No,Category,Guideline,Description,Do,Don't,Code Good,Code Bad,Severity,Docs URL,Applies To,Status,Verified At
1,Reactivity,Use $: for reactive statements,Legacy-mode automatic dependency tracking,$: only while maintaining a legacy-mode component,Use $: in new runes-mode code,$: doubled = count * 2,let doubled = $derived(count * 2),Medium,https://svelte.dev/docs/svelte/legacy-reactive-assignments,svelte legacy <=4,deprecated,2026-08-13
2,Reactivity,Trigger legacy reactivity with assignment,Legacy-mode reactivity tracks assignments,Reassign arrays or objects in legacy mode,Treat assignment as the Svelte 5 runes contract,"items = [...items, newItem]",let items = $state([]),High,https://svelte.dev/docs/svelte/legacy-let,svelte legacy <=4,deprecated,2026-08-13
3,Reactivity,Use $state in Svelte 5,Runes for explicit reactivity,let count = $state(0),Implicit reactivity in Svelte 5,let count = $state(0),let count = 0 (Svelte 5),Medium,https://svelte.dev/blog/runes,svelte 5,active,2026-08-13
4,Reactivity,Use $derived for computed values,$derived replaces $: in Svelte 5,let doubled = $derived(count * 2),$: in Svelte 5,let doubled = $derived(count * 2),$: doubled = count * 2 (Svelte 5),Medium,,svelte 5,active,2026-08-13
5,Reactivity,Use $effect for side effects,$effect replaces $: side effects,Use $effect for subscriptions,$: for side effects in Svelte 5,$effect(() => console.log(count)),$: console.log(count) (Svelte 5),Medium,,svelte 5,active,2026-08-13
6,Props,Use export let for legacy props,Declare props with export let only in legacy mode,Retain export let while maintaining legacy components,Introduce export let in new runes-mode components,export let count = 0,let { count = 0 } = $props(),High,https://svelte.dev/docs/svelte/legacy-export-let,svelte legacy <=4,deprecated,2026-08-13
7,Props,Use $props in Svelte 5,$props rune for prop access,let { name } = $props(),export let in Svelte 5,"let { name, age = 0 } = $props()",export let name; export let age = 0,Medium,,svelte 5,active,2026-08-13
8,Props,Provide prop default values,Destructure defaults from $props,Use defaults in $props destructuring,Add separate fallback mutation,let { count = 0 } = $props(),let { count } = $props(); count ??= 0,Low,https://svelte.dev/docs/svelte/$props,svelte 5,active,2026-08-13
9,Props,Use rest props in runes mode,Pass through unknown props with $props rest destructuring,Spread a rest object onto the element,Use legacy $$restProps,"let { class: className, ...rest } = $props(); <button {...rest}>",<button {...$$restProps}>,Low,https://svelte.dev/docs/svelte/$props,svelte 5,active,2026-08-13
10,Bindings,Use bind: for two-way binding,Simplified input handling,bind:value for inputs,on:input with manual update,<input bind:value={name}>,<input value={name} on:input={e => name = e.target.value}>,Low,https://svelte.dev/docs/element-directives#bind-property,svelte 5,active,2026-08-13
11,Bindings,Bind to DOM elements,Reference DOM nodes,bind:this for element reference,querySelector in onMount,<div bind:this={el}>,onMount(() => el = document.querySelector()),Medium,,svelte 5,active,2026-08-13
12,Bindings,Use bind:group for radios/checkboxes,Simplified group handling,bind:group for radio/checkbox groups,Manual checked handling,"<input type=""radio"" bind:group={selected}>","<input type=""radio"" checked={selected === value}>",Low,,svelte 5,active,2026-08-13
13,Events,Use on: for legacy event handlers,Event directive syntax for legacy components,on:click only while maintaining legacy mode,Introduce on: handlers in runes mode,<button on:click={handleClick}>,<button onclick={handleClick}>,Medium,https://svelte.dev/docs/svelte/legacy-on,svelte legacy <=4,deprecated,2026-08-13
14,Events,Forward events with on:event in legacy mode,Legacy event forwarding without a handler,on:click only for a legacy component,Use legacy forwarding in runes mode,<button on:click>,<button onclick={onclick}>,Low,https://svelte.dev/docs/svelte/legacy-on,svelte legacy <=4,deprecated,2026-08-13
15,Events,Use createEventDispatcher only in legacy components,Legacy custom component events,Retain dispatch while maintaining legacy components,Add createEventDispatcher to new runes-mode code,"dispatch('save', { data })",let { onsave } = $props(),Medium,https://svelte.dev/docs/svelte/svelte#createeventdispatcher,svelte legacy <=4,deprecated,2026-08-13
16,Lifecycle,Use onMount for initialization,Run code after component mounts,onMount for setup and data fetching,Code in script body for side effects,onMount(() => fetchData()),fetchData() in script body,High,https://svelte.dev/docs/svelte#onmount,svelte 5,active,2026-08-13
17,Lifecycle,Return cleanup from onMount,Automatic cleanup on destroy,Return function from onMount,Separate onDestroy for paired cleanup,onMount(() => { sub(); return unsub }),onMount(sub); onDestroy(unsub),Medium,,svelte 5,active,2026-08-13
18,Lifecycle,Use onDestroy sparingly,Only when onMount cleanup not possible,onDestroy for non-mount cleanup,onDestroy for mount-related cleanup,onDestroy for store unsubscribe,onDestroy(() => clearInterval(id)),Low,,svelte 5,active,2026-08-13
19,Lifecycle,Avoid beforeUpdate and afterUpdate,Legacy lifecycle hooks are unavailable in runes mode,Use $effect.pre and $effect only when synchronization is required,Use lifecycle hooks or reactive assignments for derived state,$effect.pre(() => measure()),beforeUpdate(() => measure()),Low,https://svelte.dev/docs/svelte/lifecycle-hooks,svelte 5,active,2026-08-13
20,Stores,Use writable for mutable state,Basic reactive store,writable for shared mutable state,Local variables for shared state,const count = writable(0),let count = 0 in module,Medium,https://svelte.dev/docs/svelte-store#writable,svelte 5,active,2026-08-13
21,Stores,Use readable for read-only state,External data sources,readable for derived/external data,writable for read-only data,"readable(0, set => interval(set))",writable(0) for timer,Low,https://svelte.dev/docs/svelte-store#readable,svelte 5,active,2026-08-13
22,Stores,Use derived for computed stores,Combine or transform stores,derived for computed values,Manual subscription for derived,"derived(count, $c => $c * 2)",count.subscribe(c => doubled = c * 2),Medium,https://svelte.dev/docs/svelte-store#derived,svelte 5,active,2026-08-13
23,Stores,Use $ prefix for auto-subscription,Automatic subscribe/unsubscribe,$storeName in components,Manual subscription,{$count},count.subscribe(c => value = c),High,https://svelte.dev/docs/svelte/stores,svelte 5,active,2026-08-13
24,Stores,Clean up custom subscriptions,Unsubscribe when component destroys,Return unsubscribe from onMount,Leave subscriptions open,onMount(() => store.subscribe(fn)),store.subscribe(fn) in script,High,https://svelte.dev/docs/svelte/stores,svelte 5,active,2026-08-13
25,Slots,Use slots for legacy composition,Legacy content projection with slot elements,Retain slots while maintaining legacy components,Introduce slot elements in runes-mode components,<slot>Default</slot>,{@render children()},Medium,https://svelte.dev/docs/svelte/legacy-slots,svelte legacy <=4,deprecated,2026-08-13
26,Slots,Use named slots for legacy areas,Legacy composition with multiple slot elements,Retain named slots only in legacy components,Add named slots to new runes-mode code,"<slot name=""header"">",{@render header()},Low,https://svelte.dev/docs/svelte/legacy-slots,svelte legacy <=4,deprecated,2026-08-13
27,Slots,Check slot content with $$slots in legacy mode,Legacy conditional slot rendering,Retain $$slots only in legacy components,Use $$slots in runes mode,"{#if $$slots.footer}<slot name=""footer""/>{/if}","{#if footer}{@render footer()}{/if}",Low,https://svelte.dev/docs/svelte/legacy-$$slots,svelte legacy <=4,deprecated,2026-08-13
28,Styling,Use scoped styles by default,Styles scoped to component,<style> for component styles,Global styles for component,:global() only when needed,<style> all global,Medium,https://svelte.dev/docs/svelte-components#style,svelte 5,active,2026-08-13
29,Styling,Use :global() sparingly,Escape scoping when needed,:global for third-party styling,Global for all styles,:global(.external-lib),<style> without scoping,Medium,,svelte 5,active,2026-08-13
30,Styling,Use CSS variables for theming,Dynamic styling,CSS custom properties,Inline styles for themes,"style=""--color: {color}""","style=""color: {color}""",Low,,svelte 5,active,2026-08-13
31,Transitions,Use built-in transitions,Svelte transition directives,transition:fade for simple effects,Manual CSS transitions,<div transition:fade>,<div class:fade={visible}>,Low,https://svelte.dev/docs/element-directives#transition-fn,svelte 5,active,2026-08-13
32,Transitions,Use in: and out: separately,Different enter/exit animations,in:fly out:fade for asymmetric,Same transition for both,<div in:fly out:fade>,<div transition:fly>,Low,,svelte 5,active,2026-08-13
33,Transitions,Add local modifier,Prevent ancestor trigger,transition:fade|local,Global transitions for lists,<div transition:slide|local>,<div transition:slide>,Medium,,svelte 5,active,2026-08-13
34,Actions,Use actions for DOM behavior,Reusable DOM logic,use:action for DOM enhancements,onMount for each usage,<div use:clickOutside>,onMount(() => setupClickOutside(el)),Medium,https://svelte.dev/docs/element-directives#use-action,svelte 5,active,2026-08-13
35,Actions,Return update and destroy,Lifecycle methods for actions,"Return { update, destroy }",Only initial setup,"return { update(params) {}, destroy() {} }",return destroy only,Medium,,svelte 5,active,2026-08-13
36,Actions,Pass parameters to actions,Configure action behavior,use:action={params},Hardcoded action behavior,<div use:tooltip={options}>,<div use:tooltip>,Low,,svelte 5,active,2026-08-13
37,Logic,Use {#if} for conditionals,Template conditionals,{#if} {:else if} {:else},Ternary in expressions,{#if cond}...{:else}...{/if},{cond ? a : b} for complex,Low,https://svelte.dev/docs/logic-blocks#if,svelte 5,active,2026-08-13
38,Logic,Use {#each} for lists,List rendering,{#each} with key,Map in expression,{#each items as item (item.id)},{items.map(i => `<div>${i}</div>`)},Medium,,svelte 5,active,2026-08-13
39,Logic,Always use keys in {#each},Proper list reconciliation,(item.id) for unique key,Index as key or no key,{#each items as item (item.id)},"{#each items as item, i (i)}",High,https://svelte.dev/docs/svelte/each,svelte 5,active,2026-08-13
40,Logic,Use {#await} for promises,Handle async states,{#await} for loading/error states,Manual promise handling,{#await promise}...{:then}...{:catch},{#if loading}...{#if error},Medium,https://svelte.dev/docs/logic-blocks#await,svelte 5,active,2026-08-13
41,SvelteKit,Use +page.svelte for routes,File-based routing,+page.svelte for route components,Custom routing setup,routes/about/+page.svelte,routes/About.svelte,Medium,https://kit.svelte.dev/docs/routing,svelte 5,active,2026-08-13
42,SvelteKit,Use +page.js for data loading,Load data before render,load function in +page.js,onMount for data fetching,export function load() {},onMount(() => fetchData()),High,https://kit.svelte.dev/docs/load,svelte 5,active,2026-08-13
43,SvelteKit,Use +page.server.js for server-only,Server-side data loading,+page.server.js for sensitive data,+page.js for API keys,+page.server.js with DB access,+page.js with DB access,High,https://svelte.dev/docs/kit/load#universal-vs-server,svelte 5,active,2026-08-13
44,SvelteKit,Use form actions,Server-side form handling,+page.server.js actions,API routes for forms,export const actions = { default },fetch('/api/submit'),Medium,https://kit.svelte.dev/docs/form-actions,svelte 5,active,2026-08-13
45,SvelteKit,Use $app/stores for legacy app state,Legacy SvelteKit page navigating and updated stores,Retain $app/stores only for legacy Svelte projects,Add $app/stores to new SvelteKit code,import { page } from '$app/stores',import { page } from '$app/state',Medium,https://svelte.dev/docs/kit/$app-stores,svelte legacy <=4,deprecated,2026-08-13
46,Performance,Use {#key} for forced re-render,Reset component state,{#key id} for fresh instance,Manual destroy/create,{#key item.id}<Component/>{/key},on:change={() => component = null},Low,https://svelte.dev/docs/logic-blocks#key,svelte 5,active,2026-08-13
47,Performance,Avoid unnecessary effects,Not every computation needs $effect,Use $derived for computed state and $effect only for external synchronization,Use effects for simple assignments,let doubled = $derived(count * 2),$effect(() => doubled = count * 2),Low,https://svelte.dev/docs/svelte/$effect,svelte 5,active,2026-08-13
48,Performance,Avoid legacy immutable compiler assumptions,Runes use fine-grained reactivity without the legacy immutable option,Use current runes state and measure real bottlenecks,Add immutable mode to new runes components,$state for reactive data,<svelte:options immutable/>,Low,https://svelte.dev/docs/svelte/legacy-compiler-options,svelte 5,active,2026-08-13
49,TypeScript,"Use lang=""ts"" in script",TypeScript support,"<script lang=""ts"">",JavaScript for typed projects,"<script lang=""ts"">",<script> with JSDoc,Medium,https://svelte.dev/docs/typescript,svelte 5,active,2026-08-13
50,TypeScript,Type props with an interface,Explicit prop types for $props,Destructure $props with an interface annotation,Use legacy $$Props or untyped props,"interface Props { name: string }; let { name }: Props = $props()",interface $$Props { name: string },Medium,https://svelte.dev/docs/svelte/typescript#typing-$props,svelte 5,active,2026-08-13
51,TypeScript,Type legacy events with createEventDispatcher,Type-safe events in legacy components,Retain typed dispatch only for legacy components,Add dispatcher events to runes-mode components,createEventDispatcher<{ save: Data }>(),let { onsave }: Props = $props(),Medium,https://svelte.dev/docs/svelte/svelte#createeventdispatcher,svelte legacy <=4,deprecated,2026-08-13
52,Accessibility,Use semantic elements,Proper HTML in templates,button nav main appropriately,div for everything,<button onclick={handleClick}>,<div onclick={handleClick}>,High,https://svelte.dev/docs/svelte/compiler-warnings#a11y_click_events_have_key_events,svelte 5,active,2026-08-13
53,Accessibility,Add aria to dynamic content,Accessible state changes,aria-live for updates,Silent dynamic updates,"<div aria-live=""polite"">{message}</div>",<div>{message}</div>,Medium,,svelte 5,active,2026-08-13
54,Events,Use event properties in runes mode,Svelte 5 event handlers are component or element properties,Use onclick and callback props for new code,Use on: directives or createEventDispatcher in runes mode,<button onclick={handleClick}>Save</button>,<button on:click={handleClick}>Save</button>,High,https://svelte.dev/docs/svelte/v5-migration-guide#event-changes,svelte 5,active,2026-08-13
55,SvelteKit,Use $app/state for current app state,SvelteKit exposes page navigating and updated as reactive state,Import current state from $app/state,Start new code with deprecated $app/stores,import { page } from '$app/state',import { page } from '$app/stores',High,https://svelte.dev/docs/kit/$app-state,svelte 5,active,2026-08-13