javascript - 无法理解在 JavaScript 中删除 vars 的行为

Question

问题是:

var x = 5;
window.x === x // true. x, as it seems, is a property of window
delete x; // false
delete window.x; // false;

但是

window.x = 5;
delete window.x; // true

和

window.x = 5;
delete x; // true

如何解释这种行为？

Answer 1

本质上，原因是声明的变量是使用内部 DontDelete 属性创建的，而通过赋值创建的属性则不是。

这是一篇很好的文章，解释了delete 的内部细节:Understanding delete

When declared variables and functions become properties of a Variable object — either Activation object (for Function code), or Global object (for Global code), these properties are created with DontDelete attribute. However, any explicit (or implicit) property assignment creates property without DontDelete attribute. And this is essentialy why we can delete some properties, but not others: