How can you keep Server-only Code out of the Client Environment
To prevent this sort of unintended client usage of server code, we can use the server-only package to give other developers a build-time error if they ever accidentally import one of these modules into a Client Component.
Now, any Client Component that imports getData() will receive a build-time error explaining that this module can only be used on the server.
The corresponding package client-only can be used to mark modules that contain client-only code – for example, code that accesses the window object.