code-style-svelte

Svelte Code Style

Optional snippet rendering

Use {@render snippet?.()} instead of {#if snippet}{@render snippet()}{/if}. The optional chaining form is shorter and idiomatic Svelte 5.

No $effect for setting state

NEVER use $effect to set state. If you need derived state, use $derived instead. Effects that write to state create unnecessary re-renders and are a sign of wrong reactive design.

No bare async in onMount or $effect

NEVER pass an async function directly to onMount or $effect:

// BAD — cleanup function is lost, dependency tracking breaks
onMount(async () => { ... });
$effect(async () => { ... });

// GOOD — async IIFE inside a sync wrapper
onMount(() => {
 (async () => { ... })();
 return () => { /* cleanup */ };
});

Two reasons:

1. Cleanup is lost. Both onMount and $effect expect the callback to optionally return a teardown function. An async function always returns a Promise, so the teardown never runs — intervals won't clear, subscriptions won't unsubscribe, listeners won't detach.
2. Dependency tracking breaks ($effect only). $effect tracks reactive dependencies that are read synchronously. Anything read after an await is invisible to the tracker and won't trigger re-runs.

No addEventListener / removeEventListener

NEVER use addEventListener or removeEventListener in Svelte components. Svelte 5 has multiple declarative mechanisms for event handling — on* attributes, callback props, svelte:window, svelte:document, svelte:body, and the on action from svelte/events. These handle cleanup automatically and integrate with Svelte's reactivity.

When unsure which pattern to use, invoke the Svelte MCP autofixer (mcp__svelte__svelte-autofixer) or fetch the relevant docs via mcp__svelte__get-documentation (sections: svelte-events, svelte-special-elements).

Svelte 5 parent + Svelte 4 child: use slot=, not {#snippet}

TODO: verify this is still needed after sveltejs/language-tools#2716 is resolved

When a Svelte 5 (runes mode) component renders a Svelte 4 child that uses <slot name="...">, pass content with slot="name" attribute syntax — NOT {#snippet name()}.

<!-- GOOD — slot= works in runes mode and passes svelte-check -->
<CollapsibleCard>
 <div slot="header">Header content</div>
 <div slot="body">Body content</div>
</CollapsibleCard>

<!-- BAD — runtime works but svelte-check rejects it -->
<CollapsibleCard>
 {#snippet header()}Header content{/snippet}
 {#snippet body()}Body content{/snippet}
</CollapsibleCard>

Why: svelte-check does not resolve named snippet props on legacy components (sveltejs/language-tools#2716, open as of 2026-04-09). The {#snippet} approach produces 'header' does not exist in type errors. The slot= attribute is still valid in runes-mode components and passes all checks.

When to use {#snippet}: Only when the child component is also Svelte 5 and accepts snippet props via $props().

Events from Svelte 4 children: on:click, on:change, etc. still work in a Svelte 5 parent. Use on: for Svelte 4 children, onclick/callback props for Svelte 5 children.

Verified 2026-04-09 with svelte ^5.55.2, svelte-check ^4.4.6.

HTML comments in component markup

Svelte supports standard HTML comments in component markup. <!-- stylelint-disable -->, <!-- svelte-ignore ... -->, and other tool-control comments work in .svelte files.

Component Documentation (@component)

Every new .svelte component file MUST start with a <!-- @component --> comment block on the very first line that documents the component's purpose. This follows JSDoc conventions — the @component tag is picked up by the Svelte language server (svelte2tsx) to display documentation on hover in IDEs, similar to how /** @description */ works in JSDoc for JS/TS files.

Format:

<!--
	@component
	Brief description of what this component does.
	Additional details if needed.
-->

The @component tag must appear inside the HTML comment. The description follows on subsequent lines, indented with a tab. Keep it concise but informative — someone reading this should understand the component's role without reading the implementation.

You can use other JSDoc tags inside the comment block where useful (e.g. @example, @see, @deprecated).

Excluded: pages, layouts, tests, and stories (.stories.svelte) do not need this comment.

Related skills

• frontend:code-style — General code style rules (variable naming, braces, data attributes). Applies to all code including Svelte.
• svelte:svelte-code-writer — Svelte 5 documentation lookup and code analysis. MUST invoke when creating or editing .svelte / .svelte.ts / .svelte.js files.
• svelte:svelte-core-bestpractices — Reactivity, events, styling, and integration patterns for modern Svelte.
• svelte-5:testing-svelte — Vitest + Playwright testing patterns for Svelte 5 components.