breaking: improve types for `createEventDispatcher` (#7224)

---------

Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

.github/workflows/ci.yml

@@ -46,8 +46,17 @@ jobs:
     timeout-minutes: 10
     strategy:
       matrix:
-        node-version: 14
+        include:
-        os: [ubuntu-latest, windows-latest, macOS-latest]
+          - node-version: 14
+            os: ubuntu-latest
+          - node-version: 14
+            os: windows-latest
+          - node-version: 14
+            os: macOS-latest
+          - node-version: 16
+            os: ubuntu-latest
+          - node-version: 18
+            os: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
       - uses: actions/setup-node@v3

CHANGELOG.md

@@ -2,7 +2,9 @@
 
 ## Unreleased (4.0)
 
-* Minimum supported Node version is now Node 14
+* **breaking** Minimum supported Node version is now Node 14
+* **breaking** Minimum supported TypeScript version is now 5 (it will likely work with lower versions, but we make no guarantess about that)
+* **breaking** Stricter types for `createEventDispatcher` (see PR for migration instructions) ([#7224](https://github.com/sveltejs/svelte/pull/7224))
 
 ## Unreleased (3.0)
 

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

@@ -56,6 +56,14 @@ export function onDestroy(fn: () => any) {
 	get_current_component().$$.on_destroy.push(fn);
 }
 
+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;
+}
+
 export interface DispatchOptions {
 	cancelable?: boolean;
 }
@@ -70,18 +78,21 @@ export interface DispatchOptions {
  * 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.
  * 
+ * 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
  */
-export function createEventDispatcher<EventMap extends {} = any>(): <
+export function createEventDispatcher<EventMap extends Record<string, any> = any>(): EventDispatcher<EventMap> {
-	EventKey extends Extract<keyof EventMap, string>
->(
-	type: EventKey,
-	detail?: EventMap[EventKey],
-	options?: DispatchOptions
-) => boolean {
 	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) {
@@ -95,7 +106,7 @@ export function createEventDispatcher<EventMap extends {} = any>(): <
 		}
 
 		return true;
-	};
+	}) as EventDispatcher<EventMap>;
 }
 
 /**

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": ["."]
+}