fix: handle sole empty expression tags (#10433)

* fix: handle sole empty expression tags

When there's only a single expression tag and its value evaluates to the empty string, special handling is needed to create and insert a text node
fixes #10426

* fix

* need this, too

* Update packages/svelte/src/internal/client/operations.js

---------

Co-authored-by: Rich Harris <richard.a.harris@gmail.com>

.changeset/silent-apes-report.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: handle sole empty expression tags

packages/svelte/src/internal/client/each.js

@@ -13,9 +13,8 @@ import {
 	hydrate_block_anchor,
 	set_current_hydration_fragment
 } from './hydration.js';
-import { clear_text_content, map_get, map_set } from './operations.js';
+import { clear_text_content, empty, map_get, map_set } from './operations.js';
 import { insert, remove } from './reconciler.js';
-import { empty } from './render.js';
 import {
 	destroy_signal,
 	execute_effect,

packages/svelte/src/internal/client/hydration.js

@@ -1,5 +1,6 @@
 // Handle hydration
 
+import { empty } from './operations.js';
 import { schedule_task } from './runtime.js';
 
 /** @type {null | Array<import('./types.js').TemplateNode>} */
@@ -16,9 +17,10 @@ export function set_current_hydration_fragment(fragment) {
 /**
  * Returns all nodes between the first `<!--ssr:...-->` comment tag pair encountered.
  * @param {Node | null} node
+ * @param {boolean} [insert_text] Whether to insert an empty text node if the fragment is empty
  * @returns {Array<import('./types.js').TemplateNode> | null}
  */
-export function get_hydration_fragment(node) {
+export function get_hydration_fragment(node, insert_text = false) {
 	/** @type {Array<import('./types.js').TemplateNode>} */
 	const fragment = [];
 
@@ -37,6 +39,11 @@ export function get_hydration_fragment(node) {
 				if (target_depth === null) {
 					target_depth = depth;
 				} else if (depth === target_depth) {
+					if (insert_text && fragment.length === 0) {
+						const text = empty();
+						fragment.push(text);
+						/** @type {Node} */ (current_node.parentNode).insertBefore(text, current_node);
+					}
 					return fragment;
 				} else {
 					fragment.push(/** @type {Text | Comment | Element} */ (current_node));

packages/svelte/src/internal/client/operations.js

@@ -158,6 +158,11 @@ export function clone_node(node, deep) {
 	return /** @type {N} */ (clone_node_method.call(node, deep));
 }
 
+/** @returns {Text} */
+export function empty() {
+	return document.createTextNode('');
+}
+
 /**
  * @template {Node} N
  * @param {N} node
@@ -169,7 +174,7 @@ export function child(node) {
 	if (current_hydration_fragment !== null) {
 		// Child can be null if we have an element with a single child, like `<p>{text}</p>`, where `text` is empty
 		if (child === null) {
-			const text = document.createTextNode('');
+			const text = empty();
 			node.appendChild(text);
 			return text;
 		} else {
@@ -193,7 +198,7 @@ export function child_frag(node, is_text) {
 		// if an {expression} is empty during SSR, there might be no
 		// text node to hydrate — we must therefore create one
 		if (is_text && first_node?.nodeType !== 3) {
-			const text = document.createTextNode('');
+			const text = empty();
 			current_hydration_fragment.unshift(text);
 			if (first_node) {
 				/** @type {DocumentFragment} */ (first_node.parentNode).insertBefore(text, first_node);
@@ -221,8 +226,10 @@ export function child_frag(node, is_text) {
 export function sibling(node, is_text = false) {
 	const next_sibling = next_sibling_get.call(node);
 	if (current_hydration_fragment !== null) {
+		// if a sibling {expression} is empty during SSR, there might be no
+		// text node to hydrate — we must therefore create one
 		if (is_text && next_sibling?.nodeType !== 3) {
-			const text = document.createTextNode('');
+			const text = empty();
 			if (next_sibling) {
 				const index = current_hydration_fragment.indexOf(
 					/** @type {Text | Comment | Element} */ (next_sibling)

packages/svelte/src/internal/client/render.js

@@ -4,6 +4,7 @@ import {
 	child,
 	clone_node,
 	create_element,
+	empty,
 	init_operations,
 	map_get,
 	map_set,
@@ -75,11 +76,6 @@ const all_registerd_events = new Set();
 /** @type {Set<(events: Array<string>) => void>} */
 const root_event_handles = new Set();
 
-/** @returns {Text} */
-export function empty() {
-	return document.createTextNode('');
-}
-
 /**
  * @param {string} html
  * @param {boolean} return_fragment
@@ -212,11 +208,22 @@ const space_template = template(' ', false);
 const comment_template = template('<!>', true);
 
 /**
- * @param {null | Text | Comment | Element} anchor
+ * @param {Text | Comment | Element | null} anchor
  */
 /*#__NO_SIDE_EFFECTS__*/
 export function space(anchor) {
-	return open(anchor, true, space_template);
+	/** @type {Node | null} */
+	var node = /** @type {any} */ (open(anchor, true, space_template));
+	// if an {expression} is empty during SSR, there might be no
+	// text node to hydrate (or an anchor comment is falsely detected instead)
+	//  — we must therefore create one
+	if (current_hydration_fragment !== null && node?.nodeType !== 3) {
+		node = empty();
+		// @ts-ignore in this case the anchor should always be a comment,
+		// if not something more fundamental is wrong and throwing here is better to bail out early
+		anchor.parentElement.insertBefore(node, anchor);
+	}
+	return node;
 }
 
 /**
@@ -228,6 +235,8 @@ export function comment(anchor) {
 }
 
 /**
+ * Assign the created (or in hydration mode, traversed) dom elements to the current block
+ * and insert the elements into the dom (in client mode).
  * @param {Element | Text} dom
  * @param {boolean} is_fragment
  * @param {null | Text | Comment | Element} anchor
@@ -2866,7 +2875,9 @@ export function mount(component, options) {
 	const container = options.target;
 	const block = create_root_block(options.intro || false);
 	const first_child = /** @type {ChildNode} */ (container.firstChild);
-	const hydration_fragment = get_hydration_fragment(first_child);
+	// Call with insert_text == true to prevent empty {expressions} resulting in an empty
+	// fragment array, resulting in a hydration error down the line
+	const hydration_fragment = get_hydration_fragment(first_child, true);
 	const previous_hydration_fragment = current_hydration_fragment;
 
 	/** @type {Exports} */

packages/svelte/src/internal/client/transitions.js

@@ -10,8 +10,7 @@ import {
 	ROOT_BLOCK
 } from './block.js';
 import { destroy_each_item_block, get_first_element } from './each.js';
-import { append_child } from './operations.js';
+import { append_child, empty } from './operations.js';
-import { empty } from './render.js';
 import {
 	current_block,
 	current_effect,

packages/svelte/tests/hydration/samples/if-block-empty/_after.html

@@ -0,0 +1,4 @@
+<!--ssr:0-->
+<!--ssr:1-->
+<!--ssr:if:true-->x<!--ssr:1-->
+<!--ssr:0-->

packages/svelte/tests/hydration/samples/if-block-empty/_before.html

@@ -0,0 +1,4 @@
+<!--ssr:0-->
+<!--ssr:1-->
+<!--ssr:if:true--><!--ssr:1-->
+<!--ssr:0-->

packages/svelte/tests/hydration/samples/if-block-empty/_config.js

@@ -0,0 +1,3 @@
+import { test } from '../../test';
+
+export default test({});

packages/svelte/tests/hydration/samples/if-block-empty/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	let foo = typeof window === 'undefined' ? '' : 'x';
+</script>
+
+{#if true}
+	{foo}
+{/if}

packages/svelte/tests/hydration/samples/text-empty/_after.html

@@ -0,0 +1 @@
+<!--ssr:0-->x<!--ssr:0-->

packages/svelte/tests/hydration/samples/text-empty/_before.html

@@ -0,0 +1 @@
+<!--ssr:0--><!--ssr:0-->

packages/svelte/tests/hydration/samples/text-empty/_config.js

@@ -0,0 +1,3 @@
+import { test } from '../../test';
+
+export default test({});

packages/svelte/tests/hydration/samples/text-empty/main.svelte

@@ -0,0 +1,5 @@
+<script>
+    let x = typeof window === 'undefined' ? '' : 'x'
+</script>
+
+{x}