为什么不写作

Why not writing just

if (!Sys) {
   // you cannot use Sys
}

请记住 Sys 也是一个对象。在这种方法中，你在undefined和null（JavaScript中的不同对象）之间没有区别，但在很多情况下，不必进行这种区别。



剩下的你的帖子对我来说不是很清楚，抱歉。欢迎您提出后续问题。

Remember that Sys is also an object. In this approach, you make no difference between undefined and null (different objects in JavaScript), but in many cases this difference does not have to be made.

The rest of your post is not quite clear to me, sorry. Your follow-up questions will be welcome.