How do you use the Resize Observer API in Svelte?

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The Resize Observer API is a useful browser API that allows you to watch the size of elements and receive updates whenever the size changes. I've wrapped this API to use in React with a custom hook as it can be useful to apply different styles or even swap out components depending on the size the element is taking up. So how would we go about using this API in Svelte?

Get to the code!

There's not much to it so take a look at the code and let's walk through it.

Get a reference to the element

In React, whenever you want to interact with the DOM directly you have to create a reference with the useRef hook and then access current on the ref as it changes. Svelte's API is a bit easier to do this.

  let boxEl;

<div class="box" bind:this={boxEl}></div>

We simply declare a variable then use the bind:this attribute on the element. Done. Now we can access the DOM on boxEl.

Adding the Resize Observer

We add the observer on mount with the aptly named onMount lifecycle hook in Svelte.

onMount(() => {
    const resizeObserver = new ResizeObserver(entries => {
      // We're only watching one element
      const entry =;

      //Get the block size
      boxWidth = entry.contentBoxSize[0].blockSize;


    // This callback cleans up the observer
    return () => resizeObserver.unobserve(boxEl);

The key things to observer is that we create a new ResizeObserver with a callback that will be triggered whenever the element changes. You can observer multiple elements but for our use case we only care about one, that's why I'm ignoring the other entries and just pulling out the first one - const entry =;.

I want to see the size of the element so I'm setting boxWidth = entry.contentBoxSize[0].blockSize; and Svelte will see the assignment to the variable boxWidth and infer this is a reactive variable so the UI will update.

Once it's setup, we observe the element with resizeObserver.observe(boxEl);. The last piece of the puzzle is providing a cleanup callback so we return resizeObserver.unobserve(boxEl); so that if the component were to ever be unmounted, the observer will stop watching for updates.

Wrapping up

There you have it, the Resize Observer API is a powerful web API that can come in handy. It's easy to use regardless of component framework you decide to go with.


Seth Corker

A Fullstack Software Engineer working with React and Django. My main focus is JavaScript specialising in frontend UI with React. I like to explore different frameworks and technologies in my spare time. Learning languages (programming and real life) is a blast.