feat: add support for rewriting middleware responses

Author: ascorbic

.prettierignore

@@ -21,6 +21,5 @@ node_modules
 lib
 tsconfig.json
 demos/nx-next-monorepo-demo
-plugin/src/templates/edge
 
 plugin/CHANGELOG.md

demos/middleware/middleware.ts

@@ -1,12 +1,68 @@
 import { NextResponse } from 'next/server'
-import { NextFetchEvent, NextRequest } from 'next/server'
+import type { NextRequest } from 'next/server'
 
-export function middleware(request: NextRequest, ev: NextFetchEvent) {
+/**
+ * Supercharge your Next middleware with Netlify Edge Functions
+ */
+class NetlifyResponse {
+  static async next(request: NextRequest): Promise<NetlifyNextResponse> {
+    const context = (request.geo as any).__nf_context
+    if (!context) {
+      throw new Error('NetlifyResponse can only be used with Netlify Edge Functions')
+    }
+    const response: Response = await context.next()
+    return new NetlifyNextResponse(response)
+  }
+}
+
+type NextDataTransform = <T extends Record<string, any>>(props: T) => T
+
+// A NextReponse that will pass through the Netlify origin response
+// We can't pass it through directly, because Next disallows returning a response body
+class NetlifyNextResponse extends NextResponse {
+  private originResponse: Response
+  private transforms: NextDataTransform[]
+  constructor(originResponse: Response) {
+    super()
+    this.originResponse = originResponse
+    Object.defineProperty(this, 'transforms', {
+      value: [],
+      enumerable: false,
+      writable: false,
+    })
+  }
+
+  /**
+   * Transform the page props before they are passed to the client.
+   * This works for both HTML pages and JSON data
+   */
+  transformData(transform: NextDataTransform) {
+    // The transforms are evaluated after the middleware is returned
+    this.transforms.push(transform)
+  }
+  get headers(): Headers {
+    // If we have the origin response, we should use its headers
+    return this.originResponse?.headers || super.headers
+  }
+}
+
+export async function middleware(request: NextRequest) {
   let response
   const {
     nextUrl: { pathname },
   } = request
 
+  if (pathname.startsWith('/static')) {
+    // Unlike NextResponse.next(), this actually sends the request to the origin
+    const res = await NetlifyResponse.next(request)
+    res.transformData((data) => {
+      data.pageProps.message = `This was static but has been transformed in ${request.geo.country}`
+      return data
+    })
+
+    return res
+  }
+
   if (pathname.startsWith('/cookies')) {
     response = NextResponse.next()
     response.cookies.set('netlifyCookie', 'true')