online, offline do something

 window.addEventListener('online', this.refreshOnlineStatus)

 window.addEventListener('offline', this.refreshOnlineStatus)

refreshOnlineStatus () {

      if (process.client) {

        if (typeof window.navigator.onLine === 'undefined') {

          // If the browser doesn't support connection status reports

          // assume that we are online because most apps' only react

          // when they now that the connection has been interrupted

          this.isOnline = true

        } else {

          this.isOnline = window.navigator.onLine

        }

      }

    },