web-framework-svelte

Svelte 5 Patterns

Quick Guide: Svelte 5 uses Runes for explicit reactivity. Use $state for reactive variables, $derived for computed values, $effect only as an escape hatch. Use snippets instead of slots. Use callback props instead of event dispatchers. Keep components small and composable.

---

<critical_requirements>

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST use Svelte 5 Runes syntax — NOT Svelte 4 patterns like export let, $:, or stores for component state)

(You MUST use $derived for computed values — NEVER use $effect to synchronize state)

(You MUST use snippets ({#snippet} / {@render}) instead of slots (<slot>))

(You MUST use callback props (onclick, onsomething) instead of createEventDispatcher)

(You MUST use $state.raw() for large objects/arrays that are replaced, not mutated)

(You MUST use createContext for type-safe context instead of raw setContext/getContext with string keys)

</critical_requirements>

---

Auto-detection: Svelte 5, Runes, $state, $derived, $effect, $props, $bindable, $inspect, .svelte, snippet, @render, createContext, getContext, setContext, $state.raw, $state.eager, $derived.by, $effect.pre, ClassValue

When to use:

• Building Svelte 5 components with Runes reactivity
• Managing component state with $state and computed values with $derived
• Creating reusable markup with snippets (replacing slots)
• Handling events with native event attributes and callback props
• Sharing state across components with context API
• Two-way binding with $bindable props

Key patterns covered:

• Runes: $state, $derived, $effect, $props, $bindable, $inspect
• Component composition with snippets and {@render}
• Event handling with native attributes and callback props
• Context API with createContext for type-safe cross-component state
• Class-based reactive state with $state fields
• Deep vs shallow reactivity ($state vs $state.raw)

When NOT to use:

• Meta-framework-specific patterns (routing, load functions, form actions) — use the corresponding meta-framework skill
• Svelte 4 patterns (export let, $: reactive statements, <slot>, createEventDispatcher)
• Server-side logic (use your meta-framework's server hooks and routes)

Detailed Resources:

• For decision frameworks and anti-patterns, see reference.md

Runes & Reactivity:

• examples/core.md - $state, $derived, $effect, $props, $bindable, component patterns

Component Patterns:

• examples/snippets.md - Snippet blocks, {@render}, passing snippets as props, replacing slots
• examples/events.md - Event handling, component events via callback props, event modifiers

Advanced:

• examples/advanced.md - $inspect, context API, $state.raw, $state.eager, class-based state, shared state modules

---

Philosophy

Svelte 5 introduces Runes — a set of primitives that bring explicit, fine-grained reactivity to Svelte. Unlike Svelte 4's compiler magic ($:, export let), Runes make reactivity visible and portable across .svelte files, .ts files, and class definitions.

Core principles:

1. Explicit reactivity — Runes ($state, $derived, $effect) make reactive declarations visible. No hidden compiler transformations.
2. Derived over effects — Compute values with $derived, not $effect. Effects are escape hatches, not primary tools.
3. Deep reactivity by default — $state creates deeply reactive proxies for objects/arrays. Mutations are tracked automatically.
4. Snippets replace slots — {#snippet} blocks are more powerful, typed, and composable than <slot> elements.
5. Callback props replace event dispatchers — Pass onsomething callback props instead of using createEventDispatcher.
6. Compile-time optimization — Svelte compiles components to efficient imperative code. No virtual DOM diffing at runtime.

When to use Svelte 5 Runes:

• All new Svelte components (Runes are the default in Svelte 5)
• Reactive state in .svelte.ts or .svelte.js files
• Class-based state with reactive fields
• Any computed value that depends on reactive state

When NOT to use:

• Non-reactive constants (use plain const or let)
• Server-side code that doesn't need reactivity
• Meta-framework concerns (routing, load functions, server hooks) — use the corresponding meta-framework skill
• Svelte 4 patterns — export let, $:, stores for component state, <slot>, createEventDispatcher

---

Core Patterns

Pattern 1: Reactive State with $state

Use $state to declare reactive variables. Updates to $state variables automatically trigger UI re-renders.

<!-- counter.svelte -->
<script lang="ts">
  let count = $state(0);
  const STEP = 5;

  function increment() {
    count += 1;
  }

  function incrementByStep() {
    count += STEP;
  }
</script>

<button onclick={increment}>
  Count: {count}
</button>
<button onclick={incrementByStep}>
  +{STEP}
</button>

Why good: Explicit reactive declaration, named constants for magic numbers, plain function event handlers

<!-- BAD: Svelte 4 style -->
<script>
  let count = 0; // Not explicitly reactive in Svelte 5 mode
  $: doubled = count * 2; // Svelte 4 reactive statement
</script>

Why bad: $: is Svelte 4 syntax deprecated in Svelte 5, implicit reactivity is confusing and non-portable

Deep Reactivity

$state creates deep proxies for objects and arrays — mutations are tracked automatically:

<script lang="ts">
  interface Todo {
    done: boolean;
    text: string;
  }

  let todos = $state<Todo[]>([
    { done: false, text: 'Learn Svelte 5' }
  ]);

  function addTodo(text: string) {
    todos.push({ done: false, text }); // Mutation tracked!
  }

  function toggleTodo(index: number) {
    todos[index].done = !todos[index].done; // Deep mutation tracked!
  }
</script>

Why good: No need for immutable update patterns, array methods like .push() trigger reactivity, property mutations tracked deeply

---

Pattern 2: Computed Values with $derived

Use $derived for values that depend on other reactive state. Never use $effect to synchronize state.

<script lang="ts">
  let count = $state(0);

  // Simple expression
  let doubled = $derived(count * 2);

  // Complex computation with $derived.by
  let stats = $derived.by(() => {
    const isEven = count % 2 === 0;
    const isPositive = count > 0;
    return { isEven, isPositive };
  });
</script>

<p>{count} doubled is {doubled}</p>
<p>Even: {stats.isEven}, Positive: {stats.isPositive}</p>

Why good: Automatically recalculates when dependencies change, no side effects, push-pull reactivity avoids unnecessary recalculations

<!-- BAD: Using $effect to synchronize state -->
<script lang="ts">
  let count = $state(0);
  let doubled = $state(0);

  $effect(() => {
    doubled = count * 2; // WRONG: Use $derived instead
  });
</script>

Why bad: $effect for derived state creates unnecessary reactive subscriptions, runs after DOM update (timing issues), harder to reason about data flow

---

Pattern 3: Component Props with $props

Use $props to declare component inputs. Supports destructuring, defaults, rest props, and TypeScript.

<!-- user-card.svelte -->
<script lang="ts">
  interface Props {
    name: string;
    email: string;
    role?: string;
    class?: string;
  }

  let { name, email, role = 'member', ...rest }: Props = $props();

  // Derived from props — updates when props change
  let initials = $derived(
    name.split(' ').map(n => n[0]).join('').toUpperCase()
  );
</script>

<div class="user-card" {...rest}>
  <span class="avatar">{initials}</span>
  <h3>{name}</h3>
  <p>{email}</p>
  <span class="badge">{role}</span>
</div>

Why good: Type-safe props with interface, destructuring with defaults, rest props for pass-through, derived values update with prop changes

<!-- BAD: Svelte 4 style -->
<script>
  export let name; // Svelte 4 prop declaration
  export let email;
  export let role = 'member';
</script>

Why bad: export let is Svelte 4 syntax deprecated in Svelte 5, no type safety, no rest props

---

Pattern 4: Two-Way Binding with $bindable

Use $bindable to declare props that support two-way binding with bind:. Use sparingly — prefer one-way data flow.

<!-- text-input.svelte -->
<script lang="ts">
  interface Props {
    value: string;
    placeholder?: string;
  }

  let { value = $bindable(''), placeholder = '' }: Props = $props();
</script>

<input
  bind:value={value}
  {placeholder}
  class="text-input"
/>

<!-- parent.svelte -->
<script lang="ts">
  import TextInput from './text-input.svelte';

  let searchQuery = $state('');
</script>

<TextInput bind:value={searchQuery} placeholder="Search..." />
<p>Searching for: {searchQuery}</p>

Why good: Explicit two-way binding declaration, parent controls the state, child can modify via bind:, TypeScript-safe

When to use: Form inputs, UI primitives (sliders, toggles) where two-way binding simplifies the API

When not to use: Most component communication — prefer callback props for explicit data flow

---

Pattern 5: Side Effects with $effect

Use $effect for side effects that need to run when reactive state changes. This is an escape hatch — prefer $derived for computed values and event handlers for user-triggered actions.

<script lang="ts">
  let searchQuery = $state('');
  let results = $state<string[]>([]);
  const DEBOUNCE_MS = 300;

  // Good: Side effect for external API calls
  $effect(() => {
    const query = searchQuery;

    if (!query) {
      results = [];
      return;
    }

    const timer = setTimeout(async () => {
      const response = await fetch(`/api/search?q=${encodeURIComponent(query)}`);
      results = await response.json();
    }, DEBOUNCE_MS);

    // Cleanup function runs before next effect and on unmount
    return () => clearTimeout(timer);
  });
</script>

<input bind:value={searchQuery} placeholder="Search..." />

{#each results as result}
  <p>{result}</p>
{/each}

Why good: External API call is a legitimate side effect, cleanup prevents stale requests, named constant for debounce

When NOT to Use $effect

<script lang="ts">
  let count = $state(0);

  // BAD: Synchronizing state — use $derived
  // $effect(() => { doubled = count * 2; });

  // BAD: Logging in effect — use $inspect for debugging
  // $effect(() => { console.log(count); });

  // BAD: Calling functions on change — use event handlers
  // $effect(() => { if (count > 10) showAlert(); });

  // GOOD: Use $derived for computed values
  let doubled = $derived(count * 2);
</script>

---

Pattern 6: Snippets (Replacing Slots)

Snippets are reusable markup blocks declared with {#snippet} and rendered with {@render}. They replace Svelte 4's <slot> elements.

<!-- card.svelte -->
<script lang="ts">
  import type { Snippet } from 'svelte';

  interface Props {
    title: string;
    children: Snippet;
    footer?: Snippet;
  }

  let { title, children, footer }: Props = $props();
</script>

<div class="card">
  <h2>{title}</h2>
  <div class="card-body">
    {@render children()}
  </div>
  {#if footer}
    <div class="card-footer">
      {@render footer()}
    </div>
  {/if}
</div>

<!-- usage -->
<script lang="ts">
  import Card from './card.svelte';
</script>

<Card title="Welcome">
  <p>This becomes the children snippet automatically.</p>

  {#snippet footer()}
    <button>Learn More</button>
  {/snippet}
</Card>

Why good: Type-safe with Snippet type, optional snippets with conditional rendering, children is implicit for content between tags

<!-- BAD: Svelte 4 slots -->
<div class="card">
  <slot /> <!-- Deprecated in Svelte 5 -->
  <slot name="footer" /> <!-- Use snippets instead -->
</div>

Why bad: <slot> is deprecated in Svelte 5, no type safety, less composable than snippets

---

Pattern 7: Event Handling

Svelte 5 uses native event attributes (onclick, onsubmit) instead of Svelte 4's on:click directive. Component events use callback props.

Element Events

<script lang="ts">
  let count = $state(0);

  function handleClick(event: MouseEvent) {
    count += 1;
  }

  function handleSubmit(event: SubmitEvent) {
    event.preventDefault();
    // handle form
  }
</script>

<button onclick={handleClick}>Clicked {count} times</button>

<!-- Inline handlers are fine for simple logic -->
<button onclick={() => count = 0}>Reset</button>

<form onsubmit={handleSubmit}>
  <input name="query" />
  <button type="submit">Search</button>
</form>

Component Events via Callback Props

<!-- color-picker.svelte -->
<script lang="ts">
  interface Props {
    color: string;
    onchange?: (color: string) => void;
    onreset?: () => void;
  }

  let { color, onchange, onreset }: Props = $props();

  const COLORS = ['red', 'green', 'blue', 'purple'] as const;
</script>

{#each COLORS as c}
  <button
    onclick={() => onchange?.(c)}
    class={{ selected: color === c }}
  >
    {c}
  </button>
{/each}

{#if onreset}
  <button onclick={onreset}>Reset</button>
{/if}

<!-- parent.svelte -->
<script lang="ts">
  import ColorPicker from './color-picker.svelte';

  let selectedColor = $state('red');
</script>

<ColorPicker
  color={selectedColor}
  onchange={(c) => selectedColor = c}
  onreset={() => selectedColor = 'red'}
/>

Why good: Type-safe callback props, optional with ?. call, parent controls event handling, no indirection through dispatcher

<!-- BAD: Svelte 4 event dispatcher -->
<script>
  import { createEventDispatcher } from 'svelte';
  const dispatch = createEventDispatcher();

  function handleClick() {
    dispatch('change', { color: 'red' }); // Deprecated pattern
  }
</script>

Why bad: createEventDispatcher is deprecated in Svelte 5, no type safety, requires manual event typing

---

Integration Guide

Styling integration:

• Scoped <style> blocks are the default — styles don't leak to other components
• Use :global() for global styles or CSS custom properties for parent-to-child styling
• Any CSS approach (CSS Modules, utility-first, preprocessors) works with Svelte

State management:

• $state for component-local state
• Context API (createContext) for subtree-scoped state
• Reactive classes with $state fields for shared state modules (.svelte.ts)
• Meta-framework load functions for server state

TypeScript integration:

• Full TypeScript support in <script lang="ts"> blocks
• Snippet<[ParamType]> for typed snippet props
• Interface-based prop typing with $props()
• ClassValue type from svelte/elements for type-safe class props (Svelte 5.19+)

---

<red_flags>

RED FLAGS

High Priority:

• Using export let for props — use $props() instead
• Using $: reactive statements — use $derived or $effect
• Using <slot> or <slot name="x"> — use {#snippet} and {@render}
• Using createEventDispatcher — use callback props
• Using $effect to sync state — use $derived for computed values
• Destructuring $state objects — breaks reactivity (values captured at destructure time)

Medium Priority:

• Using on:click directive — use onclick attribute
• Not using $state.raw() for large API responses — unnecessary proxy overhead
• Using setContext/getContext with string keys — use createContext for type safety
• Using class:name={condition} — use built-in class attribute object/array syntax (since 5.16)

Gotchas:

• $state proxies are not the original object — use $state.snapshot() to get a plain copy
• Destructuring $state captures values, not references — access properties directly instead
• $derived return values are NOT deeply reactive — only $state creates deep proxies
• $effect runs after DOM update — use $effect.pre() for pre-update timing
• Dependencies after await in $effect are not tracked
• Context must be set during component init — cannot call setContext in event handlers or $effect

For complete decision frameworks and the full anti-patterns list, see reference.md.

</red_flags>

---

<critical_reminders>

CRITICAL REMINDERS

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST use Svelte 5 Runes syntax — NOT Svelte 4 patterns like export let, $:, or stores for component state)

(You MUST use $derived for computed values — NEVER use $effect to synchronize state)

(You MUST use snippets ({#snippet} / {@render}) instead of slots (<slot>))

(You MUST use callback props (onclick, onsomething) instead of createEventDispatcher)

(You MUST use $state.raw() for large objects/arrays that are replaced, not mutated)

(You MUST use createContext for type-safe context instead of raw setContext/getContext with string keys)

Failure to follow these rules will produce outdated Svelte 4 code that is deprecated and will break in future versions.

</critical_reminders>