What's the difference between self and window?

self is a read-only property that can be more flexible than, and sometimes used in favor of, the window directly. This is because self's reference changes depending on the operating context (unlike window.self, which only exists if window exists). It's also great for comparisons, as others have mentioned.

For example, if you use self inside a Web Worker (which lives in its own background thread), self will actually reference WorkerGlobalScope.self. However, if you use self in a normal browser context, self will simply return a reference to Window.self (the one that has document, addEventListener(), and all the other stuff you're used to seeing).

TL;DR while the .self in window.self will not exist if window doesn't exist, using self on its own will point to Window.self in a traditional window/browser context, or WorkerGlobalScope.self in a web worker context.

As usual, MDN has a great writeup on this subject in their JavaScript docs. :)

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Side note: The usage of self here should not be confused with the common JS pattern of declaring a local variable: var self = this to maintain a reference to a context after switching.

You can read more about that here: Getting Out of Binding Situations in JavaScript.

From Javascript: The Definitive Guide:

The Window object defines a number of properties and methods that allow you to manipulate the web browser window. It also defines properties that refer to other important objects, such as the document property for the Document object. Finally, the Window object has two self-referential properties, window and self. You can use either global variable to refer directly to the Window object.

In short, both window and self are references to the Window object, which is the global object of client-side javascript.

Here's the explanation and example from the MDN page for window.self:

if (window.parent.frames[0] != window.self) {
   // this window is not the first frame in the list
}

window.self is almost always used in comparisons like in the example above, which finds out if the current window is the first subframe in the parent frameset.

Given that nobody is using framesets these days, I think it's okay to consider that there are no useful cases for self. Also, at least in Firefox, testing against window instead of window.self is equivalent.