breaking: update onMount type definition to prevent async function return (#8136)

---------

Co-authored-by: Yuichiro Yamashita <xydybaseball@gmail.com>
Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

CHANGELOG.md

@@ -6,6 +6,7 @@
 * **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))
 * **breaking** Stricter types for `Action` and `ActionReturn` (see PR for migration instructions) ([#7224](https://github.com/sveltejs/svelte/pull/7224))
+* **breaking** Stricter types for `onMount` - now throws a type error when returning a function asynchronously to catch potential mistakes around callback functions (see PR for migration instructions) ([#8136](https://github.com/sveltejs/svelte/pull/8136))
 * Add `a11y no-noninteractive-element-interactions` rule ([#8391](https://github.com/sveltejs/svelte/pull/8391))
 * Add `a11y-no-static-element-interactions`rule ([#8251](https://github.com/sveltejs/svelte/pull/8251))
 * Bind `null` option and input values consistently ([#8312](https://github.com/sveltejs/svelte/issues/8312))

src/runtime/internal/lifecycle.ts

@@ -27,11 +27,13 @@ export function beforeUpdate(fn: () => any) {
  * It must be called during the component's initialisation (but doesn't need to live *inside* the component; 
  * it can be called from an external module).
  * 
+ * If a function is returned _synchronously_ from `onMount`, it will be called when the component is unmounted.
+ * 
  * `onMount` does not run inside a [server-side component](/docs#run-time-server-side-component-api).
  * 
  * https://svelte.dev/docs#run-time-svelte-onmount
  */
-export function onMount(fn: () => any) {
+export function onMount<T>(fn: () => T extends Promise<() => any> ? "Returning a function asynchronously from onMount won't call that function on destroy" : T): void {
 	get_current_component().$$.on_mount.push(fn);
 }
 

test/types/on-mount.ts

@@ -0,0 +1,58 @@
+import { onMount } from '$runtime/index';
+
+// sync and no return
+onMount(() => {
+	console.log('mounted');
+});
+
+// sync and return value
+onMount(() => {
+	return 'done';
+});
+
+// sync and return sync
+onMount(() => {
+	return () => {
+		return 'done';
+	};
+});
+
+// sync and return async
+onMount(() => {
+	return async () => {
+		const res = await fetch('');
+		return res;
+	};
+});
+
+// async and no return
+onMount(async () => {
+	await fetch('');
+});
+
+// async and return value
+onMount(async () => {
+	const res = await fetch('');
+	return res;
+});
+
+// @ts-expect-error async and return sync
+onMount(async () => {
+	return () => {
+		return 'done';
+	};
+});
+
+// @ts-expect-error async and return async
+onMount(async () => {
+	return async () => {
+		const res = await fetch('');
+		return res;
+	};
+});
+
+// @ts-expect-error async and return any
+onMount(async () => {
+    const a: any = null as any;
+	return a;
+});