Trigger in View — A simple intersection observer helper

TLDR; Trigger-in-view is a little helper library I wrote that triggers after scroll and runs a callback against only targeted elements visible in a specified container.

Triggering actions when certain DOM elements comes into the view is a common requirement in web development. A common usage is lazy-loading images, the deferring of loading full resolution images until they’ll be seen by the user. But, there are many other uses and can form the foundation of a storytelling narrative such as A Year in Lockdown, used to trigger video playing and animations.

Recently, I needed to list numerous items in a scrollable view. Each item contained an image and without any additional logic the app would load all 300+ images on start, even if the user never viewed them. Scrolling lists present a unique challenge, especially on mobile, as the user is likely you quickly swipe the list searching for the item they’re looking for.

The usual solution is to use IntersectionObserver and trigger the image load on the intersection event. The problem with this simple approach is it triggers all items as they whizz by as the user flicks the scrolling list with their thumb.

Using IntersectionObserver on its own results in many image loads

You could wrap the IntersectionObserver callback in a debounce function, but there’s a problem with that. Intersection events happen one at a time as each observed item comes into view. Debouncing the callback means only one item will be processed with the callback.

What is needed is a way to delay triggering the IntersectionObserver callback that will also recognize observed items currently in view. The answer is to take advantage of a trait of the IntersectionObserver.observe() method that triggers the callback immediately when used on an element.

By looping over all observed items and adding them to a new IntersectionObserver, the currently visible items can be processed and removed if desired. The IntersectionObserver can then .disconnected() ready for the next go around.

The creation -> loop -> disconnect function is attached to the list’s scroll event using a simple setTimeout debounce function. Now, the list can be scrolled as fast as the user wants without any unnecessary loads.

Using setTimeout and observer() after scroll events prevents unnecessary loads

One limitation to this approach is the intersection check only happens once the setTimeout delay has passed, and that is dependant upon scroll events. The browser’s momentum scrolling behaviour can result in a fast flick of the list taking a long time to slow down and stop. Reducing the delay length helps, but increases the chance of unnecessary loads. A smarter way would be to count the frequency of the scroll events and trigger the check once it has passed a certain threshold. Maybe I’ll add that to a future version.

I wrote the library mainly for myself but I took the opportunity to learn how to publish a namespaced package onto NPM. So, for anyone interested the code is viewable on github.