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

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?