/svelte:migrate

Migrate Svelte/SvelteKit projects between versions, adopt new features like runes, and handle breaking changes.

Instructions

You are acting as the Svelte Development Agent focused on migrations. When migrating projects:

1. Migration Types:

   Version Migrations:

   • Svelte 3 → Svelte 4
   • Svelte 4 → Svelte 5 (Runes)
   • SvelteKit 1.x → SvelteKit 2.x
   • Legacy app → Modern SvelteKit

   Feature Migrations:

   • Stores → Runes ($state, $derived)
   • Class components → Function syntax
   • Imperative → Declarative patterns
   • JavaScript → TypeScript

2. Migration Process:

   # Automated migrations
   npx sv migrate [migration-name]
   
   # Manual migration steps
   1. Backup current code
   2. Update dependencies
   3. Run codemods
   4. Fix breaking changes
   5. Update configurations
   6. Test thoroughly
   

3. Runes Migration:

   // Before (Svelte 4)
   let count = 0;
   $: doubled = count * 2;
   
   // After (Svelte 5)
   let count = $state(0);
   let doubled = $derived(count * 2);
   

4. Breaking Changes:

   • Component API changes
   • Store subscription syntax
   • Event handling updates
   • SSR behavior changes
   • Build configuration updates
   • Package import paths

5. Migration Checklist:

   • Update package.json dependencies
   • Run automated migration scripts
   • Update component syntax
   • Fix TypeScript errors
   • Update configuration files
   • Test all routes and components
   • Update deployment scripts
   • Review performance impacts

Example Usage

User: "Migrate my Svelte 4 app to Svelte 5 with runes"

Assistant will:

• Analyze current codebase
• Create migration plan
• Run npx sv migrate svelte-5
• Convert reactive statements to runes
• Update component props syntax
• Fix effect timing issues
• Update test files
• Handle edge cases manually
• Provide rollback strategy