The IntersectionObserver method observe() adds an element to the set of target elements being watched by the IntersectionObserver. One observer has one set of thresholds and one root, but can watch multiple target elements for visibility changes in keeping with those.

To stop observing the element, call IntersectionObserver.unobserve().

When the visibility of the specified element crosses over one of the observer's visibility thresholds (as listed in IntersectionObserver.thresholds), the observer's callback is executed with an array of IntersectionObserverEntry objects representing the intersection changes which occurred. Note that this design allows multiple elements' intersection changes to be processed by a single call to the callback.





An element whose visibility within the root is to be monitored. This element must be a descendant of the root element (or contained within the current document, if the root is the document's viewport).

Return value

None (undefined).


// Register IntersectionObserver
const io = new IntersectionObserver(entries => {
  entries.forEach(entry => {
    // Add 'active' class if observation target is inside viewport
    if (entry.intersectionRatio > 0) {'active');
    // Remove 'active' class otherwise
    else {'active');

// Declares what to observe, and observes its properties.
const boxElList = document.querySelectorAll('.box');
boxElList.forEach((el) => {


Intersection Observer
# dom-intersectionobserver-observe

Browser compatibility

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See also