title: How to check online user in nodejs, socketio?
body: <p>How can I check user is online or not?
  I know we can track it by 'disconnect' event, but I don't know how to save it in my socket server.
  The client that i wrote in React, but it can only send accessToken.
  Thanks</p>
  
  <pre><code>socket.on('disconnect', function () {
          console.log('user disconnected');
      });
  </code></pre>