为什么FileList对象不是数组？

嗯，可能有几个原因。首先，如果它是一个数组，你可以修改它。您无法修改 FileList 实例。其次但是相关的，它可能（可能是）浏览器数据结构的视图，因此最小的功能集使实现更容易提供它。

Well, there could be several reasons. For one, if it were an array, you could modify it. You can't modify a FileList instance. Secondly but related, it could be (probably is) a view onto a browser data structure, so a minimal set of capabilities makes it easier for implementations to provide it.

2018年更新：有趣的是，规范有一个关于 FileList 的注释：

Update in 2018: Interestingly, though, the spec has a note on FileList:

FileList 接口应被视为有风险，因为Web平台的一般趋势是用ECMAScript中的 Array 平台对象替换此类接口[ECMA- 262]。特别是，这意味着排序 filelist.item（0）的语法存在风险;大多数其他程序使用 FileList 不太可能受到最终迁移到 Array 类型的影响。

The FileList interface should be considered "at risk" since the general trend on the Web Platform is to replace such interfaces with the Array platform object in ECMAScript [ECMA-262]. In particular, this means syntax of the sort filelist.item(0) is at risk; most other programmatic use of FileList is unlikely to be affected by the eventual migration to an Array type.

（我觉得很奇怪，我认为趋势是朝向可迭代，而不是数组 —例如更新 NodeList 将其标记为 iterable ，以便与扩展语法兼容， for-of 和 forEach 。）

(Which I find odd, I thought the trend was toward iterable, not Array — such as the update to NodeList marking it iterable for compatibility with spread syntax, for-of, and forEach.)

您还可以将其转换为一个数组通过 Array.from（theFileList）。

You can also convert it to an array via Array.from(theFileList).