During the life of web developer comes a moment when you have to build infinite scrolling list, react to the changes of the size of given element or its children or implement some behaviour dependant on the visibility of the object in the viewport. Observer's family can help you while working on mentioned tasks.
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1. IntersectionObserver
What
thanks to it you can asynchronously observe changes in the intersection of a target element with an ancestor element or with a top-level document's viewport.
Why
You might use it in below cases:
- Lazy-loading of images
- Implementing "infinite scrolling"
- Reporting of visibility of advertisements etc.
How
const elements = document.querySelectorAll('.elt');
const options = {
root: null, // set document viewport as root
rootMargin: '0px', // margin around root
threshold: 1.0 // 1.0 means that when 100% of the target is visible
//inside the root, then observer callback is invoked.
};
const observer = new IntersectionObserver(entries => {
entries.forEach(entry => {
if (entry.intersectionRatio > 0) {
console.log('in viewport');
} else {
console.log('out of viewport');
}
});
}, options);
elements.forEach(elt => {
observer.observe(elt);
});
Link to the docs
2. MutationObserver
What
It will be helpful if you need to watch for DOM tree changes.
Why
MutationObserver will tell you when the DOM element's child was removed, added or any of attributes of any DOM tree elements was changed.
How
const element = document.querySelector("#element");
const options = {
childList: true, // listen to listen to children being added or removed
attributes: true, // listen to attributes changes
subtree: true // omit or set to false to observe only changes to the parent node
}
const callback = (mutationList, observer) => {
mutationList.forEach((mutation) => {
switch(mutation.type) {
case 'childList':
// check mutation.addedNodes or mutation.removedNodes
break;
case 'attributes':
/* An attribute value changed on the element in
mutation.target; the attribute name is in
mutation.attributeName and its previous value is in
mutation.oldValue */
break;
}
});
}
const observer = new MutationObserver(callback);
observer.observe(element, options);
Link to the docs
3. ResizeObserver
What
It reports about changes of the dimensions of the element.
Why
It can be useful if you would like to listen to changes of the viewport (portrait vs landscape) or just you have some external content and you would like to react to it changes.
How
const elements = document.querySelectorAll('.elt');
const observer = new ResizeObserver(entries => {
entries.forEach(entry => {
const width = Math.floor(entry.contentRect.width);
const height = Math.floor(entry.contentRect.height);
// execute some logic based on width and height params
}
});
elements.forEach(elt => {
observer.observe(elt);
});
Link to the docs
Top comments (5)
Observers are really awesome, but I feel like still not enough people know about them ๐
That's true. I just knew about mutation observer. I wish I knew about it since. It would have saved me a lot of stress.
exactly, but I think sooner or later every developer is thankful that they exists
What task are they? microTask or Macrotask?
the only thing i know here is intersection, quite handy with lazy load and infintescrolling. should i use both the observer with
lazyattribute or pick one? which one you prefer?