fix: don't throw for undefined non delegated event handlers (#15087)

* fix: don't throw for `undefined` non delegated event handlers

* chore: update typings

Author: paoloricciuti

.changeset/clever-cherries-hear.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't throw for `undefined` non delegated event handlers

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

@@ -49,10 +49,10 @@ export function replay_events(dom) {
 /**
  * @param {string} event_name
  * @param {EventTarget} dom
- * @param {EventListener} handler
- * @param {AddEventListenerOptions} options
+ * @param {EventListener} [handler]
+ * @param {AddEventListenerOptions} [options]
  */
-export function create_event(event_name, dom, handler, options) {
+export function create_event(event_name, dom, handler, options = {}) {
 	/**
 	 * @this {EventTarget}
 	 */
@@ -63,7 +63,7 @@ export function create_event(event_name, dom, handler, options) {
 		}
 		if (!event.cancelBubble) {
 			return without_reactive_context(() => {
-				return handler.call(this, event);
+				return handler?.call(this, event);
 			});
 		}
 	}
@@ -108,8 +108,8 @@ export function on(element, type, handler, options = {}) {
 /**
  * @param {string} event_name
  * @param {Element} dom
- * @param {EventListener} handler
- * @param {boolean} capture
+ * @param {EventListener} [handler]
+ * @param {boolean} [capture]
  * @param {boolean} [passive]
  * @returns {void}
  */

packages/svelte/tests/runtime-runes/samples/undefined-event-handler/_config.js

@@ -0,0 +1,9 @@
+import { ok, test } from '../../test';
+
+export default test({
+	async test({ target }) {
+		const button = target.querySelector('button');
+		ok(button);
+		button.dispatchEvent(new window.MouseEvent('mouseenter'));
+	}
+});

packages/svelte/tests/runtime-runes/samples/undefined-event-handler/main.svelte

@@ -0,0 +1 @@
+<button onmouseenter={undefined}></button>