feat: take form resets into account for two way bindings (#10617)

* feat: take form resets into account for two way bindings

When resetting a form, the value of the inputs within it get out of sync with the bound value of those inputs. This PR introduces a reset listener on the parent form to reset the value in that case
closes #2659

* slightly different approach

* tweaks, test

* this is a breaking change, strictly speaking

* bind:files

* use capture phase

* tweak wording

* use promise, explain

Author: dummdidumm

.changeset/silly-ways-wash.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+feat: take form resets into account for two way bindings

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

@@ -2892,6 +2892,10 @@ export const template_visitors = {
 					break;
 				}
 
+				case 'files':
+					call_expr = b.call(`$.bind_files`, state.node, getter, setter);
+					break;
+
 				case 'this':
 					call_expr = serialize_bind_this(node.expression, context, state.node);
 					break;

packages/svelte/src/compiler/phases/bindings.js

@@ -182,8 +182,6 @@ export const binding_properties = {
 		valid_elements: ['input', 'textarea', 'select']
 	},
 	files: {
-		event: 'change',
-		type: 'set',
 		valid_elements: ['input'],
 		omit_in_ssr: true
 	}

packages/svelte/src/internal/client/dom/elements/bindings/input.js

@@ -1,6 +1,7 @@
 import { DEV } from 'esm-env';
 import { render_effect, user_effect } from '../../../reactivity/effects.js';
 import { stringify } from '../../../render.js';
+import { listen_to_event_and_reset_event } from './shared.js';
 
 /**
  * @param {HTMLInputElement} input
@@ -9,7 +10,7 @@ import { stringify } from '../../../render.js';
  * @returns {void}
  */
 export function bind_value(input, get_value, update) {
-	input.addEventListener('input', () => {
+	listen_to_event_and_reset_event(input, 'input', () => {
 		if (DEV && input.type === 'checkbox') {
 			throw new Error(
 				'Using bind:value together with a checkbox input is not allowed. Use bind:checked instead'
@@ -72,16 +73,22 @@ export function bind_group(inputs, group_index, input, get_value, update) {
 
 	binding_group.push(input);
 
-	input.addEventListener('change', () => {
-		// @ts-ignore
-		var value = input.__value;
+	listen_to_event_and_reset_event(
+		input,
+		'change',
+		() => {
+			// @ts-ignore
+			var value = input.__value;
 
-		if (is_checkbox) {
-			value = get_binding_group_value(binding_group, value, input.checked);
-		}
+			if (is_checkbox) {
+				value = get_binding_group_value(binding_group, value, input.checked);
+			}
 
-		update(value);
-	});
+			update(value);
+		},
+		// TODO better default value handling
+		() => update(is_checkbox ? [] : null)
+	);
 
 	render_effect(() => {
 		var value = get_value();
@@ -119,7 +126,7 @@ export function bind_group(inputs, group_index, input, get_value, update) {
  * @returns {void}
  */
 export function bind_checked(input, get_value, update) {
-	input.addEventListener('change', () => {
+	listen_to_event_and_reset_event(input, 'change', () => {
 		var value = input.checked;
 		update(value);
 	});
@@ -173,3 +180,17 @@ function is_numberlike_input(input) {
 function to_number(value) {
 	return value === '' ? null : +value;
 }
+
+/**
+ * @param {HTMLInputElement} input
+ * @param {() => FileList | null} get_value
+ * @param {(value: FileList | null) => void} update
+ */
+export function bind_files(input, get_value, update) {
+	listen_to_event_and_reset_event(input, 'change', () => {
+		update(input.files);
+	});
+	render_effect(() => {
+		input.files = get_value();
+	});
+}

packages/svelte/src/internal/client/dom/elements/bindings/select.js

@@ -1,4 +1,5 @@
 import { effect } from '../../../reactivity/effects.js';
+import { listen_to_event_and_reset_event } from './shared.js';
 import { untrack } from '../../../runtime.js';
 
 /**
@@ -76,7 +77,7 @@ export function init_select(select, get_value) {
 export function bind_select_value(select, get_value, update) {
 	var mounting = true;
 
-	select.addEventListener('change', () => {
+	listen_to_event_and_reset_event(select, 'change', () => {
 		/** @type {unknown} */
 		var value;
 

packages/svelte/src/internal/client/dom/elements/bindings/shared.js

@@ -25,3 +25,51 @@ export function listen(target, events, handler, call_handler_immediately = true)
 		};
 	});
 }
+
+let listening_to_form_reset = false;
+
+/**
+ * Listen to the given event, and then instantiate a global form reset listener if not already done,
+ * to notify all bindings when the form is reset
+ * @param {HTMLElement} element
+ * @param {string} event
+ * @param {() => void} handler
+ * @param {() => void} [on_reset]
+ */
+export function listen_to_event_and_reset_event(element, event, handler, on_reset = handler) {
+	element.addEventListener(event, handler);
+	// @ts-expect-error
+	const prev = element.__on_r;
+	if (prev) {
+		// special case for checkbox that can have multiple binds (group & checked)
+		// @ts-expect-error
+		element.__on_r = () => {
+			prev();
+			on_reset();
+		};
+	} else {
+		// @ts-expect-error
+		element.__on_r = on_reset;
+	}
+
+	if (!listening_to_form_reset) {
+		listening_to_form_reset = true;
+		document.addEventListener(
+			'reset',
+			(evt) => {
+				// Needs to happen one tick later or else the dom properties of the form
+				// elements have not updated to their reset values yet
+				Promise.resolve().then(() => {
+					if (!evt.defaultPrevented) {
+						for (const e of /**@type {HTMLFormElement} */ (evt.target).elements) {
+							// @ts-expect-error
+							e.__on_r?.();
+						}
+					}
+				});
+			},
+			// In the capture phase to guarantee we get noticed of it (no possiblity of stopPropagation)
+			{ capture: true }
+		);
+	}
+}

packages/svelte/tests/runtime-runes/samples/bindings-form-reset/_config.js

@@ -0,0 +1,19 @@
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const p = target.querySelector('p');
+
+		assert.htmlEqual(
+			p?.innerHTML || '',
+			`{"text":"text","checkbox":true,"radio_group":"a","checkbox_group":["a"],"select":"b","textarea":"textarea"}`
+		);
+
+		await target.querySelector('button')?.click();
+		await Promise.resolve();
+		assert.htmlEqual(
+			p?.innerHTML || '',
+			`{"text":"","checkbox":false,"radio_group":null,"checkbox_group":[],"select":"a","textarea":""}`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/bindings-form-reset/main.svelte

@@ -0,0 +1,31 @@
+<script>
+	let text = $state('text');
+	let checkbox = $state(true);
+	let radio_group = $state('a');
+	let checkbox_group = $state(['a']);
+	let select = $state('b');
+	let textarea = $state('textarea');
+</script>
+
+<p>{JSON.stringify({ text, checkbox, radio_group, checkbox_group, select, textarea })}</p>
+
+<form>
+	<input bind:value={text} />
+	
+	<input type="checkbox" bind:checked={checkbox} />
+	
+	<input type="radio" name="radio" value="a" bind:group={radio_group} />
+	<input type="radio" name="radio" value="b" bind:group={radio_group} />
+	
+	<input type="checkbox" name="checkbox" value="a" bind:group={checkbox_group} />
+	<input type="checkbox" name="checkbox" value="b" bind:group={checkbox_group} />
+
+	<select bind:value={select}>
+		<option value="a">a</option>
+		<option value="b">b</option>
+	</select>
+
+	<textarea bind:value={textarea}></textarea>
+
+	<button type="button" onclick={(e) => e.target.form.reset()}>Reset</button>
+</form>

sites/svelte-5-preview/src/routes/docs/content/03-appendix/02-breaking-changes.md

@@ -179,3 +179,7 @@ In Svelte 4, `null` and `undefined` were printed as the corresponding string. In
 ### `bind:files` values can only be `null`, `undefined` or `FileList`
 
 `bind:files` is now a two-way binding. As such, when setting a value, it needs to be either falsy (`null` or `undefined`) or of type `FileList`.
+
+### Bindings now react to form resets
+
+Previously, bindings did not take into account `reset` event of forms, and therefore values could get out of sync with the DOM. Svelte 5 fixes this by placing a `reset` listener on the document and invoking bindings where necessary.