fix: improve deduplication support for svelte during dev (#137)

* fix: improve deduplication support by adding svelte itself to optimizeDeps.include by default

* docs: improve changeset description

Author: dominikg

.changeset/seven-cows-leave.md

@@ -0,0 +1,32 @@
+---
+'@sveltejs/vite-plugin-svelte': major
+---
+
+automatically include svelte in vite config optimizeDeps.include
+
+Previously, svelte was automatically excluded. We include it now by default to improve deduplication.
+
+As a result, svelte is pre-bundled by vite during dev, which it logs when starting the devserver
+
+```shell
+Pre-bundling dependencies:
+  svelte/animate
+  svelte/easing
+  svelte/internal
+  svelte/motion
+  svelte/store
+  (...and 2 more)
+(this will be run only when your dependencies or config have changed)
+```
+
+And it's also visible in the browsers network tab, where requests for svelte imports now start with `node_modules/.vite/` during dev.
+
+Check out the [vite pre-bundling documentation](https://vitejs.dev/guide/dep-pre-bundling.html) for more information.
+
+To get the old behavior back, add the following to your vite config
+
+```js
+optimizeDeps: {
+	exclude: ['svelte'];
+}
+```

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

@@ -7,6 +7,7 @@ export const SVELTE_IMPORTS = [
 	'svelte/easing',
 	'svelte/internal',
 	'svelte/motion',
+	'svelte/ssr',
 	'svelte/store',
 	'svelte/transition',
 	'svelte'

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

@@ -179,20 +179,24 @@ export function buildExtraViteConfig(
 	options: ResolvedOptions,
 	config: UserConfig
 ): Partial<UserConfig> {
-	const allSvelteImports = [...SVELTE_IMPORTS, ...SVELTE_HMR_IMPORTS];
-
-	// exclude svelte imports from optimization unless explicitly included
-	const excludeFromOptimize = allSvelteImports.filter(
-		(x) => !config.optimizeDeps?.include?.includes(x)
-	);
-
+	// include svelte imports for optimization unless explicitly excluded
+	const include: string[] = [];
+	const exclude: string[] = ['svelte-hmr'];
+	const isSvelteExcluded = config.optimizeDeps?.exclude?.includes('svelte');
+	if (!isSvelteExcluded) {
+		log.debug(`adding bare svelte packages to optimizeDeps.include: ${SVELTE_IMPORTS.join(', ')} `);
+		include.push(...SVELTE_IMPORTS);
+	} else {
+		log.debug('"svelte" is excluded in optimizeDeps.exclude, skipped adding it to include.');
+	}
 	const extraViteConfig: Partial<UserConfig> = {
 		optimizeDeps: {
-			exclude: excludeFromOptimize
+			include,
+			exclude
 		},
 		resolve: {
 			mainFields: [...SVELTE_RESOLVE_MAIN_FIELDS],
-			dedupe: allSvelteImports
+			dedupe: [...SVELTE_IMPORTS, ...SVELTE_HMR_IMPORTS]
 		}
 		// this option is still awaiting a PR in vite to be supported
 		// see https://github.com/sveltejs/vite-plugin-svelte/issues/60