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Server-only modules

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Like a good friend, SvelteKit keeps your secrets. When writing your backend and frontend in the same repository, it can be easy to accidentally import sensitive data into your front-end code (environment variables containing API keys, for example). SvelteKit provides a way to prevent this entirely: server-only modules.

Private environment variables

The $env/static/private and $env/dynamic/private modules, which are covered in the modules section, can only be imported into modules that only run on the server, such as hooks.server.js.ts or +page.server.js.ts.

Your modules

You can make your own modules server-only in two ways:

  • adding .server to the filename, e.g. secrets.server.js.ts
  • placing them in $lib/server, e.g. $lib/server/secrets.js.ts

How it works

Any time you have public-facing code that imports server-only code (whether directly or indirectly)...

export const atlantisCoordinates = [/* redacted */];
Variable 'atlantisCoordinates' implicitly has an 'any[]' type.7005Variable 'atlantisCoordinates' implicitly has an 'any[]' type.
export { atlantisCoordinates } from '$lib/server/secrets.js';
Cannot find module '$lib/server/secrets.js' or its corresponding type declarations.2307Cannot find module '$lib/server/secrets.js' or its corresponding type declarations.
export const add = (a, b) => a + b;
Parameter 'a' implicitly has an 'any' type.
Parameter 'b' implicitly has an 'any' type.
Parameter 'a' implicitly has an 'any' type.
Parameter 'b' implicitly has an 'any' type.
    import { add } from './utils.js';

...SvelteKit will error:

Cannot import $lib/server/secrets.js into public-facing code:
- src/routes/+page.svelte
    - src/routes/utils.js
        - $lib/server/secrets.js

Even though the public-facing code — src/routes/+page.svelte — only uses the add export and not the secret atlantisCoordinates export, the secret code could end up in JavaScript that the browser downloads, and so the import chain is considered unsafe.

This feature also works with dynamic imports, even interpolated ones like await import(`./${foo}.js`), with one small caveat: during development, if there are two or more dynamic imports between the public-facing code and the server-only module, the illegal import will not be detected the first time the code is loaded.

Unit testing frameworks like Vitest do not distinguish between server-only and public-facing code. For this reason, illegal import detection is disabled when running tests, as determined by process.env.TEST === 'true'.

Further reading