gistfile1.js

// index.js
require('./a');
require('./b');

// mqtt.js
class MQTT {
  connect() {
    const connectionId = Math.random();
    console.log('connected to: ', connectionId);
    this.connectionId = connectionId;
  }
  publish(message) {
    console.log('(class) published to', message);
  }
  subscribe(topic){
    console.log('(class) subscribed to:', topic);
  }
}

const client = new MQTT();
client.connect();

module.exports = client;


// a.js
const { publish, subscribe, connectionId } = require('./mqtt');

publish('Hello from A');
subscribe('A Topic');
console.log({connectionId});


// b.js
const { publish, subscribe, connectionId } = require('./mqtt');

publish('Hello from B');
subscribe('B Topic');
console.log({connectionId});

Running index.js will give you:

connected to:  0.6625096413626022
(class) published to Hello from A
(class) subscribed to: A Topic
{ connectionId: 0.6625096413626022 }
(class) published to Hello from B
(class) subscribed to: B Topic
{ connectionId: 0.6625096413626022 }