svelte/documentation/docs/01-introduction/04-svelte-js-files.md

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title: .svelte.js and .svelte.ts files
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Besides `.svelte` files, Svelte also operates on `.svelte.js` and `.svelte.ts` files.

These behave like any other `.js` or `.ts` module, except that you can use runes. This is useful for creating reusable reactive logic, or sharing reactive state across your app.