docs: added middleware documentation (#3000)

* docs: added a middleware documentation

* docs: fixes based on eslint

docs/content/advanced/backend/batch-jobs/customize-import.md

@@ -44,9 +44,12 @@ The batch job strategy class must extend the `AbstractBatchJobStrategy` class wh
 
 For example, you can define the following class in the file you created:
 
-```typescript title=src/strategies/import.ts
-import { AbstractBatchJobStrategy, BatchJobService } from '@medusajs/medusa'
-import { EntityManager } from 'typeorm'
+```ts title=src/strategies/import.ts
+import { 
+  AbstractBatchJobStrategy, 
+  BatchJobService,
+} from "@medusajs/medusa"
+import { EntityManager } from "typeorm"
 
 class MyImportStrategy extends AbstractBatchJobStrategy {
   protected batchJobService_: BatchJobService
@@ -54,10 +57,10 @@ class MyImportStrategy extends AbstractBatchJobStrategy {
   protected transactionManager_: EntityManager
 
   processJob(batchJobId: string): Promise<void> {
-    throw new Error('Method not implemented.')
+    throw new Error("Method not implemented.")
   }
   buildTemplate(): Promise<string> {
-    throw new Error('Method not implemented.')
+    throw new Error("Method not implemented.")
   }
 }
 
@@ -78,10 +81,10 @@ Since only one batch job strategy can handle a batch job type, you can overwrite
 
 So, for example, to overwrite the product import strategy set the `batchType` property in your strategy to `product-import`:
 
-```typescript
+```ts
 class MyImportStrategy extends AbstractBatchJobStrategy {
-  static batchType = 'product-import'
-  //...
+  static batchType = "product-import"
+  // ...
 }
 ```
 

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).

docs/content/advanced/backend/price-selection-strategy/override.md

@@ -12,46 +12,51 @@ If you’re interested in learning what a price selection strategy is and how it
 
 Create a TypeScript or JavaScript file in `src/strategies` of your Medusa server project with a class that extends the `AbstractPriceSelectionStrategy` class:
 
-```typescript title=src/strategies/price.ts
-import { AbstractPriceSelectionStrategy, IPriceSelectionStrategy, PriceSelectionContext, PriceSelectionResult } from "@medusajs/medusa";
+```ts title=src/strategies/price.ts
+import { 
+  AbstractPriceSelectionStrategy, 
+  IPriceSelectionStrategy, 
+  PriceSelectionContext, 
+  PriceSelectionResult,
+} from "@medusajs/medusa"
 
-import { EntityManager } from "typeorm";
+import { EntityManager } from "typeorm"
 
-export default class MyPriceListStrategy extends AbstractPriceSelectionStrategy {
+export default class MyStrategy extends AbstractPriceSelectionStrategy {
 
   withTransaction(manager: EntityManager): IPriceSelectionStrategy {
     if (!manager) {
       return this
     }
 
-    return new MyPriceListStrategy()
+    return new MyStrategy()
   }
 
   async calculateVariantPrice(
     variant_id: string,
     context: PriceSelectionContext
   ): Promise<PriceSelectionResult> {
-    //TODO
+    // TODO
   }
 }
 ```
 
-You can use services or repositories in the strategy by adding them to the constructor and updating the parameters passed to the `MyPriceListStrategy` constructor in `withTransaction`. For example:
+You can use services or repositories in the strategy by adding them to the constructor and updating the parameters passed to the `MyStrategy` constructor in `withTransaction`. For example:
 
-```typescript
+```ts
 import { 
   AbstractPriceSelectionStrategy, 
   CustomerService, 
   IPriceSelectionStrategy, 
   PriceSelectionContext, 
-  PriceSelectionResult 
-} from "@medusajs/medusa";
+  PriceSelectionResult, 
+} from "@medusajs/medusa"
 
-export default class MyPriceListStrategy extends AbstractPriceSelectionStrategy {
+export default class MyStrategy extends AbstractPriceSelectionStrategy {
   private customerService: CustomerService
 
   constructor({
-    customerService
+    customerService,
   }) {
     super()
     this.customerService = customerService
@@ -62,11 +67,11 @@ export default class MyPriceListStrategy extends AbstractPriceSelectionStrategy
       return this
     }
 
-    return new MyPriceListStrategy({
-      customerService: this.customerService
+    return new MyStrategy({
+      customerService: this.customerService,
     })
   }
-  //...
+  // ...
 }
 ```
 
@@ -80,10 +85,10 @@ This method accepts the variant ID as a first parameter and the [context](./inde
 
 This method must return an object having the following fields:
 
-```typescript noReport
+```ts noReport
 {
-  originalPrice, //number | null
-  calculatedPrice, //number | null
+  originalPrice, // number | null
+  calculatedPrice, // number | null
   prices // MoneyAmount[]
 }
 ```

www/docs/sidebars.js

@@ -368,6 +368,11 @@ module.exports = {
           id: "advanced/backend/endpoints/add",
           label: "Create an Endpoint"
         },
+        {
+          type: "doc",
+          id: "advanced/backend/endpoints/add-middleware",
+          label: "Add a Middleware"
+        },
         {
           type: "doc",
           id: "advanced/backend/services/create-service",