fix: Keep the rewrites when next.config.js is an ES module

copyNextjsConfig loads next.config.js with a require and only falls back to
import when that throws. The require returns a module namespace rather than
the config for an ES module in two cases: since Node.js 22.12 require of an
ES module succeeds and returns a namespace, and the nsm bin registers
esbuild-runner, which transpiles an ES module to CommonJS and marks the result
with __esModule.

Reading the config off the namespace left rewrites undefined, so it was
dropped from the generated .nsm/next.config.ts. A project that relies on a
rewrite to reach its routes then 404s on every request, with no error to
explain it.

Unwrap the default export whenever a namespace is loaded, whichever way it was
loaded. Resolve rewrites into a copy of the config, since a module namespace is
frozen, and pass a file URL to the import fallback so it also works on Windows.

Extract the loading into loadNextjsConfig and cover the CommonJS object,
CommonJS function and ES module configs with tests. The sample project used by
the existing tests has a CommonJS config, so the ES module path was untested.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V6bQ6zQqwT1vq9NkrG1UTm

Author: claude

nsm/scripts/copy-nextjs-config.js

@@ -2,33 +2,56 @@ const path = require("path")
 const prettier = require("prettier")
 const fs = require("fs/promises")
 const { existsSync } = require("fs")
+const { pathToFileURL } = require("url")
 
-async function copyNextjsConfig() {
-  const nextConfigPath = path.resolve(__dirname, "../../next.config.js")
+// A module namespace is not the config, the config is its default export.
+// The require below returns a namespace for an ES module in two cases: since
+// Node.js 22.12 require of an ES module succeeds and returns one, and the nsm
+// bin registers esbuild-runner, which transpiles an ES module to CommonJS and
+// marks the result with __esModule. Reading the config off the namespace left
+// rewrites undefined, which silently dropped it from the generated config.
+const interopDefault = (value) => {
+  if (value == null) return value
+
+  const is_namespace =
+    value[Symbol.toStringTag] === "Module" || value.__esModule === true
+
+  return is_namespace && "default" in value ? value.default : value
+}
 
-  let nextConfig = {}
-  if (existsSync(nextConfigPath)) {
+async function loadNextjsConfig(nextConfigPath) {
+  let nextConfig
+  try {
+    nextConfig = interopDefault(require(nextConfigPath))
+  } catch (errorA) {
     try {
-      nextConfig = require(nextConfigPath)
-    } catch (errorA) {
-      try {
-        nextConfig = (await import(nextConfigPath)).default
-      } catch (errorB) {
-        console.error(errorA)
-        console.error(errorB)
-        throw new Error(`Failed to load ${nextConfigPath}`)
-      }
+      nextConfig = interopDefault(await import(pathToFileURL(nextConfigPath)))
+    } catch (errorB) {
+      console.error(errorA)
+      console.error(errorB)
+      throw new Error(`Failed to load ${nextConfigPath}`)
     }
+  }
 
-    if (typeof nextConfig === "function") {
-      nextConfig = await nextConfig()
-    }
+  if (typeof nextConfig === "function") {
+    nextConfig = await nextConfig()
+  }
 
-    if (typeof nextConfig.rewrites === "function") {
-      nextConfig.rewrites = await nextConfig.rewrites()
-    }
+  // A module namespace is frozen, so resolve rewrites into a copy.
+  if (typeof nextConfig.rewrites === "function") {
+    nextConfig = { ...nextConfig, rewrites: await nextConfig.rewrites() }
   }
 
+  return nextConfig
+}
+
+async function copyNextjsConfig() {
+  const nextConfigPath = path.resolve(__dirname, "../../next.config.js")
+
+  const nextConfig = existsSync(nextConfigPath)
+    ? await loadNextjsConfig(nextConfigPath)
+    : {}
+
   const nextConfigFile = await prettier.format(
     `export default ${JSON.stringify(nextConfig)}`,
     { semi: false, parser: "babel" },
@@ -40,7 +63,7 @@ async function copyNextjsConfig() {
   )
 }
 
-module.exports = { copyNextjsConfig }
+module.exports = { copyNextjsConfig, loadNextjsConfig }
 
 if (require.main === module) {
   copyNextjsConfig()

tests/assets/next-configs/commonjs-function/next.config.js

@@ -0,0 +1,13 @@
+module.exports = async () => ({
+  reactStrictMode: true,
+  async rewrites() {
+    return {
+      beforeFiles: [
+        {
+          source: "/:path*",
+          destination: "/api/:path*",
+        },
+      ],
+    }
+  },
+})

tests/assets/next-configs/commonjs/next.config.js

@@ -0,0 +1,13 @@
+module.exports = {
+  reactStrictMode: true,
+  async rewrites() {
+    return {
+      beforeFiles: [
+        {
+          source: "/:path*",
+          destination: "/api/:path*",
+        },
+      ],
+    }
+  },
+}

tests/assets/next-configs/module/next.config.js

@@ -0,0 +1,13 @@
+export default {
+  reactStrictMode: true,
+  async rewrites() {
+    return {
+      beforeFiles: [
+        {
+          source: "/:path*",
+          destination: "/api/:path*",
+        },
+      ],
+    }
+  },
+}

tests/assets/next-configs/module/package.json

@@ -0,0 +1,5 @@
+{
+  "name": "next-config-module-fixture",
+  "private": true,
+  "type": "module"
+}

tests/load-nextjs-config.test.ts

@@ -0,0 +1,40 @@
+import test from "ava"
+import { loadNextjsConfig } from "nsm/scripts/copy-nextjs-config"
+import path from "path"
+
+const configPath = (name: string) =>
+  path.resolve(__dirname, "assets", "next-configs", name, "next.config.js")
+
+const expected_rewrites = {
+  beforeFiles: [
+    {
+      source: "/:path*",
+      destination: "/api/:path*",
+    },
+  ],
+}
+
+test("loads a CommonJS config", async (t) => {
+  const config = await loadNextjsConfig(configPath("commonjs"))
+
+  t.true(config.reactStrictMode)
+  t.deepEqual(config.rewrites, expected_rewrites)
+})
+
+test("loads a CommonJS config exporting a function", async (t) => {
+  const config = await loadNextjsConfig(configPath("commonjs-function"))
+
+  t.true(config.reactStrictMode)
+  t.deepEqual(config.rewrites, expected_rewrites)
+})
+
+// The nsm bin registers esbuild-runner, which transpiles an ES module to
+// CommonJS, and require of an ES module returns a module namespace since
+// Node.js 22.12. Both give the require in loadNextjsConfig a namespace instead
+// of the config, and reading rewrites off the namespace silently dropped it.
+test("loads an ES module config", async (t) => {
+  const config = await loadNextjsConfig(configPath("module"))
+
+  t.true(config.reactStrictMode)
+  t.deepEqual(config.rewrites, expected_rewrites)
+})