# 3.14 類別(Class)

在 `javascript/practice` 資料夾下，建立 `class.html` 來練習。

## 概念

要做杯子蛋糕，就要有做杯子蛋糕的模具(杯子)，有了模具(杯子)，就可以重覆做出杯子蛋糕。

## 類別宣告

使用 `class` 來宣告，以及常搭配 `constructor` 建構子，以下就是上述概念所說的模具：

```javascript
class Book {
  constructor(book_name, price){
    //console.log("hi");
    //console.log(this);
    this.book_name = book_name;
    this.price = price;
  }
}
```

## 實體化一個書本物件

做出一個書本：

```javascript
var book1 = new Book("書本名稱一", 300); // 會自動執行 constructor
console.log(book1);
```

## 增加功能

替模具增加功能，改成以下(加上 `show_discount_price 函式`)：

```javascript
class Book {
  constructor(book_name, price){
    //console.log("hi");
    //console.log(this);
    this.book_name = book_name;
    this.price = price;
  }
  
  show_discount_price(){
    return this.book_name + " 特價 " + this.price * 0.9 + " 元";
  }
}
```

## 使用功能

```javascript
var book1 = new Book("書本名稱一", 300);
console.log(book1.show_discount_price());
```

## 實體化第二個物件

```javascript
var book1 = new Book("書本名稱一", 300);
var book2 = new Book("書本名稱二", 400);
```


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