According to the "TS to JSDoc Conversion" PR in the Svelte repo , the Svelte team will migrate from the current TypeScript to JSDoc.
Rich Harris, the founder of the front-end UI framework Svelte, confirmed the news in a reply on Twitter. He stated that this decision did not change Svelte's type safety.
The developers responsible for the Svelte compiler say that by switching to JSDoc, code can be debugged without compiling and building—simplifying compiler development. After all, the current debugging work is more complicated, and it is necessary to use the build step for debugging. In addition, using JSDoc will not affect the development security of the compiler, because its type is almost equivalent to TypeScript, and the tsc compiler can still be used to check the type.
Of course, Svelte developers (not compiler developers) will still get the type definition files as they do now. So for Svelte developers, there will be no changes in types.