Svelte 5 Migration Guide: From Implicit Reactivity to Explicit Runes

// Counter.svelte
<script lang="ts">
    // Explicit reactive state
    let count = $state(0);
    let user = $state({ name: 'Alice', role: 'admin' });

    // Deep mutation triggers update automatically
    function updateUser() {
        user.name = 'Bob';
    }

    // Reassignment updates reference
    function resetCount() {
        count = 0;
    }
</script>

<button onclick={resetCount}>
    Count: {count}
</button>
<button onclick={updateUser}>
    Update User
</button>

<script lang="ts">
    let items = $state([1, 2, 3, 4, 5]);
    
    // Computed lazily; updates only when 'items' changes
    let total = $derived(items.reduce((sum, n) => sum + n, 0));
    let hasItems = $derived(items.length > 0);

    // Derived can depend on other derived values
    let formattedTotal = $derived(`Total: ${total}`);
</script>

<p>{formattedTotal}</p>
<p>{hasItems ? 'List active' : 'Empty'}</p>

ions ($:). It enforces purity; derived functions should not have side effects.

3. $effect: Side Effects

$effect runs code when dependencies change, after DOM updates. It replaces $: statements used for side effects. Effects automatically track dependencies and provide cleanup functions.

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

    // Runs after DOM update when 'count' changes
    $effect(() => {
        console.log(`Count changed to ${count}`);
        
        // Cleanup function runs before next effect execution
        return () => {
            console.log('Cleaning up previous effect');
        };
    });

    // Effect with dependencies array (optional for explicit control)
    $effect(() => {
        // Runs once on mount and when count changes
    }, [count]);
</script>

Key Behavior: $effect does not run during server-side rendering. It is strictly for client-side side effects.

4. $props: Component Interface

In Svelte 5, props must be declared using $props. Destructuring is mandatory to access reactive props. If you do not destructure, you receive the raw props object, which is not reactive.

<script lang="ts">
    // Destructuring is required for reactivity
    interface Props {
        title: string;
        count?: number;
        onToggle?: () => void;
    }

    let { 
        title, 
        count = 0, 
        onToggle 
    }: Props = $props();

    // $bindable enables two-way binding on props
    let { value = $bindable('') }: { value?: string } = $props();
</script>

<h1>{title}</h1>
<p>{count}</p>
<input bind:value />

Critical Change: Svelte 5 removes the export let syntax. All prop declarations use $props. Default values are handled via destructuring defaults.

5. $inspect: Reactive Debugging

$inspect provides reactive debugging. It logs values whenever they change, similar to console.log but integrated with the reactivity system.

<script lang="ts">
    let count = $state(0);
    
    // Logs count whenever it changes
    $inspect(count);
    
    // Inspects deep changes with options
    $inspect(count, { deep: true });
</script>

6. Module Context and Shared State

Runes can be used in <script module> blocks to create shared state across component instances. This replaces many use cases for Svelte stores.

<script module lang="ts">
    // Shared state across all instances
    let globalCount = $state(0);

    export function incrementGlobal() {
        globalCount++;
    }
</script>

<script lang="ts">
    // Component instance can access module state
    console.log(globalCount);
</script>

Pitfall Guide

1. Forgetting to Destructure Props:

   • Mistake: Using let props = $props(); and accessing props.title.
   • Result: props is a reactive proxy, but accessing properties via dot notation does not trigger reactivity in templates or derived values.
   • Fix: Always destructure: let { title } = $props();.

2. Infinite Loops in $effect:

   • Mistake: Writing to a state variable inside an $effect that reads the same variable.
   • Result: Infinite update loop.
   • Fix: Ensure effects only write to state that they do not read, or use guards to prevent cycles.

3. Misusing $derived for Side Effects:

   • Mistake: Performing API calls or mutations inside $derived.
   • Result: Derived values are lazy and may not run when expected, or run multiple times.
   • Fix: Use $effect for side effects; $derived is strictly for pure computations.

4. Assuming $state is Shallow:

   • Mistake: Expecting $state to behave like useState in React.
   • Result: Mutating nested objects updates the UI, which may be unintended or cause performance issues with large objects.
   • Fix: Use $state.raw for large, immutable objects where deep reactivity is unnecessary.

5. Using Runes Outside Valid Context:

   • Mistake: Calling runes inside regular functions or event handlers.
   • Result: Compiler error or runtime failure. Runes are only valid in component script top-level or rune functions.
   • Fix: Encapsulate rune logic in $state, $derived, or $effect at the top level.

6. Store Migration Errors:

   • Mistake: Attempting to use Svelte 4 store syntax ($store) with Svelte 5 runes.
   • Result: Runtime errors or stale data.
   • Fix: Migrate stores to $state in module context or use svelte/store interop carefully. Prefer runes for new state management.

7. Binding Syntax Confusion:

   • Mistake: Using bind:prop without $bindable in child components.
   • Result: Two-way binding fails silently or throws errors.
   • Fix: Mark props as $bindable in the child component when using bind:prop in the parent.

Production Bundle

Action Checklist

• Audit $: Statements: Replace all reactive statements with $state, $derived, or $effect as appropriate.
• Update Prop Declarations: Convert export let to $props with destructuring; add $bindable where needed.
• Migrate Stores: Replace simple stores with $state in module context; refactor complex stores to rune-based logic.
• Verify $effect Dependencies: Check for potential infinite loops and ensure cleanup functions are implemented.
• Optimize with $state.raw: Identify large objects and apply $state.raw to reduce proxy overhead.
• Run svelte-migrate: Execute the migration tool to automate syntax updates, then review changes manually.
• Test Two-Way Bindings: Ensure bind:value and custom bindings work correctly with $bindable props.
• Update TypeScript Types: Adjust type definitions to reflect rune usage and prop interfaces.

Decision Matrix

Scenario	Recommended Approach	Why	Cost Impact
Local Component State	$state	Simple, explicit, no boilerplate	Low
Computed Values	$derived	Lazy evaluation, automatic tracking	Low
Side Effects	$effect	Runs after DOM update, cleanup support	Medium
Shared State (Simple)	$state in <script module>	Replaces stores, type-safe	Low
Shared State (Complex)	Svelte Store or External Library	Advanced patterns, SSR support	Medium
Large Immutable Data	$state.raw	Reduces proxy overhead, better performance	Low
Two-Way Binding	$props with $bindable	Explicit opt-in, safer than default	Low

Configuration Template

svelte.config.js

import adapter from '@sveltejs/adapter-auto';
import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';

/** @type {import('@sveltejs/kit').Config} */
const config = {
    preprocess: vitePreprocess(),
    kit: {
        adapter: adapter()
    },
    compilerOptions: {
        runes: true // Enable runes mode explicitly
    }
};

export default config;

vite.config.ts

import { sveltekit } from '@sveltejs/kit/vite';
import { defineConfig } from 'vite';

export default defineConfig({
    plugins: [sveltekit()],
    build: {
        target: 'es2020',
        sourcemap: true
    }
});

Quick Start Guide

1. Create Project:

   npm create svelte@latest my-svelte5-app
   cd my-svelte5-app
   npm install
   

2. Verify Runes Mode: Ensure svelte.config.js includes runes: true in compiler options.

3. Create Reactive Component: Create src/routes/+page.svelte:

   <script lang="ts">
       let count = $state(0);
       let doubled = $derived(count * 2);
   
       $effect(() => {
           console.log(`Count is now ${count}`);
       });
   </script>
   
   <h1>Svelte 5 Runes</h1>
   <p>Count: {count}</p>
   <p>Doubled: {doubled}</p>
   <button onclick={() => count++}>Increment</button>
   

4. Run Development Server:

   npm run dev
   

5. Inspect Output: Verify console logs update on increment and UI reflects derived values. Check network tab for bundle size improvements.

Svelte 5 runes represent a mature evolution of Svelte's reactivity model. By embracing explicit state management, developers gain predictable behavior, improved performance, and a robust foundation for building scalable applications. The migration requires attention to prop handling and effect management, but the long-term benefits in maintainability and bundle efficiency justify the investment.