Bug report

Describe the bug

Importing a project ESM .mjs file (the Node.js standard file extension for ESM) in a page file causes a runtime error, but only in dev mode:

Unhandled Runtime Error
ReferenceError: module is not defined

Here is the call stack:

To Reproduce

Steps to reproduce the behavior, please provide code snippets or a repository:

1. Create a basic Next.js project.

2. Create a config.mjs file containing:

   export const A = 'a';

3. In pages/index.js:

   import { A } from '../config.mjs';
   
   export default function IndexPage() {
     return A;
   }

4. In Terminal, run:

   npm run dev
   

You should then see no error in Terminal, but on the client you will see the Next.js error overlay with the error described earlier.

Expected behavior

You should be able to use ESM .mjs files in your Next.js project without runtime errors in dev mode.

System information

• Version of Next.js: v9.5.5

Additional context

Other Next.js users have also encountered this issue:

#13076