Migrating from Svelte 4 to Svelte 5: A Comprehensive Guide

Svelte 5 brings significant improvements and changes to the framework, including the new Runes API, better TypeScript support, and enhanced performance. In this guide, we’ll walk through the process of migrating your Svelte 4 application to Svelte 5.

Key Changes in Svelte 5

1. Introduction of Runes

The most significant change in Svelte 5 is the introduction of Runes. Runes are special functions that replace the reactive declarations ($:) and stores ($store) from Svelte 4.

// Svelte 4
let count = 0;
$: doubled = count * 2;

// Svelte 5
let count = $state(0);
let doubled = $derived(count * 2);

2. State Management Changes

The way we handle state has been simplified and made more explicit:

// Svelte 4
let count = 0;
function increment() {
  count += 1;
}

// Svelte 5
let count = $state(0);
function increment() {
  count += 1; // Still works the same way
}

3. Props Declaration

Props are now declared using the props rune:

// Svelte 4
export let name;
export let count = 0;

// Svelte 5
let { name, count = 0 } = $props();

Migration Steps

1. Update Dependencies

First, update your package.json:

{
  "dependencies": {
    "svelte": "^5.0.0"
  }
}

2. Update Configuration

If you’re using SvelteKit, update your svelte.config.js:

const config = {
  compilerOptions: {
    runes: true
  }
};

3. Convert Reactive Declarations

Replace all $: declarations with $derived:

// Before
$: total = items.reduce((sum, item) => sum + item.price, 0);

// After
let total = $derived(items.reduce((sum, item) => sum + item.price, 0));

4. Update Store Usage

The store contract has changed in Svelte 5:

// Svelte 4
import { writable } from 'svelte/store';
const count = writable(0);
// In component: $count

// Svelte 5
import { signal } from 'svelte';
const count = signal(0);
// In component: count.value

Common Pitfalls and Solutions

1. Lifecycle Methods

Lifecycle methods have been simplified:

// Svelte 4
onMount(() => {
  // setup
});

// Svelte 5
$effect(() => {
  // setup
  return () => {
    // cleanup
  };
});

2. Event Handlers

Event handlers remain largely the same, but with better type inference:

// Both versions work similarly
function handleClick(event) {
  console.log('clicked');
}

Testing Your Migration

After migration, thoroughly test your application:

1. Check all reactive updates
2. Verify component props
3. Test event handlers
4. Validate store interactions
5. Review lifecycle-dependent code

Performance Improvements

Svelte 5 brings several performance improvements:

• Smaller bundle sizes
• Faster component initialization
• More efficient updates
• Better memory management

Conclusion

Migrating to Svelte 5 might seem daunting at first, but the benefits are worth it. The new Runes API makes code more explicit and easier to understand, while the performance improvements make your applications faster and more efficient.

Remember to:

• Take the migration step by step
• Test thoroughly after each major change
• Keep your dependencies updated
• Review the official Svelte 5 documentation for detailed information

For more complex applications, consider using the official migration tool (when available) or migrating component by component to ensure a smooth transition.

Resources

• Official Svelte 5 Documentation
• Svelte 5 Migration Guide
• Svelte Discord Community