TL;DR

This update primarily improves the stability and correctness of reactivity, specifically regarding derived states and SSR (Server-Side Rendering) output.

Fixes Worth Knowing

• Resolved several issues with derived states, including incorrect reconnection behavior and AbortSignal handling.
• Fixed SSR transformation errors for $state.eager() and declaration tags.
• Corrected computed key transformations within keyed {#each} blocks.
• Fixed a memory leak in Tween (animation utility) when tweens were interrupted.
• Improved sourcemap chaining for preprocessors (code transformation tools).

TL;DR

Svelte now correctly removes event listeners when components are destroyed, preventing potential memory leaks and unexpected behavior.

Fixes Worth Knowing

Resolved an issue where event listeners weren’t always removed when a component unmounted, which could lead to memory leaks or incorrect functionality in dynamic applications.

Before You Upgrade

No specific action is needed; this is a bug fix release.

TL;DR

Svelte 5 introduces significant CSS scoping changes, restricting the use of :not() and :has() selectors to prevent unintended style leakage, and disallows direct state mutation within template logic.

Breaking

• State Mutation in Templates: Directly modifying state variables inside Svelte template expressions is no longer permitted. (Requires updating code to use explicit assignment.)
• CSS Selectors: :not() and :has() selectors are now scoped, potentially requiring adjustments to complex CSS rules. (May impact component styling.)
• Enumerated Types: Stronger type enforcement has been added. (May require code updates to align with stricter types.)

New

• Migration Tasks: New tooling assists with upgrading to Svelte 5, identifying and flagging potentially problematic code patterns. (Helps streamline the upgrade process.)
• Hidden Transitions: Added until-found and beforematch options for transitions. (Provides more control over transition behavior.)

Fixes Worth Knowing

• Transitions: Transitions now work correctly within dynamic components.
• Autofocus: Autofocus now applies correctly to static elements.
• Blur Dispatch: Resolved a Chromium-specific issue

TL;DR

Svelte now uses the standard CustomEvent constructor, and developers can optionally expose the Svelte version in the browser (for debugging/reporting).

Breaking

• createEvent is deprecated; use CustomEvent instead (standard web API for events).

New

• Expose Svelte version in window (can be disabled).
• Smaller bundle sizes for component destructuring.

Fixes Worth Knowing

• Source maps now work correctly with Svelte preprocessors (tools that transform code).

Before You Upgrade

• Update any code using createEvent to use the CustomEvent constructor.