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# Class

JavaScript 中的 class 是建立在 prototype 之上的，但它也有一些 class 特有的 syntax 和 semantics。

class 实际上是一种特殊的 function，它也有两种定义方式：

1. class declaration 类声明
   * 但是，它不会被提升，也有 temporal dead zone（临时死区）限制，同 `let`, `const`
2. class expression 类表达式

```js
// Declaration
class Rectangle {
    constructor(height, width) {
        this.height = height
        this.width = width
    }
}

// Expression: the class is anonymous but assigned to a variable
const Rectangle = class {
    constructor(height, width) {
        this.height = height
        this.width = width
    }
};

// Expression: the class has its own name
const Rectangle = class Rectangle2 {
    constructor(height, width) {
        this.height = height
        this.width = width
    }
}
```

class 的代码体默认运行在 strict mode。

class 元素可以通过以下三个方面来表征：

| Kind                                          | Location                  | Visibility               |
| --------------------------------------------- | ------------------------- | ------------------------ |
| <p>getter<br>setter<br>method</p><p>field</p> | <p>instance<br>static</p> | <p>public<br>private</p> |

## `constructor()`

`constructor()` 用来 create 和 initialize 一个对象。在一个 class 中，只能有一个 `constructor()`。

有两种字段：instance properties 和 class fields。

如果 instance properties 的 values 不依赖 constructor 的 arguments，则可以用 class fields。class fields 类似于 object properties 而不是 variables，所以声明时不需要使用 `const` 等关键字。如果声明时没有 default value，则默认是 `undefined`。

```javascript
class Rectangle {
    // class fields
    height = 0
    width;

    constructor(height, width) {
        // instance properties
        this.height = height
        this.width = width
    }
}
```

```javascript
class Rectangle {
    // private fields
    #height = 0
    #width
    constructor(height, width) {
        this.#height = height
        this.#width = width
    }
}
```

说明：private 特性有一个限制，那就是在同一个 class 中声明的所有 property names 必须唯一。而 public properties 则没有这个限制，当有多个 public property names 时，最后一个会 overwrites 其它的。

## static initialization blocks

## methods

可以是：

* plain function
* async function
* generator function
* async generator function

## 继承 inheritance


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