Svelte Guideline: Reactivity — Use $: for reactive statements. Description: Automatic dependency tracking. Do: $: for derived values. Don't: Manual recalculation. Good Example: $: doubled = count * 2. Bad Example: let doubled; count && (doubled = count * 2). Severity: Medium. Docs: https://svelte.dev/docs/svelte-components#script-3-$-marks-a-statement-as-reactive.