Skip to content
Swizec Teller - a geek with a hatswizec.com

Trying the new ResizeObserver and IntersectionObserver APIs

You might remember the useDimensions React Hook I opensourced a few weeks ago. 246 stars, 2 issues, and 1 merged PR make it the most useful library I ever released πŸ’ͺ Some 10 days ago I made it update measurements live. Needed to make this working for a client project πŸ‘‡ Scrollable overlay highlight
Scrollable overlay highlight
It's not perfect, but that highlight is a hole punched through a darkened canvas. It uses useDimensions for positioning and needs to update when you scroll. Works good enough. Built with hooks, live updating looks like this: techletter app screenshot 1554309442351

We use a useLayoutEffect to run our code synchronously on DOM updates. Inside, we check that the node exists, then create a debounced measure() function. Measure runs on requestAnimationFrame so we don't kill the browser even if the code falls into an infinite loop. You might get small subpixel resizes forever because of layouting edge cases. I've seen it happen. It's bad. getDimensionObject gets measurements, setDimensions is the state setter from a useState hook. You can see full code here. Window resize and scroll listeners will re-run our measure() function when things change. Scrolling will change its position, resizing might change its dimensions. There's a problem here πŸ‘‰ if the container resizes and the window doesn't, we'll have stale dimensions. When you use JavaScript or CSS to resize an element in response to user events, for example. No bueno.

IntersectionObserver to the rescue!

Last night I decided to fix that with the new IntersectionObserver API

https://www.youtube.com/watch?v=uSmd6VMEMTo
https://www.youtube.com/watch?v=uSmd6VMEMTo
After some research I realized I was wrong. IntersectionObserver doesn't do what I thought it does, but it's super neat anyway. You want to use the IntersectionObserver API when you have two DOM nodes and you want to know when their intersection changes. It's all in the name right? A common use-case is something like my react-lazyload-fadein library where you want to lazy load images juuuuust before they become visible. You could create a new intersection observer between the document and your placeholder node and wait for notifications. Super neat, not what's going to fix our resize edge case.
https://twitter.com/Swizec/status/1113313756325355520
https://twitter.com/Swizec/status/1113313756325355520

ResizeObserver, that's the thing πŸ‘Œ

ResizeObserver is an even newer API. Mozilla calls it "experimental". Support is lacking, but there are polyfills you can use. I used this one The idea here is that you can create a new observer object, give it a DOM node to observe and it's going to call your function when something changes. You can give it multiple nodes too. Something like this πŸ‘‡ techletter app screenshot 1554309485677

Create a new ResizeObserver in an effect post initial mount. Give it a method that runs when the observer fires, make it observe your React element, return the disconnect function to clean up on unmount. Perfect. Much cleaner than the hacky solution πŸ‘Œ Except it doesn't work. You can see it in the stream around the 29 minute mark. ResizeObserver was going crazy and firing as fast as we could read. Remember when I said you can run into subtle layouting edge cases where things are unstable and resize very often? That. So we gave up, reverted back to window.addEventListener, and left the edge case alone. Pretend like none of this ever happened. But it was fun to experiment :)

Cheers,
~Swizec

Did you enjoy this article?

Published on April 3rd, 2019 in Front End, Livecoding, Technical

Learned something new?
Want to become a high value JavaScript expert?

Here's how it works πŸ‘‡

Leave your email and I'll send you an Interactive Modern JavaScript Cheatsheet πŸ“–right away. After that you'll get thoughtfully written emails every week about React, JavaScript, and your career. Lessons learned over my 20 years in the industry working with companies ranging from tiny startups to Fortune5 behemoths.

Start with an interactive cheatsheet πŸ“–

Then get thoughtful letters πŸ’Œ on mindsets, tactics, and technical skills for your career.

"Man, love your simple writing! Yours is the only email I open from marketers and only blog that I give a fuck to read & scroll till the end. And wow always take away lessons with me. Inspiring! And very relatable. πŸ‘Œ"

~ Ashish Kumar

Join over 10,000 engineers just like you already improving their careers with my letters, workshops, courses, and talks. ✌️

Have a burning question that you think I can answer?Β I don't have all of the answers, but I have some! Hit me up on twitter or book a 30min ama for in-depth help.

Ready to Stop copy pasting D3 examples and create data visualizations of your own? Β Learn how to build scalable dataviz components your whole team can understand with React for Data Visualization

Curious about Serverless and the modern backend? Check out Serverless Handbook, modern backend for the frontend engineer.

Ready to learn how it all fits together and build a modern webapp from scratch? Learn how to launch a webapp and make your first πŸ’° on the side with ServerlessReact.Dev

Want to brush up on your modern JavaScript syntax?Β Check out my interactive cheatsheet: es6cheatsheet.com

By the way, just in case no one has told you it yet today: I love and appreciate you for who you are ❀️