ranges.js

function rangeFromZero(end) {
  return [...Array(end).keys()];
}

function exclusiveRange(start, end) {
  if (end === undefined) {
    return rangeFromZero(start);
  }
  if (end > start) {
    return rangeFromZero(end - start).map(x => x + start);
  }
  return [];
}

function inclusiveRange(start, end) {
  if (end === undefined) {
    return rangeFromZero(start+1);
  }
  return exclusiveRange(start, end+1);
}