fix: keep nullish `value` of spread `<option>` for `bind:value`

Closes #17395

.changeset/lucky-options-wink.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: keep nullish `value` of spread `<option>` for `bind:value`

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

@@ -350,7 +350,9 @@ function set_attributes(
 				// a very rare edge case, and removing the attribute altogether isn't possible either
 				// for the <option value={undefined}> case, so we're not losing any functionality here.
 				// @ts-ignore
-				element.value = element.__value = '';
+				element.value = '';
+				// @ts-ignore keep the nullish value so that `bind:value` on the parent <select> gets it back
+				element.__value = value;
 				current[key] = value;
 				continue;
 			}

packages/svelte/tests/runtime-runes/samples/select-spread-option-nullish-value/_config.js

@@ -0,0 +1,19 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		const select = /** @type {HTMLSelectElement} */ (target.querySelector('select'));
+		const p = /** @type {HTMLParagraphElement} */ (target.querySelector('p'));
+
+		select.selectedIndex = 1;
+		select.dispatchEvent(new Event('change'));
+		flushSync();
+		assert.equal(p.textContent, 'undefined undefined');
+
+		select.selectedIndex = 2;
+		select.dispatchEvent(new Event('change'));
+		flushSync();
+		assert.equal(p.textContent, 'object null');
+	}
+});

packages/svelte/tests/runtime-runes/samples/select-spread-option-nullish-value/main.svelte

@@ -0,0 +1,16 @@
+<script>
+	let value = $state('a');
+	const options = [
+		{ value: 'a', label: 'a' },
+		{ value: undefined, label: 'undefined' },
+		{ value: null, label: 'null' }
+	];
+</script>
+
+<select bind:value>
+	{#each options as { value, label, ...rest }}
+		<option {value} {...rest}>{label}</option>
+	{/each}
+</select>
+
+<p>{typeof value} {String(value)}</p>