Add Redirect An Unauthorized User as a Next.js TIL

Author: jbranchaud

Diff for: README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1358 TILs and counting..._
+_1359 TILs and counting..._
 
 ---
 
@@ -592,6 +592,7 @@ _1358 TILs and counting..._
 - [Define URL Redirects In The Next Config](nextjs/define-url-redirects-in-the-next-config.md)
 - [Make Environment Variable Publicly Available](nextjs/make-environment-variable-publicly-available.md)
 - [Push A Route With A URL Object](nextjs/push-a-route-with-a-url-object.md)
+- [Redirect An Unauthorized User](nextjs/redirect-an-unauthorized-user.md)
 - [Remove A Query Param From The URL](nextjs/remove-a-query-param-from-the-url.md)
 - [Ship Public Assets With A Next.js App](nextjs/ship-public-assets-with-a-nextjs-app.md)
 

Diff for: nextjs/redirect-an-unauthorized-user.md

@@ -0,0 +1,50 @@
+# Redirect An Unauthorized User
+
+With the Page Router in earlier next version, we could do a server-side
+authorization check in `getServerSideProps` and then return a
+[`redirect`](https://nextjs.org/docs/pages/api-reference/functions/get-server-side-props#redirect)
+response in order to redirect the user to a page they are authorized for.
+
+That might look something like this:
+
+```javascript
+export async function getServerSideProps(context) {
+  const session = await getServerAuthSession()
+  const ability = getAbility({user: session?.user})
+
+  if (!ability.can('create', 'Post')) {
+    return {
+      redirect: {
+        destination: '/posts',
+        permanent: false,
+      },
+    }
+  }
+ 
+  return {
+    props: {},
+  }
+}
+```
+
+We can achieve the same thing with the App Router, but with a bit less code.
+The `next/navigation` package has a [`redirect`
+function](https://nextjs.org/docs/app/api-reference/functions/redirect) that we
+can invoke directly in a component. This will redirect the user instead of
+rendering the component to HTML.
+
+```javascript
+import { redirect } from 'next/navigation'
+
+export default async function CreatePost() {
+  const session = await getServerAuthSession()
+  const ability = getAbility({user: session?.user})
+
+  if (!ability.can('create', 'Post')) {
+    redirect('/posts')
+  }
+
+  // JSX follows
+  return (...)
+}
+```