fix: ensure single script and scripts in loops execute properly (#13386)

- add a comment to lone script tags to ensure there's always a parent so we can synchronously call the replace function
- always call the replace function, not just on the first call

fixes #13378

---------

Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

packages/svelte/src/compiler/phases/3-transform/utils.js

@@ -270,6 +270,19 @@ export function clean_nodes(
 
 	var first = trimmed[0];
 
+	// Special case: Add a comment if this is a lone script tag. This ensures that our run_scripts logic in template.js
+	// will always be able to call node.replaceWith() on the script tag in order to make it run. If we don't add this
+	// and would still call node.replaceWith() on the script tag, it would be a no-op because the script tag has no parent.
+	if (trimmed.length === 1 && first.type === 'RegularElement' && first.name === 'script') {
+		trimmed.push({
+			type: 'Comment',
+			data: '',
+			parent: first.parent,
+			start: -1,
+			end: -1
+		});
+	}
+
 	return {
 		hoisted,
 		trimmed,

packages/svelte/src/internal/client/dom/template.js

@@ -72,21 +72,8 @@ export function template(content, flags) {
  */
 /*#__NO_SIDE_EFFECTS__*/
 export function template_with_script(content, flags) {
-	var first = true;
 	var fn = template(content, flags);
-
-	return () => {
-		if (hydrating) return fn();
-
-		var node = /** @type {Element | DocumentFragment} */ (fn());
-
-		if (first) {
-			first = false;
-			run_scripts(node);
-		}
-
-		return node;
-	};
+	return () => run_scripts(/** @type {Element | DocumentFragment} */ (fn()));
 }
 
 /**
@@ -151,21 +138,8 @@ export function ns_template(content, flags, ns = 'svg') {
  */
 /*#__NO_SIDE_EFFECTS__*/
 export function svg_template_with_script(content, flags) {
-	var first = true;
 	var fn = ns_template(content, flags);
-
-	return () => {
-		if (hydrating) return fn();
-
-		var node = /** @type {Element | DocumentFragment} */ (fn());
-
-		if (first) {
-			first = false;
-			run_scripts(node);
-		}
-
-		return node;
-	};
+	return () => run_scripts(/** @type {Element | DocumentFragment} */ (fn()));
 }
 
 /**
@@ -182,10 +156,11 @@ export function mathml_template(content, flags) {
  * Creating a document fragment from HTML that contains script tags will not execute
  * the scripts. We need to replace the script tags with new ones so that they are executed.
  * @param {Element | DocumentFragment} node
+ * @returns {Node | Node[]}
  */
 function run_scripts(node) {
 	// scripts were SSR'd, in which case they will run
-	if (hydrating) return;
+	if (hydrating) return node;
 
 	const is_fragment = node.nodeType === 11;
 	const scripts =
@@ -202,28 +177,17 @@ function run_scripts(node) {
 
 		clone.textContent = script.textContent;
 
-		const replace = () => {
-			// The script has changed - if it's at the edges, the effect now points at dead nodes
-			if (is_fragment ? node.firstChild === script : node === script) {
-				effect.nodes_start = clone;
-			}
-			if (is_fragment ? node.lastChild === script : node === script) {
-				effect.nodes_end = clone;
-			}
-
-			script.replaceWith(clone);
-		};
-
-		// If node === script tag, replaceWith will do nothing because there's no parent yet,
-		// waiting until that's the case using an effect solves this.
-		// Don't do it in other circumstances or we could accidentally execute scripts
-		// in an adjacent @html tag that was instantiated in the meantime.
-		if (script === node) {
-			queue_micro_task(replace);
-		} else {
-			replace();
+		// The script has changed - if it's at the edges, the effect now points at dead nodes
+		if (is_fragment ? node.firstChild === script : node === script) {
+			effect.nodes_start = clone;
+		}
+		if (is_fragment ? node.lastChild === script : node === script) {
+			effect.nodes_end = clone;
 		}
+
+		script.replaceWith(clone);
 	}
+	return node;
 }
 
 /**

packages/svelte/tests/runtime-browser/samples/head-scripts/_config.js

@@ -0,0 +1,13 @@
+import { test } from '../../assert';
+
+export default test({
+	mode: ['client'],
+	async test({ assert, window }) {
+		// wait the script to load (maybe there a better way)
+		await new Promise((resolve) => setTimeout(resolve, 1));
+		assert.htmlEqual(
+			window.document.body.innerHTML,
+			`<main><b id="r1">1</b><b id="r2">2</b><b id="r3">3</b></main>`
+		);
+	}
+});

packages/svelte/tests/runtime-browser/samples/head-scripts/main.svelte

@@ -0,0 +1,16 @@
+<script>
+	function jssrc(src) {
+		return 'data:text/javascript;base64, ' + btoa(src);
+	}
+
+	const scriptSrcs = [ 1, 2, 3 ]
+		.map(n => jssrc(`document.getElementById('r${n}').innerText = '${n}';`));
+</script>
+
+<svelte:head>
+	{#each scriptSrcs as src}
+    	<script src={src} async defer></script>
+	{/each}
+</svelte:head>
+
+<b id="r1">?</b><b id="r2">?</b><b id="r3">?</b>