svelte-core-bestpractices

title	Best practices
skill	true
name	svelte-core-bestpractices
description	Svelte best practices for writing, editing, or reviewing Svelte/SvelteKit components and modules. Use when touching Svelte reactivity, props, events, snippets, each blocks, styling, context, actions/attachments, stores, or Svelte 5 runes.

Process

1. Identify the Svelte mode before changing code.

   • Check package.json and existing components.
   • If the project already uses Svelte 5 runes, write new code in runes mode.
   • If the touched area is legacy Svelte and a runes migration is not requested, keep the local style consistent.
   • Done when the edit has one clear syntax mode for the touched files.

2. Put reactivity in the narrowest primitive that fits.

   • Local reactive state: $state.
   • Computed value: $derived or $derived.by.
   • Side effect or external subscription: $effect, {@attach ...}, createSubscriber, or an event handler, depending on the trigger.
   • Done when no $effect is being used just to compute state.

3. Keep state scoped.

   • Component-only values stay local.
   • Shared state that must be scoped to a subtree uses context.
   • Shared module state is acceptable only when cross-user leakage is impossible or irrelevant.
   • Done when SSR state cannot leak between users.

4. Review DOM boundaries.

   • Lists use keyed each blocks with stable object identity.
   • Global listeners use <svelte:window> or <svelte:document> when possible.
   • Parent-to-child styling uses CSS custom properties before :global.
   • Done when the DOM update and cleanup paths are explicit.

$state

Use $state only for values that should update a template expression, $derived, or $effect. Normal variables are fine for everything else.

Objects and arrays created with $state({ ... }) or $state([ ... ]) are deeply reactive. Mutation triggers updates, but proxying has overhead. For large values that are only reassigned, such as API responses, use $state.raw. Use $state.snapshot before passing proxy state to an external API or storing a moment-in-time copy.

Rune-enabled .svelte.js/.svelte.ts modules should export factories or objects with getters and named mutation methods. Do not export a rune variable that is reassigned internally, and avoid module-global mutable state when instances or SSR requests need isolation.

$derived

Use $derived for computed values.

let square = $derived(num * num);

Use $derived.by when the computation needs a function body.

let total = $derived.by(() => {
  return items.reduce((sum, item) => sum + item.price, 0);
});

Do not use $effect to assign ordinary computed state.

// avoid
let square;

$effect(() => {
  square = num * num;
});

Derived values are writable, but they re-evaluate when their dependencies change. If a derived expression returns an object or array, Svelte returns it as-is; it is not made deeply reactive.

$effect

Use $effect as an escape hatch for real side effects. Avoid writing Svelte state inside effects.

Choose the tighter tool when one exists:

• External library sync, such as D3: {@attach ...}.
• User-triggered work: the event handler or a function binding.
• Debug logging: $inspect.
• External reactive source: createSubscriber from svelte/reactivity.

Effects do not run on the server, so if (browser) { ... } inside an effect is unnecessary.

$props

Treat props as changing values. Values that depend on props should usually be derived.

let { type } = $props();

let color = $derived(type === 'danger' ? 'red' : 'green');

This stale version should be avoided:

let { type } = $props();
let color = type === 'danger' ? 'red' : 'green';

$bindable

Use $bindable only when a child is intentionally an input-like controller for a parent-owned value. Keep domain operations such as add, save, or delete as callback props so mutation ownership stays visible.

$inspect.trace

Use $inspect.trace(label) to debug reactivity. Put it as the first line of an $effect, $derived.by, or a function called by them. It shows which dependency triggered the update.

Events

In modern Svelte, event listeners are element attributes that start with on.

<button onclick={() => save()}>save</button>
<button {onclick}>save</button>
<button {...props}>save</button>

For global targets, prefer Svelte elements over manual lifecycle wiring.

<svelte:window onkeydown={handleKeydown} />
<svelte:document onvisibilitychange={handleVisibilityChange} />

Snippets

Use snippets for reusable markup that can be rendered with {@render ...} or passed as props.

{#snippet greeting(name)}
  <p>hello {name}!</p>
{/snippet}

{@render greeting('world')}

Declare snippets in the template. Top-level snippets can be referenced from <script>. A snippet that does not read component state can also be exported from <script module>.

Each blocks

Use keyed each blocks for lists with stable item identity.

{#each todos as todo (todo.id)}
  <TodoItem {todo} />
{/each}

The key must uniquely identify the object. Use IDs, not array indexes.

If an item will be mutated with a binding such as bind:value={item.count}, avoid destructuring the item in the each block.

CSS from JavaScript values

Use CSS custom properties through the style: directive.

<div style:--columns={columns}>...</div>

.grid {
  grid-template-columns: repeat(var(--columns), 1fr);
}

Styling child components

Use CSS custom properties as the parent-to-child styling hook.

<!-- Parent.svelte -->
<Child --color="red" />

<!-- Child.svelte -->
<h1>Hello</h1>

<style>
  h1 {
    color: var(--color);
  }
</style>

Use :global only when the child cannot expose a styling hook, such as with some library components.

<div>
  <Child />
</div>

<style>
  div :global {
    h1 {
      color: red;
    }
  }
</style>

Attachments

Prefer {@attach ...} for new reusable DOM behavior. Return cleanup for every global listener or external resource. Existing use: actions remain valid for interoperability, but do not omit their destroy() path.

Error boundaries

Use <svelte:boundary> for local descendant rendering/effect failures and clear the failure condition before calling its reset function. Do not expect it to catch errors thrown directly by event handlers. In SvelteKit, keep route failures distinct: error(...) from load is handled by the nearest +error.svelte.

Context

Use context instead of shared-module state when state should be scoped to a component subtree. This is especially important during server-side rendering, where module state can leak between users.

Prefer createContext over setContext and getContext when possible because it provides type safety.

TypeScript and tests

Type component props, callbacks, snippets, and state collections when a project uses TypeScript, but continue runtime validation at server/API boundaries. Keep verification layers distinct: pure tests for logic, component-browser tests for isolated component contracts, end-to-end tests for complete running-app flows, and builds for bundling evidence.

Async Svelte

Svelte 5.36 and higher can use await expressions and hydratable promises directly inside components. These require experimental.async in svelte.config.js and are not fully stable.

Use async Svelte only when the project has already opted into that experimental mode.

Modern replacements

For new runes-mode code, prefer these replacements:

• $state for reactive state instead of implicit let reactivity.
• $derived for computed values instead of $: assignments.
• $effect for side effects instead of $: statements.
• $props instead of export let, $$props, and $$restProps.
• onclick={...} instead of on:click={...}.
• {#snippet ...} and {@render ...} instead of <slot>, $$slots, and <svelte:fragment>.
• <DynamicComponent> instead of <svelte:component this={DynamicComponent}>.
• import Self from './ThisComponent.svelte' and <Self> instead of <svelte:self>.
• Classes with $state fields for shared reactivity instead of stores when that state is part of a domain object.
• {@attach ...} for new DOM behavior; retain use:action where interoperability requires it.
• Arrays and objects in class attributes instead of class: directives.

Completion check

Before finishing a Svelte edit or review, verify every touched component/module against this list:

• State uses the narrowest reactive primitive.
• Computed values are derived, not effect-assigned.
• Effects are limited to real side effects or external subscriptions.
• Prop-dependent values update when props change.
• Lists with identity are keyed by stable IDs.
• Global listeners have a clear cleanup path or use Svelte global elements.
• Parent-to-child styling has an explicit CSS custom property hook when needed.
• Context is used where SSR-scoped shared state matters.
• Rune modules do not export reassigned state or leak accidental globals.
• Bindings are limited to deliberate input-like shared values.
• Route errors and component error boundaries are not conflated.
• TypeScript checks are paired with runtime validation at untrusted boundaries.
• Tests target the narrowest appropriate layer.
• New code follows the project's Svelte mode.