window.alert = null;

alert('test'); // fail

delete window.alert; // true

alert('test'); // win

window is an instance of DOMWindow, and by setting something to window.alert, the correct implementation is being shadowed, i.e. when accessing alert it is first looking for it on the window object. Usually this is not found, and it then goes up the prototype chain to find the native implementation. However, when manually adding the alert property to window it finds it straight away and does not need to go up the prototype chain. Using delete window.alert you can remove the window own property and again expose the prototype implementation of alert. This may help explain:

window.hasOwnProperty('alert'); // false

window.alert = null;

window.hasOwnProperty('alert'); // true

delete window.alert;

window.hasOwnProperty('alert'); // false