fix: app dir with next dev and output: export (#47171)

This PR is a follow up to PR #47022 which broke `next dev`.

A test has been added to confirm `next dev` works as expected.

fix NEXT-825 ([link](https://linear.app/vercel/issue/NEXT-825)) ([NEXT-825](https://linear.app/vercel/issue/NEXT-825))

packages/next/src/client/components/router-reducer/fetch-server-response.ts

@@ -39,12 +39,14 @@ export async function fetchServerResponse(
 
   try {
     let fetchUrl = url
-    if (process.env.__NEXT_CONFIG_OUTPUT === 'export') {
-      fetchUrl = new URL(url) // clone
-      if (fetchUrl.pathname.endsWith('/')) {
-        fetchUrl.pathname += 'index.txt'
-      } else {
-        fetchUrl.pathname += '.txt'
+    if (process.env.NODE_ENV === 'production') {
+      if (process.env.__NEXT_CONFIG_OUTPUT === 'export') {
+        fetchUrl = new URL(url) // clone
+        if (fetchUrl.pathname.endsWith('/')) {
+          fetchUrl.pathname += 'index.txt'
+        } else {
+          fetchUrl.pathname += '.txt'
+        }
       }
     }
     const res = await fetch(fetchUrl, {
@@ -59,9 +61,11 @@ export async function fetchServerResponse(
     const contentType = res.headers.get('content-type') || ''
     let isFlightResponse = contentType === RSC_CONTENT_TYPE_HEADER
 
-    if (process.env.__NEXT_CONFIG_OUTPUT === 'export') {
-      if (!isFlightResponse) {
-        isFlightResponse = contentType.startsWith('text/plain')
+    if (process.env.NODE_ENV === 'production') {
+      if (process.env.__NEXT_CONFIG_OUTPUT === 'export') {
+        if (!isFlightResponse) {
+          isFlightResponse = contentType.startsWith('text/plain')
+        }
       }
     }
 

packages/next/src/server/dev/next-dev-server.ts

@@ -535,6 +535,7 @@ export default class DevServer extends Server {
           })
 
           if (
+            !isAppPath &&
             pageName.startsWith('/api/') &&
             this.nextConfig.output === 'export'
           ) {
@@ -1535,6 +1536,7 @@ export default class DevServer extends Server {
     staticPaths?: string[]
     fallbackMode?: false | 'static' | 'blocking'
   }> {
+    const isAppPath = Boolean(originalAppPath)
     // we lazy load the staticPaths to prevent the user
     // from waiting on them for the page to load in dev mode
 
@@ -1561,7 +1563,7 @@ export default class DevServer extends Server {
         locales,
         defaultLocale,
         originalAppPath,
-        isAppPath: !!originalAppPath,
+        isAppPath,
         requestHeaders,
         incrementalCacheHandlerPath:
           this.nextConfig.experimental.incrementalCacheHandlerPath,
@@ -1579,7 +1581,7 @@ export default class DevServer extends Server {
     )
       .then((res) => {
         const { paths: staticPaths = [], fallback } = res.value
-        if (this.nextConfig.output === 'export') {
+        if (!isAppPath && this.nextConfig.output === 'export') {
           if (fallback === 'blocking') {
             throw new Error(
               'getStaticPaths with "fallback: blocking" cannot be used with "output: export". See more info here: https://nextjs.org/docs/advanced-features/static-html-export'

test/integration/app-dir-export/test/index.test.ts

@@ -7,6 +7,9 @@ import webdriver from 'next-webdriver'
 import globOrig from 'glob'
 import {
   File,
+  findPort,
+  killApp,
+  launchApp,
   nextBuild,
   nextExport,
   startStaticServer,
@@ -20,12 +23,13 @@ const distDir = join(__dirname, '.next')
 const exportDir = join(appDir, 'out')
 const nextConfig = new File(join(appDir, 'next.config.js'))
 const slugPage = new File(join(appDir, 'app/another/[slug]/page.js'))
-const delay = 100
 
 async function runTests({
+  isDev,
   trailingSlash,
   dynamic,
 }: {
+  isDev?: boolean
   trailingSlash?: boolean
   dynamic?: string
 }) {
@@ -43,12 +47,18 @@ async function runTests({
   }
   await fs.remove(distDir)
   await fs.remove(exportDir)
-  await nextBuild(appDir)
-  await nextExport(appDir, { outdir: exportDir })
-  const app = await startStaticServer(exportDir)
-  const address = app.address()
-  const appPort = typeof address !== 'string' ? address.port : 3000
-
+  const delay = isDev ? 500 : 100
+  const appPort = await findPort()
+  let stopOrKill: () => Promise<void>
+  if (isDev) {
+    const app = await launchApp(appDir, appPort)
+    stopOrKill = async () => await killApp(app)
+  } else {
+    await nextBuild(appDir)
+    await nextExport(appDir, { outdir: exportDir })
+    const app = await startStaticServer(exportDir, null, appPort)
+    stopOrKill = async () => await stopApp(app)
+  }
   try {
     const a = (n: number) => `li:nth-child(${n}) a`
     console.log('[navigate]')
@@ -108,15 +118,20 @@ async function runTests({
       '/test.3f1a293b.png'
     )
   } finally {
-    await stopApp(app)
+    await stopOrKill()
     nextConfig.restore()
     slugPage.restore()
   }
 }
 
-describe('app dir with next export', () => {
-  it.each([{ trailingSlash: false }, { trailingSlash: true }])(
-    "should work with trailingSlash '$trailingSlash'",
+describe('app dir with output export', () => {
+  it.each([
+    { isDev: true, trailingSlash: false },
+    { isDev: true, trailingSlash: true },
+    { isDev: false, trailingSlash: false },
+    { isDev: false, trailingSlash: true },
+  ])(
+    "should work with isDev '$isDev' and trailingSlash '$trailingSlash'",
     async ({ trailingSlash }) => {
       await runTests({ trailingSlash })
     }