为什么阵列仍具有删除之后非零长度？ [英] Why does the array still have a non-zero length after deletion?

问题描述

我有以下code，输出阵列的长度，将其删除，然后输出的新长度：

 的console.log（$ scope.adviceList.activeAdvices.length）; // * 1 *
$ scope.adviceList.activeAdvices.splice（ID，1）; // * id是一个参数*
的console.log（$ scope.adviceList.activeAdvices.length）; // * 0 *的console.log（$ scope.adviceList.activeAdvices.length）; // * 1 *
删除$ scope.adviceList.activeAdvices [ID]
的console.log（$ scope.adviceList.activeAdvices.length）; // * 0 *
的console.log（$ scope.adviceList.activeAdvices）; // * [] *
 

删除后，该数组正确显示为空。它的长度，但是，仍然是1。

解决方案

删除是一个低级别操作符。它直接作用在物体上，它并不知道的阵列。使用删除根本不触发程序重新计算数组长度。

---

其他答案在这里宣称该属性的值设置为 undfined ，这是不正确。一个简单的测试表明：

  VAR A = [1];
删除一个[0];
console.dir（一）;
 

和比较，如果你真的值设置为会发生什么未定义：

  VAR A = [1];
一个[0] =未定义;
console.dir（一）;
 

有关更确凿的证据，让我们来看看在规范：

  

当[删除]中的 0 的调用与属性名的 P 的和布尔标志的扔的，下面的步骤内部方法采取：

  
  

  
1. 我们的说明的是调用的 0 的的[[GetOwnProperty]]内部方法，属性名的 P 的结果。

  
2. 如果说明的是不确定的，则返回真正。

  
3. 如果说明的[[配置]是的真正，然后结果
     ＆NBSP;＆NBSP;一个。与名称中删除自己的财产的 P 的距离的 0 的。结果
     ＆NBSP;＆NBSP;湾返回的真正

  
4. 否则，如果扔，则抛出一个类型错误例外。

  
5. 返回假。

  

无处据说属性的值设置为未定义。

---

不同浏览器的控制台可能显示阵列的不同重新presentations。在这个特殊的例子，你可以争论是否应该 [] 或 [未定义] 。

• [] （铬）似乎是有道理的，因为数组真的没有任何元素，有与数字域名没有属性。但是，当您访问。长度属性，你会得到一个 1 ，这可能会造成混淆。结果
  不久以前，Chrome的使用重新presentation像 [未定义×5] 指示长度为5，但没有元素的数组。我认为这实际上是一个很好的解决方案。




• [未定义] （火狐）是有道理的，因为数组的长度是1和访问的改编[0] 实际上返回未定义（但这样做改编[10] ），但 arr.hasOwnProperty（0）是假如果数组包含真值未定义，怎么能只由这再presentation从一个长度的空数组区分（答案：你不能）。

的底线是：不要相信的console.log 太多。如果你想要一个确切的再presentation而使用 console.dir 。

I have the following code that outputs the length of an array, deletes it, and then outputs the new length:

console.log($scope.adviceList.activeAdvices.length); // *1*
$scope.adviceList.activeAdvices.splice(id,1); // *id is a parameter*
console.log($scope.adviceList.activeAdvices.length); // *0*

console.log($scope.adviceList.activeAdvices.length); // *1*
delete $scope.adviceList.activeAdvices[id];
console.log($scope.adviceList.activeAdvices.length); // *0*
console.log($scope.adviceList.activeAdvices); // *[]*

After the deletion, the array is correctly displayed as empty. Its length, however, is still 1.

解决方案

delete is a "lower level" operator. It directly works on the object, it doesn't "know" about the array. Using delete simply doesn't trigger the routine to recompute the array length.

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Other answers here claim that the value of the property is set to undfined, which is incorrect. A simple test shows that:

var a = [1]; 
delete a[0]; 
console.dir(a);

and compare to what happens if you really set the value to undefined:

var a = [1]; 
a[0] = undefined; 
console.dir(a);

For a more solid proof, lets have a look at the specification:

When the [[Delete]] internal method of O is called with property name P and the Boolean flag Throw, the following steps are taken:

1. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with property name P.
2. If desc is undefined, then return true.
3. If desc.[[Configurable]] is true, then
      a. Remove the own property with name P from O.
      b. Return true.
4. Else if Throw, then throw a TypeError exception.
5. Return false.

Nowhere it is said that the value of the property is set to undefined.

---

The consoles of different browsers might show different representations of the array. In this particular example, you can argue whether it should be [] or [undefined].

• [] (Chrome) seems to make sense because the array really doesn't have any elements, there are no properties with numeric names. However, when you access the .length property, you would get a 1, which can be confusing.
  Not too long ago, Chrome used a representation like [undefined x 5] to indicate an array of length 5 but without elements. I think this was actually a good solution.

• [undefined] (Firefox) makes sense because the array is of length 1 and accessing arr[0] actually returns undefined (but so does arr[10]).However, arr.hasOwnProperty(0) would be false and if an array really contains the value undefined, how can it be distinguished from an empty array of length one solely by this representation (answer: you can't).

The bottom line is: Don't trust console.log too much. Rather use console.dir if you want an exact representation.

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