middleware: add request referrer support (#31343)

closes: #30353

According with spec, `'about:client'` is the default value is the user doesn't provide it.

It needs to add a test there, looks like there no unit tests for these classes 🤔

packages/next/server/web/spec-compliant/request.ts

@@ -10,6 +10,7 @@ class BaseRequest extends Body implements Request {
     credentials: RequestCredentials
     headers: Headers
     method: string
+    referrer: string
     redirect: RequestRedirect
     url: NextURL
   }
@@ -48,6 +49,7 @@ class BaseRequest extends Body implements Request {
         init.credentials || getProp(input, 'credentials') || 'same-origin',
       headers,
       method,
+      referrer: init.referrer || 'about:client',
       redirect: init.redirect || getProp(input, 'redirect') || 'follow',
       url: new NextURL(typeof input === 'string' ? input : input.url),
     }
@@ -65,6 +67,10 @@ class BaseRequest extends Body implements Request {
     return this[INTERNALS].method
   }
 
+  get referrer() {
+    return this[INTERNALS].referrer
+  }
+
   get headers() {
     return this[INTERNALS].headers
   }
@@ -97,10 +103,6 @@ class BaseRequest extends Body implements Request {
     return notImplemented('Request', 'destination')
   }
 
-  get referrer() {
-    return notImplemented('Request', 'referrer')
-  }
-
   get referrerPolicy() {
     return notImplemented('Request', 'referrerPolicy')
   }

test/unit/web-runtime/request.test.ts

@@ -35,3 +35,13 @@ it('parses and reconstructs the URL alone', async () => {
 it('throws when the URL is malformed', async () => {
   expect(() => new Request('meeeh')).toThrowError('Invalid URL')
 })
+
+it('Request.referrer is `about:client` by default', async () => {
+  const request = new Request('https://vercel.com')
+  expect(request.referrer).toBe('about:client')
+})
+
+it('Request.referrer can be customized', async () => {
+  const request = new Request('https://vercel.com', { referrer: 'client' })
+  expect(request.referrer).toBe('client')
+})