feat!: remove ssr proxy for externalized modules

docs/guide/migration.md

@@ -34,6 +34,32 @@ See the [troubleshooting guide](/guide/troubleshooting.html#vite-cjs-node-api-de
 
 ## General Changes
 
+### SSR externalized modules value now matches production
+
+In Vite 4, SSR externalized modules are wrapped with `.default` and `.__esModule` handling for better interoperability, but it doesn't match the production behaviour when loaded by the runtime environment (e.g. Node.js), causing hard-to-catch inconsistencies. By default, all direct project dependencies are SSR externalized.
+
+Vite 5 now removes the `.default` and `.__esModule` handling to match the production behaviour. In practice, this shouldn't affect properly-packaged dependencies, but if you encounter new issues loading modules, you can try these refactors:
+
+```js
+// Before:
+import { foo } from 'bar'
+
+// After:
+import _bar from 'bar'
+const { foo } = _bar
+```
+
+```js
+// Before:
+import foo from 'bar'
+
+// After:
+import * as _foo from 'bar'
+const foo = _foo.default
+```
+
+Note that these changes matches the Node.js behaviour, so you can also run the imports in Node.js to test it out. If you prefer to stick with the previous behaviour, you can set `legacy.proxySsrExternalModules` to `true`.
+
 ### `worker.plugins` is now a function
 
 In Vite 4, `worker.plugins` accepted an array of plugins (`(Plugin | Plugin[])[]`). From Vite 5, it needs to be configured as a function that returns an array of plugins (`() => (Plugin | Plugin[])[]`). This change is required so parallel worker builds run more consistently and predictably.

packages/vite/src/node/config.ts

@@ -32,9 +32,9 @@ import {
   createFilter,
   isBuiltin,
   isExternalUrl,
+  isFilePathESM,
   isNodeBuiltin,
   isObject,
-  lookupFile,
   mergeAlias,
   mergeConfig,
   normalizeAlias,
@@ -314,8 +314,16 @@ export interface ExperimentalOptions {
 
 export interface LegacyOptions {
   /**
-   * No longer needed for now, but kept for backwards compatibility.
+   * In Vite 4, SSR-externalized modules (modules not bundled and loaded by Node.js at runtime)
+   * are implicitly proxied in dev to automatically handle `default` and `__esModule` access.
+   * However, this does not correctly reflect how it works in the Node.js runtime, causing
+   * inconsistencies between dev and prod.
+   *
+   * In Vite 5, the proxy is removed so dev and prod are consistent, but if you still require
+   * the old behaviour, you can enable this option. If so, please leave your feedback at
+   * https://github.com/vitejs/vite/discussions/14697.
    */
+  proxySsrExternalModules?: boolean
 }
 
 export interface ResolvedWorkerOptions {
@@ -978,19 +986,7 @@ export async function loadConfigFromFile(
     return null
   }
 
-  let isESM = false
-  if (/\.m[jt]s$/.test(resolvedPath)) {
-    isESM = true
-  } else if (/\.c[jt]s$/.test(resolvedPath)) {
-    isESM = false
-  } else {
-    // check package.json for type: "module" and set `isESM` to true
-    try {
-      const pkg = lookupFile(configRoot, ['package.json'])
-      isESM =
-        !!pkg && JSON.parse(fs.readFileSync(pkg, 'utf-8')).type === 'module'
-    } catch (e) {}
-  }
+  const isESM = isFilePathESM(resolvedPath)
 
   try {
     const bundled = await bundleConfigFile(resolvedPath, isESM)
@@ -1076,18 +1072,6 @@ async function bundleConfigFile(
               false,
             )?.id
           }
-          const isESMFile = (id: string): boolean => {
-            if (id.endsWith('.mjs')) return true
-            if (id.endsWith('.cjs')) return false
-
-            const nearestPackageJson = findNearestPackageData(
-              path.dirname(id),
-              packageCache,
-            )
-            return (
-              !!nearestPackageJson && nearestPackageJson.data.type === 'module'
-            )
-          }
 
           // externalize bare imports
           build.onResolve(
@@ -1135,7 +1119,11 @@ async function bundleConfigFile(
               if (idFsPath && isImport) {
                 idFsPath = pathToFileURL(idFsPath).href
               }
-              if (idFsPath && !isImport && isESMFile(idFsPath)) {
+              if (
+                idFsPath &&
+                !isImport &&
+                isFilePathESM(idFsPath, packageCache)
+              ) {
                 throw new Error(
                   `${JSON.stringify(
                     id,

packages/vite/src/node/plugins/resolve.ts

@@ -26,6 +26,7 @@ import {
   isBuiltin,
   isDataUrl,
   isExternalUrl,
+  isFilePathESM,
   isInNodeModules,
   isNonDriveRelativeAbsolutePath,
   isObject,
@@ -822,8 +823,6 @@ export function tryNodeResolve(
     })
   }
 
-  const ext = path.extname(resolved)
-
   if (
     !options.ssrOptimizeCheck &&
     (!isInNodeModules(resolved) || // linked
@@ -859,12 +858,7 @@ export function tryNodeResolve(
     (!options.ssrOptimizeCheck && !isBuild && ssr) ||
     // Only optimize non-external CJS deps during SSR by default
     (ssr &&
-      !(
-        ext === '.cjs' ||
-        (ext === '.js' &&
-          findNearestPackageData(path.dirname(resolved), options.packageCache)
-            ?.data.type !== 'module')
-      ) &&
+      isFilePathESM(resolved, options.packageCache) &&
       !(include?.includes(pkgId) || include?.includes(id)))
 
   if (options.ssrOptimizeCheck) {