WindowscrollY

Window.scrollY is a read-only property of the window interface that returns the vertical scroll offset of the document in CSS pixels. It indicates how far the top edge of the viewport is from the top of the document.

In modern browsers, window.scrollY equals window.pageYOffset. It is supported by Firefox, Chrome, Edge, Safari, and most mobile browsers. Internet Explorer 11 does not support window.scrollY, but provides pageYOffset as an alternative. For cross-browser code, developers often reference window.scrollY with a fallback to window.pageYOffset.

Usage is straightforward: read the value from window.scrollY to determine the current vertical scroll position. Because it is read-only, you cannot assign to it. To change scroll position, use window.scrollTo or window.scrollBy, for example window.scrollTo({ top: 200, behavior: 'smooth' }) to animate to 200 pixels from the top.

The value updates in response to user scrolling and can also change when scripts scroll the page programmatically. It is commonly used to trigger actions such as lazy loading, animations, or UI changes tied to scroll position, and to implement features like sticky headers. The measurement is in CSS pixels and reflects the current layout, with page zoom or device pixel ratio affecting how those pixels map to the screen. See also: pageYOffset, scrollTop, scrollTo, scrollBy.