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.

The IntersectionObserver interface's read-only rootMargin property is a string with syntax similar to that of the CSS margin property. Each side of the rectangle represented by rootMargin is added to the corresponding  side in the root element's bounding box before the intersection test is performed. This lets you, for example, adjust the bounds outward so that the target element is considered 100% visible even if a certain number of pixels worth of width or height is clipped away, or treat the target as partially hidden if an edge is too close to the edge of the root's bounding box.

See "The root element and root margin" in Intersection Observer API for a more in-depth look at the root margin and how it works with the root's bounding box.


var marginString = IntersectionObserver.rootMargin;


A string, formatted similarly to the CSS margin property's value, which contains offsets for one or more sides of the root's bounding box. These offsets are added to the corresponding values in the root's bounding box before the intersection between the resulting rectangle and the target element's bounds.

The string returned by this property may not match the one specified when the IntersectionObserver was instantiated. The browser is permitted to alter the values

If rootMargin isn't specified when the object was instantiated, it defaults to the string "0px 0px 0px 0px", meaning that the intersection will be computed between the root element's unmodified bounds rectangle and the target's bounds.  "The root element and root margin" in Intersection Observer API describes how the rootMargin is used in more detail.


Specification Status Comment
Intersection Observer
The definition of 'IntersectionObserver.rootMargin' in that specification.
Editor's Draft Initial definition

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 51.0 (Yes) 55 (55)[1] 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)[1] No support No support ? ? 51.0

[1] 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.

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