IntersectionObserver.IntersectionObserver()

The IntersectionObserver() constructor creates and returns a new IntersectionObserver object. The rootMargin, if specified, is checked to ensure it's syntactically correct, the thresholds are checked to ensure that they're all in the range 0.0 and 1.0 inclusive, and the threshold list is sorted in ascending numeric order. if the threshold list is empty, it's set to the array [0.0].

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.

observer

The IntersectionObserver for which the callback is being invoked.

options Optional

An optional object which customizes the observer. If options isn't 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). You can provide any combination of the following options:

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.

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 root element and root margin in Intersection Observer API for more information on how the margin works and the syntax. 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 in Intersection Observer API for a more in-depth description of how thresholds are used. The default is a threshold of 0.0.

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

The specified rootMargin 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});