code: function onerror(err) {
      var error = new Error("probe error: " + err);
      error.transport = transport$$1.name;
  
      freezeTransport();
  
      debug$5('probe transport "%s" failed because of error: %s', name, err);
  
      self.emit("upgradeError", error);
    }
docs: Handle any error that happens while probing