Async middleware

README.md

@@ -14,7 +14,7 @@ The smol method routing and middleware for [Next.js](https://nextjs.org/) (also
 - Compatible with Express.js middleware and router => Drop-in replacement for Express.js.
 - Lightweight (~ 3KB) => Suitable for serverless environment.
 - 5x faster than Express.js with no overhead
-- Works with async handlers (with error catching)
+- Works with async handlers and middleware (with error catching)
 - TypeScript support
 
 ## Installation
@@ -57,6 +57,30 @@ For usage in pages with [`getServerSideProps`](https://nextjs.org/docs/basic-fea
 
 See an example in [nextjs-mongodb-app](https://github.com/hoangvvo/nextjs-mongodb-app) (CRUD, Authentication with Passport, and more)
 
+### Async middleware
+
+Middleware may `await next()` to perform actions after the request handler has been resolved. Like other tools which support async middleware, however, **all** middleware in the handler **must** call `next()` with `await` or `return` to execute properly.
+
+```javascript
+import nc from "next-connect";
+
+const handler = nc()
+  .use(async (req, res, next) => {
+    await next(); // Waits for .get() handler.
+
+    if (!res.writableEnded) {
+      res.end(res.body); // respond with "Hello world"!
+    }
+  })
+  .get(async (req, res) => {
+    res.body = "Hello world";
+  })
+
+export default handler;
+```
+
+
+
 ### TypeScript
 
 By default, the base interfaces of `req` and `res` are `IncomingMessage` and `ServerResponse`. When using in API Routes, you would set them to `NextApiRequest` and `NextApiResponse` by providing the generics to the factory function like so:
@@ -244,7 +268,7 @@ export async function getServerSideProps({ req, res }) {
 <details id="catch-all">
 <summary>Match multiple routes</summary>
 
-If you created the file `/api/<specific route>.js` folder, the handler will only run on that specific route. 
+If you created the file `/api/<specific route>.js` folder, the handler will only run on that specific route.
 
 If you need to create all handlers for all routes in one file (similar to `Express.js`). You can use [Optional catch all API routes](https://nextjs.org/docs/api-routes/dynamic-api-routes#optional-catch-all-api-routes).
 

src/index.d.ts

@@ -1,7 +1,7 @@
 declare module "next-connect" {
   import { IncomingMessage, ServerResponse } from "http";
 
-  type NextHandler = (err?: any) => void;
+  type NextHandler = (err?: any) => void | Promise<void>;
   type Middleware<Req, Res> = NextConnect<Req, Res> | RequestHandler<Req, Res>;
 
   type RequestHandler<Req, Res> = (

src/index.js

@@ -67,13 +67,11 @@ export default function factory({
     let i = 0;
     const len = handlers.length;
     const loop = async (next) => handlers[i++](req, res, next);
-    const next = (err) => {
-      i < len
-        ? err
-          ? onError(err, req, res, next)
-          : loop(next).catch(next)
-        : done && done(err);
-    };
+    const next = (err) => i < len
+      ? err
+        ? onError(err, req, res, next)
+        : loop(next).catch(next)
+      : done && done(err);
     next();
   };
   return nc;

test/index.test.js

@@ -61,6 +61,25 @@ describe("nc()", () => {
     return await request(app).get("/").expect("one", "1").expect("done");
   });
 
+  it("supports async middleware", async () => {
+    const handler = nc()
+      .use(async (req, res, next) => {
+        await next();
+        res.end(res.body)
+      })
+      .get(
+        (req, res) =>
+          new Promise((resolve) => {
+            setTimeout(() => {
+              res.body = "done"
+              resolve();
+            }, 1);
+          })
+      );
+    const app = createServer(handler);
+    return await request(app).get("/").expect("done");
+  });
+
   it("is a function with two argument", () => {
     assert(typeof nc() === "function" && nc().length === 2);
   });