fix: improve spreading of attributes (#11177)

* fix: improve spreading of attributes

* twak

.changeset/nervous-turkeys-end.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: improve spreading of attributes

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

@@ -104,6 +104,9 @@ export function set_attributes(element, prev, attrs, lowercase_attributes, css_h
 	var setters = map_get(setters_cache, element.nodeName);
 	if (!setters) map_set(setters_cache, element.nodeName, (setters = get_setters(element)));
 
+	// @ts-expect-error
+	var attributes = /** @type {Record<string, unknown>} **/ (element.__attributes ??= {});
+
 	for (key in next) {
 		var value = next[key];
 		if (value === prev?.[key]) continue;
@@ -140,6 +143,7 @@ export function set_attributes(element, prev, attrs, lowercase_attributes, css_h
 				}
 			}
 		} else if (value == null) {
+			attributes[key] = null;
 			element.removeAttribute(key);
 		} else if (key === 'style') {
 			element.style.cssText = value + '';

packages/svelte/tests/runtime-runes/samples/spread-props-2/_config.js

@@ -0,0 +1,23 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		flushSync(() => {
+			btn?.click();
+		});
+
+		assert.htmlEqual(target.innerHTML, `<button>Toggle</button><h1>Style: not red</h1>`);
+
+		flushSync(() => {
+			btn?.click();
+		});
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<button>Toggle</button><h1 data-red="true">Style: red</h1>`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/spread-props-2/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	let isRed = $state(true)
+
+  const attributes = $derived({
+    'data-red': isRed ? true : undefined
+  });
+</script>
+
+<button onclick={() => isRed = !isRed }>Toggle</button>
+
+<h1 {...attributes}>Style: {isRed ? 'red' : 'not red'}</h1>