allow optional parameters for createEventDispatcher

Author: ivanhofer

src/runtime/internal/lifecycle.ts

@@ -36,6 +36,8 @@ type ExtractObjectValues<Object extends Record<any, any>> = Object[keyof Object]
 type ConstructDispatchFunction<EventMap extends Record<string, any>, EventKey extends keyof EventMap> =
 	EventMap[EventKey] extends never
 	? (type: EventKey) => void
+	: undefined extends EventMap[EventKey]
+	? (type: EventKey, detail?: EventMap[EventKey]) => void
 	: (type: EventKey, detail: EventMap[EventKey]) => void
 
 type CreateDispatchFunctionMap<EventMap> = {
@@ -63,6 +65,30 @@ export function createEventDispatcher<
 	}) as EventDispatcher<EventMap>;
 }
 
+
+const dispatch = createEventDispatcher<{
+	loaded: never
+	change: string
+	valid: boolean
+}>();
+
+// @ts-expect-error
+dispatch('some-event');
+
+dispatch('loaded');
+// @ts-expect-error
+dispatch('loaded', 123);
+
+dispatch('change', 'string');
+// @ts-expect-error
+dispatch('change', 123);
+
+dispatch('valid', true);
+// @ts-expect-error
+dispatch('valid', 'string');
+dispatch('change', undefined);
+
+
 export function setContext<T>(key, context: T) {
 	get_current_component().$$.context.set(key, context);
 }