add esm

Co-authored-by: Abdón Rodríguez <[email protected]>

Author: devjiwonchoi

docs/01-app/03-api-reference/05-config/02-typescript.mdx

@@ -123,7 +123,9 @@ In Node.js versions **v22.18.0+**, `process.features.typescript` is enabled by d
 NODE_OPTIONS=--experimental-transform-types next <command>
 ```
 
-By default, Node treats `.ts` files as CommonJS. If `next.config.ts` uses ESM syntax, Node re-parses it as ESM, which works but prints a warning about performance overhead. To avoid this extra pass, use `next.config.mts` to declare ESM up front.
+#### For CommonJS Projects (Default)
+
+Although `next.config.ts` supports native ESM syntax on CommonJS projects, Node.js will still assume `next.config.ts` is a CommonJS file by default, resulting in Node.js reparsing the file as ESM when module syntax is detected. Therefore, we recommend using the `next.config.mts` file for CommonJS projects to explicitly indicate it's an ESM module:
 
 ```ts filename="next.config.mts"
 import type { NextConfig } from 'next'
@@ -139,6 +141,19 @@ const nextConfig: NextConfig = {
 export default nextConfig
 ```
 
+#### For ESM Projects
+
+If your project is configured as an ESM project by adding `"type": "module"` to your `package.json` (see [Node.js documentation](https://nodejs.org/api/packages.html#type)), you can use `next.config.ts` directly with ESM syntax:
+
+```json filename="package.json"
+{
+  "type": "module"
+  // ... other package.json fields
+}
+```
+
+> **Good to know**: When using `"type": "module"` in your `package.json`, all `.js` and `.ts` files in your project are treated as ESM modules by default. You may need to rename files with CommonJS syntax to `.cjs` or `.cts` extensions if needed.
+
 ### Statically Typed Links
 
 Next.js can statically type links to prevent typos and other errors when using `next/link`, improving type safety when navigating between pages.