Resolve ESM-only plugins passed as strings

[20syldev]

What

When a remark/rehype/recma plugin is passed as a string in @next/mdx options:

const withMDX = createMDX({
  options: {
    rehypePlugins: [['@stefanprobst/rehype-extract-toc']],
  },
})

mdx-js-loader.js resolves it with require.resolve(), which uses CJS resolution semantics. For ESM-only packages that expose an exports field with only an "import" condition (no "require" condition), Node throws ERR_PACKAGE_PATH_NOT_EXPORTED because no CJS-compatible entry exists.

Why

require.resolve() applies the "require" condition when matching exports. If the package's exports map looks like:

{
  "exports": {
    ".": {
      "import": "./dist/index.mjs"
    }
  }
}

require.resolve() finds no matching condition and throws. A dynamic import() uses ESM resolution and correctly matches the "import" condition.

Fix

Catch ERR_PACKAGE_PATH_NOT_EXPORTED from require.resolve() and fall back to a direct import(). This lets Node's ESM resolver handle the exports map, resolving the module correctly.

Code diff

 async function importPluginForPath(pluginPath, projectRoot) {
-  const path = require.resolve(pluginPath, { paths: [projectRoot] })
+  let resolvedPath
+  try {
+    resolvedPath = require.resolve(pluginPath, { paths: [projectRoot] })
+  } catch (e) {
+    if (e.code === 'ERR_PACKAGE_PATH_NOT_EXPORTED') {
+      // ESM-only package: no "require" condition in exports map.
+      // Fall back to a direct import() which uses ESM resolution.
+      return interopDefault(await import(pluginPath))
+    }
+    throw e
+  }
   return interopDefault(
-    await import(process.platform === 'win32' ? pathToFileURL(path) : path)
+    await import(process.platform === 'win32' ? pathToFileURL(resolvedPath) : resolvedPath)
   )
 }

Testing

The existing test/e2e/app-dir/mdx suite already covers the string plugin resolution happy path (packages with CJS-compatible exports).

The fix affects the fallback path only — packages whose exports map has no "require" entry — which isn't covered by the current suite. Adding a test fixture with a local ESM-only plugin would provide coverage; happy to add one if the maintainers prefer it.

Fixes #73757

[nextjs-bot]

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