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svelte/documentation/docs/99-legacy/00-legacy-overview.md

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Overview

Svelte 5 introduced some significant changes to Svelte's API, including runes, snippets and event attributes. As a result, some Svelte 3/4 features are deprecated (though supported for now, unless otherwise specified) and will eventually be removed. We recommend that you incrementally migrate your existing code.

The following pages document these features for

  • people still using Svelte 3/4
  • people using Svelte 5, but with components that haven't yet been migrated

Since Svelte 3/4 syntax still works in Svelte 5, we will distinguish between legacy mode and runes mode. Once a component is in runes mode (which you can opt into by using runes, or by explicitly setting the runes: true compiler option), legacy mode features are no longer available.

If you're exclusively interested in the Svelte 3/4 syntax, you can browse its documentation at v4.svelte.dev.