"delete" - restore native function not working for changed prototype, how then?

When you change window.open to something else, e.g. using window.open = 'something else';, then you're shadowing the open method from the prototype;

// Looking up window.open (in the prototype chain)....
window.open;           // Found, result == 'something else'
window.__proto__.open; // Not accessible any more (shadowed by previous line)

After invoking delete window.open to delete 'something else', the original method becomes visible again, because it had never disappeared from the prototype chain.

But if you've modified the open method on the prototype, e.g. window.__proto__.open = bogus;, then you cannot easily restore the old method. So, to get the "open window" behavior again, you need to either keep a reference to the original method before replacing it,

var original_open = window.open;
window.__proto__.open = 'bogus';
// .... whatever ....
// Now restore it:
window.__proto__.open = original_open;

Or borrow it from another window, e.g. using a temporary new frame:

var frame = document.createElement('iframe');
document.body.appendChild(frame);
window.__proto__.open = frame.contentWindow.open;
frame.parentNode.removeChild(frame);

This whole idea is ridiculous though: You should not break built-in methods.