 Thomas Steiner [GitHub](https://github.com/tomayac) [LinkedIn](https://www.linkedin.com/in/thomassteinerlinkedin) [Mastodon](https://toot.cafe/@tomayac) [Bluesky](https://bsky.app/profile/tomayac.com) [Homepage](https://blog.tomayac.com/)
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/API/IntersectionObserver)
[Intersection Observer](https://developer.mozilla.org/docs/Web/API/Intersection_Observer_API)
is one of those APIs that's probably universally loved, and it's usable in all
major browsers. Developers have used this API for a wide range of use cases,
including
[lazy loading images and videos](https://web.dev/explore/fast#lazy-load-images-and-video),
[notifications when elements reach `position: sticky`](https://developer.chrome.com/blog/sticky-headers),
[firing analytics events](https://github.com/ampproject/amphtml/blob/master/extensions/amp-analytics/0.1/visibility-manager.js),
and many more.
[Video](https://www.youtube.com/watch?v=kW_atFXMG98)
In its most basic form, this is what the Intersection
Observer v1 API looks like:
const onIntersection = (entries) => {
for (const entry of entries) {
if (entry.isIntersecting) {
console.log(entry);
}
}
};
const observer = new IntersectionObserver(onIntersection);
observer.observe(document.querySelector('#some-target'));
## Visibility challenges in Intersection Observer v1
With the Intersection Observer v1 API, you can learn when an element is
scrolled into the window's viewport. However, you can't determine if that
element is covered by other page content, known as *occlusion* , or if the
element appear modified by CSS, such as `transform`, `opacity`, or `filter`,
which may make the element it invisible.
For an element in the top-level document, this information can be determined by
analyzing the DOM with JavaScript, for example with
[`DocumentOrShadowRoot.elementFromPoint()`](https://developer.mozilla.org/docs/Web/API/DocumentOrShadowRoot/elementFromPoint).
In contrast, the same information cannot be obtained if the element in question is
located in a third-party iframe.
### Why is visibility important?
Unfortunately, the internet has bad actors. For example, a dishonest publisher
might use pay-per-click ads on a website. They might trick users into clicking
these ads to earn more money, at least until the ad network discovers their
scheme. Typically, such ads are served in iframes.
To trick users, the publisher could make the ad iframes completely transparent
with CSS: `iframe { opacity: 0; }`. Then, they could place these transparent
iframes over appealing content, like a cute cat video, that users want to click.
This is called *clickjacking*.
You can see such a clickjacking attack in action in the [upper
section of our demo](https://googlechrome.github.io/samples/trick-ad-click/).
Try "watching" the cat video and activate **trick mode**. The ad in the iframe
registers clicks as legitimate, even if you (unintentionally) clicked it while
the iframe was transparent.
## Improvements in Intersection Observer v2
Intersection Observer v2 can track an element's "visibility" as a human
would define it. If you set an option in the [`IntersectionObserver`
constructor](https://developer.mozilla.org/docs/Web/API/IntersectionObserver/IntersectionObserver),
the resulting
[`IntersectionObserverEntry`](https://developer.mozilla.org/docs/Web/API/IntersectionObserverEntry)
instances include a new boolean field called `isVisible`. When `isVisible`
is `true`, the browser makes sure the element is completely uncovered by other
content and has no visual effects that hide or change its display. If
`isVisible` is `false`, the browser cannot make that guarantee.
The
[spec](https://w3c.github.io/IntersectionObserver/v2/#calculate-visibility-algo)
allows *false negatives* : `isVisible` can be `false` even when the element is
truly visible and unchanged. For performance, browsers use simpler calculations,
such as bounding boxes and rectangular shapes, and don't check every pixel for
complex details like `border-radius`.
However, false positives are not permitted under any circumstances. This
means `isVisible` won't be `true` if the element is not completely visible
and unmodified.
> [!WARNING]
> **Warning:** Visibility is much harder and slower to calculate than intersection. Because of this, Intersection Observer v2 is *not for widespread use*, like Intersection Observer v1. Use it to prevent fraud, and only when you need visibility details that Intersection Observer v1 cannot provide.
### Apply these changes
The `IntersectionObserver` constructor now takes two additional configuration
properties:
- `delay` is a number that indicates the minimum delay in milliseconds between notifications from the observer, for a given target.
- `trackVisibility` is a boolean to indicate if the observer will track changes in a target's visibility.
When `trackVisibility` is `true`, `delay` must be set to `100` or a higher value
(that is, no more than one notification every 100ms).
As visibility is expensive to calculate, this is a precaution against
performance degradation and battery consumption. Responsible developers should
use the largest tolerable value for delay.
The [spec](https://w3c.github.io/IntersectionObserver/v2/#calculate-visibility-algo),
calculates visibility. Like with version 1, when observer's `trackVisibility`
attribute is `false`, the target is considered visible
In version 2, the target is considered invisible if:
- It has an effective transformation matrix, other than a
2D translation or proportional 2D upscaling.
- The target, or any element in its containing block chain, has an effective
opacity smaller than 1.0.
- The target, or any element in its containing block chain, has any filters
applied.
- If the implementation cannot guarantee that the target is completely
unoccluded by other page content.
This means current implementations are pretty conservative in guaranteeing
visibility. For example, applying a nearly unnoticeable grayscale filter
(`filter: grayscale(0.01%)`) or setting the smallest transparency
(`opacity: 0.99`) would render the element invisible.
Here is a code sample that illustrates the new API features. You can see its
click tracking logic in action in the
[second section of the demo](https://googlechrome.github.io/samples/trick-ad-click/)
Try "watching" the puppy video. Activate **trick mode** to convert yourself into
a bad actor and see how Intersection Observer v2 prevents
non-legitimate ad clicks from being tracked.
Intersection Observer v2 protects us.
> [!CAUTION]
> **Caution:** Unlike typical lazy-loading, if you use Intersection Observer to prevent clickjacking, don't set [`unobserve`](https://developer.mozilla.org/docs/Web/API/IntersectionObserver/unobserve) on the element after the first intersection.
// This is code running in the iframe.
// The iframe must be visible for at least 800ms prior to an input event
// for the input event to be considered valid.
const minimumVisibleDuration = 800;
// Keep track of when the button transitioned to a visible state.
let visibleSince = 0;
const button = document.querySelector('#callToActionButton');
button.addEventListener('click', (event) => {
if ((visibleSince > 0) &&
(performance.now() - visibleSince >= minimumVisibleDuration)) {
trackAdClick();
} else {
rejectAdClick();
}
});
const observer = new IntersectionObserver((changes) => {
for (const change of changes) {
// ⚠️ Feature detection
if (typeof change.isVisible === 'undefined') {
// The browser doesn't support v2, fallback to v1 behavior.
change.isVisible = true;
}
if (change.isIntersecting && change.isVisible) {
visibleSince = change.time;
} else {
visibleSince = 0;
}
}
}, {
threshold: [1.0],
// 🆕 Track the actual visibility of the element
trackVisibility: true,
// 🆕 Set a minimum delay between notifications
delay: 100
}));
// Require that the entire iframe be visible.
observer.observe(document.querySelector('#ad'));
## Additional resources
- Working Draft of the [Intersection Observer](https://www.w3.org/TR/intersection-observer/).
- Intersection Observer v2 on [Chrome Platform Status](https://www.chromestatus.com/feature/5878481493688320).
## Acknowledgements
Thanks to [Simeon Vincent](https://twitter.com/dotproto),
[Yoav Weiss](https://twitter.com/yoavweiss), and [Mathias Bynens](https://twitter.com/mathias)
for reviewing, as well as [Stefan Zager](https://twitter.com/stefanzager)
for reviewing and implementing the feature in Chrome.