Question

Why does the DOM have an object called self and another called window when they are the same thing? To add to the confusion window has a property called self so:

window === window.self === self

Why is it like this? Which one should I use?

Answer 1

self is defined by the javascript environment and points to the [global] object (but is not part of the spec, so might not be there), while window is part of the DOM specification.
In most browsers the window is used as the [global] object, but this is not always so.

That self == window.self is not strange as they are the same object – when self is being looked up, it is found as a property of the global object (window). So it is in fact the same as window.self == window.self.

As noted elsewhere, to reliable reference the [global] object, you should define it your sef by running var global = this; in the global execution context.