docs, test

src/runtime/internal/lifecycle.ts

@@ -27,6 +27,8 @@ 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

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;
+});