scrollY

scrollY is a property of the Window interface in web browsers that indicates the vertical scroll offset of the document in CSS pixels. It represents how far the document's content has been scrolled from the top of the initial viewport.

The value is read-only and updates as the user scrolls or as scripts programmatically adjust the scroll position. In modern browsers, window.scrollY is equivalent to window.pageYOffset, and both report the current vertical offset. The property is 0 when the document is at the top.

It is expressed in CSS pixels and is affected by the page's zoom level. DevicePixelRatio does not change the value itself, but the CSS pixel unit scales with zoom, so the numeric value reflects the zoomed scroll distance.

Usage: Read window.scrollY to determine how far the user has scrolled, which can drive UI behavior such as revealing a sticky header or lazy-loading content. You can also move the document by calling window.scrollTo({ top: value, behavior: 'smooth' }) or window.scrollBy({ top: delta, behavior: 'smooth' }).

Compatibility: scrollY is widely supported in modern browsers. It is a standard part of the CSSOM View Module, with older browsers providing an equivalent window.pageYOffset property.

See also window.scrollTo, window.scrollBy, pageYOffset, and the CSSOM View Module.