Why does the DOM have both window and self?

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 开发者_如何学编程self so:

window === window.self === self

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

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.

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");
}

The global self is a shortcut to window.self. They are both the very same object.

As to why there are both window and self, in a web page context, self is always equal to window (as far as I know). But I guess this doesn't have to always be that way - Javascript/ECMAScript is not limited to use in a web page where there is a DOM.

As to what to use, both should be safe to use in a normal web page context AFAIK. See @Nick's comment for good arguments for using window, though.