Next.js: How to mark a JavaScript module as server only

Use the server-onlypackage. For example:

import 'server-only'
 
export async function getData() {
  const res = await fetch('https://external-service.com/data', {
    headers: {
      authorization: process.env.API_KEY,
    },
  })
 
  return res.json()
}

Now, if you try to use that module in a client component, you should get a build error.