Get 'undefined' if 'window.undefined' is overwritten

Question

It appears that window.undefined is writable, i.e. it can be set to something else than its default value (which is, unsurprisingly, undefined).

The point is however that whenever I refer to undefined, it refers to window.undefined (as window can be removed in cases like this).

So how do I actually get access to an undefined 'instance', so to say? How would I be able to set another variable to undefined, if window.undefined has been changed?

If I code:

window.undefined = 'foo'; // This code might have been executed by someone/something
var blah = undefined; // blah is not undefined, but equals to 'foo' instead...

How could I possibly solve this?

Answer 1

The "standard" solution to this problem is to use the built in void operator. Its only purpose is to return undefined:

var my_undefined = void 0;

In addition to this, yhere are other ways to get undefined:

Functions return undefined if you don't return anything so you could do something like

this_is_undefined = (function(){}());

You also get undefined if you don't pass enough arguments to a function. So a common idiom is

function foo(arg1, arg2, undefined){ //undefined is the last argument
    //Use `undefined` here without worrying.
    //It is a local variable so no one else can overwrite it
}
foo(arg1, arg2);
//since you didn't pass the 3rd argument,
//the local variable `undefined` in foo is set to the real `undefined`.

This kind is particularly good for cases when you define and call the function at the same time so you don't have any risk of forgetting and passing the wrong number of arguments latter.

Answer 2

In addition to the other solutions, you can do the void 0 trick, which always returns undefined irrespective of the window property.

window.undefined = 'foo';
var blah = void 0;

alert( blah );  // undefined