new TypeError("Super expression must either be null or a function");
    }
    subClass.prototype = Object.create(superClass && superClass.prototype, {
        constructor: {
            value: subClass,
            writable: true,
            configurable: true
        }
    });
    if (superClass) _setPrototypeOf(subClass, superClass);
}

前面的和ES5继承没有区别，也是用了Object.create实现子类的prototype继承父类的prototype，但是最后多了一个操作

_setPrototypeOf(subClass, superClass);

看一下对应的函数定义

function _setPrototypeOf(o, p) {
    _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) {
        o.__proto__ = p;
        return o;
    };
    return _setPrototypeOf(o, p);
}

意图很明显了，就是为了实现子类继承父类！

从这里我们看出差别了，ES6的继承除了子类的原型继承以外，还多了子类的自身继承父类，但是为什么这么做呢？

（这里笔者也没有想明白为什么要这么做，欢迎留言补充解惑！）

2，_possibleConstructorReturn (这里等同于super关键字)

function _possibleConstructorReturn(self, call) {
    if (call && (_typeof(call) === "object" || typeof call === "function")) {
        return call;
    } return _assertThisInitialized(self);
}

function _assertThisInitialized(self) {
    if (self === void 0) {
        throw new ReferenceError("this hasn't been initialised - super() hasn't been called");
    } return self;
}

看一下用法

_this = _possibleConstructorReturn(this, _getPrototypeOf(Sub).call(this, name));

这里的 _getPrototypeOf(Sub) 实际上就是Super，因为上面使用了_inherits，所以已经继承了类，因此这里就是等同于在Sub的内部使用了Super.call(this, name)；从而实现了属性的继承，因此和ES5是一样的。

不过综上来看，确实ES6的继承要简单很多，主要是写法上比较简单，不用手动去实现原型的继承以及属性的复制，但是本质上其实是一样的。

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