simplify type, add tests

package.json

@@ -90,7 +90,7 @@
   },
   "types": "types/runtime/index.d.ts",
   "scripts": {
-    "test": "npm run test:unit && npm run test:integration",
+    "test": "npm run test:unit && npm run test:integration && ECHO \"manually check that there are no type errors in test/types by opening the files in there\"",
     "test:integration": "mocha --exit",
     "test:unit": "mocha --config .mocharc.unit.js --exit",
     "quicktest": "mocha --exit",

src/runtime/internal/lifecycle.ts

@@ -56,25 +56,14 @@ 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>
+export interface EventDispatcher<EventMap extends Record<string, any>> {
+	<Type extends keyof EventMap>(
+		...args: [EventMap[Type]] extends [never] ? [type: Type, parameter?: null | undefined, options?: DispatchOptions] :
+		null extends EventMap[Type] ? [type: Type, parameter?: EventMap[Type], options?: DispatchOptions] :
+		undefined extends EventMap[Type] ? [type: Type, parameter?: EventMap[Type], options?: DispatchOptions] :
+		[type: Type, parameter: EventMap[Type], options?: DispatchOptions]): boolean;
 }
 
-type EventDispatcher<EventMap extends Record<string, any>> = UnionToIntersection<ExtractObjectValues<CreateDispatchFunctionMap<EventMap>>>
-
 export interface DispatchOptions {
 	cancelable?: boolean;
 }
@@ -87,7 +76,16 @@ export interface DispatchOptions {
  * [CustomEvent](https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent). 
  * These events do not [bubble](https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Building_blocks/Events#Event_bubbling_and_capture).
  * The `detail` argument corresponds to the [CustomEvent.detail](https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/detail) 
- * property and can contain any type of data. 
+ * property and can contain any type of data.
+ * 
+ * The event dispatcher can be typed to narrow the allowed event names and the type of the `detail` argument:
+ * ```ts
+ * const dispatch = createEventDispatcher<{
+ *  loaded: never; // does not take a detail argument
+ *  change: string; // takes a detail argument of type string, which is required
+ *  optional: number | null; // takes an optional detail argument of type number
+ * }>();
+ * ```
  * 
  * https://svelte.dev/docs#run-time-svelte-createeventdispatcher
  */

test/types/create-event-dispatcher.ts

@@ -0,0 +1,43 @@
+import { createEventDispatcher } from '$runtime/internal/lifecycle';
+
+const dispatch = createEventDispatcher<{
+   loaded: never
+   change: string
+   valid: boolean
+   optional: number | null
+}>();
+
+// @ts-expect-error: dispatch invalid event
+dispatch('some-event');
+
+dispatch('loaded');
+dispatch('loaded', null);
+dispatch('loaded', undefined);
+dispatch('loaded', undefined, { cancelable: true });
+// @ts-expect-error: no detail accepted
+dispatch('loaded', 123);
+
+// @ts-expect-error: detail not provided
+dispatch('change');
+dispatch('change', 'string');
+dispatch('change', 'string', { cancelable: true });
+// @ts-expect-error: wrong type of detail
+dispatch('change', 123);
+// @ts-expect-error: wrong type of detail
+dispatch('change', undefined);
+
+dispatch('valid', true);
+dispatch('valid', true, { cancelable: true });
+// @ts-expect-error: wrong type of detail
+dispatch('valid', 'string');
+
+dispatch('optional');
+dispatch('optional', 123);
+dispatch('optional', 123, { cancelable: true });
+dispatch('optional', null);
+dispatch('optional', undefined);
+dispatch('optional', undefined, { cancelable: true });
+// @ts-expect-error: wrong type of optional detail
+dispatch('optional', 'string');
+// @ts-expect-error: wrong type of option
+dispatch('optional', undefined, { cancelabled: true });

test/types/tsconfig.json

@@ -0,0 +1,16 @@
+{
+	"extends": "../../tsconfig.json",
+	"compilerOptions": {
+        "rootDir": "../..",
+		"baseUrl": "../../",
+		"paths": {
+			"$runtime/*": ["src/runtime/*"]
+		},
+		// enable strictest options
+		"allowUnreachableCode": false,
+		"noFallthroughCasesInSwitch": true,
+		"noImplicitReturns": true,
+		"strict": true,
+	},
+	"include": ["."]
+}