feat: improve types for createEventDispatcher

src/runtime/internal/lifecycle.ts

@@ -56,6 +56,25 @@ export function onDestroy(fn: () => any) {
 	get_current_component().$$.on_destroy.push(fn);
 }
 
+type UnionToIntersection<U> = (U extends any ? (k: U) => void : never) extends (k: infer I) => void
+	? I
+	: never
+
+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 | null
+	? (type: EventKey, detail?: null, options?: DispatchOptions) => boolean
+	: null extends EventMap[EventKey]
+	? (type: EventKey, detail?: EventMap[EventKey], options?: DispatchOptions) => boolean
+	: (type: EventKey, detail: EventMap[EventKey], options?: DispatchOptions) => boolean
+
+type CreateDispatchFunctionMap<EventMap> = {
+	[Key in keyof EventMap]: ConstructDispatchFunction<EventMap, Key>
+}
+
+type EventDispatcher<EventMap extends Record<string, any>> = UnionToIntersection<ExtractObjectValues<CreateDispatchFunctionMap<EventMap>>>
+
 export interface DispatchOptions {
 	cancelable?: boolean;
 }
@@ -72,16 +91,10 @@ export interface DispatchOptions {
  * 
  * https://svelte.dev/docs#run-time-svelte-createeventdispatcher
  */
-export function createEventDispatcher<EventMap extends {} = any>(): <
-	EventKey extends Extract<keyof EventMap, string>
->(
-	type: EventKey,
-	detail?: EventMap[EventKey],
-	options?: DispatchOptions
-) => boolean {
+export function createEventDispatcher<EventMap extends Record<string, any> = any>(): EventDispatcher<EventMap> {
 	const component = get_current_component();
 
-	return (type: string, detail?: any, { cancelable = false } = {}): boolean => {
+	return ((type: string, detail?: any, { cancelable = false } = {}): boolean => {
 		const callbacks = component.$$.callbacks[type];
 
 		if (callbacks) {
@@ -95,9 +108,19 @@ export function createEventDispatcher<EventMap extends {} = any>(): <
 		}
 
 		return true;
-	};
+	}) as EventDispatcher<EventMap>;
 }
 
+// no type restrictions
+const dispatch1 = createEventDispatcher()
+dispatch1('click')
+dispatch1('something')
+dispatch1('something', true)
+
+const dispatch2 = createEventDispatcher<{ click: never }>()
+dispatch2('click')
+dispatch2('click', true)
+
 /**
  * Associates an arbitrary `context` object with the current component and the specified `key` 
  * and returns that object. The context is then available to children of the component 
@@ -140,7 +163,7 @@ export function getAllContexts<T extends Map<any, any> = Map<any, any>>(): T {
  * https://svelte.dev/docs#run-time-svelte-hascontext
  */
 export function hasContext(key): boolean {
-	return get_current_component().$$.context.has(key);	
+	return get_current_component().$$.context.has(key);
 }
 
 // TODO figure out if we still want to support