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?

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?

• javascript
• undefined

• 1 Fix the code that's overwriting undefined? – John Kugelman Commented Feb 16, 2011 at 19:29
• My guess would be store it before you change it. var undef = window.undefined; window.undefined='foo'; car myvar = undef; – Brad Christie Commented Feb 16, 2011 at 19:29
• His point is This code might have been executed by someone/something. Where to take the value undefined from then? – Martin Hennings Commented Feb 16, 2011 at 19:31
• @John: No I don't really mean that - I'd like to make my code safe and stable and not reliable on window.undefined. E.g., someone else could set window.undefined and then my code gets executed. Then is there any way to still get undefined? – pimvdb Commented Feb 16, 2011 at 19:31
• 1 Maybe it's worth pointing out that modern browsers with full support of ECMA 5 do no longer allow you to override reserved keywords like e.g. undefined so this could not be a case then. – Pawel Sledzikowski Commented May 14, 2013 at 10:35

4 Answers 4

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.

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

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

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

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?