The Window.self read-only property returns the window itself, as a WindowProxy. It can be used with dot notation on a window object (that is, window.self) or standalone (self). The advantage of the standalone notation is that a similar notation exists for non-window contexts, such as in Web Workers. By using self, you can refer to the global scope in a way that will work not only in a window context (self will resolve to window.self) but also in a worker context (self will then resolve to WorkerGlobalScope.self).

A WindowProxy object.

Uses of window.self like the following could just as well be replaced by window.

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

Furthermore, when executing in the active document of a browsing context, window is a reference to the current global object and thus all of the following are equivalent:

const w1 = window;
const w2 = self;
const w3 = window.window;
const w4 = window.self;
// w1, w2, w3, w4 all strictly equal, but only w2 will function in workers

• Its Worker equivalent, WorkerGlobalScope.self.