fix: emit a connect_error event upon connection failure

Related: socketio/socket.io#3734

lib/socket.ts

@@ -84,6 +84,7 @@ export class Socket extends Emitter {
     this.subs = [
       on(io, "open", this.onopen.bind(this)),
       on(io, "packet", this.onpacket.bind(this)),
+      on(io, "error", this.onerror.bind(this)),
       on(io, "close", this.onclose.bind(this)),
     ];
   }
@@ -204,6 +205,18 @@ export class Socket extends Emitter {
     }
   }
 
+  /**
+   * Called upon engine or manager `error`.
+   *
+   * @param err
+   * @private
+   */
+  private onerror(err: Error) {
+    if (!this.connected) {
+      super.emit("connect_error", err);
+    }
+  }
+
   /**
    * Called upon engine `close`.
    *

test/socket.ts

@@ -50,6 +50,31 @@ describe("socket", function () {
     }, 300);
   });
 
+  it("fire a connect_error event when the connection cannot be established", (done) => {
+    const socket = io("http://localhost:9823", {
+      forceNew: true,
+      timeout: 100,
+    });
+    socket.on("connect_error", () => {
+      socket.close();
+      done();
+    });
+  });
+
+  it("doesn't fire a connect_error event when the connection is already established", (done) => {
+    const socket = io({ forceNew: true });
+    socket.on("connect", () => {
+      socket.io.engine.close(true);
+    });
+    socket.on("connect_error", () => {
+      done(new Error("should not happen"));
+    });
+    setTimeout(() => {
+      socket.close();
+      done();
+    }, 300);
+  });
+
   it("should change socket.id upon reconnection", (done) => {
     const socket = io({ forceNew: true });
     socket.on("connect", () => {