原型继承：如何获取 instanceof？

Question

看了这个video之后，我了解到了一种用Javascript实现继承的好方法：原​​型继承。此方法使用 Object.create/Object.assign 基于另一个实例创建新对象。这似乎很有趣，因为很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);

通过经典继承，我可以使用instanceof 来了解给定对象是否在其链中具有给定的proto（例如musician instanceof Human）。

这就是 proto 链的样子，看起来与使用 new 运算符创建的链非常相似。

问题 如何通过原型继承实现同样的目标？

Answer 1

您的尝试有两个主要问题：

• instanceof 的 RHS 必须是一个函数，因此您的类型必须建立在函数之上。
• 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。

以下应该按您的预期工作。

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。唯一创建新对象的地方就是这一行：

    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()。

您的代码中没有任何内容继承自 human 或 musician。

Answer 3

@JLRishe 的回答有效。但是，如果您想避免使用函数并继续使用对象，您可以实现自己的 instanceof 版本。这段代码 sn-p 也可以：

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));