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.
; const monitor = ; // watch for new elements added to the DOMmonitor; // watch for elements removed from the DOMmonitor; // 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-v2monitor; // watch for elements that are no longer visible on the pagemonitor; // watch for when elements are resizedmonitor;
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:
Getting callback details
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 = ; monitor; monitor; monitor;
Stopping monitors
You can cancel a monitor by calling its cancel
method. This cancels all callbacks registered against that monitor:
const monitor = ;const divAdded = monitor;const divRemoved = monitor; // stops the monitor, including divAdded and divRemovedmonitor;
You can also cancel a specific callback:
const monitor = ;const divAdded = monitor;const divRemoved = monitor; // cancels divAdded, doesn't effect divRemoveddivAdded;
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 = ; // cancels the callback after its first callmonitor;
IFrames
Monitors support an iframes
option to include monitoring elements within iframes of the same origin. For example:
const monitor = ; monitor;
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 = ;monitor;
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 = ;monitor
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
different from arrive.js
?
How is arrive.js
is an excellent library and was the inspiration for this project. However, there are some differences:
arrive.js
does not support theIntersectionObserver
orResizeObserver
.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 newMutationObserver
for each callback, which can be slow.arrive.js
requires jquery for observing elements from different documents. For example:
// works! documentbody; // does not work :-(frames0documentbody; // works, but you need jquery ;