Implement isFallback Router Property

This adds a `isFallback` property to detect if the page is being rendered in "fallback" mode or normal mode.
Accessed via the `useRouter()` hook.

---

Closes vercel#10527

packages/next/client/index.js

@@ -206,6 +206,7 @@ export default async ({ webpackHMR: passedWebpackHMR } = {}) => {
     Component,
     wrapApp,
     err: initialErr,
+    isFallback,
     subscription: ({ Component, props, err }, App) => {
       render({ App, Component, props, err })
     },

packages/next/client/router.ts

@@ -29,7 +29,14 @@ const singletonRouter: SingletonRouterBase = {
 }
 
 // Create public properties and methods of the router in the singletonRouter
-const urlPropertyFields = ['pathname', 'route', 'query', 'asPath', 'components']
+const urlPropertyFields = [
+  'pathname',
+  'route',
+  'query',
+  'asPath',
+  'components',
+  'isFallback',
+]
 const routerEvents = [
   'routeChangeStart',
   'beforeHistoryChange',

packages/next/next-server/lib/router/router.ts

@@ -49,6 +49,7 @@ export type NextRouter = BaseRouter &
     | 'prefetch'
     | 'beforePopState'
     | 'events'
+    | 'isFallback'
   >
 
 type RouteInfo = {
@@ -122,6 +123,7 @@ export default class Router implements BaseRouter {
   events: MittEmitter
   _wrapApp: (App: ComponentType) => any
   isSsr: boolean
+  isFallback: boolean
 
   static events: MittEmitter = mitt()
 
@@ -137,6 +139,7 @@ export default class Router implements BaseRouter {
       Component,
       err,
       subscription,
+      isFallback,
     }: {
       subscription: Subscription
       initialProps: any
@@ -145,6 +148,7 @@ export default class Router implements BaseRouter {
       App: ComponentType
       wrapApp: (App: ComponentType) => any
       err?: Error
+      isFallback: boolean
     }
   ) {
     // represents the current component key
@@ -180,6 +184,8 @@ export default class Router implements BaseRouter {
     // back from external site
     this.isSsr = true
 
+    this.isFallback = isFallback
+
     if (typeof window !== 'undefined') {
       // in order for `e.state` to work on the `onpopstate` event
       // we have to register the initial route upon initialization
@@ -580,6 +586,8 @@ export default class Router implements BaseRouter {
     as: string,
     data: RouteInfo
   ): void {
+    this.isFallback = false
+
     this.route = route
     this.pathname = pathname
     this.query = query

packages/next/next-server/server/render.tsx

@@ -46,14 +46,22 @@ class ServerRouter implements NextRouter {
   query: ParsedUrlQuery
   asPath: string
   events: any
+  isFallback: boolean
   // TODO: Remove in the next major version, as this would mean the user is adding event listeners in server-side `render` method
   static events: MittEmitter = mitt()
 
-  constructor(pathname: string, query: ParsedUrlQuery, as: string) {
+  constructor(
+    pathname: string,
+    query: ParsedUrlQuery,
+    as: string,
+    { isFallback }: { isFallback: boolean }
+  ) {
     this.route = pathname.replace(/\/$/, '') || '/'
     this.pathname = pathname
     this.query = query
     this.asPath = as
+
+    this.isFallback = isFallback
   }
   push(): any {
     noRouter()
@@ -394,7 +402,9 @@ export async function renderToHTML(
 
   // @ts-ignore url will always be set
   const asPath: string = req.url
-  const router = new ServerRouter(pathname, query, asPath)
+  const router = new ServerRouter(pathname, query, asPath, {
+    isFallback: isFallback,
+  })
   const ctx = {
     err,
     req: isAutoExport ? undefined : req,

test/integration/prerender/pages/catchall/[...slug].js

@@ -1,4 +1,10 @@
+import { useRouter } from 'next/router'
+
 export async function unstable_getStaticProps({ params: { slug } }) {
+  if (slug[0] === 'delayby3s') {
+    await new Promise(resolve => setTimeout(resolve, 3000))
+  }
+
   return {
     props: {
       slug,
@@ -18,4 +24,10 @@ export async function unstable_getStaticPaths() {
   }
 }
 
-export default ({ slug }) => <p id="catchall">Hi {slug?.join('/')}</p>
+export default ({ slug }) => {
+  const { isFallback } = useRouter()
+  if (isFallback) {
+    return <p id="catchall">fallback</p>
+  }
+  return <p id="catchall">Hi {slug.join('/')}</p>
+}

test/integration/prerender/test/index.test.js

@@ -386,9 +386,20 @@ const runTests = (dev = false) => {
     }
     // Production will render fallback for a "lazy" route
     else {
-      const browser = await webdriver(appPort, '/catchall/notreturnedinpaths')
-      const text = await browser.elementByCss('#catchall').text()
-      expect(text).toMatch(/Hi.*?notreturnedinpaths/)
+      const html = await renderViaHTTP(appPort, '/catchall/notreturnedinpaths')
+      const $ = cheerio.load(html)
+      expect($('#catchall').text()).toBe('fallback')
+
+      // hydration
+      const browser = await webdriver(appPort, '/catchall/delayby3s')
+
+      const text1 = await browser.elementByCss('#catchall').text()
+      expect(text1).toBe('fallback')
+
+      await new Promise(resolve => setTimeout(resolve, 4000))
+
+      const text2 = await browser.elementByCss('#catchall').text()
+      expect(text2).toMatch(/Hi.*?delayby3s/)
     }
   })