整列した日付の配列を月ごとに分ける

eachMonth.js

const { isLastDayOfMonth } = require("date-fns");

const eachMonth = (ds, ms = []) => {
  if (ds.length === 0) return ms;
  const i = ds.findIndex(isLastDayOfMonth);
  if (i < 0) {
    return eachMonth([], [...ms, ds]);
  } else {
    return eachMonth(ds.slice(i + 1), [...ms, ds.slice(0, i + 1)]);
  }
};

module.exports = eachMonth;