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Decide whether to use Svelte 5 runes ($state, $derived, $effect) or legacy $: statements for new components. Compare trade-offs, see a working runes-only example, and learn how to validate reactivity in mixed codebases.
A decision guide for Svelte 5 developers comparing runes, legacy stores, and component context for shared reactive state, with SSR validation steps.
Learn how to identify and fix stale UI in Svelte after data mutations, with separate steps for legacy reactive statements and Svelte 5 runes mode.
Use $state, $derived, and $effect for component-local reactive state in Svelte 5, and keep stores for cross-component state.
Learn how Svelte 5’s $state, $derived, and $effect runes replace the $: label syntax, enable module‑scope reactive state, and reduce unnecessary re‑renders in large UIs.
Svelte 5 replaces implicit $: reactive statements with explicit runes. Learn when to use $state, $derived, and $effect, with a worked migration and the mistakes that cause loops.
Svelte 5 runes replace the implicit $: magic with explicit $state, $derived, and $effect. Here's how to migrate a real component, the mistake most people make, and the trade-offs.
Symptom After upgrading a project to Svelte 5, some components were migrated to runes ( $state , $derived , $effect ) while others still use legacy $: reactive statements. Everything compiles, but side effects appear to run earlier or later than before in components that sit at the boundary between the two styles — values update correctly, only the ordering
The goal is to migrate a component to Svelte 5’s runes reactivity gradually, keeping existing legacy reactive statements ( $: ) for parts of the logic that are not yet converted, while using $state , $derived , or $effect for new features. However, the Svelte 5 compiler emits the error “Legacy reactive statements are not allowed in components using runes” wh
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