From 87868a2db2303bb20657a4e19fc2e9b151f7ceb6 Mon Sep 17 00:00:00 2001
From: Edoardo Ranghieri <edoardo@ranghieri.com>
Date: Mon, 12 Aug 2024 04:33:26 +0200
Subject: [PATCH] feat(middleware): support standalone middleware definition
 via exported `createMiddleware` function

---
 packages/next-safe-action/src/index.ts |  2 +-
 packages/next-safe-action/src/utils.ts | 22 ++++++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/packages/next-safe-action/src/index.ts b/packages/next-safe-action/src/index.ts
index 7c6756c1..967caa29 100644
--- a/packages/next-safe-action/src/index.ts
+++ b/packages/next-safe-action/src/index.ts
@@ -10,7 +10,7 @@ import {
 	formatValidationErrors,
 } from "./validation-errors";
 
-export { ActionMetadataError, DEFAULT_SERVER_ERROR_MESSAGE } from "./utils";
+export { ActionMetadataError, createMiddleware, DEFAULT_SERVER_ERROR_MESSAGE } from "./utils";
 export {
 	ActionValidationError,
 	flattenBindArgsValidationErrors,
diff --git a/packages/next-safe-action/src/utils.ts b/packages/next-safe-action/src/utils.ts
index c923c9ec..aaa41115 100644
--- a/packages/next-safe-action/src/utils.ts
+++ b/packages/next-safe-action/src/utils.ts
@@ -1,3 +1,5 @@
+import type { MiddlewareFn } from "./index.types";
+
 export const DEFAULT_SERVER_ERROR_MESSAGE = "Something went wrong while executing the operation.";
 
 export const isError = (error: unknown): error is Error => error instanceof Error;
@@ -28,3 +30,23 @@ export class ActionMetadataError extends Error {
 		this.name = "ActionMetadataError";
 	}
 }
+
+/**
+ * Creates a standalone middleware function. It accepts a generic object with optional `serverError`, `ctx` and `metadata`
+ * properties, if you need one or all of them to be typed. The type for each property that is passed as generic is the
+ * **minimum** shape required to define the middleware function, but it can also be larger than that.
+ *
+ * {@link https://next-safe-action.dev/docs/safe-action-client/middleware#create-standalone-middleware-with-createmiddleware See docs for more information}
+ */
+export const createMiddleware = <BaseData extends { serverError?: any; ctx?: object; metadata?: any }>() => {
+	return {
+		define: <NextCtx extends object>(
+			middlewareFn: MiddlewareFn<
+				BaseData extends { serverError: infer SE } ? SE : any,
+				BaseData extends { metadata: infer MD } ? MD : any,
+				BaseData extends { ctx: infer Ctx extends object } ? Ctx : object,
+				NextCtx
+			>
+		) => middlewareFn,
+	};
+};