App files ending with page registred as page files (#42996)

When building (`next/build/index.ts`) files ending with `page.js` got
picked up by the RegEx which meant files like `not-a-page.js` ended up
as a page. This change makes sure the segment actually starts with
`page`.

Dev had the same issue and also missed taking `pageExtensions` into
account.

Fixes #42972

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

packages/next/build/index.ts

@@ -493,7 +493,7 @@ export default async function build(
           .traceAsyncFn(() =>
             recursiveReadDir(
               appDir,
-              new RegExp(`page\\.(?:${config.pageExtensions.join('|')})$`)
+              new RegExp(`^page\\.(?:${config.pageExtensions.join('|')})$`)
             )
           )
       }

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

@@ -405,7 +405,7 @@ export default class DevServer extends Server {
           })
 
           if (isAppPath) {
-            if (!isLayoutsLeafPage(fileName)) {
+            if (!isLayoutsLeafPage(fileName, this.nextConfig.pageExtensions)) {
               continue
             }
 

packages/next/server/lib/find-page-file.ts

@@ -64,9 +64,9 @@ export async function findPageFile(
 }
 
 // Determine if the file is leaf node page file under layouts,
-// The filename should start with 'page', it can be either shared,
-// client, or server components with allowed page file extension.
-// e.g. page.js, page.server.js, page.client.tsx, etc.
-export function isLayoutsLeafPage(filePath: string) {
-  return /[\\/]?page\.((server|client)\.?)?[jt]sx?$/.test(filePath)
+// The filename should start with 'page' and end with one of the allowed extensions
+export function isLayoutsLeafPage(filePath: string, pageExtensions: string[]) {
+  return new RegExp(`(^page|/page)\\.(?:${pageExtensions.join('|')})$`).test(
+    filePath
+  )
 }

test/e2e/app-dir/app/app/catch-all/[...slug]/not-a-page.js

@@ -0,0 +1,3 @@
+export default function NotAPage() {
+  return <h1 id="not-a-page">Not a page</h1>
+}

test/e2e/app-dir/app/app/catch-all/[...slug]/page.js

@@ -1,4 +1,5 @@
 import Widget from './components/widget'
+import NotAPage from './not-a-page'
 
 export default function Page({ params }) {
   return (
@@ -7,6 +8,7 @@ export default function Page({ params }) {
         hello from /catch-all/{params.slug.join('/')}
       </h1>
       <Widget />
+      <NotAPage />
     </>
   )
 }

test/e2e/app-dir/index.test.ts

@@ -726,6 +726,7 @@ describe('app dir', () => {
           const html = await renderViaHTTP(next.url, `/catch-all/${route}`)
           const $ = cheerio.load(html)
           expect($('#text').attr('data-params')).toBe(route)
+          expect($('#not-a-page').text()).toBe('Not a page')
 
           // Components under catch-all should not be treated as route that errors during build.
           // They should be rendered properly when imported in page route.

test/unit/find-page-file.test.ts

@@ -46,16 +46,22 @@ describe('findPageFile', () => {
 })
 
 describe('isLayoutsLeafPage', () => {
+  const pageExtensions = ['tsx', 'ts', 'jsx', 'js']
   it('should determine either server or client component page file as leaf node page', () => {
-    expect(isLayoutsLeafPage('page.js')).toBe(true)
-    expect(isLayoutsLeafPage('./page.server.js')).toBe(true)
-    expect(isLayoutsLeafPage('./page.server.jsx')).toBe(true)
-    expect(isLayoutsLeafPage('./page.client.ts')).toBe(true)
-    expect(isLayoutsLeafPage('./page.client.tsx')).toBe(true)
+    expect(isLayoutsLeafPage('page.js', pageExtensions)).toBe(true)
+    expect(isLayoutsLeafPage('./page.js', pageExtensions)).toBe(true)
+    expect(isLayoutsLeafPage('./page.jsx', pageExtensions)).toBe(true)
+    expect(isLayoutsLeafPage('/page.ts', pageExtensions)).toBe(true)
+    expect(isLayoutsLeafPage('/path/page.tsx', pageExtensions)).toBe(true)
   })
 
   it('should determine other files under layout routes as non leaf node', () => {
-    expect(isLayoutsLeafPage('./page.component.jsx')).toBe(false)
-    expect(isLayoutsLeafPage('layout.js')).toBe(false)
+    expect(isLayoutsLeafPage('./not-a-page.js', pageExtensions)).toBe(false)
+    expect(isLayoutsLeafPage('not-a-page.js', pageExtensions)).toBe(false)
+    expect(isLayoutsLeafPage('./page.component.jsx', pageExtensions)).toBe(
+      false
+    )
+    expect(isLayoutsLeafPage('layout.js', pageExtensions)).toBe(false)
+    expect(isLayoutsLeafPage('page', pageExtensions)).toBe(false)
   })
 })