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Compilers

Projects may have source files that are not JavaScript or TypeScript, and thus require compilation (or transpilation, or pre-processing, you name it). Files like .mdx, .astro, .vue and .svelte may also import other source files and external dependencies. So ideally, these files are included when linting the project. That’s why Knip supports compilers.

Built-in compilers

Knip has built-in “compilers” for the following file extensions:

  • .astro
  • .mdx
  • .svelte
  • .vue

Note that “compilers” is quoted, as they are not real compilers, but regular expressions to collect import statements from files with those extensions. This allows Knip to build the dependency graph.

On the other hand, real compilers may expose their own challenges in the context of Knip. For instance, the Svelte compiler keeps exports intact, while they might represent component properties. This results in those exports being reported as unused by Knip.

In short, the built-in functions seem to do a decent job, but you can override them however you like.

Custom compilers

Additional custom compilers can be added, and built-in compilers can be overridden. Since compilers are functions, the Knip configuration file must be a dynamic .js or .ts file.

Interface

The compiler function interface is straightforward. Text in, text out:

(source: string, filename: string) => string;

This may also be an async function.

Svelte

In a project with Svelte, the compiler is automatically enabled, but you may have unresolved imports starting with $app/:

Terminal window
Unresolved imports (5)
$app/stores src/routes/Header.svelte:1:9
$app/environment src/routes/about/+page.ts:1:9

In this case, you can manually add the $app path alias:

knip.json
{
"paths": {
"$app/*": ["node_modules/@sveltejs/kit/src/runtime/app/*"]
}
}

CSS

Here’s an example, minimal compiler for CSS files:

knip.ts
export default {
compilers: {
css: (text: string) => [...text.matchAll(/(?<=@)import[^;]+/g)].join('\n'),
},
};

You may wonder why the CSS compiler is not included by default. It’s currently not clear if it should be included. And if so, what would be the best way to determine it should be enabled, and what syntax(es) it should support.

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