# 组合式继承

```javascript
function SuperClass(name) {
  this.name = name; 
  this.books = ['Js','CSS'];
}
SuperClass.prototype.getBooks = function() {
    console.log(this.books);
}
function SubClass(name,time) {
  SuperClass.call(this,name); // 拥有 name和 books 就相当于super
  this.time = time;
}
SubClass.prototype = new SuperClass();

SubClass.prototype.getTime = function() {
  console.log(this.time);
}
```

如上，我们就解决了之前说到的一些问题。

组合继承最大的问题是，无论什么情况下，都会调用两次超类型构造函数：一次是在创建子类型原型的时候，另一次是在子类型构造函数内部。

{% hint style="info" %}
SuperClass.call 很重要

在 ES6 class 中，子类必须在 `constructor` 方法中调用 `super` 方法，否则新建实例时会报错。这是因为**子类没有自己的 `this` 对象，而是继承父类的 `this` 对象，然后对其进行加工。如果不调用 `super` 方法，子类就得不到 `this` 对象。**

也正是因为这个原因，在子类的构造函数中，只有调用 `super` 之后，才可以使用 `this` 关键字，否则会报错。
{% endhint %}


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://mm.ricky.moe/javascript/js-concept/ji-cheng/zu-he-shi-ji-cheng.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.