Svelte Adds Official TypeScript Support
As explained by TypeScript team member Orta Therox:
Before this update, using TypeScript with Svelte required combining many different tools. TypeScript support in Svelte allows TypeScript within script blocks, type checking of Svelte components, auto-completion hunts when authoring components and their markup, and TypeScript understanding the Svelte component API.
To get started with TypeScript with Svelte, developers should install the Svelte template and Svelte TypeScript app and run a TypeScript setup script:
npx degit sveltejs/template svelte-typescript-app cd svelte-typescript-app node scripts/setupTypeScript.js
There is also now a Svelte TypeScript VSCode extension available. It is also possible to add TypeScript support to existing Svelte projects. TypeScript support for the Sapper framework on top of Svelte is currently under development.