javascript中类似数组的对象 [英] Array-like object in javascript

问题描述

查看Closure库中的dom.js源代码，我发现了这个（在 goog.dom.getElementsByTagNameAndClass _ 中）：

Looking through the dom.js source from the Closure library I found this (in goog.dom.getElementsByTagNameAndClass_):

if (opt_class) {
var arrayLike = {};
var len = 0;
for (var i = 0, el; el = els[i]; i++) {
  var className = el.className;
  // Check if className has a split function since SVG className does not.
  if (typeof className.split == 'function' &&
      goog.array.contains(className.split(' '), opt_class)) {
    arrayLike[len++] = el;
  }
}
arrayLike.length = len;
return arrayLike;
}

在普通数组上执行此操作有什么好处？

What would be the benefit of doing this over a regular array?

推荐答案

代码的作者使用空的JavaScript对象作为对象数组的基础，即可以通过索引访问的数组有一个长度属性。

The author of the code used empty JavaScript object as a basis of a array like object, i.e. the one that can be accessed by index and has a length property.

我可以想到两个原因：

1. 内存使用 - 如果数组由分配n个元素的实现支持，当它达到它的限制时，它会增加一些因素来增加它的容量，因此浪费 capacity - length of memory


2. cpu time - 实现者选择插入速度超过随机访问速度 - 此方法的返回更有可能比随机访问顺序迭代，并在插入时调整数组大小导致具有cpu惩罚的allocate-copy-deallocation

1. memory use - if array is backed by implementation that allocates n elements, when it reaches it's limits it grows by some factor to increase it's capacity, and thusly wastes capacity - length of memory
2. cpu time - implementor choose insertion speed over random access speed -- a return from this method is more likely to be iterated over sequentially than random accessed, and resizing the array in insertion causes allocation-copy-deallocation which has a cpu penalty

我打赌在其他JavaSc中会找到类似的代码ript库，它是基准测试的结果，并且可以在不同的浏览器中找到最适合的解决方案。

I'm betting that similar code would be found in other JavaScript libraries, and that it's result of benchmarking and finding the best to fit solution across different browsers.

在Justin发表评论之后编辑

进一步谷歌搜索后，似乎类似数组的对象在JavaScript开发人员中很常见：checkout JavaScript：David Flanagan的权威指南，它有一个整体关于类数组对象的子章节。另外这些 < a href =http://shifteleven.com/articles/2007/06/28/array-like-objects-in-javascript =nofollow noreferrer>伙计们提及它们。

Upon further googling it appears that array-like objects are common among JavaScript developers: checkout JavaScript: the definitive guide by David Flanagan, it has a whole sub-chapter on Array-like objects. Also these guys mention them.

没有提到为什么要选择类似数组的数组对象。这可能是一个很好的问题。

No mention of why should one prefer array-like vs array object. This could be a good SO question.

所以第三个选项可能是关键：符合JavaScript API的规范。

So a third option could be the key: compliance with norms of JavaScript API's.

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