如何在JavaScript中定义未定义？

Question

There's this IIFE trick to get around people that have defined undefined to something. It goes something like this:

(function (undefined) {
    // ...
})();

How do I become this person? Is it even possible to define undefined anymore?

If I open my console (FF or Chrome), it will look something like this:

undefined = 5  // => 5
undefined      // => undefined

Answer 1

在现代浏览器中，您不必为未定义的值赋值。

Answer 2

Well I agree - there is no way to define it. And I will try to explain why:

console.log(Object.getOwnPropertyDescriptor(window,"undefined"))

which, of course, prints the following:

Object {value: undefined, writable: false, enumerable: false, configurable: false}

According to MDN if the property writable is false then you cannot change it. Quite obvious, isn't it?

If you try to change your scope (do it inside of the function), you will be the owner of the scope and perhaps you can define the name as you like. But as far as you are on window scope - no way.