Monitoring
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);
monitor.added('div', div => console.log('div added:', div));
monitor.removed('.ad', ad => console.log('advert removed:', ad));
monitor.appeared('#content', content => console.log('content is visible:', content));
monitor.disappeared('img', img => console.log('img is no longer visible:', img));
monitor.resized('textarea', textarea => console.log('textarea resized:', textarea));
Installation
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>
Getting callback details
Callbacks also recieve the observer entry that triggered the callback. This table shows the type of entry each methods recieves:
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`);
});
Stopping monitors
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);
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);
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);
monitor.added('div', div => {
console.log('div added!');
return false;
});
IFrames
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!');
}
});
Existing elements
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});
Performance
Monitors reuse their observers so you should avoid declaring new monitors for the same element. For example:
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');
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:
monitoring(document.body).added('#content', content => {
monitoring(content).added('.my_class', div =>
console.log('found our class in the content div!');
);
return false;
});
How is monitoring
different from arrive.js
?
arrive.js
is an excellent library and was the inspiration for this project. However, there are some differences:
document.body.arrive('div', console.log);
frames[0].document.body.arrive('div', console.log);
$(frames[0].document.body).arrive('div', console.log);