Svelte template directives

Svelte Template Directives

@attach (Svelte 5.29+)

The reactive alternative to use: actions. Re-runs when dependencies change, passes through components via spread, supports cleanup functions.

<script>
	import ImageZoom from 'js-image-zoom';

	function useZoom(options) {
		return (element) => {
			new ImageZoom(element, options);
			return () => console.log('cleanup');
		};
	}
</script>

<!-- Re-runs if options changes (use: wouldn't!) -->
<figure {@attach useZoom({ width: 400 })}>
	<img src="photo.jpg" alt="zoomable" />
</figure>

Quick Reference

Directive	Purpose	Reactive?
{@attach}	DOM manipulation, 3rd-party	Yes
{@html}	Render raw HTML strings	Yes
{@render}	Render snippets	Yes
{let ...} / {const ...}	Local declarations in markup	N/A
{@debug}	Pause debugger on value change	N/A
{#each (key)}	Keyed iteration (always key!)	Yes
<svelte:window>	Window event listeners	N/A

@attach vs use: Actions

Feature	use:	@attach
Re-runs on arg change	No	Yes
Composable	Limited	Fully
Pass through props	Manual	Auto via spread
Convert legacy	N/A	fromAction()

Reference Files

• attach-patterns.md - Real-world @attach examples
• other-directives.md - @html, @render, declaration tags, @debug

Notes

• @attach requires Svelte 5.29+
• Use fromAction from svelte/attachments to convert legacy actions
• Attachments pass through wrapper components when you spread props
• Always use keyed each blocks — never use index as key
• Use <svelte:window>/<svelte:document> for global events, not $effect
• Use {const ...} / {let ...} declaration tags for local markup variables; {@const ...} is legacy syntax.
• Last verified: 2026-05-31

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