fix: skip prebundle non-js nested dependencies (#234)

Author: bluwy

.changeset/kind-ducks-swim.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/vite-plugin-svelte': patch
+---
+
+Skip prebundle non-js nested dependencies

packages/e2e-tests/_test_dependencies/scss-only/main.scss

@@ -0,0 +1,3 @@
+#foo {
+	color: red;
+}

packages/e2e-tests/_test_dependencies/scss-only/package.json

@@ -0,0 +1,11 @@
+{
+  "version": "1.0.0",
+  "private": true,
+  "name": "e2e-test-dep-scss-only",
+  "main": "main.scss",
+  "files": [
+    "package.json",
+    "main.scss"
+  ],
+  "type": "commonjs"
+}

packages/e2e-tests/_test_dependencies/svelte-nested/package.json

@@ -13,6 +13,7 @@
   },
   "dependencies": {
     "e2e-test-dep-svelte-simple": "workspace:*",
-    "e2e-test-dep-cjs-and-esm": "workspace:*"
+    "e2e-test-dep-cjs-and-esm": "workspace:*",
+    "e2e-test-dep-scss-only": "workspace:*"
   }
 }

packages/e2e-tests/_test_dependencies/svelte-nested/src/components/Message.svelte

@@ -2,6 +2,7 @@
 	export let id = 'id';
 	export let message = '';
 	import { cjs_and_esm } from 'e2e-test-dep-cjs-and-esm';
+	import 'e2e-test-dep-scss-only';
 </script>
 
 <div {id}>{message}</div>

packages/vite-plugin-svelte/src/utils/dependencies.ts

@@ -184,7 +184,12 @@ export function needsOptimization(dep: string, localRequire: NodeRequire): boole
 	const pkg = depData.pkg;
 	// only optimize if is cjs, using the below as heuristic
 	// see https://github.com/sveltejs/vite-plugin-svelte/issues/162
-	return pkg.main && !pkg.module && !pkg.exports;
+	const isCjs = pkg.main && !pkg.module && !pkg.exports;
+	if (!isCjs) return false;
+	// ensure entry is js so vite can prebundle it
+	// see https://github.com/sveltejs/vite-plugin-svelte/issues/233
+	const entryExt = path.extname(pkg.main);
+	return !entryExt || entryExt === '.js' || entryExt === '.cjs';
 }
 
 interface DependencyData {