This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the specification changes.
IntersectionObserver interface's read-only
root property identifies the
Element whose bounds are treated as the bounding box of the viewport for the element which is the observer's target. If the
null, then the bounds of the actual document viewport are used.
var root =IntersectionObserver.root;
Element object whose bounding box is used as the bounds of the viewport for the purposes of determining how much of the target element is visible. The intersection of this bounding rectangle, offset by any margins specified in the options passed to the
IntersectionObserver() constructor, the target element's bounds, minus the bounds of every element or other object which overlaps the target element, is considered to be the visible area of the target element.
null, then the bounds of the document's viewport (that is, the visible area of the document) are used as the root.
The definition of 'IntersectionObserver' in that specification.
|Editor's Draft||Initial definition|
|Feature||Chrome||Edge||Firefox (Gecko)||Internet Explorer||Opera||Safari (WebKit)|
|Basic support||51.0||(Yes)||55 (55)||No support||?||?|
|Feature||Android||Android Webview||Edge||Firefox Mobile (Gecko)||Firefox OS||IE Mobile||Opera Mobile||Safari Mobile||Chrome for Android|
|Basic support||No support||51.0||(Yes)||55.0 (55)||No support||No support||?||?||51.0|
 This feature has been implemented since Gecko 53.0 (Firefox 53.0 / Thunderbird 53.0 / SeaMonkey 2.50) behind the preference
dom.IntersectionObserver.enabled, which was
false by default. Enabled by default beginning in Firefox 55. See bug 1243846.