Migrating to Svelte 5 can be done incrementally. Here's an overview of the process.
Migration Phases
┌─────────────────────────────────────────────────────────┐
│ PHASE 1: Preparation │
│ • Update dependencies (svelte@5, sveltekit@2) │
│ • Run the migration script │
│ • Fix any breaking changes │
└───────────────────────────┬─────────────────────────────┘
↓
┌─────────────────────────────────────────────────────────┐
│ PHASE 2: Get Running │
│ • App works in "legacy mode" │
│ • Old syntax still supported │
│ • No Runes yet │
└───────────────────────────┬─────────────────────────────┘
↓
┌─────────────────────────────────────────────────────────┐
│ PHASE 3: Incremental Adoption │
│ • Migrate components one by one to Runes │
│ • Convert events, slots, stores │
│ • Test thoroughly │
└───────────────────────────┬─────────────────────────────┘
↓
┌─────────────────────────────────────────────────────────┐
│ PHASE 4: Full Runes │
│ • All components use Runes │
│ • Legacy imports removed │
│ • Modern Svelte 5 codebase │
└─────────────────────────────────────────────────────────┘
What Changes in Svelte 5?
| Old (Svelte 4) | New (Svelte 5) |
|---|---|
let count = 0 (magic reactivity) | let count = $state(0) |
$: doubled = count * 2 | let doubled = $derived(count * 2) |
$: { console.log(count) } | $effect(() => { console.log(count) }) |
export let name | let { name } = $props() |
on:click | onclick |
<slot /> | {@render children()} |
createEventDispatcher() | Callback props |
$$restProps | let { ...rest } = $props() |
Good News: Backward Compatibility
Svelte 5 supports "legacy mode" - your Svelte 4 code still works! You can migrate gradually.