feat: add action failure helper (#12878)

closes #12611

Exposes a helper method to check if a variable is an instanceof ActionFailure

.changeset/fresh-guests-knock.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': minor
+---
+
+feat: add helper to identify `ActionFailure` objects

packages/kit/src/exports/index.js

@@ -190,3 +190,12 @@ export function fail(status, data) {
 	// @ts-expect-error unique symbol missing
 	return new ActionFailure(status, data);
 }
+
+/**
+ * Checks whether this is an action failure thrown by {@link fail}.
+ * @param {unknown} e The object to check.
+ * @return {e is import('./public.js').ActionFailure}
+ */
+export function isActionFailure(e) {
+	return e instanceof ActionFailure;
+}

packages/kit/types/index.d.ts

@@ -1826,6 +1826,11 @@ declare module '@sveltejs/kit' {
 	 * @param data Data associated with the failure (e.g. validation errors)
 	 * */
 	export function fail<T extends Record<string, unknown> | undefined = undefined>(status: number, data: T): ActionFailure<T>;
+	/**
+	 * Checks whether this is an action failure thrown by {@link fail}.
+	 * @param e The object to check.
+	 * */
+	export function isActionFailure(e: unknown): e is ActionFailure<undefined>;
 	export type LessThan<TNumber extends number, TArray extends any[] = []> = TNumber extends TArray['length'] ? TArray[number] : LessThan<TNumber, [...TArray, TArray['length']]>;
 	export type NumericRange<TStart extends number, TEnd extends number> = Exclude<TEnd | LessThan<TEnd>, LessThan<TStart>>;
 	export const VERSION: string;