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
js
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
js
// 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
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