javascript - 返回值 `delete` 的真正含义是什么？

Question

根据 this MDN page , delete 关键字

Returns false only if the property exists and cannot be deleted. It returns true in all other cases.

但是，我看到 delete 返回 true 的情况，尽管属性没有被删除:

delete Window
delete alert
delete dir
delete console
delete 2
delete null
delete {}.x
...

事实上，window 的几乎所有属性都在 delete 时返回 true，正如在 about:blank 中运行以下脚本所见>:

for(a in window) { if(delete window[a]) { console.log(a); } }

但是，window 的大多数属性实际上并没有被删除。 delete返回值的真正含义是什么？为什么它为未删除的属性返回 true？

(注意:我会对解释 delete 行为的 Chromium 代码的引用感兴趣。)

Answer 1

窗口是一个host object ，其语义由宿主环境定义的，例如浏览器。 delete 应用于宿主对象的属性时比应用于 native 对象时更复杂。

Host objects may support these internal properties with any implementation-dependent behaviour as long as it is consistent with the specific host object restrictions stated in this document.

Section 11.4.1 - The delete operator说

If IsUnresolvableReference(ref) then,
  If IsStrictReference(ref) is true, throw a SyntaxError exception.
  Else, return true.

因此当宿主对象不支持删除或修改属性时，它会返回一个无法解析的引用或一个假装被删除的引用。这两种方法都会导致 true 在非严格模式下返回。