withastro · tony-sull · Apr 22, 2022 · Apr 20, 2022 · Apr 22, 2022 · Apr 22, 2022
diff --git a/.changeset/strong-dogs-boil.md b/.changeset/strong-dogs-boil.md
@@ -0,0 +1,5 @@
+---
+'@astrojs/svelte': patch
+---
+
+`@astrojs/svelte` integration supports custom svelte compiler options
diff --git a/packages/astro/test/fixtures/svelte-component/astro.config.mjs b/packages/astro/test/fixtures/svelte-component/astro.config.mjs
@@ -3,5 +3,7 @@ import svelte from '@astrojs/svelte';
 
 // https://astro.build/config
 export default defineConfig({
-	integrations: [svelte()],
+	integrations: [svelte({
+		extensions: ['.svelte', '.sve']
+	})],
 });
diff --git a/packages/astro/test/fixtures/svelte-component/src/components/Custom.sve b/packages/astro/test/fixtures/svelte-component/src/components/Custom.sve
@@ -0,0 +1,5 @@
+<script lang="ts">
+  export let message: string;
+</script>
+
+<div id="svelte-custom-ext">{ message }</div>
diff --git a/packages/astro/test/fixtures/svelte-component/src/pages/typescript.astro b/packages/astro/test/fixtures/svelte-component/src/pages/typescript.astro
@@ -1,5 +1,9 @@
 ---
 import TypeScript from '../components/TypeScript.svelte'
+
+// Using a custom extension to verify svelte options
+// in astro.config.mjs are passed properly to the svelte integration
+import Custom from '../components/Custom.sve'
 ---
 <html lang="en">
   <head>
@@ -20,6 +24,7 @@ import TypeScript from '../components/TypeScript.svelte'
   <body>
     <main>
       <TypeScript message="Hello, TypeScript" client:load />
+      <Custom message="Hello, Custom Extensions" client:idle />
     </main>
   </body>
 </html>
diff --git a/packages/astro/test/svelte-component.test.js b/packages/astro/test/svelte-component.test.js
@@ -22,6 +22,13 @@ describe('Svelte component', () => {
 
 			expect($('#svelte-ts').text()).to.equal('Hello, TypeScript');
 		});
+
+		it('Works with custom Svelte config', async () => {
+			const html = await fixture.readFile('/typescript/index.html');
+			const $ = cheerio.load(html);
+
+			expect($('#svelte-custom-ext').text()).to.equal('Hello, Custom Extensions');
+		});
 	});
 
 	if (isWindows) return;

diff --git a/packages/integrations/svelte/README.md b/packages/integrations/svelte/README.md
@@ -63,3 +63,31 @@ Also check our [Astro Integration Documentation][astro-integration] for more on
 
 [astro-integration]: https://docs.astro.build/en/guides/integrations-guide/
 [astro-ui-frameworks]: https://docs.astro.build/en/core-concepts/framework-components/#using-framework-components
+
+## Options
+
+This integration is powered by `@sveltejs/vite-plugin-svelte`. To customize the Svelte compiler, options can be provided to the integration. See the `@sveltejs/vite-plugin-svelte` [docs](https://github.com/sveltejs/vite-plugin-svelte/blob/HEAD/docs/config.md) for more details.
+
+### Default options
+
+A few of the default options passed to the Svelte compiler are required to build properly for Astro and cannot be overridden.
+
+```js
+const defaultOptions = {
+  emitCss: true,
+  compilerOptions: { dev: isDev, hydratable: true },
+  preprocess: [
+    preprocess({
+      less: true,
+      sass: { renderSync: true },
+      scss: { renderSync: true },
+      stylus: true,
+      typescript: true,
+    }),
+  ],
+};
+```
+
+The `emitCss`, `compilerOptions.dev`, and `compilerOptions.hydratable` cannot be overridden.
+
+Providing your own `preprocess` options **will** override the defaults - make sure to enable the preprocessor flags needed for your project.
diff --git a/packages/integrations/svelte/src/index.ts b/packages/integrations/svelte/src/index.ts
@@ -1,6 +1,7 @@
 import type { AstroIntegration, AstroRenderer } from 'astro';
 import { svelte } from '@sveltejs/vite-plugin-svelte';
 import preprocess from 'svelte-preprocess';
+import type { Options } from '@sveltejs/vite-plugin-svelte';
 
 function getRenderer(): AstroRenderer {
 	return {
@@ -10,38 +11,61 @@ function getRenderer(): AstroRenderer {
 	};
 }
 
-function getViteConfiguration(isDev: boolean) {
+function getViteConfiguration(isDev: boolean, options?: Options | OptionsCallback) {
+	const defaultOptions = {
+		emitCss: true,
+		compilerOptions: { dev: isDev, hydratable: true },
+		preprocess: [
+			preprocess({
+				less: true,
+				sass: { renderSync: true },
+				scss: { renderSync: true },
+				stylus: true,
+				typescript: true,
+			}),
+		],
+	};
+
+	let resolvedOptions: Partial<Options>;
+
+	if (!options) {
+		resolvedOptions = defaultOptions;
+	} else if (typeof options === 'function') {
+		resolvedOptions = options(defaultOptions);
+	} else {
+		resolvedOptions = {
+			...options,
+			...defaultOptions,
+			compilerOptions: {
+				...options.compilerOptions,
+				// Always use dev and hydratable from defaults
+				...defaultOptions.compilerOptions,
+			},
+			// Ignore default preprocessor if the user provided their own
+			preprocess: options.preprocess ?? defaultOptions.preprocess,
+		};
+	}
+
 	return {
 		optimizeDeps: {
 			include: ['@astrojs/svelte/client.js', 'svelte', 'svelte/internal'],
 			exclude: ['@astrojs/svelte/server.js'],
 		},
 		plugins: [
-			svelte({
-				emitCss: true,
-				compilerOptions: { dev: isDev, hydratable: true },
-				preprocess: [
-					preprocess({
-						less: true,
-						sass: { renderSync: true },
-						scss: { renderSync: true },
-						stylus: true,
-						typescript: true,
-					}),
-				],
-			}),
+			svelte(resolvedOptions),
 		],
 	};
 }
 
-export default function (): AstroIntegration {
+type OptionsCallback = (defaultOptions: Options) => Options;
+export default function (options?: Options | OptionsCallback): AstroIntegration {
 	return {
 		name: '@astrojs/svelte',
 		hooks: {
 			// Anything that gets returned here is merged into Astro Config
 			'astro:config:setup': ({ command, updateConfig, addRenderer }) => {
 				addRenderer(getRenderer());
-				updateConfig({ vite: getViteConfiguration(command === 'dev') });
+				updateConfig({ vite: getViteConfiguration(command === 'dev', options) });
 			},
 		},
 	};