Svelte 5 Migration: Technical Reference

Performance Benefits

Bundle Size Reductions

• Potential: 30-50% reduction with proper optimization
• Reality: Initial bundle may increase by 3-4KB (signals runtime)
• Actual case: 340KB → 180KB after migration + server-side optimization
• Comparison: React + ReactDOM = 42KB gzipped vs Svelte 5 = 3-4KB

Performance Thresholds

• UI Breaking Point: 1000+ spans makes distributed transaction debugging impossible
• Mobile Impact: Significant on slow networks due to reduced JavaScript parsing
• Compilation Advantage: Direct DOM manipulation vs React's virtual DOM overhead

Migration Strategies

Approach	Success Rate	Time Investment	Failure Modes
Auto-migration tool	80% conversion	1-2 days + weeks fixing	Breaks stores, complex state, lifecycle hooks
Manual component-by-component	High	1-2 weeks large apps	Requires patience, planning
Incremental by feature	High	2-6 months	Coordination complexity
Full rewrite	Variable	1-3 months	Everything breaks simultaneously

Critical Failure Scenarios

Auto-Migration Tool Limitations

Handles Successfully:

• Simple $: reactive statements → $derived
• Basic prop destructuring → $props()
• Straightforward store subscriptions
• Simple event handlers

Fails On:

• Complex state management (store-to-store communication)
• Custom store implementations
• Interdependent lifecycle hooks
• Performance-critical reactive statements

Common Breaking Points

1. Store Conversion Error: Tool converts all stores to runes (incorrect)
2. TypeScript Configuration: Fails with strictNullChecks: false
3. Development Server: Crashes every 20 minutes during migration
4. HMR Breakage: Hot module reload fails on runes syntax changes
5. Memory Leaks: $effect() without proper cleanup

Resource Requirements

Time Investment Reality

• Marketing Claim: 2-week sprint
• Actual Duration: 3-6 months for production apps
• Team Learning Curve: 3 weeks complaints + 2-4 weeks adaptation
• Stability Period: 2-3 months post-migration

Expertise Requirements

• Understanding of signals-based reactivity model
• Bundle optimization knowledge
• Server-side load function implementation
• Component lazy loading patterns

Configuration That Works

Bundle Optimization

// Server-side data processing
// +page.server.js
export async function load() {
  const result = await heavyDataProcessing(data);
  return { processedData: result };
}

// Lazy loading heavy components
import { browser } from '$app/environment';
let HeavyChart;
if (browser && shouldShowChart) {
  HeavyChart = await import('./HeavyChart.svelte');
}

Memory Leak Prevention

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

Build Performance

• Node memory: --max-old-space-size=4096
• TypeScript requirement: 5.0+
• Bundle analysis: rollup-plugin-visualizer

Decision Criteria

Migrate When:

• Performance affects conversion rates
• Targeting mobile users on slow networks
• React state management complexity is excessive
• Team has 3-6 months available
• Bundle size directly impacts business metrics

Don't Migrate When:

• Need to hire React developers quickly
• App already performs adequately
• Heavy dependency on React-specific libraries
• Team lacks time for 3-6 month commitment
• Ecosystem libraries don't support Svelte 5

Runes Implementation Patterns

Correct Usage

• $state(): Component-local reactive variables
• $derived(): Computed values (prefer over $effect())
• $effect(): Side effects only (use sparingly)

Avoid

• Converting global stores to runes
• Overusing $effect() for reactive computations
• Mixing stores and runes inefficiently

Store vs Runes Decision Matrix

Keep as Stores:

• Global application state
• Cross-component communication
• Data persisting between routes

Convert to Runes:

• Component-local state
• Derived values
• Side effects and cleanup

Performance Monitoring

Critical Metrics

• Bundle Size: Target 30-50% reduction
• Core Web Vitals: FCP <2s, LCP <3s, CLS near zero
• Load Times: Focus on 3G network performance
• Developer Productivity: Measure after 2-3 month learning curve

Build Performance Issues

• Dev Server Crashes: ECONNRESET, ENOMEM errors common
• Build Time Increase: Large apps see slower compilation
• HMR Instability: Requires frequent restarts during migration

Ecosystem Compatibility

Library Support Status

• Reality: 50% of Svelte ecosystem hasn't updated for Svelte 5
• Workaround: Custom wrapper components for React/Vue libraries
• Check: SvelteAdd for Svelte 5 compatibility before migration

TypeScript Issues

• Common Error: "Property 'runes' does not exist on type 'CompileOptions'"
• Solution: Upgrade TypeScript 5.0+, update Svelte language tools
• Debug Method: Delete node_modules, reinstall

Migration Sequence

Phase 1: Low-Risk Components

• Buttons, icons, form inputs
• Stateless utility components
• Components without complex state

Phase 2: Feature Modules

• Complete pages/sections
• Related data handling components
• Complete Redux slice replacements

Phase 3: Critical Infrastructure

• Routing components
• Global state management
• Build configuration

Risk Mitigation

Pre-Migration Validation

• npx sv migrate svelte-5 --dry-run to preview changes
• Ecosystem library compatibility audit
• Team training on runes concepts
• Performance baseline establishment

Post-Migration Monitoring

• Bundle size tracking in CI
• Real user performance monitoring
• Memory leak detection
• Development workflow stability

Operational Intelligence

Hidden Costs

• Auth System Breakage: Reported breaking twice during migration
• Team Productivity Loss: 2-3 months for full adaptation
• Debugging Complexity: Traditional React tools don't work
• Library Wrapper Development: Custom components for incompatible libraries

Success Indicators

• Reduced bug rates from compile-time checks
• Improved mobile performance metrics
• Decreased state management complexity
• Faster load times on slow networks

This migration requires significant investment but delivers measurable performance improvements when executed properly with adequate time allocation and team preparation.