Using Svelte to create a scroll video effect

In this tutorial I’ll show you how svelte’s bind command can be used to create a cool scroll video effect with very little code. The effect we’ll be creating plays a video when the user scrolls up or down. On my blog is a gif of how this looks on the NYTimes website

Note: the effect itself is a lot smoother than the gifs 😂. Another example can be found here). The effect is also sometimes used on product landing pages. On my blog is a gif of how this looks on Apple’s website for the iMac Pro

It gives web pages a very dynamic look and I was surprised to learn how easy you can accomplish this with svelte.

Setting up svelte

Following the instructions on the svelte.dev homepage we run the following commands to get started with svelte:

npx degit sveltejs/template scroll-video
npm install
npm run dev

A new svelte project is now running on: http://localhost:5000

We start with an empty App.svelte file, so let’s delete all the code in that file first.

Scrolling formula

When the user scrolls down the page, we want the video the play forward. To accomplish this we’ll set the current video position using the following formula:

currentTime = duration * scrollY / total scroll height

where:

• currentTime = the play position in the video
• duration = total playing time of the video
• scrollY = the current vertical scroll position

Binding variables

Now that we figured out what the formula is going to be, let’s see how we can read these values by using svelte’s variable binding.

We can get the vertical scrolling position with the following code:

<script>
  let scrollY
</script>

<svelte:window bind:scrollY />

Here we are binding the vertical scroll position of the window to a variable scrollY. Before we can continue, we have to make sure our window is heigh enough to allow for scrolling. Let’s add a div and some CSS for that:

<script>
  let scrollY
</script>

<svelte:window bind:scrollY />

<div class="scroll-container" />

<style>
  .scroll-container {
    height: 5000px;
  }
</style>

Notice how you can combine HTML, CSS and JavaScript within a svelte component. Almost like you would in a regular HTML file!

To verify if this works you might be tempted to add a console.log just below the line let scrollY;. If you try that however you’ll see only one 0 being logged. The reason for this is that any code inside the <script> will run only once (when initialising the component). If you want to keep executing the console.log as the scroll values updates, make it reactive using the following notation:

$: console.log(scrollY)

This may look a bit funky (and it is! I’m not even sure if it’s still JavaScript) but you’ll get used to it.

After adding the console.log with the dollar sign, you should see the scroll position logged on every update. Pretty cool for just 2 lines of code!

Similarly we can use bind on <video> to get the other values:

<script>
  // … existing code
  let time let duration</script>

<!-- …existing code -->

<video bind:currentTime="{time}" bind:duration preload="metadata" muted src="https://static01.nyt.com/newsgraphics/2019/10/23/turkey-syria-video-upload/71ab097907156ca46fb7ffd4d21dfbd119fb47e8/syria-turkey-reconstruct-7-800.mp4" type="video/mp4"/>

The cool thing about these bindings is that they are bidirectional. This means that the time variable not only holds the current playback time, but that if we change it, we’re also updating the current playback position of the video! This is also known as two-way data binding which was initially popularized by the original AngularJS.

In the next section we’ll combine this with the formula we made earlier.

Updating video playback on scroll

Before we can continue we first have to make sure the video is always displayed fullscreen on the background while we are scrolling. Let’s add a container div and some CSS to accomplish that:

<!-- .... -->

<div class="video-container"> <video
    bind:currentTime="{time}"
    bind:duration
    preload="metadata"
    muted
    src="https://int.nyt.com/newsgraphics/2020/beirut-explosion-video/main/warehouse-800.mp4"
    type="video/mp4"
  />
</div>
<style>
  .video-container { position: absolute; top: 0; bottom: 0; overflow: hidden; } .video-container video { min-width: 100%; min-height: 100%; width: auto; height: auto; position: fixed; top: 50%; left: 50%; transform: translate(-50%, -50%); } /* ... */
</style>

Any changes in the scroll position should immediately be reflected in a change in the playback position. In other words: the changes in the time variable should react to the changes in the scroll position. To accomplish this we have to make sure our formule code is reactive. Just as with the console.log we use the $: notation to do this:

<script>
  let time = 0
  let duration

  let scrollY = 0

  $: {
    const totalScroll =
      document.documentElement.scrollHeight - window.innerHeight
    time = duration * (scrollY / totalScroll)
  }
</script>

Result

After adding the last piece of code the video scroll effect should work. To make the scroll effect more visible I’ve also added a heading and some content blocks. You can see the complete feature in the Svelte REPL below.

Notes on performance

The NYTimes is using optimized, small size videos on their pages. And with good reason: I’ve been doing experiments with larger (high definition) videos and it’s impossible to get the performance right on all browsers: Safari performs surprisingly well, but Firefox and Chrome quickly start to stutter when the video gets larger.

Apple is actually using a different technique to accomplish the effect on their pages: They’ve split up the video in separate frames (images) and they show a different frame when the scroll position changes. This technique performs surprisingly well, even for high definition video content. It does, however, come at a cost: All those frames have to be downloaded individually, adding up to huge download sizes. This is costly not just for mobile end users, but also if you’re hosting them.

If you want to use this technique and keep things simple I suggest using a small and optimized video just like the NYTimes does.

For anyone interested in the performance aspects of this effect: I’ve tweeted about this here (with a demo site): https://twitter.com/vnglst/status/1307681819635183616