svelte-scroll-tracker

A lightweight, dependency-free, and performant Svelte 5 component for tracking scroll progress of an element within the viewport.

This component is intentionally barebones: it generates only a single CSS progress variable (--scroll-progress), making it extremely flexible and easy to apply to any CSS animation, effect, or logic you want. You have full control over how to use the progress value in your styles or scripts.

Why is it performant?

• Uses a single IntersectionObserver to detect when the element enters or leaves the viewport, minimizing unnecessary work.
• Throttles scroll event handling using requestAnimationFrame, ensuring updates are efficient and do not overload the browser during rapid scrolling.
• Only updates the CSS variable when the element is in view, reducing unnecessary DOM writes.
• No external dependencies or heavy libraries—just native browser APIs and Svelte reactivity.

Install

npm install svelte-scroll-tracker

Usage

CSS Variable Usage

<script lang="ts">
  import ScrollTracker from 'svelte-scroll-tracker';
</script>

<ScrollTracker
  startThreshold={0}
  endThreshold={0.5}
  debug={false}
>
  <div style="height: 400px;">
    Scroll me!
    <div>Progress: {Math.round(100 * $css('--scroll-progress'))}%</div>
  </div>
</ScrollTracker>

• The component sets a CSS custom property --scroll-progress (from 0 to 1) on its root element as you scroll.
• You can customize startThreshold, endThreshold, and debug via props.
• No external dependencies; works out of the box with Svelte 5.

Programmatic Usage

You can also use the progress value programmatically via the children snippet slot:

<ScrollTracker>
  {#snippet children(progress)}
    static content
    {#if progress > 0.5}
      dynamic content
    {/if}
  {/snippet}
</ScrollTracker>

Props

| Prop | Type | Default | Description | |-----------------|---------|---------|--------------------------------------------------| | startThreshold | number | 0 | When to start mapping progress (0-1) | | endThreshold | number | 0.5 | When to end mapping progress (0-1) | | debug | boolean | false | Enable debug logging |