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).

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

Answer 3

Actually, comparing anything with undefined is not good idea at all. Should use typeof operator instead:

function isUndefined ( variant ) { return typeof variant === 'undefined' }

Answer 4

It's should be enough to just declare a variable without assigning it to anything:

var local_undefined;
alert(typeof local_undefined);

But why on Earth is it possible to change undefined value? Does anyone know history behind this?