mqttclient.js

import mqtt from 'mqtt';

export function mqclient(options) {
  return new Promise((resolve, reject) => {
    fetch('https://your-lambda-endpoint', {
      method: 'get'
    }).then((response) => {
      const client = mqtt.connect(response.body.url, options);
      resolve(client);
    }).catch(err => reject(err));
  });
}

mqclient()
  .then(client => {
    client.on('connect', () => {
      // do something
      // eg: client.publish('topic', 'message')
    });

    client.on('close', () => {
      // do something
    });

    client.on('message', () => {
      // do something
    })
  })
  .catch(console.log);