fix: don't emit an empty bundle

packages/plugin-legacy/src/index.ts

@@ -268,24 +268,26 @@ function viteLegacyPlugin(options: Options = {}): Plugin[] {
         )
       }
 
-      isDebug &&
-        console.log(
-          `[@vitejs/plugin-legacy] legacy polyfills:`,
+      if (legacyPolyfills.size || !options.externalSystemJS) {
+        isDebug &&
+          console.log(
+            `[@vitejs/plugin-legacy] legacy polyfills:`,
+            legacyPolyfills,
+          )
+
+        await buildPolyfillChunk(
+          config.mode,
           legacyPolyfills,
+          bundle,
+          facadeToLegacyPolyfillMap,
+          // force using terser for legacy polyfill minification, since esbuild
+          // isn't legacy-safe
+          config.build,
+          'iife',
+          opts,
+          options.externalSystemJS,
         )
-
-      await buildPolyfillChunk(
-        config.mode,
-        legacyPolyfills,
-        bundle,
-        facadeToLegacyPolyfillMap,
-        // force using terser for legacy polyfill minification, since esbuild
-        // isn't legacy-safe
-        config.build,
-        'iife',
-        opts,
-        options.externalSystemJS,
-      )
+      }
     },
   }
 

playground/legacy/__tests__/no-polyfills-no-systemjs/no-polyfills-no-systemjs.spec.ts

@@ -0,0 +1,16 @@
+import { expect, test } from 'vitest'
+import { isBuild, page, untilUpdated, viteTestUrl } from '~utils'
+
+test.runIf(isBuild)('includes only a single script tag', async () => {
+  await page.goto(viteTestUrl + '/no-polyfills-no-systemjs.html')
+
+  await untilUpdated(
+    () => page.getAttribute('#vite-legacy-entry', 'data-src'),
+    /.\/assets\/index-legacy-(.+)\.js/,
+    true,
+  )
+
+  expect(await page.locator('script').count()).toBe(1)
+  expect(await page.locator('#vite-legacy-polyfill').count()).toBe(0)
+  expect(await page.locator('#vite-legacy-entry').count()).toBe(1)
+})

playground/legacy/no-polyfills-no-systemjs.html

@@ -0,0 +1,3 @@
+<meta charset="UTF-8" />
+<main></main>
+<script type="module" src="./no-polyfills-no-systemjs.js"></script>

playground/legacy/no-polyfills-no-systemjs.js

@@ -0,0 +1 @@
+document.querySelector('main').innerHTML = '👋'

playground/legacy/package.json

@@ -9,6 +9,7 @@
     "build:custom-filename": "vite --config ./vite.config-custom-filename.js build  --debug legacy",
     "build:multiple-output": "vite --config ./vite.config-multiple-output.js build",
     "build:no-polyfills": "vite --config ./vite.config-no-polyfills.js build",
+    "build:no-polyfills-no-systemjs": "vite --config ./vite.config-no-polyfills-no-systemjs.js build",
     "debug": "node --inspect-brk ../../packages/vite/bin/vite",
     "preview": "vite preview"
   },

playground/legacy/vite.config-no-polyfills-no-systemjs.js

@@ -0,0 +1,30 @@
+import path from 'node:path'
+import legacy from '@vitejs/plugin-legacy'
+import { defineConfig } from 'vite'
+
+export default defineConfig({
+  base: './',
+  plugins: [
+    legacy({
+      renderModernChunks: false,
+      polyfills: false,
+      externalSystemJS: true,
+    }),
+    {
+      name: 'remove crossorigin attribute',
+      transformIndexHtml: (html) => html.replaceAll('crossorigin', ''),
+      enforce: 'post',
+    },
+  ],
+
+  build: {
+    rollupOptions: {
+      input: {
+        index: path.resolve(__dirname, 'no-polyfills-no-systemjs.html'),
+      },
+    },
+  },
+  testConfig: {
+    baseRoute: '/no-polyfills-no-systemjs/',
+  },
+})