feat(middleware): support standalone middleware definition via export…

…ed `createMiddleware` function

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,

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,
+	};
+};