fix(typings): properly type the adapter attribute

Related: #3796

lib/index.ts

@@ -37,6 +37,8 @@ type ParentNspNameMatchFn = (
   fn: (err: Error | null, success: boolean) => void
 ) => void;
 
+type AdapterConstructor = typeof Adapter | ((nsp: Namespace) => Adapter);
+
 interface EngineOptions {
   /**
    * how many ms without a pong packet to consider the connection closed
@@ -152,7 +154,7 @@ interface ServerOptions extends EngineAttachOptions {
    * the adapter to use
    * @default the in-memory adapter (https://github.com/socketio/socket.io-adapter)
    */
-  adapter: any;
+  adapter: AdapterConstructor;
   /**
    * the parser to use
    * @default the default parser (https://github.com/socketio/socket.io-parser)
@@ -207,7 +209,7 @@ export class Server<
     ParentNspNameMatchFn,
     ParentNamespace<ListenEvents, EmitEvents, ServerSideEvents>
   > = new Map();
-  private _adapter?: typeof Adapter;
+  private _adapter?: AdapterConstructor;
   private _serveClient: boolean;
   private opts: Partial<EngineOptions>;
   private eio;
@@ -360,10 +362,11 @@ export class Server<
    * @return self when setting or value when getting
    * @public
    */
-  public adapter(): typeof Adapter | undefined;
-  public adapter(v: typeof Adapter): this;
-  public adapter(v?: typeof Adapter): typeof Adapter | undefined | this;
-  public adapter(v?: typeof Adapter): typeof Adapter | undefined | this {
+  public adapter(): AdapterConstructor | undefined;
+  public adapter(v: AdapterConstructor): this;
+  public adapter(
+    v?: AdapterConstructor
+  ): AdapterConstructor | undefined | this {
     if (!arguments.length) return this._adapter;
     this._adapter = v;
     for (const nsp of this._nsps.values()) {

lib/namespace.ts

@@ -102,6 +102,7 @@ export class Namespace<
    * @private
    */
   _initAdapter(): void {
+    // @ts-ignore
     this.adapter = new (this.server.adapter()!)(this);
   }
 

test/socket.io.test-d.ts

@@ -1,8 +1,9 @@
 "use strict";
-import { Server, Socket } from "..";
+import { Namespace, Server, Socket } from "..";
 import type { DefaultEventsMap } from "../lib/typed-events";
 import { createServer } from "http";
 import { expectError, expectType } from "tsd";
+import { Adapter } from "socket.io-adapter";
 
 // This file is run by tsd, not mocha.
 
@@ -275,4 +276,25 @@ describe("server", () => {
       });
     });
   });
+
+  describe("adapter", () => {
+    it("accepts arguments of the correct types", () => {
+      const io = new Server({
+        adapter: (nsp) => new Adapter(nsp),
+      });
+      io.adapter(Adapter);
+
+      class MyCustomAdapter extends Adapter {
+        constructor(nsp, readonly opts) {
+          super(nsp);
+        }
+      }
+      io.adapter((nsp) => new MyCustomAdapter(nsp, { test: "123" }));
+    });
+
+    it("does not accept arguments of wrong types", () => {
+      const io = new Server();
+      expectError(io.adapter((nsp) => "nope"));
+    });
+  });
 });