chore: throw error when trying to use next.config.mts/.cjs/.cts (not …

…supported yet) (#70376) ## Why? We should update our documentation and also throw an error if somebody tries to use the `.mts` ( ES Module TypeScript) extension with `next.config`. We currently don't support this extension. - Fixes: #70201 --------- Co-authored-by: JJ Kasper <jj@jjsweb.site>
vercel · Sep 24, 2024 · ac9a514 · ac9a514
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diff --git a/docs/02-app/02-api-reference/05-next-config-js/index.mdx b/docs/02-app/02-api-reference/05-next-config-js/index.mdx
@@ -37,6 +37,8 @@ const nextConfig = {
 export default nextConfig
 ```
 
+> **Good to know**: `next.config` with the `.cjs`, `.cts`, or `.mts` extensions are currently **not** supported.
+
 ## Configuration as a Function
 
 You can also use a function:

diff --git a/packages/next/src/server/config.ts b/packages/next/src/server/config.ts
@@ -1178,9 +1178,12 @@ export default async function loadConfig(
     const configBaseName = basename(CONFIG_FILES[0], extname(CONFIG_FILES[0]))
     const unsupportedConfig = findUp.sync(
       [
+        `${configBaseName}.cjs`,
+        `${configBaseName}.cts`,
+        `${configBaseName}.mts`,
+        `${configBaseName}.json`,
         `${configBaseName}.jsx`,
         `${configBaseName}.tsx`,
-        `${configBaseName}.json`,
       ],
       { cwd: dir }
     )