feat: new ssr.fileExtension option

docs/config/ssr-options.md

@@ -32,3 +32,9 @@ Build target for the SSR server.
 - **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.
+
+## ssr.fileExtension
+
+- **Type:** `'js' | 'cjs' | 'mjs'`
+
+File extension of generated bundles. When not specified, it will infer from root package.json's "type" field.

packages/vite/src/node/build.ts

@@ -21,7 +21,7 @@ import commonjsPlugin from '@rollup/plugin-commonjs'
 import type { RollupCommonJSOptions } from 'types/commonjs'
 import type { RollupDynamicImportVarsOptions } from 'types/dynamicImportVars'
 import type { TransformOptions } from 'esbuild'
-import type { InlineConfig, ResolvedConfig } from './config'
+import type { InlineConfig, JsExt, ResolvedConfig } from './config'
 import { isDepsOptimizerEnabled, resolveConfig } from './config'
 import { buildReporterPlugin } from './plugins/reporter'
 import { buildEsbuildPlugin } from './plugins/esbuild'
@@ -451,8 +451,10 @@ async function doBuild(
       const format = output.format || (cjsSsrBuild ? 'cjs' : 'es')
       const jsExt =
         (ssr && config.ssr?.target !== 'webworker') || libOptions
-          ? resolveOutputJsExtension(format, getPkgJson(config.root)?.type)
+          ? config.ssr?.fileExtension ||
+            resolveOutputJsExtension(format, getPkgJson(config.root)?.type)
           : 'js'
+
       return {
         dir: outDir,
         // Default format is 'es' for regular and for SSR builds
@@ -606,8 +608,6 @@ function getPkgName(name: string) {
   return name?.startsWith('@') ? name.split('/')[1] : name
 }
 
-type JsExt = 'js' | 'cjs' | 'mjs'
-
 function resolveOutputJsExtension(
   format: ModuleFormat,
   type: string = 'commonjs'

packages/vite/src/node/config.ts

@@ -230,6 +230,8 @@ export interface ExperimentalOptions {
   importGlobRestoreExtension?: boolean
 }
 
+export type JsExt = 'js' | 'cjs' | 'mjs'
+
 export interface ResolveWorkerOptions {
   format: 'es' | 'iife'
   plugins: Plugin[]

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

@@ -1,3 +1,5 @@
+import type { JsExt } from '..'
+
 export type SSRTarget = 'node' | 'webworker'
 export type SSRFormat = 'esm' | 'cjs'
 
@@ -7,7 +9,7 @@ export interface SSROptions {
   /**
    * Define the target for the ssr build. The browser field in package.json
    * is ignored for node but used if webworker is the target
-   * Default: 'node'
+   * @default 'node'
    */
   target?: SSRTarget
   /**
@@ -18,6 +20,11 @@ export interface SSROptions {
    * @experimental
    */
   format?: SSRFormat
+  /**
+   * File extension of generated bundles
+   * Will infer from root package.json's "type" field if not specified
+   */
+  fileExtension?: JsExt
 }
 
 export interface ResolvedSSROptions extends SSROptions {