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IntersectionObserver: observe() method

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.

Note: the observer callback will always fire the first render cycle after observe() is called, even if the observed element has not yet moved with respect to the viewport. This means that, for example, an element that is outside the viewport when observe() is called on it will result in the callback being immediately called with at least one entry with intersecting set to false. An element inside the viewport will result in the callback being immediately called with at least one entry with intersecting set to true.

Syntax

observe(targetElement)

Parameters

targetElement: 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).

Examples

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

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

Specifications

Specification
Intersection Observer # dom-intersectionobserver-observe

Browser compatibility