fix: only set 'connected' to true after middleware execution

The Socket instance is only considered connected when the "connection"
event is emitted, and not during the middleware(s) execution.

```js
io.use((socket, next) => {
  console.log(socket.connected); // prints "false"
  next();
});

io.on("connection", (socket) => {
  console.log(socket.connected); // prints "true"
});
```

Related: #4129

Author: darrachequesne

Diff for: lib/socket.ts

@@ -126,8 +126,7 @@ export class Socket<
    */
   public data: Partial<SocketData> = {};
 
-  public connected: boolean;
-  public disconnected: boolean;
+  public connected: boolean = false;
 
   private readonly server: Server<
     ListenEvents,
@@ -163,8 +162,6 @@ export class Socket<
     } else {
       this.id = base64id.generateId(); // don't reuse the Engine.IO id because it's sensitive information
     }
-    this.connected = true;
-    this.disconnected = false;
     this.handshake = this.buildHandshake(auth);
   }
 
@@ -342,6 +339,7 @@ export class Socket<
    */
   _onconnect(): void {
     debug("socket connected - writing packet");
+    this.connected = true;
     this.join(this.id);
     if (this.conn.protocol === 3) {
       this.packet({ type: PacketType.CONNECT });
@@ -489,7 +487,6 @@ export class Socket<
     this.nsp._remove(this);
     this.client._remove(this);
     this.connected = false;
-    this.disconnected = true;
     this.emitReserved("disconnect", reason);
     return;
   }
@@ -631,6 +628,13 @@ export class Socket<
     run(0);
   }
 
+  /**
+   * Whether the socket is currently disconnected
+   */
+  public get disconnected() {
+    return !this.connected;
+  }
+
   /**
    * A reference to the request that originated the underlying Engine.IO Socket.
    *

Diff for: test/socket.io.ts

@@ -2726,6 +2726,25 @@ describe("socket.io", () => {
         if (++count === 2) done();
       });
     });
+
+    it("should only set `connected` to true after the middleware execution", (done) => {
+      const httpServer = createServer();
+      const io = new Server(httpServer);
+
+      const clientSocket = client(httpServer, "/");
+
+      io.use((socket, next) => {
+        expect(socket.connected).to.be(false);
+        expect(socket.disconnected).to.be(true);
+        next();
+      });
+
+      io.on("connection", (socket) => {
+        expect(socket.connected).to.be(true);
+        expect(socket.disconnected).to.be(false);
+        success(io, clientSocket, done);
+      });
+    });
   });
 
   describe("socket middleware", () => {