Example of PeerJS DataConnection

example.js

/* global Peer */

document.addEventListener('DOMContentLoaded', () => { 
  initiator();
  another();
}, false);

// initiator side
const initiator = () => {
  const peer = new Peer('initiatorId', { debug: 3 });

  peer.on('open', (id) => {
    const conn = peer.connect('calleId');

    conn.on('error', (error) => {
      console.error(error);
    });

    conn.on('open', () => {
      // now you can send data via conn
      conn.send('*_*');
    });

    conn.on('data', (data) => {
      console.log('from callee', data);
    });
  });
};

// callee side
const another = () => {
  const peer = new Peer('calleId', { debug: 3 });

  peer.on('open', (id) => {
    peer.on('connection', (conn) => {
      conn.on('error', (error) => {
        console.error(error);
      });

      conn.on('open', (data) => {
        conn.send('another *_*');
      });

      conn.on('data', (data) => {
        console.log('from initiator', data);
      });
    });
  });
};

I have a question about the callee side. Let's say I need to respond to initiator based on what the initiator has sent to me, and not some independent message like _.

In this example, whatever the initiator has sent is just logged locally, and once the connection to initiator is open, a callee can send data.

I real-life application case, such as mine, I need to apply some sort of synchronization on callee's side, meaning
When I have data from initiator, I prepare an answer based on that data, but I don't send it until the 'open' even has fired.

Can you suggest an elegant way to synchronize?

P.S.
Yes, the 'data' is fired before 'open' 100% of times in my case.
If conn.send happens before 'open', peerjs doesn't raise any error, the initiator simply doesn't get a response data.