fix: handle dynamic selects with falsy select values (#9471)

when options are added later, we need to ensure the select value still stays in sync
fixes #9412

Author: dummdidumm

Diff for: .changeset/lazy-spiders-think.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: handle dynamic selects with falsy select values

Diff for: packages/svelte/src/internal/client/render.js

@@ -881,9 +881,10 @@ export function bind_window_size(type, update) {
 	const callback = () => update(window[type]);
 	listen_to_events(window, ['resize'], callback);
 }
+
 /**
  * Finds the containing `<select>` element and potentially updates its `selected` state.
- * @param {Element} dom
+ * @param {HTMLOptionElement} dom
  * @returns {void}
  */
 export function selected(dom) {
@@ -902,10 +903,15 @@ export function selected(dom) {
 			const select_value = select.__value;
 			// @ts-ignore
 			const option_value = dom.__value;
-			// @ts-ignore
-			dom.selected = select_value === option_value;
-			// @ts-ignore
+			const selected = select_value === option_value;
+			dom.selected = selected;
 			dom.value = option_value;
+			// Handle the edge case of new options being added to a select when its state is "nothing selected"
+			// and keeping the selection state in sync (the DOM auto-selects the first option on insert)
+			// @ts-ignore
+			if (select.__value === null) {
+				/** @type {HTMLSelectElement} */ (select).value = '';
+			}
 		}
 	});
 }
@@ -957,13 +963,16 @@ export function bind_select_value(dom, get_value, update) {
 		}
 		update(value);
 	});
-	render_effect(() => {
+	// Needs to be an effect, not a render_effect, so that in case of each loops the logic runs after the each block has updated
+	effect(() => {
 		const value = get_value();
 		if (value == null && !mounted) {
 			/** @type {HTMLOptionElement | null} */
 			let selected_option = value === undefined ? dom.querySelector(':checked') : null;
 			if (selected_option === null) {
 				dom.value = '';
+				// @ts-ignore
+				dom.__value = null;
 			}
 			const options = dom.querySelectorAll('option');
 			for (const option of options) {

Diff for: packages/svelte/tests/runtime-legacy/samples/binding-select-initial-value-undefined-3/_config.js

@@ -0,0 +1,27 @@
+import { ok, test } from '../../test';
+
+export default test({
+	skip_if_ssr: 'permanent',
+
+	html: `
+		<p>selected: a</p>
+
+		<select>
+			<option disabled=''>x</option>
+			<option value="a">a</option>
+			<option value="b">b</option>
+			<option value="c">c</option>
+		</select>
+	`,
+
+	test({ assert, component, target }) {
+		assert.equal(component.selected, 'a');
+		const select = target.querySelector('select');
+		ok(select);
+		const options = [...target.querySelectorAll('option')];
+
+		// first enabled option should be selected
+		assert.equal(select.value, 'a');
+		assert.ok(options[1].selected);
+	}
+});

Diff for: packages/svelte/tests/runtime-legacy/samples/binding-select-initial-value-undefined-3/main.svelte

@@ -0,0 +1,12 @@
+<script>
+	export let selected;
+</script>
+
+<p>selected: {selected}</p>
+
+<select bind:value={selected}>
+	<option disabled>x</option>
+	{#each ["a", "b", "c"] as val}
+		<option>{val}</option>
+	{/each}
+</select>

Diff for: packages/svelte/tests/runtime-legacy/samples/binding-select-unmatched-2/_config.js

@@ -0,0 +1,112 @@
+import { ok, test } from '../../test';
+
+// // same test as the previous one, but with a dynamic each block
+export default test({
+	html: `
+		<p>selected: </p>
+
+		<select>
+			<option value="a">a</option>
+			<option value="b">b</option>
+			<option value="c">c</option>
+		</select>
+
+		<p>selected: </p>
+	`,
+
+	async test({ assert, component, target }) {
+		const select = target.querySelector('select');
+		ok(select);
+
+		const options = [...target.querySelectorAll('option')];
+
+		assert.equal(component.selected, null);
+
+		// no option should be selected since none of the options matches the bound value
+		assert.equal(select.value, '');
+		assert.equal(select.selectedIndex, -1);
+		assert.ok(!options[0].selected);
+
+		component.selected = 'a'; // first option should now be selected
+		assert.equal(select.value, 'a');
+		assert.ok(options[0].selected);
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<p>selected: a</p>
+
+			<select>
+				<option value="a">a</option>
+				<option value="b">b</option>
+				<option value="c">c</option>
+			</select>
+
+			<p>selected: a</p>
+		`
+		);
+
+		component.selected = 'd'; // doesn't match an option
+
+		// now no option should be selected again
+		assert.equal(select.value, '');
+		assert.equal(select.selectedIndex, -1);
+		assert.ok(!options[0].selected);
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<p>selected: d</p>
+
+			<select>
+				<option value="a">a</option>
+				<option value="b">b</option>
+				<option value="c">c</option>
+			</select>
+
+			<p>selected: d</p>
+		`
+		);
+
+		component.selected = 'b'; // second option should now be selected
+		assert.equal(select.value, 'b');
+		assert.ok(options[1].selected);
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<p>selected: b</p>
+
+			<select>
+				<option value="a">a</option>
+				<option value="b">b</option>
+				<option value="c">c</option>
+			</select>
+
+			<p>selected: b</p>
+		`
+		);
+
+		component.selected = undefined; // also doesn't match an option
+
+		// now no option should be selected again
+		assert.equal(select.value, '');
+		assert.equal(select.selectedIndex, -1);
+		assert.ok(!options[0].selected);
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+			<p>selected: </p>
+
+			<select>
+				<option value="a">a</option>
+				<option value="b">b</option>
+				<option value="c">c</option>
+			</select>
+
+			<p>selected: </p>
+		`
+		);
+	}
+});

Diff for: packages/svelte/tests/runtime-legacy/samples/binding-select-unmatched-2/main.svelte

@@ -0,0 +1,14 @@
+<script>
+	// set as null so no option will be selected by default
+	export let selected = null;
+</script>
+
+<p>selected: {selected}</p>
+
+<select bind:value={selected}>
+	{#each ['a', 'b', 'c'] as letter}
+		<option>{letter}</option>
+	{/each}
+</select>
+
+<p>selected: {selected}</p>