fix(rsc): harden CommonJS namespace interop

Author: TheAlexLichter

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

@@ -44,7 +44,7 @@ if (true) {
     expect(await testTransform(input)).toMatchInlineSnapshot(`
       "let __filename = "/test.js"; let __dirname = "/";
       let exports = {}; const module = { exports };
-      function __cjs_interop__(m) {return m.__cjs_module_runner_transform || "default" in m && Object.keys(m).every((k) => k === "default" || m[k] === m.default[k]) ? m.default : m;}
+      function __cjs_interop__(m) {return m.__cjs_module_runner_transform || "module.exports" in m && m["module.exports"] === m.default || "default" in m && Object.keys(m).every((k) => k === "default" || m.default != null && m[k] === m.default[k]) ? m.default : m;}
       if (true) {
         module.exports = (__cjs_interop__(await import('./cjs/use-sync-external-store.production.js')));
       } else {
@@ -69,7 +69,7 @@ if (true) {
     expect(await testTransform(input)).toMatchInlineSnapshot(`
       "let __filename = "/test.js"; let __dirname = "/";
       let exports = {}; const module = { exports };
-      function __cjs_interop__(m) {return m.__cjs_module_runner_transform || "default" in m && Object.keys(m).every((k) => k === "default" || m[k] === m.default[k]) ? m.default : m;}
+      function __cjs_interop__(m) {return m.__cjs_module_runner_transform || "module.exports" in m && m["module.exports"] === m.default || "default" in m && Object.keys(m).every((k) => k === "default" || m.default != null && m[k] === m.default[k]) ? 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() { 
@@ -100,7 +100,7 @@ function test() {
     expect(await testTransform(input)).toMatchInlineSnapshot(`
       "let __filename = "/test.js"; let __dirname = "/";
       let exports = {}; const module = { exports };
-      function __cjs_interop__(m) {return m.__cjs_module_runner_transform || "default" in m && Object.keys(m).every((k) => k === "default" || m[k] === m.default[k]) ? m.default : m;}
+      function __cjs_interop__(m) {return m.__cjs_module_runner_transform || "module.exports" in m && m["module.exports"] === m.default || "default" in m && Object.keys(m).every((k) => k === "default" || m.default != null && m[k] === m.default[k]) ? 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"));
@@ -196,6 +196,12 @@ function test() {
         },
         "depPrimitive": "[ok]",
         "dualLib": "ok",
+        "interop": {
+          "defaultOnlyNull": true,
+          "markerCallable": true,
+          "markerFalsy": true,
+          "nullDefaultNamespace": true,
+        },
         "testExternalFalsyPrimitive": {
           "ok": true,
         },

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

@@ -11,16 +11,28 @@ import { buildScopeTree } from './scope'
 // Runtime helper to handle CJS/ESM interop when transforming require() to import()
 // Unwrap .default for modules
 // 1. if it was transformed by this plugin (marked with __cjs_module_runner_transform)
-// 2. if all named exports point to .default properties (common CJS pattern)
+// 2. if Node marks .default as the exact CommonJS module.exports value
+//    https://nodejs.org/api/esm.html#commonjs-namespaces
+// 3. if all named exports point to .default properties (common CJS pattern)
 //    this is particularly important for Node built-in modules consumptions;
 //    where the built-in modules are not transformed by this plugin but still follow the CJS export pattern
 //    see [getESMFacade](https://github.com/nodejs/node/blob/f200685d9930404d610a52d9e06513bf0a821ed4/lib/internal/bootstrap/realm.js#L347-L360)
 //
-// This ensures we don't incorrectly unwrap .default on genuine ESM modules
+// Without a "module.exports" export, require(esm) returns the module namespace
+// object and exposes an ESM default export as its .default property:
+// https://nodejs.org/api/modules.html#loading-ecmascript-modules-using-require
+// https://tc39.es/ecma262/#sec-module-namespace-exotic-objects
+//
+// This ensures we don't incorrectly unwrap .default on genuine ESM modules.
+// The m.default != null guard allows genuine namespaces to have a nullish
+// default alongside named exports without dereferencing the nullish value.
 function __cjs_interop__(m: any) {
   return m.__cjs_module_runner_transform ||
+    ('module.exports' in m && m['module.exports'] === m.default) ||
     ('default' in m &&
-      Object.keys(m).every((k) => k === 'default' || m[k] === m.default[k]))
+      Object.keys(m).every(
+        (k) => k === 'default' || (m.default != null && m[k] === m.default[k]),
+      ))
     ? m.default
     : m
 }

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

@@ -6,6 +6,7 @@ import testExternalFalsyPrimitive from './external-falsy-primitive.cjs'
 import depFnRequire from './function-require.cjs'
 import depFn from './function.cjs'
 import cjsGlobals from './globals.cjs'
+import interop from './interop.cjs'
 import testNodeBuiltins from './node-builtins.cjs'
 import depPrimitive from './primitive.cjs'
 
@@ -20,4 +21,5 @@ export {
   cjsGlobals,
   testNodeBuiltins,
   testExternalFalsyPrimitive,
+  interop,
 }

packages/plugin-rsc/src/transforms/fixtures/cjs/interop.cjs

@@ -0,0 +1,12 @@
+// Node require(esm) interop semantics:
+// https://nodejs.org/api/modules.html#loading-ecmascript-modules-using-require
+const callable = require('./marker-callable.mjs')
+const falsy = require('./marker-falsy.mjs')
+const defaultOnlyNull = require('./null-default-only.mjs')
+const genuineEsm = require('./null-default.mjs')
+
+exports.markerCallable = callable() === 'ok'
+exports.markerFalsy = falsy === false
+exports.defaultOnlyNull = defaultOnlyNull === null
+exports.nullDefaultNamespace =
+  genuineEsm.default === null && genuineEsm.named === 'ok'

packages/plugin-rsc/src/transforms/fixtures/cjs/marker-callable.mjs

@@ -0,0 +1,5 @@
+const value = () => 'ok'
+
+// Node uses this named export to return the value directly from require(esm).
+// https://nodejs.org/api/modules.html#loading-ecmascript-modules-using-require
+export { value as default, value as 'module.exports' }

packages/plugin-rsc/src/transforms/fixtures/cjs/marker-falsy.mjs

@@ -0,0 +1,3 @@
+const value = false
+
+export { value as default, value as 'module.exports' }

packages/plugin-rsc/src/transforms/fixtures/cjs/null-default-only.mjs

@@ -0,0 +1 @@
+export default null

packages/plugin-rsc/src/transforms/fixtures/cjs/null-default.mjs

@@ -0,0 +1,2 @@
+export const named = 'ok'
+export default null