Request/response in Nodejs with postal and promises

ReqRes.js

import postal from 'postal'
import { v1 } from 'uuid'

export default class ReqRes {
  static channel = 'reqres'
  static request (topic, payload) {
    var replyTo = {
      channel: ReqRes.channel,
      topic: topic + v1()
    }

    var result = new Promise((resolve, reject) => {
      postal.subscribe({
        channel: replyTo.channel,
        topic: replyTo.topic,
        callback: function (msg, envelope) {
          resolve(msg)
        }
      }).once()

      postal.publish({
        channel: ReqRes.channel,
        topic,
        replyTo,
        data: payload
      })
    })

    return result
  }

  static respondTo (topic, handler) {
    postal.subscribe({
      channel: ReqRes.channel,
      topic,
      callback: function (msg, envelope) {
        let replyTo = envelope.replyTo

        handler(msg)
        .then(function (response) {
          postal.publish({
            channel: ReqRes.channel,
            topic: replyTo.topic,
            data: response
          })
        })
      }
    })
  }
}

ReqRes.test.js

import chai, { expect } from 'chai';
import ReqRes from 'ReqRes';

chai.config.includeStack = true;

describe('## ReqRes', () => {
  it('should request and respond', done => {
    ReqRes.respondTo('testRequest', data => {
      expect(data.payload).to.be.true
      return Promise.resolve({ payload: 42 })
    })


    ReqRes.request('testRequest', { payload: true })
    .then(response => {
      expect(response.payload).to.equal(42)
      done()
    })
  })
})