svelte/site/content/faq/500-what-about-typescript-support.md

question
What about TypeScript support?

You need to install a preprocessor such as svelte-preprocess. You can run type checking from the command line with svelte-check.

To declare the type of a reactive variable in a Svelte template, you should use the following syntax:

let x: number;
$: x = count + 1;

To import a type or interface make sure to use TypeScript's type modifier:

import type { SomeInterface } from './SomeFile';

You must use the type modifier because svelte-preprocess doesn't know whether an import is a type or a value — it only transpiles one file at a time without knowledge of the other files and therefore can't safely erase imports which only contain types without this modifier present.