问题描述
在下面的CoffeeScript程序，创建阵列这台在其构造两个位置的子类：
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code会改变 sp_array 的长度属性，因为它有效地设置它的立场。但是，输出我得到的是：
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，我创建了另一个类的实例推值而不是将它们设置的：
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，我得到预期的结果，但有一个的实际的长度数组中的属性（虽然我会想象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，为什么设置的位置在数组中不改变其长度是多少？
So, why does setting the position in the array does not change its length?
的这个问题是。的
This question is related to this one for which I posted this answer.
推荐答案
最简单的解释是：长度魔法
长度显然不能像普通的财产，因为当你插入/它的对象上删除其他属性（，相反，设置<$改变其价值C $ C>长度= 0 将删除其他属性）;但没有什么特别之处的标识的长度。这意味着你可以很容易地编写 foo.length ='酒吧'，世界将不停地转动。只有在阵列它有它的特殊性。
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，你可能期望时，当你延长的阵列的构造函数，你得到一个数组，但这样做您？那么，在某种意义上，你做的：
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，对于长度，你不这样做的目的。所有扩展关键词在这里所做的就是创建一个原型链，而阵列的原型的有一个明显的非魔法长度属性：
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
这就是你得到你的 PushValuesArray 实例。可悲的是，有没有办法复制在自己的对象长度魔法。你唯一的选择是要么写一个函数来代替（比如尺寸（）），或修改阵列原型你想和使用真正的数组来代替的方法。
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.
要总结：子类化阵列不会让你很远这就是为什么，例如，jQuery有一个非常类似数组。其对象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从数组原型继承：
—but doesn't actually make those objects inherit from the Array prototype:
$('body') instanceof Array # false
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