fix: form action submit function callback return type (#13420)

* fix: add promise return type to enhance form action callback

* remove unnecessary formatting change

* fix: Dangling promise is now awaited

* reapply fix

---------

Co-authored-by: S. Elliott Johnson <[email protected]>

Author: McAllaster

.changeset/clean-dancers-type.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: add promise return type to the `enhance` action callback

packages/kit/src/exports/public.d.ts

@@ -1484,7 +1484,7 @@ export type SubmitFunction<
 			 * @param invalidateAll Set `invalidateAll: false` if you don't want the action to call `invalidateAll` after submission.
 			 */
 			update: (options?: { reset?: boolean; invalidateAll?: boolean }) => Promise<void>;
-	  }) => void)
+	  }) => MaybePromise<void>)
 >;
 
 /**

packages/kit/src/runtime/app/forms.js

@@ -205,7 +205,7 @@ export function enhance(form_element, submit = () => {}) {
 			result = { type: 'error', error };
 		}
 
-		callback({
+		await callback({
 			action,
 			formData: form_data,
 			formElement: form_element,

packages/kit/test/apps/basics/src/routes/actions/enhance/+page.svelte

@@ -37,7 +37,9 @@
 	use:enhance={({ controller }) => {
 		previous?.abort();
 		previous = controller;
-		return () => count++;
+		return () => {
+			count++;
+		};
 	}}
 >
 	<button class="form2">Submit</button>

packages/kit/types/index.d.ts

@@ -1466,7 +1466,7 @@ declare module '@sveltejs/kit' {
 				 * @param invalidateAll Set `invalidateAll: false` if you don't want the action to call `invalidateAll` after submission.
 				 */
 				update: (options?: { reset?: boolean; invalidateAll?: boolean }) => Promise<void>;
-		  }) => void)
+		  }) => MaybePromise<void>)
 	>;
 
 	/**