resolver.js

const Moment = require('moment')
const MomentRange = require('moment-range')
const moment = MomentRange.extendMoment(Moment);
const fetch = require('node-fetch')

const getItems = () => { 
  return fetch(MY_API_LINK, {
    method: 'post',
    headers: {
      'Content-Type': 'application/json',
      'Authorization': MY_API_KEY
    },
    body: '{"query":"#Trytowriteyourqueryherequery{allReservations{idarrivalDatedepartureDate}}"}', 
  })
}

module.exports = function sum(event) {
  getItems()
    .then(response => {
        console.log(response)
        const data = event.data
        const rStartDate = moment("2018-04-21T10:00:00.000Z")
        const rEndDate = moment("2018-04-27T10:00:00.000Z")
        const queryRange = moment.range(data.a, data.b)
        const reservationRange = moment.range(rStartDate, rEndDate)
        const overlap = queryRange.overlaps(reservationRange)
        console.log(overlap)
        return {
          data: {
            overlapping: overlap,
            response: response
          }
        }
    })
  }