fix(rsc/cjs): unwrap default based on __cjs_module_runner_transform marker (#905)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: hi-ogawa <4232207+hi-ogawa@users.noreply.github.com>
Co-authored-by: Hiroshi Ogawa <hi.ogawa.zz@gmail.com>

Author: Copilot

packages/plugin-rsc/package.json

@@ -56,6 +56,7 @@
     "@types/react": "^19.1.16",
     "@types/react-dom": "^19.1.9",
     "@vitejs/plugin-react": "workspace:*",
+    "@vitejs/test-dep-cjs-and-esm": "./test-dep/cjs-and-esm",
     "react": "^19.1.1",
     "react-dom": "^19.1.1",
     "react-server-dom-webpack": "^19.1.1",

packages/plugin-rsc/src/plugins/cjs.ts

@@ -60,6 +60,7 @@ export function cjsModuleRunnerPlugin(): Plugin[] {
           output.append(`
 ;__vite_ssr_exportAll__(module.exports);
 export default module.exports;
+export const __cjs_module_runner_transform = true;
 `)
           return {
             code: output.toString(),

packages/plugin-rsc/src/transforms/cjs.test.ts

@@ -38,10 +38,11 @@ if (true) {
 `
     expect(await testTransform(input)).toMatchInlineSnapshot(`
       "let exports = {}; const module = { exports };
+      function __cjs_interop__(m) { return m.__cjs_module_runner_transform ? m.default : m; }
       if (true) {
-        module.exports = ((await import('./cjs/use-sync-external-store.production.js')).default);
+        module.exports = (__cjs_interop__(await import('./cjs/use-sync-external-store.production.js')));
       } else {
-        module.exports = ((await import('./cjs/use-sync-external-store.development.js')).default);
+        module.exports = (__cjs_interop__(await import('./cjs/use-sync-external-store.development.js')));
       }
       "
     `)
@@ -57,8 +58,9 @@ if (true) {
 `
     expect(await testTransform(input)).toMatchInlineSnapshot(`
       "let exports = {}; const module = { exports };
-      const __cjs_to_esm_hoist_0 = (await import("react")).default;
-      const __cjs_to_esm_hoist_1 = (await import("react-dom")).default;
+      function __cjs_interop__(m) { return m.__cjs_module_runner_transform ? m.default : m; }
+      const __cjs_to_esm_hoist_0 = __cjs_interop__(await import("react"));
+      const __cjs_to_esm_hoist_1 = __cjs_interop__(await import("react-dom"));
       "production" !== process.env.NODE_ENV && (function() { 
         var React = __cjs_to_esm_hoist_0;
         var ReactDOM = __cjs_to_esm_hoist_1;
@@ -82,12 +84,13 @@ function test() {
 `
     expect(await testTransform(input)).toMatchInlineSnapshot(`
       "let exports = {}; const module = { exports };
-      const __cjs_to_esm_hoist_0 = (await import("te" + "st")).default;
-      const __cjs_to_esm_hoist_1 = (await import("test")).default;
-      const __cjs_to_esm_hoist_2 = (await import("test")).default;
-      const x1 = ((await import("te" + "st")).default);
-      const x2 = ((await import("test")).default)().test;
-      console.log(((await import("test")).default))
+      function __cjs_interop__(m) { return m.__cjs_module_runner_transform ? m.default : m; }
+      const __cjs_to_esm_hoist_0 = __cjs_interop__(await import("te" + "st"));
+      const __cjs_to_esm_hoist_1 = __cjs_interop__(await import("test"));
+      const __cjs_to_esm_hoist_2 = __cjs_interop__(await import("test"));
+      const x1 = (__cjs_interop__(await import("te" + "st")));
+      const x2 = (__cjs_interop__(await import("test")))().test;
+      console.log((__cjs_interop__(await import("test"))))
 
       function test() {
         const y1 = __cjs_to_esm_hoist_0;
@@ -130,6 +133,7 @@ function test() {
               output.append(`
 ;__vite_ssr_exportAll__(module.exports);
 export default module.exports;
+export const __cjs_module_runner_transform = true;
 `)
               return {
                 code: output.toString(),
@@ -156,6 +160,7 @@ export default module.exports;
           "value": 3,
         },
         "depNamespace": {
+          "__cjs_module_runner_transform": true,
           "a": "a",
           "b": "b",
           "default": {
@@ -164,6 +169,7 @@ export default module.exports;
           },
         },
         "depPrimitive": "[ok]",
+        "dualLib": "ok",
       }
     `)
   })

packages/plugin-rsc/src/transforms/cjs.ts

@@ -3,6 +3,14 @@ import MagicString from 'magic-string'
 import { analyze } from 'periscopic'
 import { walk } from 'estree-walker'
 
+// TODO:
+// replacing require("xxx") into import("xxx") affects Vite's resolution.
+
+// Runtime helper to handle CJS/ESM interop when transforming require() to import()
+// Only unwrap .default for modules that were transformed by this plugin (marked with __cjs_module_runner_transform)
+// This ensures we don't incorrectly unwrap .default on genuine ESM modules
+const CJS_INTEROP_HELPER = `function __cjs_interop__(m) { return m.__cjs_module_runner_transform ? m.default : m; }`
+
 export function transformCjsToEsm(
   code: string,
   ast: Program,
@@ -13,6 +21,7 @@ export function transformCjsToEsm(
   const parentNodes: Node[] = []
   const hoistedCodes: string[] = []
   let hoistIndex = 0
+
   walk(ast, {
     enter(node) {
       parentNodes.push(node)
@@ -39,10 +48,14 @@ export function transformCjsToEsm(
         }
 
         if (isTopLevel) {
-          // top-level scope `require` to dynamic import
+          // top-level scope `require` to dynamic import with interop
           // (this allows handling react development/production re-export within top-level if branch)
-          output.update(node.start, node.callee.end, '((await import')
-          output.appendRight(node.end, ').default)')
+          output.update(
+            node.start,
+            node.callee.end,
+            '(__cjs_interop__(await import',
+          )
+          output.appendRight(node.end, '))')
         } else {
           // hoist non top-level `require` to top-level
           const hoisted = `__cjs_to_esm_hoist_${hoistIndex}`
@@ -51,7 +64,7 @@ export function transformCjsToEsm(
             node.arguments[0]!.end,
           )
           hoistedCodes.push(
-            `const ${hoisted} = (await import(${importee})).default;\n`,
+            `const ${hoisted} = __cjs_interop__(await import(${importee}));\n`,
           )
           output.update(node.start, node.end, hoisted)
           hoistIndex++
@@ -65,6 +78,9 @@ export function transformCjsToEsm(
   for (const hoisted of hoistedCodes.reverse()) {
     output.prepend(hoisted)
   }
+  if (output.hasChanged()) {
+    output.prepend(`${CJS_INTEROP_HELPER}\n`)
+  }
   // https://nodejs.org/docs/v22.19.0/api/modules.html#exports-shortcut
   output.prepend(`let exports = {}; const module = { exports };\n`)
   return { output }

packages/plugin-rsc/src/transforms/fixtures/cjs/dual-lib.cjs

@@ -0,0 +1,2 @@
+const lib = require('@vitejs/test-dep-cjs-and-esm')
+module.exports = lib.ok

packages/plugin-rsc/src/transforms/fixtures/cjs/entry.mjs

@@ -4,11 +4,13 @@ import depFn from './function.cjs'
 import depPrimitive from './primitive.cjs'
 import depExports from './exports.cjs'
 import depFnRequire from './function-require.cjs'
+import dualLib from './dual-lib.cjs'
 export {
   depDefault,
   depNamespace,
   depFn,
   depPrimitive,
   depExports,
   depFnRequire,
+  dualLib,
 }

packages/plugin-rsc/test-dep/cjs-and-esm/index.cjs

@@ -0,0 +1 @@
+exports.ok = 'ok'

packages/plugin-rsc/test-dep/cjs-and-esm/index.mjs

@@ -0,0 +1 @@
+export const ok = 'ok'

packages/plugin-rsc/test-dep/cjs-and-esm/package.json

@@ -0,0 +1,11 @@
+{
+  "name": "@vitejs/test-dep-cjs-and-esm",
+  "private": true,
+  "type": "module",
+  "exports": {
+    ".": {
+      "require": "./index.cjs",
+      "default": "./index.mjs"
+    }
+  }
+}