NextResponse: add .json static method (#31483)

closes: #31196

This new API was suggested in a previous version of this feature:
#31024 (comment)

Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>

packages/next/server/web/spec-extension/response.ts

@@ -63,6 +63,12 @@ export class NextResponse extends Response {
     return this.cookie(name, '', { expires: new Date(1), path: '/', ...opts })
   }
 
+  static json(body: any) {
+    return new NextResponse(JSON.stringify(body), {
+      headers: { 'content-type': 'application/json' },
+    })
+  }
+
   static redirect(url: string | NextURL, status = 302) {
     if (!REDIRECTS.has(status)) {
       throw new RangeError(

test/unit/web-runtime/next-response.test.ts

@@ -0,0 +1,54 @@
+/* eslint-env jest */
+
+import { Blob, File, FormData } from 'next/dist/compiled/formdata-node'
+import { Crypto } from 'next/dist/server/web/sandbox/polyfills'
+import { Response } from 'next/dist/server/web/spec-compliant/response'
+import { Headers } from 'next/dist/server/web/spec-compliant/headers'
+import * as streams from 'web-streams-polyfill/ponyfill'
+
+beforeAll(() => {
+  global['Blob'] = Blob
+  global['crypto'] = new Crypto()
+  global['File'] = File
+  global['FormData'] = FormData
+  global['Headers'] = Headers
+  global['ReadableStream'] = streams.ReadableStream
+  global['TransformStream'] = streams.TransformStream
+  global['Response'] = Response
+})
+
+afterAll(() => {
+  delete global['Blob']
+  delete global['crypto']
+  delete global['File']
+  delete global['Headers']
+  delete global['FormData']
+  delete global['ReadableStream']
+  delete global['TransformStream']
+})
+
+const toJSON = async (response) => ({
+  body: await response.json(),
+  contentType: response.headers.get('content-type'),
+})
+
+it('automatically parses and formats JSON', async () => {
+  const { NextResponse } = await import(
+    'next/dist/server/web/spec-extension/response'
+  )
+
+  expect(await toJSON(NextResponse.json({ message: 'hello!' }))).toMatchObject({
+    contentType: 'application/json',
+    body: { message: 'hello!' },
+  })
+
+  expect(await toJSON(NextResponse.json(null))).toMatchObject({
+    contentType: 'application/json',
+    body: null,
+  })
+
+  expect(await toJSON(NextResponse.json(''))).toMatchObject({
+    contentType: 'application/json',
+    body: '',
+  })
+})