为什么在 Array 的子类中设置位置不会改变它的长度?我不应该继承数组吗?

问题描述

在下面的 CoffeeScript 程序中，我创建了一个 Array 的子类，它在其构造函数中设置了两个位置:

In the CoffeeScript program below, I create a subclass of Array which sets two positions in its constructor:

class SetPositionsArray extends Array
  constructor: (x,y) ->
    @[0] = x
    @[1] = y

  myLength: ->
    @length

sp_array = new SetPositionsArray 1, 2

console.log "sp_array: "
console.log sp_array
console.log "sp_array[0]: "
console.log sp_array[0]
console.log "sp_array[1]: "
console.log sp_array[1]
console.log "sp_array.length: "
console.log sp_array.length
console.log "sp_array.myLength(): "
console.log sp_array.myLength()

我希望这段代码能改变 sp_array 的 length 属性，因为它有效地设置了位置.但是，我得到的输出是:

I would hope that this code would change the length property of sp_array, since it effectively sets positions on it. However, the output I get is:

$ coffee sp.coffee
sp_array: 
[ 1, 2 ]
sp_array[0]: 
1
sp_array[1]: 
2
sp_array.length: 
0
sp_array.myLength(): 
0

即长度为0.

然后，我创建了另一个类，它在实例中推送值而不是设置它们:

Then, I created another class which pushes values in the instance instead of setting them:

class PushValuesArray extends Array
  constructor: (x,y) ->
    @push x
    @push y

  myLength: ->
    @length


pv_array = new PushValuesArray 1, 2


console.log "pv_array: "
console.log pv_array
console.log "pv_array[0]: "
console.log pv_array[0]
console.log "pv_array[1]: "
console.log pv_array[1]
console.log "pv_array.length: "
console.log pv_array.length
console.log "pv_array.myLength(): "
console.log pv_array.myLength()

在这种情况下，我得到了预期的结果，除了数组中有一个 actual length 属性(虽然我想这将是一些内部细节):

In this case, I get the expected result, except that there is an actual length attribute in the array (while I would imagine that it would be some internal detail):

$ coffee pv.coffee
pv_array: 
[ 1, 2, length: 2 ]
pv_array[0]: 
1
pv_array[1]: 
2
pv_array.length: 
2
pv_array.myLength(): 
2

那么，为什么设置数组中的位置不会改变它的长度呢?

So, why does setting the position in the array does not change its length?

这个问题与我发布的 这个有关stackoverflow.com/questions/7421781/coffeescript-how-to-return-a-array-from-class/7422121#7422121">这个答案.

This question is related to this one for which I posted this answer.

推荐答案

最简单的解释是:length是魔法.

The simplest explanation is: length is magic.

length 显然不像普通属性，因为当您在其对象上插入/删除其他属性时它会更改其值(并且相反，设置 length = 0 将删除其他属性)；但是 identifier 长度"并没有什么特别之处.这意味着您可以轻松编写 foo.length = 'bar'，世界将继续转动.只有在数组上才有它的特殊性.

length obviously doesn't behave like an ordinary property, since it changes its value when you insert/delete other properties on its object (and, conversely, setting length = 0 will delete other properties); but there's nothing special about the identifier "length". That means you can easily write foo.length = 'bar', and the world will keep turning. Only on arrays does it have its special nature.

现在，您可能期望当您 extend Array 构造函数时，您会得到一个数组 - 但您会吗?好吧，从某种意义上说，你这样做了:

Now, you might expect when when you extend the Array constructor, that you get an array—but do you? Well, in one sense you do:

class PushValuesArray extends Array
(new PushValuesArray) instanceof Array  # true

不幸的是，出于 length 的目的，您没有这样做.extends 关键字在这里所做的只是创建一个原型链，而数组 prototype 具有一个明显不神奇的 length 属性:

Unfortunately, for the purpose of length, you don't. All the extends keyword does here is create a prototype chain, and the Array prototype has a distinctly non-magical length property:

Array::length  # 0

这就是您在 PushValuesArray 实例上获得的全部内容.遗憾的是，没有办法在您自己的对象上复制 length 魔法.您唯一的选择是改写一个函数(例如，size())，或者使用您想要的方法修改 Array 原型并改用真正的数组.

That's all you get on your PushValuesArray instance. Sadly, there's no way to duplicate that length magic on your own objects. Your only option is to either write a function instead (say, size()), or modify the Array prototype with the methods you want and use true arrays instead.

总结一下:子类化 Array 不会让你走得太远.这就是为什么，例如，jQuery 在它的对象上有一个非常类似于数组的 API——

To sum up: Subclassing Array won't get you very far. That's why, for instance, jQuery has a very array-like API on its objects—

$('body').length  # 1
$('body')[0]      # [object HTMLBodyElement]

——但实际上并没有使这些对象继承自 Array 原型:

—but doesn't actually make those objects inherit from the Array prototype:

$('body') instanceof Array  # false

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