docs: add usage with express-session and TypeScript

Related: socketio/socket.io#3890

docs/categories/08-Miscellaneous/faq.md

@@ -27,30 +27,4 @@ Take a look at [socket.io-client.java](https://github.com/nkzawa/socket.io-clien
 
 ## Usage with [express-session](https://www.npmjs.com/package/express-session)
 
-```js
-const express = require('express');
-const session = require('express-session');
-const app = express();
-
-const server = require('http').createServer(app);
-const io = require('socket.io')(server);
-
-const sessionMiddleware = session({ secret: 'keyboard cat', cookie: { maxAge: 60000 }});
-// register middleware in Express
-app.use(sessionMiddleware);
-// register middleware in Socket.IO
-io.use((socket, next) => {
-  sessionMiddleware(socket.request, {}, next);
-  // sessionMiddleware(socket.request, socket.request.res, next); will not work with websocket-only
-  // connections, as 'socket.request.res' will be undefined in that case
-});
-
-io.on('connection', (socket) => {
-  const session = socket.request.session;
-  session.connections++;
-  session.save();
-});
-
-const port = process.env.PORT || 3000;
-server.listen(port, () => console.log('server listening on port ' + port));
-```
+Please check [this guide](/how-to/use-with-express-session).

src/pages/how-to/use-with-express-session.md

@@ -222,3 +222,77 @@ io.on("connection", (socket) => {
   });
 });
 ```
+
+## With TypeScript
+
+To add proper typings to the session details, you will need to extend the `IncomingMessage` object from the Node.js "http" module.
+
+Which gives, in the [first case](#1st-use-case-socketio-only-retrieves-the-session-context):
+
+```ts
+import { Request, Response, NextFunction } from "express";
+import { Session } from "express-session";
+
+declare module "http" {
+    interface IncomingMessage {
+        session: Session & {
+            authenticated: boolean
+        }
+    }
+}
+
+io.use((socket, next) => {
+    sessionMiddleware(socket.request as Request, {} as Response, next as NextFunction);
+});
+```
+
+And in the [second case](#2nd-use-case-socketio-can-also-create-the-session-context):
+
+```ts
+import { Request, Response } from "express";
+import { Session } from "express-session";
+import { IncomingMessage } from "http";
+
+declare module "http" {
+    interface IncomingMessage {
+        cookieHolder?: string,
+        session: Session & {
+            count: number
+        }
+    }
+}
+
+const io = new Server(httpServer, {
+    allowRequest: (req, callback) => {
+        // with HTTP long-polling, we have access to the HTTP response here, but this is not
+        // the case with WebSocket, so we provide a dummy response object
+        const fakeRes = {
+            getHeader() {
+                return [];
+            },
+            setHeader(key: string, values: string[]) {
+                req.cookieHolder = values[0];
+            },
+            writeHead() {},
+        };
+        sessionMiddleware(req as Request, fakeRes as unknown as Response, () => {
+            if (req.session) {
+                // trigger the setHeader() above
+                fakeRes.writeHead();
+                // manually save the session (normally triggered by res.end())
+                req.session.save();
+            }
+            callback(null, true);
+        });
+    },
+});
+
+io.engine.on("initial_headers", (headers: { [key: string]: string }, req: IncomingMessage) => {
+    if (req.cookieHolder) {
+        headers["set-cookie"] = req.cookieHolder;
+        delete req.cookieHolder;
+    }
+});
+```
+
+Reference: [TypeScript's Declaration Merging](https://www.typescriptlang.org/docs/handbook/declaration-merging.html)