Undefined env config values cause validation warnings

[michaldudak]

Link to the code that reproduces this issue

https://codesandbox.io/p/sandbox/xenodochial-antonelli-9x7v6w?file=%2Fnext.config.js%3A8%2C1

To Reproduce

1. Run next dev
2. See the warnings in the console

Current vs. Expected behavior

Prior to 13.5.5, an env object with undefined values would not cause any validation warnings. In 13.5.5 (and higher) a message is shown saying

⚠ Invalid next.config.js options detected: 
⚠     "env.FOO" is missing, expected string

In our case, we set some env vars in the CI environment and leave them out in local development.

Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 10 Pro
Binaries:
  Node: 18.17.0
  npm: N/A
  Yarn: N/A
  pnpm: 8.6.12
Relevant Packages:
  next: 13.5.6
  eslint-config-next: 13.5.6
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.2.2
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Not sure

Additional context

I believe the bug was introduced with #56383

[michaldudak]

I just verified: updating the config schema in https://github.com/vercel/next.js/blob/v13.5.5/packages/next/src/server/config-schema.ts#L219 to z.record(z.string(), z.union([z.string(), z.undefined()])).optional(), does solve the issue.

[timneutkens]

Feel free to open a PR with a test 👍

[oliviertassinari]

Oh, interesting, I landed here by luck, wondering why there are warnings for undefined variables.

It's to be noted that it's only because Next.js raises warnings for undefined variables that I could spot dead env variables, e.g. mui/mui-x#11181. But there are env variables that are only relevant when in a preview environment, e.g. COMMIT_REF in our case to link the right CodeSandbox CI npm dependency PR build, so raising warnings in a local environment mode feels off.

👍 to change this.

[github-actions]

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