svelte

name	svelte
description	Svelte compile-time framework with reactive declarations. Use for .svelte files.

Svelte

Svelte is a component framework that compiles your code to tiny, framework-less vanilla JS. Svelte 5 (2025) introduces "Runes" for explicit reactivity.

When to Use

• High Performance defaults: Svelte apps are tiny and fast by default.
• Embedded Apps: Great for widgets/embeds because of small bundle size.
• Simplicity: HTML, CSS, and JS in one file, with very little boilerplate.

Quick Start (Runes)

<script>
  let count = $state(0);
  let double = $derived(count * 2);

  function increment() {
    count += 1;
  }
</script>

<button onclick={increment}>
  Count: {count} / Double: {double}
</button>

Core Concepts

Runes

Svelte 5 reactivity markers.

• $state: Declares reactive state.
• $derived: Declares a value that updates when dependencies change.
• $effect: Runs code when dependencies change (side effects).
• $props: Declares component props.

Snippets

Reusable chunks of markup within a component.

{#snippet figure(src, caption)}
  <figure>
    <img {src} alt={caption} />
    <figcaption>{caption}</figcaption>
  </figure>
{/snippet}

{@render figure(imageSrc, "A nice image")}

Best Practices (2025)

Do:

• Use Runes: Migrate from let + $ syntax to $state and $derived for explicit reactivity.
• Use onclick: Svelte 5 prefers native attributes (onclick) over on:click directives.
• Use Snippets: Replace slots with Snippets for better type safety and flexibility.

Don't:

• Don't rely on auto-reactivity (Legacy): In Svelte 5 settings, opting into Runes disables the "magic" assignment tracking of Svelte 3/4. This is good for predictability.

References

• Svelte 5 Preview
• Svelte Documentation