generator.js

// imperative way
function rangeMonths(start, end) {
  const out = []
  for(let curr = moment(start), mEnd = moment(end); curr.isBefore(mEnd); curr.add(1, 'day'))
    if (curr.date() === 1)
      out.push(curr.format())
  return out
}

const months = rangeMonths('2015-01-01', '2015-10-01')

// generator
function* rangeMonths(start, end) {
  for(let curr = moment(start), mEnd = moment(end); curr.isBefore(mEnd); curr.add(1, 'day'))
    if (curr.date() === 1)
      yield curr.format()
}

const months = into([], map(identity), rangeMonths('2015-01-01', '2015-10-01'))