feat: Adding ssrFormat flag to overwrite default output format

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
@@ -399,12 +412,7 @@ async function doBuild(
     const buildOutputOptions = (output: OutputOptions = {}): OutputOptions => {
       return {
         dir: outDir,
-        format:
-          ssr &&
-          JSON.parse(fs.readFileSync(resolve('package.json'), 'utf-8')).type !=
-            'module'
-            ? 'cjs'
-            : 'es',
+        format: ssr ? ssrFormat : 'es',
         exports: ssr ? 'named' : 'auto',
         sourcemap: options.sourcemap,
         name: libOptions ? libOptions.name : undefined,

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)`