TL;DR

Svelte now supports variable declarations directly within your templates, allowing for more concise and readable component logic.

New

• Template Declarations: You can now use let and const to declare variables inside Svelte templates. (Declarative programming: defining variables where they’re used)

Fixes Worth Knowing

• Improved parsing and stability of declaration tags, resolving errors related to duplicate definitions, whitespace, and type attributes.
• Fixed issues with asynchronous code, ensuring awaited values are correctly handled in template expressions and effects.
• Resolved several bugs related to event handling and component lifecycle, improving overall reliability.

Before You Upgrade

• Review your usage of asynchronous code and effects to ensure they behave as expected with the improved handling of awaited values.

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.