fix: ensure input value is correctly set during hydration (#12083)

* fix: ensure input value is correctly set during hydration

* fix: ensure input value is correctly set during hydration

* address feedback

* fix typos

* early return, var

* fix test

* Update packages/svelte/src/internal/client/dom/elements/attributes.js

Co-authored-by: Rich Harris <rich.harris@vercel.com>

* update changeset

* tweak names

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/moody-toys-relax.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: preserve current input values when removing defaults

packages/svelte/src/compiler/phases/3-transform/client/visitors/template.js

@@ -2030,7 +2030,7 @@ export const template_visitors = {
 		}
 
 		if (needs_input_reset && node.name === 'input') {
-			context.state.init.push(b.stmt(b.call('$.remove_input_attr_defaults', context.state.node)));
+			context.state.init.push(b.stmt(b.call('$.remove_input_defaults', context.state.node)));
 		}
 
 		if (needs_content_reset && node.name === 'textarea') {

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -16,29 +16,40 @@ import { queue_idle_task, queue_micro_task } from '../task.js';
 /**
  * The value/checked attribute in the template actually corresponds to the defaultValue property, so we need
  * to remove it upon hydration to avoid a bug when someone resets the form value.
- * @param {HTMLInputElement} dom
+ * @param {HTMLInputElement} input
  * @returns {void}
  */
-export function remove_input_attr_defaults(dom) {
-	if (hydrating) {
-		let already_removed = false;
-		// We try and remove the default attributes later, rather than sync during hydration.
-		// Doing it sync during hydration has a negative impact on performance, but deferring the
-		// work in an idle task alleviates this greatly. If a form reset event comes in before
-		// the idle callback, then we ensure the input defaults are cleared just before.
-		const remove_defaults = () => {
-			if (already_removed) return;
-			already_removed = true;
-			const value = dom.getAttribute('value');
-			set_attribute(dom, 'value', null);
-			set_attribute(dom, 'checked', null);
-			if (value) dom.value = value;
-		};
-		// @ts-expect-error
-		dom.__on_r = remove_defaults;
-		queue_idle_task(remove_defaults);
-		add_form_reset_listener();
-	}
+export function remove_input_defaults(input) {
+	if (!hydrating) return;
+
+	var already_removed = false;
+
+	// We try and remove the default attributes later, rather than sync during hydration.
+	// Doing it sync during hydration has a negative impact on performance, but deferring the
+	// work in an idle task alleviates this greatly. If a form reset event comes in before
+	// the idle callback, then we ensure the input defaults are cleared just before.
+	var remove_defaults = () => {
+		if (already_removed) return;
+		already_removed = true;
+
+		// Remove the attributes but preserve the values
+		if (input.hasAttribute('value')) {
+			var value = input.value;
+			set_attribute(input, 'value', null);
+			input.value = value;
+		}
+
+		if (input.hasAttribute('checked')) {
+			var checked = input.checked;
+			set_attribute(input, 'checked', null);
+			input.checked = checked;
+		}
+	};
+
+	// @ts-expect-error
+	input.__on_r = remove_defaults;
+	queue_idle_task(remove_defaults);
+	add_form_reset_listener();
 }
 
 /**

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

@@ -21,7 +21,7 @@ export { element } from './dom/blocks/svelte-element.js';
 export { head } from './dom/blocks/svelte-head.js';
 export { action } from './dom/elements/actions.js';
 export {
-	remove_input_attr_defaults,
+	remove_input_defaults,
 	set_attribute,
 	set_attributes,
 	set_custom_element_data,

packages/svelte/tests/hydration/samples/input-value-changed/_config.js

@@ -0,0 +1,16 @@
+import { test } from '../../test';
+
+export default test({
+	server_props: {
+		name: 'server'
+	},
+
+	props: {
+		name: 'browser'
+	},
+
+	test(assert, target) {
+		const input = target.querySelector('input');
+		assert.equal(input?.value, 'browser');
+	}
+});

packages/svelte/tests/hydration/samples/input-value-changed/_expected.html

@@ -0,0 +1 @@
+<!--[--><input type="text"><!--]-->

packages/svelte/tests/hydration/samples/input-value-changed/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	const { name } = $props();
+</script>
+
+<input type="text" value={name} />

packages/svelte/tests/snapshot/samples/state-proxy-literal/_expected/client/index.svelte.js

@@ -16,11 +16,11 @@ export default function State_proxy_literal($$anchor) {
 	var fragment = root();
 	var input = $.first_child(fragment);
 
-	$.remove_input_attr_defaults(input);
+	$.remove_input_defaults(input);
 
 	var input_1 = $.sibling($.sibling(input, true));
 
-	$.remove_input_attr_defaults(input_1);
+	$.remove_input_defaults(input_1);
 
 	var button = $.sibling($.sibling(input_1, true));
 
@@ -30,4 +30,4 @@ export default function State_proxy_literal($$anchor) {
 	$.append($$anchor, fragment);
 }
 
-$.delegate(["click"]);
+$.delegate(["click"]);