1.0.5 • Public • Published


    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:
    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 '';
      const monitor = monitoring(document.body);

    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 = 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

    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

    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!');

    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});


    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:

    // 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);




    npm i monitoring

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    • kylejking