Why does the DOM have both window and self?

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.

Answer 2

When you're calling self it is window.self, so same thing there like any other global property (e.g. location is really window.location).

The reason it's there? Usually for checks like this:

if(window.top != window.self) {
  alert("We're in a frame");
}