正确地扩展数组，保留子类的实例 [英] Extending an Array properly, keeping the instance of subclass

问题描述

我写了一个类试图用自定义类扩展本地的Javascript 数组类，让我们称之为 MyClass 。这基本上是它的样子：

I've written a class trying to extend the native Javascript Array class with a custom class, let's call it MyClass. This is basically what it looks like:

class MyClass extends Array

  constructor: (obj) -> @push.apply @, obj

  first: -> @slice 0, 1

实例化类没有问题。在控制台中执行此操作：

Instantiating the class is no problem. Running this in the console:

var myInstance = new MyClass(["1", "2"])
> ["1", "2"]
myInstance instanceof MyClass
> true
myInstance instanceof Array
> true

正常工作。

问题是如果我运行：

myInstance.first()
> ["1"] // as expected
myInstance.first() instanceof MyClass
> false // not expected
myInstance.first() instanceof Array
> true

返回的值不再是MyClass的实例。

the returned value is no longer an instance of MyClass.

我也尝试了 @__ proto __。first = @first 在构造函数和 first： - > @ slice.call @，0，1 。但是没有成功。

I've also tried @__proto__.first = @first in the constructor function and first: -> @slice.call @, 0, 1. But with no success.

为什么 myInstance.first（）instanceof MyClass return true ？

Why doesn't myInstance.first() instanceof MyClass return true?

推荐答案

为什么不 myInstance.first（）instanceof MyClass return true？

Why doesn't myInstance.first() instanceof MyClass return true?

因为 调用 slice 和 Array.prototype.slice 总是返回 Array 。您将需要使用一个方法将其覆盖，再次将它包装在 MyClass 中：

Because first calls slice, and Array.prototype.slice does always return an Array. You will need to overwrite it with a method that wraps it in a MyClass again:

class MyClass extends Array

  constructor: (obj) -> @push.apply @, obj

  slice: () -> new MyClass super
  splice: () -> new MyClass super
  concat: () -> new MyClass super
  filter: () -> new MyClass super
  map: () -> new MyClass super

  first: -> @slice 0, 1

请注意， subclassing Array 无效。

And notice that subclassing Array does not work.

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