⚠️ This polyfill was originally hosted in the IntersectionObserver spec repo. Refer to that repo for commit history.
IntersectionObserver
polyfill
This library polyfills the native IntersectionObserver
API in unsupporting browsers. See the API documentation for usage information.
Installation
You can install the polyfill via npm or by downloading a zip of this repository:
npm install intersection-observer
Adding the polyfill to your site
The examples below show various ways to add the IntersectionObserver
polyfill to your site. Be sure to include the polyfill prior to referencing it anywhere in your JavaScript code.
Using <script>
tags in the HTML:
<script src="path/to/intersection-observer.js"></script>
<script src="app.js"></script>
Using a module loader (e.g. Browserify or Webpack):
require('intersection-observer');
require('./foo.js');
require('./bar.js');
Configuring the polyfill
It's impossible to handle all possible ways a target element could intersect with a root element without resorting to constantly polling the document for intersection changes.
To avoid this extra work and performance penalty, the default configuration of the polyfill optimizes for the most common IntersectionObserver
use cases, which primarily include target elements intersecting with a root element due to:
- User scrolling.
- Resizing the window.
- Changes to the DOM.
All of the above can be handled without polling the DOM.
There are, however, additional use cases that the default configuration will not detect. These include target elements intersecting with a root element due to:
- CSS changes on
:hover
, :active
, or :focus
states. - CSS changes due to transitions or animations with a long initial delay.
- Resizable
<textarea>
elements that cause other elements to move around. - Scrolling of non-document elements in browsers that don't support the event capture phase.
If you need to handle any of these use-cases, you can configure the polyfill to poll the document by setting the POLL_INTERVAL
property. This can be set either globally or on a per-instance basis.
Enabling polling for all instances:
To enable polling for all instances, set a value for POLL_INTERVAL
on the IntersectionObserver
prototype:
IntersectionObserver.prototype.POLL_INTERVAL = 100;
Enabling polling for individual instances:
To enable polling on only specific instances, set a POLL_INTERVAL
value on the instance itself:
var io = new IntersectionObserver(callback);
io.POLL_INTERVAL = 100;
io.observe(someTargetElement);
Note: the POLL_INTERVAL
property must be set prior to calling the .observe
method, or the default configuration will be used.
Ignoring DOM changes
You can also choose to not check for intersections when the DOM changes by setting an observer's USE_MUTATION_OBSERVER
property to false
(either globally on the prototype or per-instance)
IntersectionObserver.prototype.USE_MUTATION_OBSERVER = false;
var io = new IntersectionObserver(callback);
io.USE_MUTATION_OBSERVER = false;
This is recommended in cases where the DOM will update frequently but you know those updates will have no effect on the position or your target elements.
iframe support
Same-origin iframes
Same-origin iframes are supported by the polyfill out of the box.
Cross-origin iframes
Additional code and configuration are required to support cross-origin iframes,
both on the iframe and host sides.
The setup is as following:
- The host and iframe will establish a messaging channel.
- The host will setup its own IntersectionObserver instance for the
cross-origin iframe element. It can either use the this polyfill or any other
approach. For each IntersectionObserverEntry for the iframe it will forward
intersection data to the iframe via messaging.
- The iframe will load the polyfill and configure it by calling the
_setupCrossOriginUpdater()
method. It will call the provided callback
whenever it receives the intersection data from the the parent via messaging.
A hypothetical host code:
function forwardIntersectionToIframe(iframe) {
createMessagingChannel(iframe, function(port) {
var io = new IntersectionObserver(function() {
port.postMessage({
boundingClientRect: serialize(boundingClientRect),
intersectionRect: serialize(intersectionRect)
});
}, {threshold: [0, 0.1, ..., 1]});
io.observe(iframe);
});
}
Notice that the host should provide a threshold
argument for the desired
level of precision. Otherwise, the iframe side may not update as frequently as
desired.
A hypothetical iframe code:
createMessagingChannel(parent, function(port) {
if (IntersectionObserver._setupCrossOriginUpdater) {
var crossOriginUpdater = IntersectionObserver._setupCrossOriginUpdater();
port.onmessage = function(event) {
crossOriginUpdater(
deserialize(event.data.boundingClientRect),
deserialize(event.data.intersectionRect)
);
};
}
});
Limitations
This polyfill does not support the proposed v2 additions, as these features are not currently possible to do with JavaScript and existing web APIs.
Browser support
The polyfill has been tested and known to work in the latest version of all browsers.
Legacy support is also possible in very old browsers by including a shim for ES5 as well as the window.getComputedStyle
method. The easiest way to load the IntersectionObserver polyfill and have it work in the widest range of browsers is via polyfill.io, which will automatically include dependencies where necessary:
<script src="https://polyfill.io/v3/polyfill.min.js?features=IntersectionObserver"></script>
With these polyfills, IntersectionObserver
has been tested and known to work in the following browsers:
Running the tests
To run the test suite for the IntersectionObserver
polyfill, open the intersection-observer-test.html
page in the browser of your choice.
If you run the tests in a browser that supports IntersectionObserver
natively, the tests will be run against the native implementation. If it doesn't, the tests will be run against the polyfill.