next.config.js that doesn't export anything doesn't throw error

[timneutkens]

Bug report

Describe the bug

When you have an empty next.config.js no error is thrown, which can cause confusion.

To Reproduce

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

1. yarn create next-app --example hello-world
2. Add next.config.js with contents:

{
  experimental: { basePath: '/docs' }
}

3. Run yarn dev, everything works but obviously the option isn't applied.

Expected behavior

Next.js should throw an error when there is no object exported from next.config.js.

System information

• OS: [e.g. macOS, Windows] - macOS, but will happen on both.
• Version of Next.js: [e.g. 6.0.2] - latest canary

Additional context

Just a small DX improvement.

[dmitrika]

Looking at #9921 and at https://nodejs.org/dist/latest-v6.x/docs/api/modules.html#modules_module_require_id looks like we cannot distinguish using require when there is an empty object or nothing is exported.

[timneutkens]

@dmitrika yeah unfortunately it's tricky to detects, would improve DX quite a bit though, we got multiple users that didn't export their config.

Potentially we could show a warning along the lines of "Detected next.config.js, no exported configuration found"

[balazsorban44]

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