window, document, screen

window is the main JavaScript object root, aka the global object in a browser, and it can also be treated as the root of the document object model. You can access it as window.

window.screen or just screen is a small information object about physical screen dimensions.

window.document or just document is the main object of the potentially visible (or better yet: rendered) document object model/DOM.

Since window is the global object, you can reference any properties of it with just the property name - so you do not have to write down window. - it will be figured out by the runtime.

(source: stackoverflow)