feat: emit an Error object upon middleware error

This commit restores the ability to send additional data in the
middleware functions, which was removed during the rewrite to
Typescript ([1]).

The only difference with the previous implementation is that the client
will now emit a "connect_error" (previously, "error") event with an
actual Error object, with both the message and an optional "data"
attribute.

```js
// server-side
io.use((socket, next) => {
  const err = new Error("not authorized");
  err.data = { content: "Please retry later" };
  next(err);
});

// client-side
socket.on("connect_error", err => {
  console.log(err.message); // not authorized
  console.log(err.data.content); // Please retry later
});
```

[1]: a5581a9

lib/client.ts

@@ -93,7 +93,9 @@ export class Client {
         this._packet({
           type: PacketType.CONNECT_ERROR,
           nsp: name,
-          data: "Invalid namespace"
+          data: {
+            message: "Invalid namespace"
+          }
         });
       }
     });

lib/index.ts

@@ -7,7 +7,7 @@ import path from "path";
 import engine from "engine.io";
 import { Client } from "./client";
 import { EventEmitter } from "events";
-import { Namespace } from "./namespace";
+import { ExtendedError, Namespace } from "./namespace";
 import { ParentNamespace } from "./parent-namespace";
 import { Adapter, Room, SocketId } from "socket.io-adapter";
 import * as parser from "socket.io-parser";
@@ -593,7 +593,7 @@ export class Server extends EventEmitter {
    * @public
    */
   public use(
-    fn: (socket: Socket, next: (err?: Error) => void) => void
+    fn: (socket: Socket, next: (err?: ExtendedError) => void) => void
   ): Server {
     this.sockets.use(fn);
     return this;

lib/namespace.ts

@@ -8,6 +8,10 @@ import { Adapter, Room, SocketId } from "socket.io-adapter";
 
 const debug = debugModule("socket.io:namespace");
 
+export interface ExtendedError extends Error {
+  data?: any;
+}
+
 export class Namespace extends EventEmitter {
   public readonly name: string;
   public readonly sockets: Map<SocketId, Socket> = new Map();
@@ -18,7 +22,9 @@ export class Namespace extends EventEmitter {
   readonly server: Server;
 
   /** @private */
-  _fns: Array<(socket: Socket, next: (err: Error) => void) => void> = [];
+  _fns: Array<
+    (socket: Socket, next: (err: ExtendedError) => void) => void
+  > = [];
 
   /** @private */
   _rooms: Set<Room> = new Set();
@@ -60,7 +66,7 @@ export class Namespace extends EventEmitter {
    * @public
    */
   public use(
-    fn: (socket: Socket, next: (err?: Error) => void) => void
+    fn: (socket: Socket, next: (err?: ExtendedError) => void) => void
   ): Namespace {
     this._fns.push(fn);
     return this;
@@ -73,7 +79,7 @@ export class Namespace extends EventEmitter {
    * @param {Function} fn - last fn call in the middleware
    * @private
    */
-  private run(socket: Socket, fn: (err: Error) => void) {
+  private run(socket: Socket, fn: (err: ExtendedError) => void) {
     const fns = this._fns.slice(0);
     if (!fns.length) return fn(null);
 
@@ -129,7 +135,11 @@ export class Namespace extends EventEmitter {
     this.run(socket, err => {
       process.nextTick(() => {
         if ("open" == client.conn.readyState) {
-          if (err) return socket._error(err.message);
+          if (err)
+            return socket._error({
+              message: err.message,
+              data: err.data
+            });
 
           // track socket
           this.sockets.set(socket.id, socket);

test/socket.io.ts

@@ -522,7 +522,7 @@ describe("socket.io", () => {
       srv.listen(() => {
         const socket = client(srv, "/doesnotexist");
         socket.on("connect_error", err => {
-          expect(err).to.be("Invalid namespace");
+          expect(err.message).to.be("Invalid namespace");
           done();
         });
       });
@@ -814,7 +814,7 @@ describe("socket.io", () => {
           sio.of(/^\/dynamic-\d+$/);
           sio.of((name, query, next) => next(null, "/dynamic-101" === name));
           socket.on("connect_error", err => {
-            expect(err).to.be("Invalid namespace");
+            expect(err.message).to.be("Invalid namespace");
             done();
           });
         });
@@ -2185,7 +2185,30 @@ describe("socket.io", () => {
           done(new Error("nope"));
         });
         socket.on("connect_error", err => {
-          expect(err).to.be("Authentication error");
+          expect(err.message).to.be("Authentication error");
+          done();
+        });
+      });
+    });
+
+    it("should pass an object", done => {
+      const srv = createServer();
+      const sio = new Server(srv);
+      sio.use((socket, next) => {
+        const err = new Error("Authentication error");
+        // @ts-ignore
+        err.data = { a: "b", c: 3 };
+        next(err);
+      });
+      srv.listen(() => {
+        const socket = client(srv);
+        socket.on("connect", () => {
+          done(new Error("nope"));
+        });
+        socket.on("connect_error", err => {
+          expect(err).to.be.an(Error);
+          expect(err.message).to.eql("Authentication error");
+          expect(err.data).to.eql({ a: "b", c: 3 });
           done();
         });
       });