Support Svelte 5 (experimental) (#9098)

Co-authored-by: Nate Moore <natemoo-re@users.noreply.github.com>

5 files changed, 107 insertions, 9 deletions

diff --git a/packages/integrations/svelte/README.md b/packages/integrations/svelte/README.md
index 5df61fab0..25513790b 100644
--- a/packages/integrations/svelte/README.md
+++ b/packages/integrations/svelte/README.md
@@ -1,6 +1,6 @@
 # @astrojs/svelte 🧡
 
-This **[Astro integration][astro-integration]** enables server-side rendering and client-side hydration for your [Svelte](https://svelte.dev/) components.
+This **[Astro integration][astro-integration]** enables server-side rendering and client-side hydration for your [Svelte](https://svelte.dev/) components. It supports Svelte 3, 4, and 5 (experimental).
 
 ## Installation
 
diff --git a/packages/integrations/svelte/client-v5.js b/packages/integrations/svelte/client-v5.js
new file mode 100644
index 000000000..b3d2e1964
--- /dev/null
+++ b/packages/integrations/svelte/client-v5.js
@@ -0,0 +1,43 @@
+import { mount } from 'svelte';
+
+export default (element) => {
+	return async (Component, props, slotted) => {
+		if (!element.hasAttribute('ssr')) return;
+
+		let children = undefined;
+		let $$slots = undefined;
+		for (const [key, value] of Object.entries(slotted)) {
+			if (key === 'default') {
+				children = createSlotDefinition(key, value);
+			} else {
+				$$slots ??= {};
+				$$slots[key] = createSlotDefinition(key, value);
+			}
+		}
+
+		const [, destroy] = mount(Component, {
+			target: element,
+			props: {
+				...props,
+				children,
+				$$slots,
+			},
+		});
+
+		element.addEventListener('astro:unmount', () => destroy(), { once: true });
+	};
+};
+
+function createSlotDefinition(key, children) {
+	/**
+	 * @param {Comment} $$anchor A comment node for slots in Svelte 5
+	 */
+	return ($$anchor, _$$slotProps) => {
+		const parent = $$anchor.parentNode;
+		const el = document.createElement('div');
+		el.innerHTML = `<astro-slot${
+			key === 'default' ? '' : ` name="${key}"`
+		}>${children}</astro-slot>`;
+		parent.insertBefore(el.children[0], $$anchor);
+	};
+}
diff --git a/packages/integrations/svelte/package.json b/packages/integrations/svelte/package.json
index 8cf1c9d87..65c8522f9 100644
--- a/packages/integrations/svelte/package.json
+++ b/packages/integrations/svelte/package.json
@@ -24,13 +24,17 @@
     "./editor": "./dist/editor.cjs",
     "./*": "./*",
     "./client.js": "./client.js",
+    "./client-v5.js": "./client-v5.js",
     "./server.js": "./server.js",
+    "./server-v5.js": "./server-v5.js",
     "./package.json": "./package.json"
   },
   "files": [
     "dist",
     "client.js",
-    "server.js"
+    "client-v5.js",
+    "server.js",
+    "server-v5.js"
   ],
   "scripts": {
     "build": "astro-scripts build \"src/index.ts\" && astro-scripts build \"src/editor.cts\" --force-cjs --no-clean-dist && tsc",
@@ -38,7 +42,7 @@
     "dev": "astro-scripts dev \"src/**/*.ts\""
   },
   "dependencies": {
-    "@sveltejs/vite-plugin-svelte": "^2.4.5",
+    "@sveltejs/vite-plugin-svelte": "^2.5.2",
     "svelte2tsx": "^0.6.20"
   },
   "devDependencies": {
@@ -49,7 +53,7 @@
   },
   "peerDependencies": {
     "astro": "^3.0.0",
-    "svelte": "^3.55.0 || ^4.0.0"
+    "svelte": "^3.55.0 || ^4.0.0 || ^5.0.0-next.1"
   },
   "engines": {
     "node": ">=18.14.1"
diff --git a/packages/integrations/svelte/server-v5.js b/packages/integrations/svelte/server-v5.js
new file mode 100644
index 000000000..105b843fb
--- /dev/null
+++ b/packages/integrations/svelte/server-v5.js
@@ -0,0 +1,42 @@
+import { render } from 'svelte/server';
+
+function check(Component) {
+	// Svelte 5 generated components always accept these two props
+	const str = Component.toString();
+	return str.includes('$$payload') && str.includes('$$props');
+}
+
+function needsHydration(metadata) {
+	// Adjust how this is hydrated only when the version of Astro supports `astroStaticSlot`
+	return metadata.astroStaticSlot ? !!metadata.hydrate : true;
+}
+
+async function renderToStaticMarkup(Component, props, slotted, metadata) {
+	const tagName = needsHydration(metadata) ? 'astro-slot' : 'astro-static-slot';
+
+	let children = undefined;
+	let $$slots = undefined;
+	for (const [key, value] of Object.entries(slotted)) {
+		if (key === 'default') {
+			children = () => `<${tagName}>${value}</${tagName}>`;
+		} else {
+			$$slots ??= {};
+			$$slots[key] = () => `<${tagName} name="${key}">${value}</${tagName}>`;
+		}
+	}
+
+	const { html } = render(Component, {
+		props: {
+			...props,
+			children,
+			$$slots,
+		},
+	});
+	return { html };
+}
+
+export default {
+	check,
+	renderToStaticMarkup,
+	supportsAstroStaticSlot: true,
+};
diff --git a/packages/integrations/svelte/src/index.ts b/packages/integrations/svelte/src/index.ts
index a9d4f37c9..5348f4c93 100644
--- a/packages/integrations/svelte/src/index.ts
+++ b/packages/integrations/svelte/src/index.ts
@@ -1,14 +1,17 @@
 import type { Options } from '@sveltejs/vite-plugin-svelte';
 import { svelte, vitePreprocess } from '@sveltejs/vite-plugin-svelte';
+import { VERSION } from 'svelte/compiler';
 import type { AstroIntegration, AstroRenderer } from 'astro';
 import { fileURLToPath } from 'node:url';
 import type { UserConfig } from 'vite';
 
+const isSvelte5 = Number.parseInt(VERSION.split('.').at(0)!) >= 5;
+
 function getRenderer(): AstroRenderer {
 	return {
 		name: '@astrojs/svelte',
-		clientEntrypoint: '@astrojs/svelte/client.js',
-		serverEntrypoint: '@astrojs/svelte/server.js',
+		clientEntrypoint: isSvelte5 ? '@astrojs/svelte/client-v5.js' : '@astrojs/svelte/client.js',
+		serverEntrypoint: isSvelte5 ? '@astrojs/svelte/server-v5.js' : '@astrojs/svelte/server.js',
 	};
 }
 
@@ -37,9 +40,15 @@ async function getViteConfiguration({
 }: ViteConfigurationArgs): Promise<UserConfig> {
 	const defaultOptions: Partial<Options> = {
 		emitCss: true,
-		compilerOptions: { dev: isDev, hydratable: true },
+		compilerOptions: { dev: isDev },
 	};
 
+	// `hydratable` does not need to be set in Svelte 5 as it's always hydratable by default
+	if (!isSvelte5) {
+		// @ts-ignore ignore Partial type above
+		defaultOptions.compilerOptions.hydratable = true;
+	}
+
 	// Disable hot mode during the build
 	if (!isDev) {
 		defaultOptions.hot = false;
@@ -69,8 +78,8 @@ async function getViteConfiguration({
 
 	return {
 		optimizeDeps: {
-			include: ['@astrojs/svelte/client.js'],
-			exclude: ['@astrojs/svelte/server.js'],
+			include: [isSvelte5 ? '@astrojs/svelte/client-v5.js' : '@astrojs/svelte/client.js'],
+			exclude: [isSvelte5 ? '@astrojs/svelte/server-v5.js' : '@astrojs/svelte/server.js'],
 		},
 		plugins: [svelte(resolvedOptions)],
 	};