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@kensingtontech/monitoring
Advanced tools
An efficient, simple, and lightweight library to watch the DOM for elements added, removed, appeared, disappeared, or resized.
A simple, efficient, and lightweight ES module to monitor the DOM for when elements are added, removed, have appeared, have disappeared, or are resized. Internally, this library uses the MutationObserver, IntersectionObserver, and ResizeObserver APIs.
import monitoring from 'monitoring';
const monitor = monitoring(document.body);
// watch for new elements added to the DOM
monitor.added('div', div => console.log('div added:', div));
// watch for elements removed from the DOM
monitor.removed('.ad', ad => console.log('advert removed:', ad));
// watch for elements to become appear on the page
// what it means to appear or be visible is complicated, details here:
// https://developers.google.com/web/updates/2019/02/intersectionobserver-v2
monitor.appeared('#content', content => console.log('content is visible:', content));
// watch for elements that are no longer visible on the page
monitor.disappeared('img', img => console.log('img is no longer visible:', img));
// watch for when elements are resized
monitor.resized('textarea', textarea => console.log('textarea resized:', textarea));
Add to your project using NPM:
$ npm install monitoring --save
You can add monitoring
directly in your site or download the latest minified version from jsdelivr:
<script type="module">
import monitoring from 'https://cdn.jsdelivr.net/npm/monitoring/dist/monitoring-latest.min.mjs';
const monitor = monitoring(document.body);
...
</script>
Callbacks also recieve the observer entry that triggered the callback. This table shows the type of entry each methods recieves:
Method | Entry type |
---|---|
added | MutationObserverEntry |
removed | MutationObserverEntry |
appeared | IntersectionObserverEntry |
disappeared | IntersectionObserverEntry |
resized | ResizeObserverEntry |
Here are some examples of how the entry information can be used:
const monitor = monitoring(document.body);
monitor.added('div', (div, entry) => {
console.log(`div added along with ${entry.addedNodes.length} other nodes`);
});
monitor.appeared('img', (img, entry) => {
console.log(`An image is ${entry.intersectionRatio*100}% visible`);
});
monitor.resized('textarea', (textarea, entry) => {
console.log(`textarea is now ${entry.contentRect.width} pixels wide`);
});
You can cancel a monitor by calling its cancel
method. This cancels all callbacks registered against that monitor:
const monitor = monitoring(document.body);
const divAdded = monitor.added('div', console.log);
const divRemoved = monitor.added('div', console.log);
// stops the monitor, including divAdded and divRemoved
monitor.cancel();
You can also cancel a specific callback:
const monitor = monitoring(document.body);
const divAdded = monitor.added('div', console.log);
const divRemoved = monitor.added('div', console.log);
// cancels divAdded, doesn't effect divRemoved
divAdded.cancel();
Finally, you can cancel by returning false
from within the callback. Note: Your callback must return false
, not a falsey value like null
or undefined
.
const monitor = monitoring(document.body);
// cancels the callback after its first call
monitor.added('div', div => {
console.log('div added!');
return false;
});
Monitors support an iframes
option to include monitoring elements within iframes of the same origin. For example:
const monitor = monitoring(document.body, {iframes: true});
monitor.added('div', div => {
if (div.ownerDocument != document) {
console.log('new div in an iframe!');
}
});
By default, the added
method will return all existing elements in the DOM that match the given selector and monitor for new ones. You can ignore existing elements by setting the existing
option to false
:
const monitor = monitoring(document.body);
monitor.added('.my_class', my_callback, {existing: false});
Monitors reuse their observers so you should avoid declaring new monitors for the same element. For example:
// this only uses one monitor for two callback - do this :-)
const monitor = monitoring(document.body);
monitor.added('div.my_class', div => console.log('my_class added');
monitor.removed('div.my_class', div => console.log('my_class removed');
// this uses two monitors, one for each callback - don't do this :-(
monitoring(document.body).added('div.my_class', div => console.log('my_class added');
monitoring(document.body).removed('div.my_class', div => console.log('my_class removed');
Observers were designed to be an efficient alternative to polling the DOM. However, monitoring large chunks of the DOM, like document
or document.body
is still expensive. I recommend monitoring the smallest portion of the DOM necessary and cancelling as soon as the monitor is no longer needed.
Here is an example of using a monitor to find more specific elements for monitoring:
// monitor the document body for a #content div
monitoring(document.body).added('#content', content => {
// monitor the #content div for our class
monitoring(content).added('.my_class', div =>
console.log('found our class in the content div!');
);
// this cancels the document body monitor
return false;
});
monitoring
different from arrive.js
?arrive.js
is an excellent library and was the inspiration for this project. However, there are some differences:
arrive.js
does not support the IntersectionObserver
or ResizeObserver
.arrive.js
does not support traversing into iframes.arrive.js
is not an ES6 module, which makes it difficult to incorporate into things like webpack.arrive.js
pollutes the DOM by decorating all matched elements and a number of prototypes.arrive.js
uses recursive node matching, which can be slow.arrive
creates a new MutationObserver
for each callback, which can be slow.arrive.js
requires jquery for observing elements from different documents. For example:// works!
document.body.arrive('div', console.log);
// does not work :-(
frames[0].document.body.arrive('div', console.log);
// works, but you need jquery
$(frames[0].document.body).arrive('div', console.log);
FAQs
An efficient, simple, and lightweight library to watch the DOM for elements added, removed, appeared, disappeared, or resized.
We found that @kensingtontech/monitoring demonstrated a not healthy version release cadence and project activity because the last version was released a year ago. It has 1 open source maintainer collaborating on the project.
Did you know?
Socket for GitHub automatically highlights issues in each pull request and monitors the health of all your open source dependencies. Discover the contents of your packages and block harmful activity before you install or update your dependencies.
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