---

name: svelte-code-writer description: Expert guidance for writing proper Svelte 5 code with runes-based reactivity. Use when writing Svelte 5 components, migrating from Svelte 4, or troubleshooting Svelte 5 syntax. Covers $state, $derived, $effect, $props, $bindable, event handling, snippets, TypeScript integration, and common pitfalls.

Svelte 5 Code Writer

Quick Reference

Svelte 5 uses runes (functions starting with $) for explicit reactivity:

Copy<script lang="ts">
  let count = $state(0); // Reactive state
  let doubled = $derived(count * 2); // Computed value
  let { name, age } = $props(); // Component props

  $effect(() => {
    // Side effects
    console.log(`Count is ${count}`);
    return () => console.log("cleanup");
  });
</script>

<button onclick={() => count++}>
  <!-- Event attributes -->
  {count} × 2 = {doubled}
</button>

Core Workflow

1. Choose the right rune:

   • Computing from state? → Use $derived
   • Managing reactive values? → Use $state
   • Side effects (DOM, network, etc.)? → Use $effect
   • Component props? → Use $props

2. Apply the pattern (see references below for details)

3. Add TypeScript types for props and state

Key Patterns

State: $state

Copy<script lang="ts">
  let count = $state(0); // Primitive
  let user = $state({ name: "Alice" }); // Object (deeply reactive)
  let data = $state.raw(largeDataset); // Non-deeply-reactive (performance)
</script>

Computed Values: $derived

Use $derived for values computed from other state (90% of cases):

Copy<script lang="ts">
  let count = $state(0);
  let doubled = $derived(count * 2); // Simple

  let total = $derived.by(() => {
    // Complex
    return items.reduce((sum, item) => sum + item.value, 0);
  });
</script>

Side Effects: $effect

Use $effect only for side effects, not derivations, try to avoid reassign state in it:

Copy<script lang="ts">
  $effect(() => {
    // DOM manipulation, network calls, subscriptions
    const interval = setInterval(() => console.log(count), 1000);
    return () => clearInterval(interval); // Cleanup
  });
</script>

Props: $props

Copy<script lang="ts">
  interface Props {
    required: string;
    optional?: number;
    callback: (value: string) => void;
  }

  let { required, optional = 42, callback }: Props = $props();
</script>

Two-Way Binding: $bindable

Copy<script lang="ts">
  let { value = $bindable("") } = $props();
</script>

<input bind:value />

Event Handling

Events are properties (not directives):

Copy<button onclick={() => count++}>Click</button>
<button onclick={handleClick}>Click</button>

<!-- Component callbacks -->
<Child onsubmit={(data) => console.log(data)} />

Snippets (Replacing Slots)

Copy<!-- Child -->
<script lang="ts">
  import type { Snippet } from "svelte";

  let {
    header,
    children,
  }: {
    header?: Snippet;
    children?: Snippet;
  } = $props();
</script>

<!-- Parent -->
<Child>
  {#snippet header()}
    <h1>Title</h1>
  {/snippet}

  Default content here
</Child>

{@render header?.()}
{@render children?.()}

Common Pitfalls

❌ Don't synchronize state with $effect:

Copylet doubled = $state(0);
$effect(() => { doubled = count * 2; });  // Wrong!

✅ Use $derived instead:

Copylet doubled = $derived(count * 2); // Correct!

❌ Don't mutate non-bindable props:

Copylet {count} = $props(); count++; // Warning!

✅ Use callbacks or $bindable:

Copylet {(count, onIncrement)} = $props(); onIncrement(); // Correct!

Migration from Svelte 4

Detailed References

For comprehensive details, see:

• RUNES.md - Complete runes reference with examples
• PATTERNS.md - Component patterns and best practices
• PITFALLS.md - Common mistakes and how to fix them
• MIGRATION.md - Detailed migration guide from Svelte 4
• TYPESCRIPT.md - TypeScript patterns and typing

Load these references only when you need detailed information beyond the quick reference above.