"Scrollability" in Svelte

I ran into a fun (and incredible satisfying) problem when making this blog that I thought would be worth sharing. I wanted to have a “scroll to top” button at the bottom of most “content” style pages (eg. a blog post or portfolio page), but not on pages that didn’t actually have any scrollable content.

Let me illustrate with an example. Here is a page that’s pretty long, with a lot of content. And here is a page that’s pretty short (but hopefully not for long). It looks a little goofy if the latter page has a “scroll to top” button, but there’s a good reason to have one on the former page.

So, how do we go about doing this? The simplest way is using methods from the DOM API, specifically on window and document. Intuitively, we want to check if the height of the window is smaller than the height of the page itself. If it is, the page is scrollable, and we can show our button.

Let’s take a look at our trusty friend, MDN.

“The Element.scrollHeight read-only property is a measurement of the height of an element’s content, including content not visible on the screen due to overflow.” MDN

“The Window.innerHeight read-only property returns the interior height of the window in pixels, including the height of the horizontal scroll bar, if present.” MDN

Sounds like what we need.

First draft

Now that we have the properties we need, let’s put it all together:

const isPageScrollable = () => {
	const pageHeight = document.body.scrollHeight;
	const windowHeight = window.innerHeight;

	return pageHeight > windowHeight;
};

We can use this function in a component in a manner that looks something like the following:

<script>
	import { isPageScrollable } from '$lib/utils';
	import { BackToTop } from '$lib/components';

	let scrollable = isPageScrollable();
</script>

{#if scrollable}
	<BackToTop />
{/if}

This looks correct, but if we try using it we’ll run into a problem in our console output:

ReferenceError: document is not defined

This is because the isScrollable function is running before the component mounts, and document doesn’t exist at that point.

Some fixes

Let’s take stock of some of the problems in our implementation and see how to fix them.

• It doesn’t work! Currently, without using Svelte’s onMount function, the scrollability (neat word) calculation doesn’t work properly.
• The logic of showing/hiding based on the scrollability of a page is generic enough that I may want to use it somewhere else in the future. This should probably be captured into its own component.
• If the user resizes their browser (just to test if my blog is robust™ and responsive™, of course), they would be shocked to find that the button does not respond accordingly.
• The current implementation doesn’t account for any offset at the bottom of the page.

Let’s fix these glaring issues by making a generic component that’s responsive and flexible.

<!-- /src/utils/components/ScrollableWrapper.svelte -->
<script>
	import { isPageScrollable } from '$lib/utils';
	import { onMount } from 'svelte';

	export let offset;
	let scrollable;

	onMount(() => {
		window.addEventListener('resize', async () => {
			scrollable = await isPageScrollable(offset);
		});
	});
</script>

{#if scrollable}
	<slot />
{/if}

This is a good chunk of code, so let’s break it down:

• We import our isPageScrollable function from a generic utils.ts file, where it’s being stored now that we’re organizing our project nicely.
• We import onMount from Svelte. This is a lifecycle function that runs when the component is mounted to the DOM, which should help solve our document related issues.
• We take in a prop called offset to allow any consumers of this component to tweak the scrollability threshold. This is useful if you want to show the button a little earlier than the bottom of the page.
• We create a scrollable variable that will be used to determine whether or not to show our button.
• We add an event listener to the window object that will run our isPageScrollable function whenever the window is resized. This makes it so that even if the user resizes the page, the button will still be shown/hidden accordingly.
• Finally, we wrap the child component that’s passed in to <slot /> with a conditional block to render or not render based on the value of scrollable.

Now that we have our component (let’s call it <ScrollableWrapper />), we can use it in our blog overview page like this:

<!-- /src/routes/blog/+layout.svelte -->
<script>
	import { ScrollableWrapper, BackToTop } from '$lib/components';
</script>

<slot />

<ScrollableWrapper offset={100}>
	<BackToTop />
</ScrollableWrapper>

Now this does work the way we are expecting.

Quick note

Attaching an event listener to the window to track the scroll event works, but a more Svelte-y (?) way of doing this is using the <svelte:window> element. We can also track the window innerHeight this way, since binding directly to innerHeight is an option in a <svelte:window>. Here’s how that might look:

<script>
	import { isPageScrollable } from '$lib/utils';
	import { onMount } from 'svelte';

	let pageHeight: number;
	let innerHeight: number;
	let scrollable: boolean;

	onMount(() => {
		pageHeight = document.body.scrollHeight;
	});

	const onResize = () => {
		pageHeight = document.body.scrollHeight;
	};

	$: scrollable = isPageScrollable(pageHeight, innerHeight, 100);
</script>

<svelte:window bind:innerHeight on:resize={onResize} />

{#if scrollable}
	<slot />
{/if}

You might also notice a few other changes here. First, we still use onMount to track the initial scrollHeight of the body, since otherwise we would run into the ReferenceError from earlier. Next, the scrollable variable is moved to a reactive assignment, that reruns whenever pageHeight or innerHeight change, which happens whenever the window is resized.

More fixes

There are a couple of other things that could be fixed here as well, both for flexibility and performance reasons.

The first is the resize event handler. It currently runs every time the window is resized, which is definitely overkill (for reference, I triggered ~100 events by resizing for less than a second). We can fix this by using a debounce function that bunches updates together into more manageable chunks. lodash includes an implementation of a debounce function you can use (but writing one yourself is pretty fun!)

Another, smaller change that could be made is adding an else block to the conditional to display a different component if the page is not scrollable. I personally left this out in my implementation, but it can definitely be added if you need that functionality.

I hope you enjoyed this little dive into a component I had fun writing in Svelte. The source is available here. Go nuts.