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);

// 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));

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);

// 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; 
});

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:

// 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;
});

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:

• 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);