feat: listen to the "offline" event

The connection will be closed once the "offline" event is emitted by
the browser, in order not to wait for the heartbeat mechanism to detect
the disconnection.

Reference: https://developer.mozilla.org/en-US/docs/Web/API/Window/offline_event

Related: socketio/socket.io-client#1433

Author: darrachequesne

lib/socket.js

@@ -102,6 +102,12 @@ class Socket extends Emitter {
         },
         false
       );
+      if (this.hostname !== "localhost") {
+        this.offlineEventListener = () => {
+          this.onClose("transport close");
+        };
+        addEventListener("offline", this.offlineEventListener, false);
+      }
     }
 
     this.open();
@@ -622,6 +628,10 @@ class Socket extends Emitter {
       // ignore further transport communication
       this.transport.removeAllListeners();
 
+      if (typeof removeEventListener === "function") {
+        removeEventListener("offline", this.offlineEventListener, false);
+      }
+
       // set ready state
       this.readyState = "closed";