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Observer APIs in a nutshell

I’ve played with the various HTML5 Observer APIs (IntersectionObserver, ResizeObserver and MutationObserver) a little over the last few years—for example using ResizeObserver in a container query solution for responsive grids. But in all honesty their roles, abilities and differences haven’t yet fully stuck in my brain. So I’ve put together a brief explainer for future reference.

Intersection Observer

Lets you watch for when an element of your choice intersects with a root element of your choice—typically the viewport—and then take action in response.

So you might watch for a div that’s way down the page entering the viewport as a result of the user scrolling, then act upon that by applying a class which animates that div’s opacity from 0 to 1 to make it fade in.

Here’s how it works:

  • Instantiate a new IntersectionObserver object, passing in firstly a callback function and secondly an options array which specifies your root element (usually the viewport, or a specific subsection of it).
  • Call observe on your instance, passing in the element you want to watch. If you have multiple elements to watch, you could call observe repeatedly in a loop through the relevant NodeList.
  • in the callback function add the stuff you want to happen in response to “intersecting” and “no longer intersecting” events.

Mutation Observer

Lets you watch for changes to the attributes or content of DOM elements then take action in response.

You might use this if you have code that you want to run if and when an element changes because of another script.

Here’s how it works:

  • Your typical starting point is that you already have one or more event listeners which modify the DOM in response to an event.
  • Instantiate a new MutationObserver object, passing in a callback function.
  • The callback function will be called every time the DOM is changed.
  • Call observe on your instance, passing in as first argument the element to watch and as second argument a config object specifying what type of changes you’re interested in, for example you might only care about changes to specific attributes.
  • your callback function provides an array of MutationRecord objects—one for each change that has just taken place—which you can loop through and act upon.

Resize Observer

Lets you watch for an element meeting a given size threshold then take action in response.

For example you might add a class of wide to a given container only when it is wider than 60em so that new styles are applied. This is a way of providing container query capability while we wait for that to land in CSS.

Or you might load additional, heavier-weight media in response to a certain width threshold because you feel you can assume a device type that indicates the user is on wifi. Adding functionality rather than applying styles is something we could not achieve with CSS alone.

Given that Container Query support is coming in CSS and that we can usually get by without it in the meantime, I don’t think it’s something I need so desperately that I’ll keep reaching for JavaScript to achieve it. However that’s not the only use for ResizeObserver.

It’s also worth remembering that it’s not all about width: ResizeObserver can also be used to detect and respond to changes to an element’s height. An example might be watching for changes to a chat window’s height—something that’s liable to happen as new messages appear—and then ensuring the focus stays at the bottom, on the latest message.


Steve Griffiths has some great video tutorials on these topics.

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