Server-only module referenced by client -- how to fix?

[ivan-kleshnin]

I'm following the docs' section on protected routes. An authenticate helper is suggested there.

As I'm trying to move BE-only logic (e.g. sessionStorage.getSession) to a helper – instead of having it in the loader – the following error appears:

12:13:05 PM [vite] Internal server error: Server-only module referenced by client

 '~/services/auth.server' imported by route 'app/routes/dashboard.tsx'

  React Router automatically removes server-code from these exports:
    `loader`, `action`, `unstable_middleware`, `headers`

WORKS

export async function loader({request}: Route.LoaderArgs) {
  let session = await sessionStorage.getSession(request.headers.get("cookie"))
  let user = session.get("user")
 ...

FAILS

export async function authenticate(request: Request) {
  let session = await sessionStorage.getSession(request.headers.get("cookie"))
  let user = session.get("user")
  ...
}

export async function loader({request}: Route.LoaderArgs) {
  let user = await authenticate(request)
 ...

I've never worked with Vite (trying to migrate from Next) previously, so I'm not sure if it's a bug or a gotcha with some fix available.
There is no function referencing sessionStorage besides what is shown.

• How to fix the above?
• How Vite decides what modules are "Server-only"?

[ivan-kleshnin]

I think I've found the solution myself. A helper touching the "BE" logic must not be exported from the module.
The second question remains, though.

[sergiodxa]

How to fix the above?

Always create those functions in a separate file, not in route files, route files contain route code only.

And you can name the file .server.ts to tell the compiler it's a server-only module

How Vite decides what modules are "Server-only"?

The compiler knows what exports from route are server-only (like a loader or action) and removes them when creating the client code, then it removes any unused code left after removing the server-only exports

If you add that authenticate function in your route and export it, the bundler will leave it there because something may depend on it