docs: added middleware documentation (#3000)

* docs: added a middleware documentation

* docs: fixes based on eslint

docs/content/advanced/backend/endpoints/add-middleware.md

@@ -0,0 +1,72 @@
+# How to Add a Middleware
+
+In this document, you’ll learn how to add a middleware to an existing or custom route in Medusa.
+
+## Overview
+
+As the Medusa server is built on top of [Express](https://expressjs.com/), Express’s features can be utilized during your development with Medusa.
+
+One feature in particular is adding a [middleware](http://expressjs.com/en/guide/using-middleware.html#using-middleware). A middleware is a function that has access to the request and response objects.
+
+A middleware can be used to perform an action when an endpoint is called or modify the response, among other usages.
+
+You can add a middleware to an existing route in the Medusa server, a route in a plugin, or your custom routes.
+
+---
+
+## Add a Middleware
+
+Adding a middleware is very similar to adding a custom endpoint. The middleware must be created either in the `src/api/index.ts` entry point, or other TypeScript or JavaScript files imported into `src/api/index.ts`.
+
+:::info
+
+Learn more about creating custom endpoints in [this documentation](./add.md).
+
+:::
+
+The following code snippet is an example of adding a middleware:
+
+```ts title=src/api/index.ts
+import { Router } from "express"
+
+export default () => {
+  const router = Router()
+
+  router.use("/store/products", (req, res, next) => {
+      // / perform an action when retrieving products
+      next()
+  })
+
+  return router
+}
+```
+
+This code snippet adds a middleware to the [List Products](/api/store/#tag/Product/operation/GetProducts) endpoint. In the middleware function, you can perform any action.
+
+Then, you must call the `next` method received as a third parameter in the middleware function to ensure that the endpoint executes after the middleware.
+
+:::info
+
+You can learn more about Middlewares and their capabilities in [Express’s documentation](http://expressjs.com/en/guide/using-middleware.html#using-middleware).
+
+:::
+
+---
+
+## Building Files
+
+Similar to custom endpoints, you must transpile the files under `src` into the `dist` directory for the server to load them.
+
+To do that, run the following command before running the Medusa server:
+
+```bash npm2yarn
+npm run build
+```
+
+---
+
+## See Also
+
+- [Create an Endpoint](./add.md)
+- [Store API reference](/api/store)
+- [Admin API reference](/api/admin)

docs/content/advanced/backend/endpoints/add.md

@@ -295,6 +295,7 @@ npm run build
 
 ## See Also
 
+- [Add a Middleware](./add-middleware.md)
 - [Storefront API Reference](/api/store)
 - [Admin API Reference](/api/admin)
 - [Create a Service](./../services/create-service.md).