# 类式继承+原型继承

## 类式继承

```javascript
function SuperClass() {
  this.superValue = true;
}
SuperClass.prototype.getSuperValue = function() {
  return this.superValue;
}

function SubClass() {
  this.subValue = false;
}
SubClass.prototype = new SuperClass();

SubClass.prototype.getSubValue = function() {
  return this.subValue;
}

var instance = new SubClass();

console.log(instance instanceof SuperClass)//true
console.log(instance instanceof SubClass)//true
console.log(SubClass instanceof SuperClass)//false
console.log(SubClass.prototype instanceof SuperClass)//true
```

虽然实现起来清晰简洁，但是这种继承方式有两个缺点：

* 由于子类通过其原型prototype对父类实例化，继承了父类，所以说父类中如果共有属性是引用类型，就会在子类中被所有的实例所共享，因此一个子类的实例更改子类原型从父类构造函数中继承的共有属性就会直接影响到其他的子类
* 由于子类实现的继承是靠其原型prototype对父类进行实例化实现的，因此在创建父类的时候，是无法向父类传递参数的。因而在实例化父类的时候也无法对父类构造函数内的属性进行初始化

## 原型继承

```javascript
function inheritObject(o) {
    //声明一个过渡对象
  function F() { }
  //过渡对象的原型继承父对象
  F.prototype = o;
  //返回过渡对象的实例，该对象的原型继承了父对象
  return new F();
}
```

原型式继承大和类式继承非常的相似，它只是对类式继承的一个封装，其中的过渡对象就相当于类式继承的子类，只不过在原型继承中作为一个普通的过渡对象存在，目的是为了创建要返回的新的实例对象。


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