vinext check should flag __dirname/__filename and suggest ESM path APIs

[southpolesteve]

Summary

vinext check should detect server-side uses of __dirname / __filename and tell users to migrate to an ESM-native path pattern.

Real-world case

This came up while testing the Platformatic Watt demo app (ecommerce branch):

• https://github.com/platformatic/k8s-watt-performance-demo/blob/ecommerce/next/src/lib/db.ts#L56-L71

That file uses __dirname to search for the app's data/ directory:

private findDataDir(): string {
  const candidates = [
    path.join(process.cwd(), 'data'),
    path.join(process.cwd(), '..', 'data'),
    path.join(__dirname, '..', 'data'),
    path.join(__dirname, 'data'),
  ]

  for (const dir of candidates) {
    if (fs.existsSync(path.join(dir, 'games.json'))) {
      return dir
    }
  }

  return path.join(process.cwd(), 'data')
}

The app works under Next.js, but this usage was the actual blocker under vinext/Vite SSR. vinext check did not flag it, so the compatibility report missed the real runtime risk.

Why this should be checked

__dirname / __filename are CommonJS globals, not ESM globals. Since vinext runs server modules through a Vite/ESM pipeline, these identifiers are a migration hazard even in otherwise normal Node-targeted server code.

This is not a Cloudflare-specific problem. It is a general Vite/ESM compatibility issue that users are likely to hit when migrating existing Next.js apps.

Proposed behavior

When vinext check scans app/server code and finds free uses of __dirname or __filename, report a warning with a migration hint.

Suggested guidance:

• Prefer ESM path APIs instead of __dirname / __filename
• For portable ESM: fileURLToPath(import.meta.url) + dirname(...)
• On Node 22+: import.meta.dirname / import.meta.filename are also available

Example warning text:

__dirname / __filename are CommonJS globals and may not work under vinext's ESM server runtime. Prefer fileURLToPath(import.meta.url) / dirname(...) (or import.meta.dirname / import.meta.filename on Node 22+).

Why this matters

Even if vinext adds more runtime compatibility here, the check command should still surface legacy CJS path usage and steer users toward the ESM-native pattern up front.