feat: Supports ES SSR build based on module type

packages/playground/esm-ssr/__tests__/build.spec.ts

@@ -0,0 +1,13 @@
+import fs from 'fs'
+import path from 'path'
+import { isBuild } from '../../testUtils'
+
+if (isBuild) {
+  test('outputs ESM', () => {
+    const content = fs.readFileSync(
+      path.join(__dirname, '..', 'dist', 'App.js'),
+      'utf8'
+    )
+    expect(/^import/.test(content)).toBeTruthy()
+  })
+}

packages/playground/esm-ssr/package.json

@@ -0,0 +1,17 @@
+{
+  "name": "test-esm-ssr",
+  "private": true,
+  "version": "0.0.0",
+  "type": "module",
+  "scripts": {
+    "build": "vite build --ssr src/App.jsx"
+  },
+  "dependencies": {
+    "preact": "^10.5.13",
+    "preact-render-to-string": "^5.1.19"
+  },
+  "devDependencies": {
+    "@preact/preset-vite": "2.0.1",
+    "vite": "^2.2.3"
+  }
+}

packages/playground/esm-ssr/src/App.jsx

@@ -0,0 +1,23 @@
+import { render } from 'preact-render-to-string';
+
+export function App() {
+  return (
+    <>
+      <p>Hello Vite + Preact!</p>
+      <p>
+        <a
+          class="link"
+          href="https://preactjs.com/"
+          target="_blank"
+          rel="noopener noreferrer"
+        >
+          Learn Preact
+        </a>
+      </p>
+    </>
+  )
+}
+
+export async function prerender() {
+	return await render(<App />);
+}

packages/playground/esm-ssr/vite.config.js

@@ -0,0 +1,6 @@
+import { defineConfig } from 'vite'
+import preact from '@preact/preset-vite'
+
+export default defineConfig({
+  plugins: [preact.default()]
+})

packages/vite/src/node/build.ts

@@ -164,6 +164,11 @@ export interface BuildOptions {
    * `rollupOptions.input`.
    */
   ssr?: boolean | string
+  /**
+   * When present, overrides the module format for the project to instead output
+   * the SSR in another format.
+   */
+  ssrFormat?: 'cjs' | 'es' | null
   /**
    * Generate SSR manifest for determining style links and asset preload
    * directives in production.
@@ -220,6 +225,7 @@ export function resolveBuildOptions(raw?: BuildOptions): ResolvedBuildOptions {
     manifest: false,
     lib: false,
     ssr: false,
+    ssrFormat: null,
     ssrManifest: false,
     brotliSize: true,
     chunkSizeWarningLimit: 500,
@@ -332,6 +338,13 @@ async function doBuild(
     )
   }
 
+  const ssrFormat =
+    options.ssrFormat ||
+    (JSON.parse(fs.readFileSync(resolve('package.json'), 'utf-8')).type ===
+    'module'
+      ? 'es'
+      : 'cjs')
+
   const outDir = resolve(options.outDir)
 
   // inject ssr arg to plugin load/transform hooks
@@ -396,10 +409,10 @@ async function doBuild(
   try {
     const pkgName = libOptions && getPkgName(config.root)
 
-    const buildOuputOptions = (output: OutputOptions = {}): OutputOptions => {
+    const buildOutputOptions = (output: OutputOptions = {}): OutputOptions => {
       return {
         dir: outDir,
-        format: ssr ? 'cjs' : 'es',
+        format: ssr ? ssrFormat : 'es',
         exports: ssr ? 'named' : 'auto',
         sourcemap: options.sourcemap,
         name: libOptions ? libOptions.name : undefined,
@@ -443,10 +456,10 @@ async function doBuild(
       const output: OutputOptions[] = []
       if (Array.isArray(outputs)) {
         for (const resolvedOutput of outputs) {
-          output.push(buildOuputOptions(resolvedOutput))
+          output.push(buildOutputOptions(resolvedOutput))
         }
       } else {
-        output.push(buildOuputOptions(outputs))
+        output.push(buildOutputOptions(outputs))
       }
 
       const watcherOptions = config.build.watch
@@ -494,7 +507,7 @@ async function doBuild(
 
     const generate = (output: OutputOptions = {}) => {
       return bundle[options.write ? 'write' : 'generate'](
-        buildOuputOptions(output)
+        buildOutputOptions(output)
       )
     }
 

packages/vite/src/node/cli.ts

@@ -114,6 +114,10 @@ cli
     '--ssr [entry]',
     `[string] build specified entry for server-side rendering`
   )
+  .option(
+    '-ssrFormat [format]',
+    '[string] SSR module format, either "cjs" or "es"'
+  )
   .option(
     '--sourcemap',
     `[boolean] output source maps for build (default: false)`