Return days relative to first day

getRelativeDays.js

import moment from 'moment'
import flow from 'lodash/flow'
import map from 'lodash/fp/map'

/**
* Returns days relative to first one, e. g.: 0, 2, 9, 10, 11
*
* @params {array} dates
* @returns {array}
*/
export const getRelativeDays = dates => {
  const initial = moment(dates.shift())

  return flow([
    map(date => moment(date).diff(initial, 'days')),
    days => {
      days.splice(0, 0, 0)

      return days
    }
  ])(dates)
}