Types of headers, rewrites and redirects methods don't support synchronous returns

[wellwelwel]

Link to the code that reproduces this issue

https://github.com/wellwelwel/next-reproduction

To Reproduce

To reproduce the issues (missing types), simply clone the above repository and run:

npm i
npm test

• After npm i, you can also notice the issues visually in the configs.ts file.
• I've opened a PR to fix it in fix(types): adjust headers, rewrites, and redirects return types #74546.

Current vs. Expected behavior

The expectation is that both return types will be accepted when using synchronous returns in the configuration file, as shown in the images:

• Please note that this is a type (TypeScript) issue only. There is no conflict regarding the execution (JavaScript) of both ways.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.1.0: Thu Oct 10 21:03:11 PDT 2024; root:xnu-11215.41.3~2/RELEASE_ARM64_T6020
  Available memory (MB): 65536
  Available CPU cores: 24
Binaries:
  Node: 23.5.0
  npm: 11.0.0
  Yarn: 4.4.1
  pnpm: N/A
Relevant Packages:
  next: 15.1.3 // Latest available version is detected (15.1.3).
  eslint-config-next: N/A
  react: 18.3.1
  react-dom: 18.3.1
  typescript: 5.7.2
Next.js Config:
  output: N/A

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

TypeScript

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

Other (Deployed)

Additional context

Note

For some reason, the #74545 was closed immediately, but the link to repro is valid. Trying one more time to see if it's a false positive 🙌

[eps1lon]

Thank you for flagging. Though this is intentional behavior. Simply convert these functions to async functions to resolve this on your end.

[wellwelwel]

Thank you for flagging. Though this is intentional behavior. Simply convert these functions to async functions to resolve this on your end.

Thanks for the quick reply, @eps1lon 🙋🏻‍♂️

For everyone who, like me, is using Biome instead of ESLint, here's a workaround to add in biome.json:

{
  "overrides": [
    {
      "include": ["next.config.ts"],
      "linter": {
        "rules": {
          "suspicious": {
            "useAwait": "off"
          }
        }
      }
    }
  ]
}

[github-actions]

This closed issue has been automatically locked because it had no new activity for 2 weeks. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.