Why can you delete an overridden property of window?
javascript
Solution
In Chrome, the `window.alert` function is part of the prototype of the `DOMWindow` class, it's not a property of `window` itself.
Hence when you overwrite `window.alert` you're adding a new property to `window`, but the version in the prototype continues to exist, but is hidden.
When you `delete window.alert` the function in the prototype is re-exposed.
Here's some console output showing that the function is in the prototype:
> window.constructor.prototype
DOMWindow
> window.constructor.prototype.alert
function alert() { [native code] }
Firefox behaves similarly, albeit with different class names.
Problem
The `delete` operator removes a property from an object. If I set a property on `window`, I can delete it: ``` window.myProp = 10; delete window.myProp; ``` As the article I so often refer others to when it comes to the behaviour of the `delete` operator states, this is because property assignment does not set the `DontDelete` attribute (as opposed to variable declaration, which does). That article also states the following (emphasis added): Note that it is during property creation that attributes are determined (i.e. none are set). Later assignments don’t modify attributes of existing property. It’s important to understand this distinction. Bearing that in mind, why can I override an existing property of window, `alert`, and then delete it to return to the original value? Am I missing something obvious? I rarely use the `delete` operator so that may well be the case. For example: ``` window.alert = function() {}; alert("Hi!"); //Nothing happens delete window.alert; alert("Hello?"); //Alerts 'Hello?' ``` Here's a fiddle to demonstrate that (only tested in Chome, pretty sure IE will not behave this way but don't have access to anything but Chrome right now).