Upgrading from Svelte 4 to Svelte 5 Runes

Writing

Writing

Writing

Introduction

Introduction

What Are Runes?

What Are Runes?

Key Differences from Svelte 4

Key Differences from Svelte 4

Migration Guide

Migration Guide

Common Migration Patterns

Common Migration Patterns

Best Practices for Runes Mode

Best Practices for Runes Mode

Performance Benefits

Performance Benefits

Conclusion

Conclusion

Aug 22, 2025

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Introduction

I recently migrated a large codebase from Svelte 4 to Svelte 5 and decided to share my learnings to ease familiarity with the new syntax. The migration was both challenging and rewarding, revealing just how much more explicit and predictable Svelte 5’s approach to reactivity really is.

Svelte 5 introduces a revolutionary new way of handling reactivity through runes which is a more explicit and predictable system that replaces the implicit reactivity of previous versions. While the transition might seem daunting at first, runes offer better performance, clearer mental models, and more consistent behavior across your application.

In this guide, we’ll walk through everything you need to know to successfully upgrade your Svelte 4 projects to Svelte 5 runes mode.

What Are Runes?

Runes are Svelte 5’s new reactive primitives that make state management explicit and predictable. Instead of relying on assignments and $: labels for reactivity, runes provide clear functions that declare reactive state, derived values, and effects.

The core runes include:

  • $state() - for reactive state

  • $derived() - for computed values

  • $effect() - for side effects

  • $props() - for component props

  • $bindable() - for two-way binding

Key Differences from Svelte 4

State Management

Svelte 4:

let count = 0;let doubled = count * 2; // Not reactive!// Reactive statement needed$: doubled = count * 2;

Svelte 5 Runes:

let count = $state(0);let doubled = $derived(count * 2); // Always reactive

Component Props

Svelte 4:

export let title;export let count = 0;

Svelte 5 Runes:

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

Effects and Side Effects

Svelte 4:

$: {  console.log('Count changed:', count);}

Svelte 5 Runes:

$effect(() => {  console.log('Count changed:', count);});

Migration Guide

For the complete step-by-step migration process, including automated migration tools and detailed conversion instructions, follow the official Svelte 5 Migration Guide. The official documentation provides:

  • Automated migration tools to handle much of the conversion

  • Comprehensive breaking changes list

  • Detailed migration steps for each feature

  • Troubleshooting common migration issues

Quick Overview of the Migration Process:

  1. Use the automated migrator: Svelte provides tools to automatically convert most of your code

  2. Update dependencies: Upgrade to Svelte 5 and compatible versions of related packages

  3. Enable runes mode: Configure your project to use the new reactivity system

  4. Handle manual conversions: Address any remaining code that needs manual updating

  5. Test thoroughly: Ensure all functionality works as expected after migration

The official guide is regularly updated with the latest migration strategies and handles edge cases that might not be covered in third-party tutorials.

Common Migration Patterns

Event Handlers

Event handler syntax has changed from on: to direct properties:

Svelte 4:

<button on:click={increment}>Click me</button><input on:input={handleInput} on:keydown={handleKeydown}

Svelte 5:

<button onclick={increment}>Click me</button><input oninput={handleInput} onkeydown={handleKeydown}

Event handlers can now directly modify state:

let count = $state(0);function increment() {  count++; // Direct mutation works with $state}

Lifecycle Functions

Lifecycle functions like onMount still work, but many use cases can be replaced with $effect():

Before:

import { onMount } from 'svelte';onMount(() => {  // Setup code  return () => {    // Cleanup code  };});

After (when you need reactivity):

$effect(() => {  // Setup code that runs when dependencies change  return () => {    // Cleanup code  };});

Best Practices for Runes Mode

Keep State Local When Possible

// Good: Local statelet count = $state(0);// Better for shared state: Move to separate .svelte.js file

Use Derived State for Computations

let items = $state([]);let completedItems = $derived(items.filter(item => item.completed));let progress = $derived(completedItems.length / items.length);

Be Careful with Object Mutations

let user = $state({ name: 'John', age: 30 });// This works and triggers reactivityuser.age = 31;// This also worksuser = { ...user, age: 31 };

Use Effects Sparingly

Only use $effect() for genuine side effects like DOM manipulation, API calls, or localStorage updates.

Common Gotchas and Solutions

Arrays and Objects

Runes work with mutations, but be consistent:

let items = $state([]);// All of these work:items.push(newItem);           // Direct mutationitems = [...items, newItem];   // Replacementitems[0] = updatedItem;        // Index assignment

Async Effects

Use $effect with async functions carefully - they can easily cause infinite loops:

let items = $state([]);$effect(() => {  async function fetchData() {    const response = await fetch('/api/data');    const data = await response.json();    items = data; // This triggers the effect again!  }    fetchData();});
// Use $effect.pre to avoid dependency on items$effect.pre(() => {  async function fetchData() {    const response = await fetch('/api/data');    const data = await response.json();    items = data;  }    fetchData();});

Cleanup Effects

Always clean up subscriptions and intervals:

$effect(() => {  const interval = setInterval(() => {    count++;  }, 1000);    return () => clearInterval(interval);});

Performance Benefits

Svelte 5 runes provide several performance improvements:

  • More predictable reactivity: No more wondering if something will trigger updates

  • Better tree-shaking: Unused reactive code is eliminated more effectively

  • Reduced bundle size: The new reactivity system is more efficient

  • Improved debugging: Clearer reactive dependencies make issues easier to track


Conclusion

Migrating from Svelte 4 to Svelte 5 runes mode represents a significant shift in how we think about reactivity in Svelte applications. While it requires updating your mental model and refactoring existing code, the benefits are substantial:

  • More explicit and predictable reactivity

  • Better performance and smaller bundle sizes

  • Clearer separation between state, derived values, and effects

  • More consistent behavior across different scenarios

The migration process, while requiring careful attention, is straightforward when following the patterns outlined in this guide. Start with smaller components and gradually work your way through your application, testing thoroughly at each step.

Svelte 5’s runes mode sets the framework up for future enhancements while providing a more robust foundation for building reactive user interfaces. The initial investment in migration will pay dividends in maintainability, performance, and developer experience.

Thanks for reading!