Fix mixed module swc compilation for app router

packages/next/src/build/handle-externals.ts

@@ -28,8 +28,9 @@ export function isResourceInPackages(
   resource: string,
   packageNames?: string[],
   packageDirMapping?: Map<string, string>
-) {
-  return packageNames?.some((p: string) =>
+): boolean {
+  if (!packageNames) return false
+  return packageNames.some((p: string) =>
     packageDirMapping && packageDirMapping.has(p)
       ? resource.startsWith(packageDirMapping.get(p)! + path.sep)
       : resource.includes(

packages/next/src/build/swc/options.ts

@@ -209,6 +209,22 @@ function getStyledComponentsOptions(
   }
 }
 
+/*
+Output module type
+
+For app router where server components is enabled, we prefer to bundle es6 modules,
+Use output module es6 to make sure:
+- the esm module is present
+- if the module is mixed syntax, the esm + cjs code are both present
+
+For pages router will remain untouched
+*/
+function getModuleOptions(
+  esm: boolean | undefined = false
+): { module: { type: 'es6' } } | {} {
+  return esm ? { module: { type: 'es6' } } : {}
+}
+
 function getEmotionOptions(
   emotionConfig: undefined | boolean | EmotionConfig,
   development: boolean
@@ -319,6 +335,7 @@ export function getLoaderSWCOptions({
   relativeFilePathFromRoot,
   serverComponents,
   isReactServerLayer,
+  esm,
 }: {
   filename: string
   development: boolean
@@ -338,6 +355,7 @@ export function getLoaderSWCOptions({
   supportedBrowsers: string[] | undefined
   swcCacheDir: string
   relativeFilePathFromRoot: string
+  esm?: boolean
   serverComponents?: boolean
   isReactServerLayer?: boolean
 }) {
@@ -412,6 +430,7 @@ export function getLoaderSWCOptions({
           node: process.versions.node,
         },
       },
+      ...getModuleOptions(esm),
     }
   } else {
     const options = {
@@ -423,7 +442,7 @@ export function getLoaderSWCOptions({
               type: 'commonjs',
             },
           }
-        : {}),
+        : getModuleOptions(esm)),
       disableNextSsg: !isPageFile,
       isDevelopment: development,
       isServerCompiler: isServer,

packages/next/src/build/webpack-config-rules/resolve.ts

@@ -12,20 +12,20 @@ export const edgeConditionNames = [
 ]
 
 const mainFieldsPerCompiler: Record<
-  CompilerNameValues | 'app-router-server',
+  CompilerNameValues | 'server-esm',
   string[]
 > = {
   // For default case, prefer CJS over ESM on server side. e.g. pages dir SSR
   [COMPILER_NAMES.server]: ['main', 'module'],
   [COMPILER_NAMES.client]: ['browser', 'module', 'main'],
   [COMPILER_NAMES.edgeServer]: edgeConditionNames,
-  // For app router since everything is bundled, prefer ESM over CJS
-  'app-router-server': ['module', 'main'],
+  // For bundling-all strategy, prefer ESM over CJS
+  'server-esm': ['module', 'main'],
 }
 
 export function getMainField(
-  pageType: 'app' | 'pages',
-  compilerType: CompilerNameValues
+  compilerType: CompilerNameValues,
+  preferEsm: boolean
 ) {
   if (compilerType === COMPILER_NAMES.edgeServer) {
     return edgeConditionNames
@@ -34,7 +34,7 @@ export function getMainField(
   }
 
   // Prefer module fields over main fields for isomorphic packages on server layer
-  return pageType === 'app'
-    ? mainFieldsPerCompiler['app-router-server']
+  return preferEsm
+    ? mainFieldsPerCompiler['server-esm']
     : mainFieldsPerCompiler[COMPILER_NAMES.server]
 }

packages/next/src/build/webpack-config.ts

@@ -437,17 +437,26 @@ export default async function getBaseWebpackConfig(
     }
   }
 
+  // RSC loaders, prefer ESM, set `esm` to true
   const swcServerLayerLoader = getSwcLoader({
     serverComponents: true,
     isReactServerLayer: true,
+    esm: true,
   })
   const swcClientLayerLoader = getSwcLoader({
     serverComponents: true,
     isReactServerLayer: false,
+    esm: true,
+  })
+  // Default swc loaders for pages doesn't prefer ESM.
+  const swcDefaultLoader = getSwcLoader({
+    serverComponents: true,
+    isReactServerLayer: false,
+    esm: false,
   })
 
   const defaultLoaders = {
-    babel: useSWCLoader ? swcClientLayerLoader : babelLoader!,
+    babel: useSWCLoader ? swcDefaultLoader : babelLoader!,
   }
 
   const swcLoaderForServerLayer = hasAppDir
@@ -621,7 +630,7 @@ export default async function getBaseWebpackConfig(
         }
       : undefined),
     // default main fields use pages dir ones, and customize app router ones in loaders.
-    mainFields: getMainField('pages', compilerType),
+    mainFields: getMainField(compilerType, false),
     ...(isEdgeServer && {
       conditionNames: edgeConditionNames,
     }),
@@ -736,8 +745,13 @@ export default async function getBaseWebpackConfig(
   const shouldIncludeExternalDirs =
     config.experimental.externalDir || !!config.transpilePackages
 
-  function createLoaderRuleExclude(skipNodeModules: boolean) {
-    return (excludePath: string) => {
+  const codeCondition = {
+    test: /\.(tsx|ts|js|cjs|mjs|jsx)$/,
+    ...(shouldIncludeExternalDirs
+      ? // Allowing importing TS/TSX files from outside of the root dir.
+        {}
+      : { include: [dir, ...babelIncludeRegexes] }),
+    exclude: (excludePath: string) => {
       if (babelIncludeRegexes.some((r) => r.test(excludePath))) {
         return false
       }
@@ -748,17 +762,8 @@ export default async function getBaseWebpackConfig(
       )
       if (shouldBeBundled) return false
 
-      return skipNodeModules && excludePath.includes('node_modules')
-    }
-  }
-
-  const codeCondition = {
-    test: /\.(tsx|ts|js|cjs|mjs|jsx)$/,
-    ...(shouldIncludeExternalDirs
-      ? // Allowing importing TS/TSX files from outside of the root dir.
-        {}
-      : { include: [dir, ...babelIncludeRegexes] }),
-    exclude: createLoaderRuleExclude(true),
+      return excludePath.includes('node_modules')
+    },
   }
 
   let webpackConfig: webpack.Configuration = {
@@ -1281,7 +1286,7 @@ export default async function getBaseWebpackConfig(
                   ],
                 },
                 resolve: {
-                  mainFields: getMainField('app', compilerType),
+                  mainFields: getMainField(compilerType, true),
                   conditionNames: reactServerCondition,
                   // If missing the alias override here, the default alias will be used which aliases
                   // react to the direct file path, not the package name. In that case the condition
@@ -1416,15 +1421,15 @@ export default async function getBaseWebpackConfig(
                     issuerLayer: [WEBPACK_LAYERS.appPagesBrowser],
                     use: swcLoaderForClientLayer,
                     resolve: {
-                      mainFields: getMainField('app', compilerType),
+                      mainFields: getMainField(compilerType, true),
                     },
                   },
                   {
                     test: codeCondition.test,
                     issuerLayer: [WEBPACK_LAYERS.serverSideRendering],
                     use: swcLoaderForClientLayer,
                     resolve: {
-                      mainFields: getMainField('app', compilerType),
+                      mainFields: getMainField(compilerType, true),
                     },
                   },
                 ]

packages/next/src/build/webpack/loaders/next-swc-loader.ts

@@ -44,6 +44,7 @@ export interface SWCLoaderOptions {
   swcCacheDir: string
   serverComponents?: boolean
   isReactServerLayer?: boolean
+  esm?: boolean
 }
 
 async function loaderTransform(
@@ -69,6 +70,7 @@ async function loaderTransform(
     swcCacheDir,
     serverComponents,
     isReactServerLayer,
+    esm,
   } = loaderOptions
   const isPageFile = filename.startsWith(pagesDir)
   const relativeFilePathFromRoot = path.relative(rootDir, filename)
@@ -92,6 +94,7 @@ async function loaderTransform(
     relativeFilePathFromRoot,
     serverComponents,
     isReactServerLayer,
+    esm,
   })
 
   const programmaticOptions = {

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

@@ -210,6 +210,24 @@ createNextDescribe(
       })
     })
 
+    describe('mixed syntax external modules', () => {
+      it('should handle mixed module with next/dynamic', async () => {
+        const browser = await next.browser('/mixed/dynamic')
+        expect(await browser.elementByCss('#component').text()).toContain(
+          'mixed-syntax-esm'
+        )
+      })
+
+      it('should handle mixed module in server and client components', async () => {
+        const $ = await next.render$('/mixed/import')
+        expect(await $('#server').text()).toContain('server:mixed-syntax-esm')
+        expect(await $('#client').text()).toContain('client:mixed-syntax-esm')
+        expect(await $('#relative-mixed').text()).toContain(
+          'relative-mixed-syntax-esm'
+        )
+      })
+    })
+
     it('should export client module references in esm', async () => {
       const html = await next.render('/esm-client-ref')
       expect(html).toContain('hello')

test/e2e/app-dir/app-external/app/mixed/dynamic/page.js

@@ -0,0 +1,12 @@
+'use client'
+
+import dynamic from 'next/dynamic'
+
+const Dynamic = dynamic(
+  () => import('mixed-syntax-esm').then((mod) => mod.Component),
+  { ssr: false }
+)
+
+export default function Page() {
+  return <Dynamic />
+}

test/e2e/app-dir/app-external/app/mixed/import/client.js

@@ -0,0 +1,7 @@
+'use client'
+
+import { value } from 'mixed-syntax-esm'
+
+export function Client() {
+  return 'client:' + value
+}

test/e2e/app-dir/app-external/app/mixed/import/mixed-mod.mjs

@@ -0,0 +1,5 @@
+;(module) => {
+  module.exports = {}
+}
+
+export const value = 'relative-mixed-syntax-esm'

test/e2e/app-dir/app-external/app/mixed/import/page.js

@@ -0,0 +1,15 @@
+import { value } from 'mixed-syntax-esm'
+import { Client } from './client'
+import { value as relativeMixedValue } from './mixed-mod.mjs'
+
+export default function Page() {
+  return (
+    <>
+      <p id="server">{'server:' + value}</p>
+      <p id="client">
+        <Client />
+      </p>
+      <p id="relative-mixed">{relativeMixedValue}</p>
+    </>
+  )
+}

test/e2e/app-dir/app-external/node_modules_bak/mixed-syntax-esm/index.mjs

@@ -0,0 +1,14 @@
+import React from 'react'
+;(module) => {
+  module.exports = {}
+}
+
+export const value = 'mixed-syntax-esm'
+
+export function Component() {
+  return /*#__PURE__*/ React.createElement(
+    'p',
+    { id: 'component' },
+    'mixed-syntax-esm'
+  )
+}

test/e2e/app-dir/app-external/node_modules_bak/mixed-syntax-esm/package.json

@@ -0,0 +1,3 @@
+{
+  "exports": "./index.mjs"
+}

test/e2e/app-dir/third-parties/app/gtm/page.js

@@ -9,7 +9,7 @@ const Page = () => {
   }
 
   return (
-    <div class="container">
+    <div className="container">
       <GoogleTagManager gtmId="GTM-XYZ" />
       <h1>GTM</h1>
       <button id="gtm-send" onClick={onClick}>