feat: default esm SSR build, simplified externalization

docs/config/ssr-options.md

@@ -24,3 +24,11 @@ Prevent listed dependencies from being externalized for SSR. If `true`, no depen
 - **Default:** `node`
 
 Build target for the SSR server.
+
+## ssr.format
+
+- **Type:** `'esm' | 'cjs'`
+- **Default:** `esm`
+- **Experimental**
+
+Build format for the SSR server. Since Vite v3 the SSR build generates ESM by default. `'cjs'` can be selected to generate a CJS build, but it isn't recommended. The option is left marked as experimental to give users more time to update to ESM. CJS builds requires complex externalization heuristics that aren't present in the ESM format.

docs/vite.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from 'vite'
+
+export default defineConfig({
+  ssr: {
+    format: 'cjs'
+  }
+})

packages/vite/src/node/build.ts

@@ -31,7 +31,10 @@ import { manifestPlugin } from './plugins/manifest'
 import type { Logger } from './logger'
 import { dataURIPlugin } from './plugins/dataUri'
 import { buildImportAnalysisPlugin } from './plugins/importAnalysisBuild'
-import { resolveSSRExternal, shouldExternalizeForSSR } from './ssr/ssrExternal'
+import {
+  cjsShouldExternalizeForSSR,
+  cjsSsrResolveExternals
+} from './ssr/ssrExternal'
 import { ssrManifestPlugin } from './ssr/ssrManifestPlugin'
 import type { DepOptimizationMetadata } from './optimizer'
 import {
@@ -373,27 +376,14 @@ async function doBuild(
     ssr ? config.plugins.map((p) => injectSsrFlagToHooks(p)) : config.plugins
   ) as Plugin[]
 
-  // inject ssrExternal if present
   const userExternal = options.rollupOptions?.external
   let external = userExternal
-  if (ssr) {
-    // see if we have cached deps data available
-    let knownImports: string[] | undefined
-    const dataPath = path.join(getDepsCacheDir(config), '_metadata.json')
-    try {
-      const data = JSON.parse(
-        fs.readFileSync(dataPath, 'utf-8')
-      ) as DepOptimizationMetadata
-      knownImports = Object.keys(data.optimized)
-    } catch (e) {}
-    if (!knownImports) {
-      // no dev deps optimization data, do a fresh scan
-      knownImports = await findKnownImports(config)
-    }
-    external = resolveExternal(
-      resolveSSRExternal(config, knownImports),
-      userExternal
-    )
+
+  // In CJS, we can pass the externals to rollup as is. In ESM, we need to
+  // do it in the resolve plugin so we can add the resolved extension for
+  // deep node_modules imports
+  if (ssr && config.ssr?.format === 'cjs') {
+    external = await cjsSsrResolveExternal(config, userExternal)
   }
 
   if (isDepsOptimizerEnabled(config) && !ssr) {
@@ -431,10 +421,12 @@ async function doBuild(
 
   try {
     const buildOutputOptions = (output: OutputOptions = {}): OutputOptions => {
+      const cjsSsrBuild = ssr && config.ssr?.format === 'cjs'
       return {
         dir: outDir,
-        format: ssr ? 'cjs' : 'es',
-        exports: ssr ? 'named' : 'auto',
+        // Default format is 'es' for regular and for SSR builds
+        format: cjsSsrBuild ? 'cjs' : 'es',
+        exports: cjsSsrBuild ? 'named' : 'auto',
         sourcemap: options.sourcemap,
         name: libOptions ? libOptions.name : undefined,
         generatedCode: 'es2015',
@@ -696,26 +688,51 @@ export function onRollupWarning(
   }
 }
 
-function resolveExternal(
-  ssrExternals: string[],
+async function cjsSsrResolveExternal(
+  config: ResolvedConfig,
   user: ExternalOption | undefined
-): ExternalOption {
+): Promise<ExternalOption> {
+  // see if we have cached deps data available
+  let knownImports: string[] | undefined
+  const dataPath = path.join(getDepsCacheDir(config), '_metadata.json')
+  try {
+    const data = JSON.parse(
+      fs.readFileSync(dataPath, 'utf-8')
+    ) as DepOptimizationMetadata
+    knownImports = Object.keys(data.optimized)
+  } catch (e) {}
+  if (!knownImports) {
+    // no dev deps optimization data, do a fresh scan
+    knownImports = await findKnownImports(config)
+  }
+  const ssrExternals = cjsSsrResolveExternals(config, knownImports)
+
   return (id, parentId, isResolved) => {
-    if (shouldExternalizeForSSR(id, ssrExternals)) {
+    const isExternal = cjsShouldExternalizeForSSR(id, ssrExternals)
+    if (isExternal) {
       return true
     }
     if (user) {
-      if (typeof user === 'function') {
-        return user(id, parentId, isResolved)
-      } else if (Array.isArray(user)) {
-        return user.some((test) => isExternal(id, test))
-      } else {
-        return isExternal(id, user)
-      }
+      return resolveUserExternal(user, id, parentId, isResolved)
     }
   }
 }
 
+function resolveUserExternal(
+  user: ExternalOption,
+  id: string,
+  parentId: string | undefined,
+  isResolved: boolean
+) {
+  if (typeof user === 'function') {
+    return user(id, parentId, isResolved)
+  } else if (Array.isArray(user)) {
+    return user.some((test) => isExternal(id, test))
+  } else {
+    return isExternal(id, user)
+  }
+}
+
 function isExternal(id: string, test: string | RegExp) {
   if (typeof test === 'string') {
     return id === test

packages/vite/src/node/config.ts

@@ -230,6 +230,8 @@ export interface ExperimentalOptions {
 
 export type SSRTarget = 'node' | 'webworker'
 
+export type SSRFormat = 'esm' | 'cjs'
+
 export interface SSROptions {
   external?: string[]
   noExternal?: string | RegExp | (string | RegExp)[] | true
@@ -239,6 +241,14 @@ export interface SSROptions {
    * Default: 'node'
    */
   target?: SSRTarget
+  /**
+   * Define the format for the ssr build. Since Vite v3 the SSR build generates ESM by default.
+   * `'cjs'` can be selected to generate a CJS build, but it isn't recommended. This option is
+   * left marked as experimental to give users more time to update to ESM. CJS builds requires
+   * complex externalization heuristics that aren't present in the ESM format.
+   * @experimental
+   */
+  format?: SSRFormat
 }
 
 export interface ResolveWorkerOptions {

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

@@ -41,7 +41,10 @@ import {
 } from '../utils'
 import type { ResolvedConfig } from '../config'
 import type { Plugin } from '../plugin'
-import { shouldExternalizeForSSR } from '../ssr/ssrExternal'
+import {
+  cjsShouldExternalizeForSSR,
+  shouldExternalizeForSSR
+} from '../ssr/ssrExternal'
 import { transformRequest } from '../server/transformRequest'
 import {
   getDepsCacheDir,
@@ -359,10 +362,11 @@ export function importAnalysisPlugin(config: ResolvedConfig): Plugin {
           }
           // skip ssr external
           if (ssr) {
-            if (
-              server._ssrExternals &&
-              shouldExternalizeForSSR(specifier, server._ssrExternals)
-            ) {
+            if (config.ssr?.format === 'cjs') {
+              if (cjsShouldExternalizeForSSR(specifier, server._ssrExternals)) {
+                continue
+              }
+            } else if (shouldExternalizeForSSR(specifier, config)) {
               continue
             }
             if (isBuiltin(specifier)) {

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

@@ -3,6 +3,7 @@ import type { ResolvedConfig } from '../config'
 import { isDepsOptimizerEnabled } from '../config'
 import type { Plugin } from '../plugin'
 import { getDepsOptimizer } from '../optimizer'
+import { shouldExternalizeForSSR } from '../ssr/ssrExternal'
 import { jsonPlugin } from './json'
 import { resolvePlugin } from './resolve'
 import { optimizedDepsBuildPlugin, optimizedDepsPlugin } from './optimizedDeps'
@@ -61,7 +62,11 @@ export async function resolvePlugins(
       packageCache: config.packageCache,
       ssrConfig: config.ssr,
       asSrc: true,
-      getDepsOptimizer: () => getDepsOptimizer(config)
+      getDepsOptimizer: () => getDepsOptimizer(config),
+      shouldExternalize:
+        isBuild && config.build.ssr && config.ssr?.format !== 'cjs'
+          ? (id) => shouldExternalizeForSSR(id, config)
+          : undefined
     }),
     htmlInlineProxyPlugin(config),
     cssPlugin(config),

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

@@ -83,6 +83,7 @@ export interface InternalResolveOptions extends ResolveOptions {
   scan?: boolean
   // Resolve using esbuild deps optimization
   getDepsOptimizer?: () => DepsOptimizer | undefined
+  shouldExternalize?: (id: string) => boolean | undefined
 }
 
 export function resolvePlugin(baseOptions: InternalResolveOptions): Plugin {
@@ -105,6 +106,7 @@ export function resolvePlugin(baseOptions: InternalResolveOptions): Plugin {
       const depsOptimizer = baseOptions.getDepsOptimizer?.()
 
       const ssr = resolveOpts?.ssr === true
+
       if (id.startsWith(browserExternalId)) {
         return id
       }
@@ -258,7 +260,10 @@ export function resolvePlugin(baseOptions: InternalResolveOptions): Plugin {
 
       // bare package imports, perform node resolve
       if (bareImportRE.test(id)) {
+        const external = options.shouldExternalize?.(id)
+
         if (
+          !external &&
           asSrc &&
           depsOptimizer &&
           !ssr &&
@@ -270,7 +275,13 @@ export function resolvePlugin(baseOptions: InternalResolveOptions): Plugin {
 
         if (
           targetWeb &&
-          (res = tryResolveBrowserMapping(id, importer, options, false))
+          (res = tryResolveBrowserMapping(
+            id,
+            importer,
+            options,
+            false,
+            external
+          ))
         ) {
           return res
         }
@@ -282,7 +293,8 @@ export function resolvePlugin(baseOptions: InternalResolveOptions): Plugin {
             options,
             targetWeb,
             depsOptimizer,
-            ssr
+            ssr,
+            external
           ))
         ) {
           return res
@@ -523,7 +535,8 @@ export function tryNodeResolve(
   options: InternalResolveOptions,
   targetWeb: boolean,
   depsOptimizer?: DepsOptimizer,
-  ssr?: boolean
+  ssr?: boolean,
+  externalize?: boolean
 ): PartialResolvedId | undefined {
   const { root, dedupe, isBuild, preserveSymlinks, packageCache } = options
 
@@ -591,7 +604,8 @@ export function tryNodeResolve(
 
   let resolveId = resolvePackageEntry
   let unresolvedId = pkgId
-  if (unresolvedId !== nestedPath) {
+  const isDeepImport = unresolvedId !== nestedPath
+  if (isDeepImport) {
     resolveId = resolveDeepImport
     unresolvedId = '.' + nestedPath.slice(pkgId.length)
   }
@@ -616,15 +630,25 @@ export function tryNodeResolve(
     return
   }
 
+  const processResult = (resolved: PartialResolvedId) => {
+    if (!externalize) {
+      return resolved
+    }
+    const resolvedExt = path.extname(resolved.id)
+    const resolvedId =
+      isDeepImport && path.extname(id) !== resolvedExt ? id + resolvedExt : id
+    return { ...resolved, id: resolvedId, external: true }
+  }
+
   // link id to pkg for browser field mapping check
   idToPkgMap.set(resolved, pkg)
-  if (isBuild && !depsOptimizer) {
+  if ((isBuild && !depsOptimizer) || externalize) {
     // Resolve package side effects for build so that rollup can better
     // perform tree-shaking
-    return {
+    return processResult({
       id: resolved,
       moduleSideEffects: pkg.hasSideEffects(resolved)
-    }
+    })
   }
 
   if (
@@ -940,7 +964,8 @@ function tryResolveBrowserMapping(
   id: string,
   importer: string | undefined,
   options: InternalResolveOptions,
-  isFilePath: boolean
+  isFilePath: boolean,
+  externalize?: boolean
 ) {
   let res: string | undefined
   const pkg = importer && idToPkgMap.get(importer)
@@ -953,10 +978,11 @@ function tryResolveBrowserMapping(
         isDebug &&
           debug(`[browser mapped] ${colors.cyan(id)} -> ${colors.dim(res)}`)
         idToPkgMap.set(res, pkg)
-        return {
+        const result = {
           id: res,
           moduleSideEffects: pkg.hasSideEffects(res)
         }
+        return externalize ? { ...result, external: true } : result
       }
     } else if (browserMappedPath === false) {
       return browserExternalId

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

@@ -12,8 +12,10 @@ import { arraify } from '../utils'
 export function ssrRequireHookPlugin(config: ResolvedConfig): Plugin | null {
   if (
     config.command !== 'build' ||
+    !config.build.ssr ||
     !config.resolve.dedupe?.length ||
     config.ssr?.noExternal === true ||
+    config.ssr?.format !== 'cjs' ||
     isBuildOutputEsm(config)
   ) {
     return null

packages/vite/src/node/server/index.ts

@@ -22,7 +22,7 @@ import {
   resolveHostname
 } from '../utils'
 import { ssrLoadModule } from '../ssr/ssrModuleLoader'
-import { resolveSSRExternal } from '../ssr/ssrExternal'
+import { cjsSsrResolveExternals } from '../ssr/ssrExternal'
 import {
   rebindErrorStacktrace,
   ssrRewriteStacktrace
@@ -330,7 +330,7 @@ export async function createServer(
             ...Object.keys(depsOptimizer.metadata.discovered)
           ]
         }
-        server._ssrExternals = resolveSSRExternal(config, knownImports)
+        server._ssrExternals = cjsSsrResolveExternals(config, knownImports)
       }
       return ssrLoadModule(
         url,