Revert "Remove `config` export (#8895)" (#8982)

This reverts commit 67f7cf56.

packages/next/build/babel/plugins/next-page-config.ts

@@ -55,18 +55,11 @@ export default function nextPageConfig({
                 path: NodePath<BabelTypes.ExportNamedDeclaration>,
                 state: any
               ) {
-                // Skip if the file will be dropped
-                if (state.bundleDropped) {
+                if (state.bundleDropped || !path.node.declaration) {
                   return
                 }
-
-                // Bail out of `export { a, b, c };` case.
-                // We should probably support this.
-                if (!path.node.declaration) {
-                  return
-                }
-
                 const { declarations, id } = path.node.declaration as any
+                const config: PageConfig = {}
 
                 // drop SSR Exports for client bundles
                 if (
@@ -85,11 +78,7 @@ export default function nextPageConfig({
                 if (!declarations) {
                   return
                 }
-
-                const config: PageConfig = {}
-                for (let dIndex = 0; dIndex < declarations.length; ++dIndex) {
-                  const declaration = declarations[dIndex]
-
+                for (const declaration of declarations) {
                   if (declaration.id.name !== 'config') {
                     continue
                   }
@@ -113,11 +102,6 @@ export default function nextPageConfig({
                       config[name] = prop.value.value
                     }
                   }
-
-                  declarations.splice(dIndex, 1)
-                  if (declarations.length === 0) {
-                    path.remove()
-                  }
                 }
 
                 if (config.amp === true) {

test/integration/page-config/next.config.js

-module.exports = { generateBuildId: () => 'test-page-config' }

test/integration/page-config/pages/check-client-remove.js

-import { config as hello } from '../something'
-import { config as world } from '../config'
-
-export const config = { _unknownKey: 'THIS_VALUE_SHOULD_BE_GONE' }
-
-export default () => (
-  <p>
-    {hello} {world}
-  </p>
-)

test/integration/page-config/test/index.test.js

@@ -13,16 +13,6 @@ describe('Page Config', () => {
   it('builds without error when export const config is used outside page', async () => {
     const { stderr } = await nextBuild(appDir, undefined, { stderr: true })
     expect(stderr).not.toMatch(/Failed to compile\./)
-
-    const clientFileOutput = await fs.readFile(
-      join(
-        appDir,
-        '.next/static/test-page-config',
-        'pages/check-client-remove.js'
-      ),
-      'utf8'
-    )
-    expect(clientFileOutput).not.toMatch('THIS_VALUE_SHOULD_BE_GONE')
   })
 
   it('shows valid error on invalid page config', async () => {