典型继承:如何获得实例？

Question

在观看了这个视频之后，我了解了用Javascript实现继承的一种很好的方法:原型继承。此方法使用Object.create/Object.Ascription基于另一个实例创建一个新对象。这似乎非常有趣，因为它是非常容易理解正在发生的事情。例如：

​

const human = {
  species: 'human',
  create: function(values) {
    const instance = Object.create(this);
    Object.assign(instance, values);
    return instance;
  },
  sayName: function() {
    console.log(`Hi, my name is ${this.name}`);
  },
  saySpecies: function() {
    console.log(`Hi, I'm a ${this.species}`);
  }
}

const musician = human.create({
  species: 'musician',
  playInstrument: function() {
    console.log(`I'm a musician and I play ${this.instrument}`);
  }
});

const aHuman = human.create({ name: 'Paul' });
aHuman.sayName();
aHuman.saySpecies();

const aMusician = musician.create({ name: 'David', instrument: 'Guitar' });
aMusician.sayName();
aMusician.saySpecies();
aMusician.playInstrument();

// how to check if musician is an instance of human?
console.log(musician instanceof human);

​

使用经典继承，我可以使用instanceof来知道给定对象的链中是否有给定的proto (例如musician instanceof Human)。

这就是proto链的样子，它看起来非常类似于用new操作符创建的链。

​

​

问题如何用原型继承来实现相同的东西？

Answer 1

你的尝试有两个主要问题：

• instanceof的RHS必须是一个函数，所以您的类型必须构建在函数之上。
• 您没有区分静态/类型方法和实例方法。create是一种类型方法。sayName是一个实例方法。

以下内容应如您所期望的那样起作用。

​

function Human(){}

Object.assign(Human, {
  create: function(values) {
    let newObj = Object.create(this.prototype);
    return Object.assign(newObj, values);
  }
});

Object.assign(Human.prototype, {
  species: 'human',
  sayName: function() {
    console.log(`Hi, my name is ${this.name}`);
  },
  saySpecies: function() {
    console.log(`Hi, I'm a ${this.species}`);
  }
});

function Musician(){}

Object.assign(Musician, Human);

Musician.prototype = Human.create({
  species: 'musician',
  playInstrument: function() {
    console.log(`I'm a musician and I play ${this.instrument}`);
  }
});

const aHuman = Human.create({ name: 'Paul' });
aHuman.sayName();
aHuman.saySpecies();

const aMusician = Musician.create({ name: 'David', instrument: 'Guitar' });
aMusician.sayName();
aMusician.saySpecies();
aMusician.playInstrument();

// how to check if musician is an instance of human?
console.log(aMusician instanceof Human);

​

Answer 2

您的代码不使用继承。创建新对象的唯一位置是在这一行中：

    return Object.assign({}, this, values);
//                       ^^ here

{}创建一个继承自Object.prototype的新对象。这就像在做Object.create(Object.prototype)或new Object()。

代码中没有任何东西是从human或musician继承的。

Answer 3

@JLRishe的答案有效。但是，如果您想避免使用函数并继续使用对象，则可以实现您自己版本的instanceof。这个代码片段也能工作：

​

const human = {
  species: 'human',
  create: function(values) {
    let newObj = Object.create(this);
    return Object.assign(newObj, values);
  },
  instanceof: function(obj) {
    if (!Object.getPrototypeOf(this)) {
      return false;
    }
    let isPrototypeOf = (
      this === obj ||
      Object.getPrototypeOf(this) === obj ||
      this === Object.getPrototypeOf(obj) ||
      Object.getPrototypeOf(this) === obj.prototype
    );
    if (!isPrototypeOf) {
      isPrototypeOf = this.instanceof.call(Object.getPrototypeOf(this), obj);
    }
    return isPrototypeOf;
  },
  sayName: function() {
    console.log(`Hi, my name is ${this.name}`);
  },
  saySpecies: function() {
    console.log(`Hi, I'm a ${this.species}`);
  }
}

const musician = human.create({
  species: 'musician',
  playInstrument: function() {
    console.log(`I'm a musician and I play ${this.instrument}`);
  }
});

const aHuman = human.create({ name: 'Paul' });
aHuman.sayName();
aHuman.saySpecies();

const aMusician = musician.create({ name: 'David', instrument: 'Guitar' });
aMusician.sayName();
aMusician.saySpecies();
aMusician.playInstrument();

var test = {};

console.log(human.instanceof(human));
console.log(musician.instanceof(human));
console.log(musician.instanceof(Object));
console.log(aHuman.instanceof(human));
console.log(aMusician.instanceof(human));
console.log(aHuman.instanceof(musician));
console.log(aHuman.instanceof(Object));
console.log(aMusician.instanceof(musician));
console.log(musician.instanceof(test));
console.log(human.instanceof(test));
console.log(aMusician.instanceof(test));
console.log(aHuman.instanceof(test));

​