Svelte Guideline: Reactivity — Use $derived for computed values. Description: $derived replaces $: in Svelte 5. Do: let doubled = $derived(count * 2). Don't: $: in Svelte 5. Good Example: let doubled = $derived(count * 2). Bad Example: $: doubled = count * 2 (Svelte 5). Severity: Medium. Docs: .