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

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