The release of Svelte 5 marks a major milestone for the popular web framework, described by its developers as the “most significant release in the project’s history.” Following 18 months of development, Svelte 5 introduces a comprehensive rewrite aimed at boosting performance, minimizing bundle sizes, and enhancing reliability—all while retaining near-complete backwards compatibility with Svelte 4 to ensure a smooth upgrade process for developers.

A standout feature in Svelte 5 is the introduction of “runes,” a more explicit approach to declaring reactive state. This innovation addresses previous limitations in Svelte’s reactivity system, where changes to individual properties could inadvertently impact entire objects, now allowing for finer control over reactivity.

The Svelte team explained, “With much of the processing handled during the build phase with npm run build, Svelte apps are compact and fast. But beyond that, Svelte emphasizes an enjoyable, efficient app-building experience.”

Key updates include built-in TypeScript support, removing the need for external preprocessors, and expanded component composition capabilities. Additionally, event handlers are now treated as standard props, offering library developers greater flexibility, especially for large applications.

Svelte 5 also introduces a revamped slot mechanism, replacing the previous syntax with the powerful {#snippet …} feature, alongside numerous performance optimizations and bug fixes.

To assist with migration, developers can upgrade entire apps with the command npx sv migrate svelte-5 or handle components individually through the Svelte extension for VS Code. Notably, Svelte 4 applications remain fully operational, supporting a phased transition to the new syntax.

The release also brings a new Command Line Interface (CLI), “sv,” and the Svelte team has announced a forthcoming SvelteKit version designed to harness Svelte 5’s enhanced capabilities.

Looking forward, Svelte 5 provides a foundation for future improvements that were previously out of reach with earlier versions.