svelte5-gotchas

The $state is not defined error in Svelte 5 occurs when runes are used in unsupported file types or when migrating Svelte 4 patterns incorrectly. Runes must be used within .svelte or .svelte.ts files to ensure proper compilation and state management.

To migrate dynamic components from Svelte 4 to Svelte 5, replace the deprecated <svelte:component> tag with direct component instantiation syntax. This ensures compatibility with Svelte 5 features and avoids deprecation warnings during the migration process.

You can use Svelte 5 runes in TypeScript files only if the file uses the .svelte.ts extension. Standard .ts files do not support runes, which will cause runtime errors when attempting to use reactive state wrappers.

In Svelte 5, {@const} tags must be placed as direct children of specific block types like {#if}, {#each}, or component tags. Placing {@const} incorrectly outside these blocks causes syntax issues and breaks the runes compilation process.

The best way to avoid deprecated patterns during Svelte 5 migration is to adopt runes-based syntax consistently. Replace Svelte 4 reactive declarations and ensure all reactive state uses $state wrappers correctly across .svelte files.

Svelte 5 supports Svelte 4 reactive syntax in legacy mode, but mixing it with runes causes drift and runtime errors. Migrating fully to runes-based syntax is required to maintain compatibility and avoid deprecated pattern warnings.