feat: expose current offset to allow deduplication

Related: socketio/socket.io-client@655dce9

lib/socket.ts

@@ -217,6 +217,22 @@ export class Socket<
    * });
    */
   public connected: boolean = false;
+  /**
+   * The identifier of the current packet. Multiple retries of the same packet will have the same identifier, in order
+   * to allow deduplication (only process a packet once).
+   *
+   * @example
+   * io.on("connection", (socket) => {
+   *   socket.on("my-event", async (payload, callback) => {
+   *     const offset = socket.currentOffset;
+   *
+   *     await insertInDatabase(payload, offset);
+   *
+   *     callback();
+   *   })
+   * });
+   */
+  public currentOffset: string;
 
   /**
    * The session ID, which must not be shared (unlike {@link id}).
@@ -652,6 +668,7 @@ export class Socket<
     if (null != packet.id) {
       debug("attaching ack callback to event");
       args.push(this.ack(packet.id));
+      this.currentOffset = `${this.id}-${packet.id}`;
     }
 
     if (this._anyListeners && this._anyListeners.length) {

test/socket.ts

@@ -1105,4 +1105,28 @@ describe("socket", () => {
       socket3.on("disconnect", partialDone);
     });
   });
+
+  // TODO: enable once a new version of the socket.io-client package is released
+  // it("should retry with the same packet ID", (done) => {
+  //   const io = new Server(0);
+  //   let counter = 0;
+  //
+  //   io.on("connection", (socket) => {
+  //     socket.on("my-event", (cb) => {
+  //       expect(socket.currentOffset).to.eql(socket.id + "-0");
+  //       if (++counter === 3) {
+  //         cb();
+  //
+  //         success(done, io, clientSocket);
+  //       }
+  //     });
+  //   });
+  //
+  //   const clientSocket = createClient(io, "/", {
+  //     retries: 10,
+  //     ackTimeout: 20,
+  //   });
+  //
+  //   clientSocket.emit("my-event");
+  // });
 });