Svelte 5 Migration Guide

The bundle size improvements are real

• Svelte 5 can cut your JavaScript payload in half compared to Svelte 4.

React apps? Still bloated as hell.

But here's the catch: that smaller bundle isn't automatic.

You'll need to understand what changed and optimize accordingly.

What Actually Changes

Svelte 5 uses signals under the hood, which means:

• Smaller bundles (30-50% reduction possible with proper optimization)
• Faster load times (compile-time optimizations work)
• Less JavaScript to parse on mobile devices

The JS Framework Benchmark shows Svelte 5 beating React by significant margins.

But benchmarks are bullshit if your real app doesn't see the benefits.

The Reality of Bundle Sizes

Your bundle might actually get bigger after upgrading. The signals runtime adds about 3-4KB, and if you're using runes wrong, you'll see size increases.

To actually get smaller bundles:

• Use server load functions to move heavy libraries server-side
• Lazy load components that aren't immediately visible
• Tree shake dependencies properly (most teams don't)
• Replace full library imports with specific functions

Svelte 5 runtime is around 3-4KB, React with ReactDOM is like 42KB gzipped.

Our bundle went from something like 340KB down to around 180KB or so after migrating and moving shit to server load functions. Your mileage will vary based on how much crap you're shipping.

Compilation vs Virtual DOM Bullshit

React's virtual DOM is overhead. Svelte compiles to vanilla JS that directly manipulates the DOM. This matters for:

• Complex dashboards with hundreds of components
• Real-time applications with frequent updates
• Mobile apps where Java

Script parsing kills performance

But Svelte's not magic.

You can still write slow Svelte code, especially if you overuse $effect() where simple reactive statements would work.

The Runes Learning Curve

Runes replace reactive statements ($:).

The mental model is different:

• $state() for reactive variables
• $derived() for computed values
• $effect() for side effects (use sparingly)

I think our team spent like 3 weeks complaining about runes syntax before it clicked.

New developers pick it up faster than React hooks, but existing Svelte developers need time to unlearn 4 years of muscle memory.

When Migration Makes Sense

Don't migrate if:

• You need to hire React developers quickly
• Your bundle size is already optimized and fast enough
• You're using tons of React-specific libraries
• Your team doesn't have 3-6 months for migration

Do migrate if:

• Performance affects your conversion rates
• You're targeting mobile users on slow networks
• Your React app is drowning in state management complexity
• Your team wants to try something that doesn't suck as much

The migration isn't free. Budget for broken things, confused team members, and ecosystem libraries that don't support Svelte 5 yet. We thought it'd take like 2 weeks. It ended up taking 4 months and broke our auth system twice.

So what are your actual options? Let's compare the migration strategies that work.

Those strategies above sound nice on paper, but here's what actually happens when you start migrating.

What the Migration Tool Actually Does

Works fine for:

• Converting simple $: reactive statements to $derived
• Basic component prop destructuring to $props()
• Straightforward store subscriptions
• Simple event handlers

Completely fucks up:

• Complex state management (if you have stores talking to other stores, good luck)
• Custom store implementations
• Lifecycle hooks that depend on each other
• Reactive statements with complex dependencies
• Performance-critical code

Most teams report the tool gets like 80% there, but the other 20% takes way longer than expected. The tool will confidently convert $: reactiveValue = computeStuff() to broken runes that throw Cannot read properties of undefined at runtime.

The Reality of Incremental Migration

You can't just migrate one component at a time without planning. Here's what actually works:

Start with the boring stuff:

• Buttons, icons, form inputs
• Components that don't have state
• Utility components that just render props

Then tackle feature modules:

• Complete pages or sections
• Everything that talks to the same data
• Replace the whole Redux slice at once

Save the scary stuff for last:

• Routing components
• Global state management
• Build configuration changes

This takes like 3-6 months for real apps, not the 2-week sprint your PM wants. The complexity scales with app size and team experience.

Bundle Size Optimization (The Hard Part)

That "smaller bundle" promise? Your bundle might actually get bigger at first. The signals runtime adds around 3-4KB and you'll probably use runes wrong initially. Legacy code patterns are inefficient in Svelte 5.

To actually get smaller bundles:

// Move heavy stuff server-side
// +page.server.js
export async function load() {
  // Process data here instead of client-side
  const result = await heavyDataProcessing(data);
  return { processedData: result };
}

// Lazy load heavy components
import { browser } from '$app/environment';

let HeavyChart;
if (browser && shouldShowChart) {
  HeavyChart = await import('./HeavyChart.svelte');
}

Use rollup-plugin-visualizer to see what's actually in your bundle. Most teams are shocked by how much crap they're shipping.

What Actually Breaks During Migration

Store-to-runes conversion:
The migration tool will convert every store to a rune, which is wrong. Keep stores for:

• Global app state
• Cross-component communication
• Data that needs to persist between route changes

Convert to runes for:

• Component-local state
• Derived values
• Effects and side effects

The $effect() trap:
New Svelte developers overuse $effect(). Most reactive code should use $derived() or simple reactive statements. If you're using $effect() for everything, you're doing it wrong.

Ecosystem libraries:
Half the Svelte ecosystem hasn't updated for Svelte 5 yet. Check compatibility before migrating, or you'll spend days writing wrapper components.

TypeScript configuration:
Svelte 5 breaks if you have strictNullChecks: false in TypeScript. The error message just says "invalid syntax" which helps exactly nobody. We spent like 2 days debugging this bullshit.

Performance Monitoring That Actually Matters

Traditional React debugging tools don't work with Svelte. You need:

Bundle analysis in CI:
Alert when bundles exceed size thresholds. One new dependency can blow up your bundle size.

Core Web Vitals:

• First Contentful Paint under 2 seconds
• Largest Contentful Paint under 3 seconds
• Cumulative Layout Shift near zero

Real user monitoring:
Synthetic tests lie. Monitor actual user performance, especially on mobile networks.

Common Gotchas That Will Bite You

Memory leaks with effects:
$effect() can create memory leaks if you don't clean up properly. Use the cleanup function:

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

Build performance:
Large apps see slower build times. Increase Node memory: --max-old-space-size=4096

Development server crashes:
The dev server crashes like every 20 minutes during migration. Keep npm run dev in a terminal you can restart quickly. HMR breaks on runes syntax changes and requires full restarts. Super fucking annoying.

Success Metrics (Be Realistic)

Don't expect immediate improvements. Performance gains happen gradually as you:

• Optimize bundle splitting
• Replace heavy dependencies
• Fix inefficient reactive patterns
• Update outdated libraries

Measure:

• Bundle size reduction (aim for like 30-50% after optimization)
• Load time improvements on slow networks
• Developer productivity after the learning curve (like 2-3 months)
• Bug rates (should decrease due to compile-time checks)

The migration is worth it if performance matters to your business. If your app is already fast enough and your team is productive, maybe stick with what works.

But when things inevitably break during migration, here are the most common issues teams hit.