Upgrading from Svelte 4 to Svelte 5
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Upgrading from Svelte 4 to Svelte 5 involves a few key steps and some awareness of the changes introduced in Svelte 5, which focuses on the new runes-based reactivity model and improved compiler internals.
Here’s a clear path to upgrading:
🧭 1. Understand the Key Changes in Svelte 5
Before upgrading, know what’s changed:
Runes: New way of declaring reactivity ($state, effect, etc.) instead of $: syntax and let bindings.
Svelte 4 reactivity is still supported (with compatibility mode), but moving to runes is recommended.
Stores are now opt-in and no longer built-in via magic $store syntax unless you enable compatibility.
Some compiler internals have changed, and tooling like preprocessors may need updates.
🛠 2. Check Compatibility of Your Dependencies
Ensure your Svelte dependencies (e.g. svelte, @sveltejs/kit, preprocessors, Vite plugins) support Svelte 5.
Check your SvelteKit version if you're using it – upgrade to the latest version that supports Svelte 5 (currently alpha/beta depending on the release timing).
⚙️ 3. Install Svelte 5
In your project folder, run:
npm install svelte@next
Or if next is no longer pointing to v5:
npm install svelte@5
If using SvelteKit, upgrade:
npm install @sveltejs/kit@next
🧪 4. Enable Compatibility Mode (if needed)
Add a Svelte config if you don’t already have one (svelte.config.js):
import { vitePreprocess } from '@sveltejs/kit/vite';
import { defineConfig } from 'vite';
export default {
preprocess: vitePreprocess(),
compilerOptions: {
// Enables support for classic reactivity like $: and $store
legacy: true,
},
};
You can keep legacy: true during the transition, and remove it when you refactor to use runes.
🔁 5. Gradually Adopt Runes
You can mix classic and rune-based reactivity. Here's an example:
Old Svelte 4 style:
<script>
let count = 0;
$: doubled = count * 2;
</script>
New Svelte 5 with runes:
<script>
import { $state, effect } from 'svelte';
const state = $state({ count: 0, doubled: 0 });
effect(() => {
state.doubled = state.count * 2;
});
</script>
✅ 6. Test Everything Thoroughly
Re-run unit tests if you have them.
Manually test UI components and reactivity.
Watch for runtime warnings or errors related to deprecated features.
🚀 7. Refactor and Remove Legacy Mode
Once everything works and you’ve converted most components to runes, remove legacy: true and ensure the app still functions.
🧰 Optional Tools & Help
Svelte REPL (v5) to test runes.
Svelte Discord for real-time help.
Upgrade helper: [https://svelte.dev/docs/upgrade-guide
](https://svelte.dev/docs/upgrade-guide)
Final Tip:
Start small – upgrade a test branch, convert one component at a time, and keep commits atomic.
Would you like a checklist or a sample conversion of one of your components to Svelte 5 runes?
