IntersectionObserver: IntersectionObserver() constructor

callback

A function which is called when the percentage of the target element is visible crosses a threshold. The callback receives as input two parameters:

entries

An array of IntersectionObserverEntry objects, each representing one threshold which was crossed, either becoming more or less visible than the percentage specified by that threshold. You should not assume the number of entries, because multiple threshold-crossing events may be reported in a single callback invocation. The entries are dispatched using a queue, so they should be ordered by the time they were generated, but you should preferably use IntersectionObserverEntry.time to correctly order them.

observer

The IntersectionObserver for which the callback is being invoked.

options Optional

An optional object which customizes the observer.

You can provide any combination (or none) of the following options:

delay

A number specifying the minimum permitted delay between notifications from the observer, in milliseconds.

The delay is used to limit the rate at which notifications will be provided when tracking visibility, as this is a computationally intensive operation. The recommendation when tracking visibility is that you set the delay to the largest tolerable value.

When trackVisibility is true the minimum value is 100. The browser will set the value to 100 if any smaller value is used, or if the value is not specified. The default value is 0.

root

An Element or Document object which is an ancestor of the intended target, whose bounding rectangle will be considered the viewport. Any part of the target not visible in the visible area of the root is not considered visible. If not specified, the observer uses the document's viewport as the root, with no margin, and a 0% threshold (meaning that even a one-pixel change is enough to trigger a callback).

rootMargin

A string which specifies a set of offsets to add to the root's bounding_box when calculating intersections, effectively shrinking or growing the root for calculation purposes. The syntax is approximately the same as that for the CSS margin property; see The intersection root and root margin for more information on how the margin works and the syntax. The default is "0px 0px 0px 0px".

scrollMargin

A string that specifies the offsets to add to every scroll container on path to the target when calculating intersections, effectively shrinking or growing the clip rectangles used to calculate intersections. This allows, for example, better observation of targets inside nested scroll containers that are currently clipped away by the scroll containers. The syntax is approximately the same as that for the CSS margin property. The default is "0px 0px 0px 0px".

threshold

Either a single number or an array of numbers between 0.0 and 1.0, specifying a ratio of intersection area to total bounding box area for the observed target. A value of 0.0 means that even a single visible pixel counts as the target being visible. 1.0 means that the entire target element is visible. See Thresholds for a more in-depth description of how thresholds are used. The default is a threshold of "0".

trackVisibility

A boolean indicating whether the observer should track visibility.

When true, the browser will check that the target does not have compromised visibility when calculating intersections; for example, that it hasn't been covered by other elements or potentially been distorted or hidden by a filter, reduced opacity, or some transform.

Tracking visibility is an expensive operation, and should only be done when necessary. A delay should also be set when this value is true. The default is false.

A new IntersectionObserver which can be used to watch for the visibility of a target element within the specified root crossing through any of the specified visibility thresholds.

Call its observe() method to begin watching for the visibility changes on a given target.

SyntaxError DOMException

The specified rootMargin or scrollMargin is invalid.

RangeError

One or more of the values in threshold is outside the range 0.0 to 1.0.

This example creates a new intersection observer which calls the function myObserverCallback every time the visible area of the element being observed changes by at least 10%.

let observer = new IntersectionObserver(myObserverCallback, { threshold: 0.1 });