Add TS types for NextMiddleware

docs/api-reference/next/server.md

@@ -11,12 +11,11 @@ These native Web API objects are extended to give you more control over how you
 The function signature:
 
 ```ts
-import type { NextRequest, NextFetchEvent } from 'next/server'
+import { NextMiddleware } from 'next/server'
 
-export type Middleware = (
-  request: NextRequest,
-  event: NextFetchEvent
-) => Promise<Response | undefined> | Response | undefined
+export const middleware: NextMiddleware = function (request, event) {
+  return new Response('Hello world')
+}
 ```
 
 The function can be a default export and as such, does **not** have to be named `middleware`. Though this is a convention. Also note that you only need to make the function `async` if you are running asynchronous code.

packages/next/server.d.ts

@@ -1,3 +1,4 @@
 export { NextFetchEvent } from 'next/dist/server/web/spec-extension/fetch-event'
 export { NextRequest } from 'next/dist/server/web/spec-extension/request'
 export { NextResponse } from 'next/dist/server/web/spec-extension/response'
+export { NextMiddleware } from 'next/server/web/types'

packages/next/server/web/adapter.ts

@@ -1,4 +1,4 @@
-import type { RequestData, FetchEventResult } from './types'
+import type { NextMiddleware, RequestData, FetchEventResult } from './types'
 import { DeprecationError } from './error'
 import { fromNodeHeaders } from './utils'
 import { NextFetchEvent } from './spec-extension/fetch-event'
@@ -7,7 +7,7 @@ import { NextResponse } from './spec-extension/response'
 import { waitUntilSymbol } from './spec-compliant/fetch-event'
 
 export async function adapter(params: {
-  handler: (request: NextRequest, event: NextFetchEvent) => Promise<Response>
+  handler: NextMiddleware
   page: string
   request: RequestData
 }): Promise<FetchEventResult> {

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

@@ -63,7 +63,7 @@ export class NextResponse extends Response {
     return this.cookie(name, '', { expires: new Date(1), path: '/', ...opts })
   }
 
-  static redirect(url: string | NextURL, status = 302) {
+  static redirect(url: string | NextURL | URL, status = 302) {
     if (!REDIRECTS.has(status)) {
       throw new RangeError(
         'Failed to execute "redirect" on "response": Invalid status code'

packages/next/server/web/types.ts

@@ -1,4 +1,7 @@
 import type { I18NConfig } from '../config-shared'
+import type { NextRequest } from '../web/spec-extension/request'
+import type { NextFetchEvent } from '../web/spec-extension/fetch-event'
+import type { NextResponse } from './spec-extension/response'
 
 export interface NodeHeaders {
   [header: string]: string | string[] | undefined
@@ -29,3 +32,10 @@ export interface FetchEventResult {
   response: Response
   waitUntil: Promise<any>
 }
+
+type NextMiddlewareResult = NextResponse | Response | null
+
+export type NextMiddleware = (
+  request: NextRequest,
+  event: NextFetchEvent
+) => NextMiddlewareResult | Promise<NextMiddlewareResult>

test/integration/middleware-typescript/next.config.js

@@ -0,0 +1,6 @@
+module.exports = {
+  i18n: {
+    locales: ['en', 'fr', 'nl'],
+    defaultLocale: 'en',
+  },
+}

test/integration/middleware-typescript/pages/interface/[id]/index.tsx

@@ -0,0 +1,3 @@
+export default function Index() {
+  return <p className="title">Dynamic route</p>
+}

test/integration/middleware-typescript/pages/interface/_middleware.ts

@@ -0,0 +1,14 @@
+import { NextMiddleware } from 'next/server'
+
+export const middleware: NextMiddleware = function (request) {
+  return new Response(null, {
+    headers: {
+      'req-url-basepath': request.nextUrl.basePath,
+      'req-url-pathname': request.nextUrl.pathname,
+      'req-url-params': JSON.stringify(request.page.params),
+      'req-url-page': request.page.name,
+      'req-url-query': request.nextUrl.searchParams.get('foo'),
+      'req-url-locale': request.nextUrl.locale,
+    },
+  })
+}

test/integration/middleware-typescript/pages/interface/static.tsx

@@ -0,0 +1,3 @@
+export default function Index() {
+  return <p className="title">Static route</p>
+}

test/integration/middleware-typescript/pages/redirects/_middleware.ts

@@ -0,0 +1,16 @@
+import { NextMiddleware, NextResponse } from 'next/server'
+
+export const middleware: NextMiddleware = async function (request) {
+  const url = request.nextUrl
+
+  if (url.searchParams.get('foo') === 'bar') {
+    url.pathname = '/redirects/new-home'
+    url.searchParams.delete('foo')
+    return Response.redirect(url)
+  }
+
+  if (url.pathname === '/redirects/old-home') {
+    url.pathname = '/redirects/new-home'
+    return NextResponse.redirect(url)
+  }
+}

test/integration/middleware-typescript/pages/redirects/index.tsx

@@ -0,0 +1,7 @@
+export default function Home() {
+  return (
+    <div>
+      <p className="title">Home Page</p>
+    </div>
+  )
+}

test/integration/middleware-typescript/pages/redirects/new-home.tsx

@@ -0,0 +1,3 @@
+export default function NewHome() {
+  return <p className="title">New Home</p>
+}

test/integration/middleware-typescript/pages/redirects/old-home.tsx

@@ -0,0 +1,3 @@
+export default function OldHome() {
+  return <p className="title">Old Home</p>
+}

test/integration/middleware-typescript/pages/responses/_middleware.ts

@@ -0,0 +1,42 @@
+import { NextMiddleware, NextResponse } from 'next/server'
+
+export const middleware: NextMiddleware = async function (request, ev) {
+  // eslint-disable-next-line no-undef
+  const { readable, writable } = new TransformStream()
+  const url = request.nextUrl
+  const writer = writable.getWriter()
+  const encoder = new TextEncoder()
+  const next = NextResponse.next()
+
+  // Header based on query param
+  if (url.searchParams.get('set-header') === 'true') {
+    next.headers.set('x-set-header', 'valid')
+  }
+
+  // Streams a basic response
+  if (url.pathname === '/responses/stream-a-response') {
+    ev.waitUntil(
+      (async () => {
+        writer.write(encoder.encode('this is a streamed '))
+        writer.write(encoder.encode('response'))
+        writer.close()
+      })()
+    )
+
+    return new Response(readable)
+  }
+
+  if (url.pathname === '/responses/bad-status') {
+    return new Response('Auth required', {
+      headers: { 'WWW-Authenticate': 'Basic realm="Secure Area"' },
+      status: 401,
+    })
+  }
+
+  // Sends response
+  if (url.pathname === '/responses/send-response') {
+    return new NextResponse(JSON.stringify({ message: 'hi!' }))
+  }
+
+  return next
+}

test/integration/middleware-typescript/pages/responses/index.tsx

@@ -0,0 +1,9 @@
+import Link from 'next/link'
+
+export default function Home({ message }) {
+  return (
+    <div>
+      <p className="title">Hello Middleware</p>
+    </div>
+  )
+}

test/integration/middleware-typescript/pages/rewrites/_middleware.ts

@@ -0,0 +1,19 @@
+import { NextMiddleware, NextResponse } from 'next/server'
+
+export const middleware: NextMiddleware = async function (request) {
+  const url = request.nextUrl
+
+  if (url.pathname === '/') {
+    let bucket = request.cookies.bucket
+    if (!bucket) {
+      bucket = Math.random() >= 0.5 ? 'a' : 'b'
+      const response = NextResponse.rewrite(`/rewrites/${bucket}`)
+      response.cookie('bucket', bucket, { maxAge: 10000 })
+      return response
+    }
+
+    return NextResponse.rewrite(`/rewrites/${bucket}`)
+  }
+
+  return null
+}

test/integration/middleware-typescript/pages/rewrites/a.tsx

@@ -0,0 +1,3 @@
+export default function Home() {
+  return <p className="title">Welcome Page A</p>
+}

test/integration/middleware-typescript/pages/rewrites/b.tsx

@@ -0,0 +1,3 @@
+export default function Home() {
+  return <p className="title">Welcome Page B</p>
+}

test/integration/middleware-typescript/test/index.test.js

@@ -0,0 +1,16 @@
+/* eslint-env jest */
+
+import { join } from 'path'
+import { nextBuild } from 'next-test-utils'
+
+const appDir = join(__dirname, '..')
+describe('TypeScript Middleware', () => {
+  describe('next build', () => {
+    it('should not fail to build middleware', async () => {
+      const { stderr, code } = await nextBuild(appDir, [], { stderr: true })
+      expect(stderr).not.toMatch(/Failed to compile/)
+      expect(stderr).not.toMatch(/is not assignable to type/)
+      expect(code).toBe(0)
+    })
+  })
+})