fix: not-found.tsx with output: export

test/integration/app-dir-export/app/not-found.js

@@ -0,0 +1,3 @@
+export default function NotFound() {
+  return <h1>My custom not found page</h1>
+}

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

@@ -4,7 +4,7 @@ import { join } from 'path'
 import {
   appDir,
   distDir,
-  expectedFiles,
+  expectedWhenTrailingSlashTrue,
   exportDir,
   getFiles,
   nextConfig,
@@ -36,7 +36,7 @@ describe('app dir with output export (next dev / next build)', () => {
     await fs.remove(distDir)
     await fs.remove(exportDir)
     await nextBuild(appDir)
-    expect(await getFiles()).toEqual(expectedFiles)
+    expect(await getFiles()).toEqual(expectedWhenTrailingSlashTrue)
     let stdout = ''
     let stderr = ''
     await nextExportDefault(appDir, {
@@ -51,13 +51,13 @@ describe('app dir with output export (next dev / next build)', () => {
       '- warn "next export" is no longer needed when "output: export" is configured in next.config.js'
     )
     expect(stdout).toContain('Export successful. Files written to')
-    expect(await getFiles()).toEqual(expectedFiles)
+    expect(await getFiles()).toEqual(expectedWhenTrailingSlashTrue)
   })
   it('should error when "next export -o <dir>" is used with config', async () => {
     await fs.remove(distDir)
     await fs.remove(exportDir)
     await nextBuild(appDir)
-    expect(await getFiles()).toEqual(expectedFiles)
+    expect(await getFiles()).toEqual(expectedWhenTrailingSlashTrue)
     let stdout = ''
     let stderr = ''
     let error = undefined
@@ -132,7 +132,7 @@ describe('app dir with output export (next dev / next build)', () => {
     )
     try {
       await nextBuild(appDir)
-      expect(await getFiles(outputDir)).toEqual(expectedFiles)
+      expect(await getFiles(outputDir)).toEqual(expectedWhenTrailingSlashTrue)
     } finally {
       nextConfig.restore()
       await fs.remove(distDir)

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

@@ -27,7 +27,7 @@ export const nextConfig = new File(join(appDir, 'next.config.js'))
 const slugPage = new File(join(appDir, 'app/another/[slug]/page.js'))
 const apiJson = new File(join(appDir, 'app/api/json/route.js'))
 
-export const expectedFiles = [
+export const expectedWhenTrailingSlashTrue = [
   '404.html',
   '404/index.html',
   '_next/static/media/test.3f1a293b.png',
@@ -49,6 +49,27 @@ export const expectedFiles = [
   'robots.txt',
 ]
 
+const expectedWhenTrailingSlashFalse = [
+  '404.html',
+  '_next/static/media/test.3f1a293b.png',
+  '_next/static/test-build-id/_buildManifest.js',
+  '_next/static/test-build-id/_ssgManifest.js',
+  'another.html',
+  'another.txt',
+  'another/first.html',
+  'another/first.txt',
+  'another/second.html',
+  'another/second.txt',
+  'api/json',
+  'api/txt',
+  'favicon.ico',
+  'image-import.html',
+  'image-import.txt',
+  'index.html',
+  'index.txt',
+  'robots.txt',
+]
+
 export async function getFiles(cwd = exportDir) {
   const opts = { cwd, nodir: true }
   const files = ((await glob('**/*', opts)) as string[])
@@ -74,19 +95,19 @@ export async function runTests({
   dynamicApiRoute?: string
   expectedErrMsg?: string
 }) {
-  if (trailingSlash) {
+  if (trailingSlash !== undefined) {
     nextConfig.replace(
       'trailingSlash: true,',
       `trailingSlash: ${trailingSlash},`
     )
   }
-  if (dynamicPage) {
+  if (dynamicPage !== undefined) {
     slugPage.replace(
       `const dynamic = 'force-static'`,
       `const dynamic = ${dynamicPage}`
     )
   }
-  if (dynamicApiRoute) {
+  if (dynamicApiRoute !== undefined) {
     apiJson.replace(
       `const dynamic = 'force-static'`,
       `const dynamic = ${dynamicApiRoute}`
@@ -156,7 +177,6 @@ export async function runTests({
       await check(() => browser.elementByCss('h1').text(), 'Home')
       expect(await browser.elementByCss(a(3)).text()).toBe('another first page')
       await browser.elementByCss(a(3)).click()
-
       await check(() => browser.elementByCss('h1').text(), 'first')
       expect(await browser.elementByCss(a(1)).text()).toBe('Visit another page')
       await browser.elementByCss(a(1)).click()
@@ -188,8 +208,14 @@ export async function runTests({
       expect(res2.status).toBe(200)
       expect(await res2.text()).toEqual('this is plain text')
 
-      if (!isDev && trailingSlash) {
-        expect(await getFiles()).toEqual(expectedFiles)
+      if (!isDev) {
+        if (trailingSlash) {
+          expect(await getFiles()).toEqual(expectedWhenTrailingSlashTrue)
+        } else {
+          expect(await getFiles()).toEqual(expectedWhenTrailingSlashFalse)
+        }
+        const html404 = await fs.readFile(join(exportDir, '404.html'), 'utf8')
+        expect(html404).toContain('<h1>My custom not found page</h1>')
       }
     }
   } finally {