为何在Next.js API路由中无法使用类型为NextResponse的res？

The error happens because of a core difference in how response handling works between Next.js's App Router (your current setup) and the older Pages Router:

1. res in App Router isn't an Express-style response object
   In App Router API routes, when you receive res: NextResponse as a parameter, this is an immutable response instance—not the mutable, writable response stream used in Pages Router. The json() method on a NextResponse instance doesn't accept arguments (it’s not designed to send new data); the json() method you’re trying to use is actually the static method NextResponse.json(), which creates a new response object from your data.

2. Wrong vs Correct Usage

   • Incorrect (triggers your error):

     export async function POST(req: NextRequest, res: NextResponse) {
       // Error: Expected 0 arguments, but got 2
       res.json({ message: "Success" }, { status: 200 });
     }
     

   • Correct (standard App Router pattern):

     import { NextRequest, NextResponse } from "next/server";
     
     export async function POST(req: NextRequest) {
       // Return a new NextResponse directly
       return NextResponse.json({ message: "Success" }, { status: 200 });
     }
     

   • If you need to modify an existing res instance (rare for most use cases), clone it with your desired changes:

     export async function POST(req: NextRequest, res: NextResponse) {
       return res.clone({
         body: JSON.stringify({ message: "Success" }),
         status: 200
       });
     }
     

3. Key Takeaway
   App Router API routes are built to return response objects directly instead of mutating a res stream. The Express-style res.json(data) pattern only works in Pages Router; in App Router, use the static NextResponse.json() method to generate and return your JSON response.

内容的提问来源于stack exchange，提问作者in43sh