Replaced JavaScript with TypeScript.

pages/api/hello.js

@@ -1,5 +0,0 @@
-// Next.js API route support: https://nextjs.org/docs/api-routes/introduction
-
-export default function handler(req, res) {
-  res.status(200).json({ name: 'John Doe' })
-}

pages/api/hello.ts

@@ -0,0 +1,13 @@
+// Next.js API route support: https://nextjs.org/docs/api-routes/introduction
+import type { NextApiRequest, NextApiResponse } from 'next'
+
+type Data = {
+  name: string
+}
+
+export default function handler(
+  req: NextApiRequest,
+  res: NextApiResponse<Data>
+) {
+  res.status(200).json({ name: 'John Doe' })
+}