Add TS types for NextMiddleware

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/types.ts

@@ -1,4 +1,6 @@
 import type { I18NConfig } from '../config-shared'
+import type { NextRequest } from '../web/spec-extension/request'
+import type { NextFetchEvent } from '../web/spec-extension/fetch-event'
 
 export interface NodeHeaders {
   [header: string]: string | string[] | undefined
@@ -29,3 +31,8 @@ export interface FetchEventResult {
   response: Response
   waitUntil: Promise<any>
 }
+
+export type NextMiddleware = (
+  request: NextRequest,
+  event: NextFetchEvent
+) => Promise<Response | null>