Kexi's blog 
JavaScript Observer APIs
Observers can be helpful to watch certain activities happening in the browser and respond accordingly. For example, we can observe, if child element has been added or removed from the parent DOM element, if a video is displayed within the viewport and enable autoplay, if the size/dimensions of a box element has changed and so on. These are different types of observer APIs in JavaScript.
Mutation Observer API
The MutationObserver interface provides the ability to watch for changes being made to the DOM tree.
The MutationObserver() constructor creates and returns a new observer which invokes a specified callback when DOM events occur. The callback function takes an array of MutationRecord objects, describing each change that occurred.
DOM observation does not begin immediately; the observe(target, options) method must be called first to establish which portion of the DOM to watch (target) and what kinds of changes to watch for (options). To be more specific, the options object describe which DOM mutations should be reported to mutationObserver’s callback. At a minimum, one of childList, attributes, and characterData must be true. Otherwise, a TypeError exception will be thrown.
childList: Set to true to monitor the target node for the addition of new child nodes or removal of existing child nodes. (detect changes in the direct children of node)attributes: Set to true to watch for changes to the value of attributes on the node being monitored. (detect attribute changes of node)characterData: Set to true to monitor the specified target node for changes to the character data (text content of a text node) contained within the node. (observe the changes of text content)subtree: Set to true to extend monitoring to the entire subtree of nodes rooted at target. All of the other properties are then extended to all of the nodes in the subtree instead of applying solely to the target node. (detect changes in all descendants of the node)
const config = { attributes: true, childList: true, subtree: true }
const callback = (mutationList, observer) => {
for (const mutation of mutationList) {
if (mutation.type === "childList") {
console.log("A child node has been added or removed.")
} else if (mutation.type === "attributes") {
console.log(`The ${mutation.attributeName} attribute was modified.`)
}
}
}
const observer = new MutationObserver(callback)
observer.observe(document.getElementById("test"), config)
// Later, you can stop observing
observer.disconnect()// Message box: automatically scroll to the bottom of the container
export default function AutomaticScroller({ children, className }) {
const ref = useRef(null);
useEffect(() => {
const mutationObserver = new MutationObserver(() => {
if (ref.current) {
ref.current.scroll({
behavior: "smooth",
top: ref.current.scrollHeight,
});
}
});
if (ref.current) {
mutationObserver.observe(ref.current, {
childList: true,
subtree: true, // observe all descendants
});
}
return () => {
mutationObserver.disconnect();
};
}, []);
return (
<div ref={ref} className={className}>
{children}
</div>
);
}Intersection Observer API
The IntersectionObserver interface provides a way to asynchronously observe changes in the intersection of a target element with an ancestor element or with a top-level document’s viewport. The ancestor element or viewport is referred to as the root.
The IntersectionObserver(callback, options) constructor creates and returns a new IntersectionObserver object.
The callback function receives a list of IntersectionObserverEntry objects. It is a list because you can use one observer for many elements, and the callback will be called whenever any of them intersect or stop intersecting. Each entry has detailed visibility info, such as target, isIntersecting, boundingClientRect, intersectionRatio, and so on.
The options object customizes the observer, for exmple, we can set the root or threshold property. If not specified, the observer uses the document’s viewport as the root, with no margin, and a 0% threshold.
const options = {
root: null, // The element to use as viewport (null = browser viewport)
rootMargin: "0px", // Adjust the root's bounding box ("top right bottom left")
threshold: 0.5, // Percentage of target visibility to trigger callback (0-1)
};Then, we call the observe() method on the IntersectionObserver object, telling it to observe intersection changes for the target element (whose visibility within the root is to be monitored.)
const observer = new IntersectionObserver(entries => {
entries.forEach(entry => {
const intersecting = entry.isIntersecting
entry.target.style.backgroundColor = intersecting ? "blue" : "orange"
})
})
observer.observe(document.getElementById("test"))const images = document.querySelectorAll('.lazyload')
function handleIntersection(entries) {
entries.map((entry) => {
if (entry.isIntersecting) {
entry.target.src = entry.target.dataset.src
entry.target.classList.add('loaded')
// `unobserve()` instructs the IntersectionObserver to stop observing the specified target element.
// `disconnect()` stops watching all of its target elements for visibility changes.
observer.unobserve(entry.target)
}
})
}
const observer = new IntersectionObserver(handleIntersection)
images.forEach(image => observer.observe(image))The thresholds, if specified, accepts a value between 0 and 1 and represents the percentage of the element that must be visible before isIntersecting becomes true. By default this is set to 0 which means as soon as any part of the element is visible it will be considered intersecting. You can also pass an array to threshold which means “Call the callback every time the visible ratio of the target passes any of these points.”
const observer = new IntersectionObserver(entries => {
entires.forEach(entry => {
entry.target.innerText = `${Math.round(entry.intersectionRatio * 100)}%`
})
},
{ threshold: [0, 0.25, 0.5, 0.75, 1] }
)There is also a rootMargin option, which specifies a set of offsets to add to the root’s bounding box when calculating intersections, effectively shrinking or growing the root. The default is “0px 0px 0px 0px”. For example, set it to “0px 0px 100px 0px” means that the observer will call the callback when element is still 100px below the viewport.
Resize Observer API
The Resize Observer API provides a performant mechanism by which we can monitor an element for changes to its size, with notifications being delivered to the observer each time the size changes.
All the APIs with the Observer suffix we mentioned above share a simple API design. You create a new ResizeObserver object using the ResizeObserver() constructor, then use observe() method to make it look for changes to a specific element’s size. A callback function set up inside the constructor then runs every time the size changes, providing access to the new dimensions and allowing you to do anything you like in response to those changes.
const observer = new ResizeObserver(entries => {
for (let entry of entries) {
// new size of the element
const { width, height } = entry.contentRect
console.log('Element:', entry.target)
console.log(`Element size: ${width}px x ${height}px`)
}
})
observer.observe(document.getElementById("test"))