findDateRanges: Find and group date-ranges in an Array of dates

findDateRanges.js

/**
 * @function findDateRanges
 * @description Find and group date-ranges in an Array of dates
 *
 * @param {Array} sourceCollection - containing ISODateStrings in any order
 * @param {?Array} destinationCollection - defaults to []
 *
 * @returns {Array} - ^destinationCollection passed or new Array
 *
 * @example findDateRanges([
 *            '2001-01-07T07:07:07.070Z', 
 *            '2001-01-04T04:04:04.040Z',
 *            '2001-01-04T04:02:04.016Z',
 *            '2001-01-02T02:02:02.020Z',
 *            '2001-01-01T01:01:01.010Z'
 *          ])  -->  [  // As Date Objects:
 *            [ 1/1/2001, 1/2/2001 ],
 *            [ 1/4/2001, 1/4/2001 ],
 *            [ 1/7/2001 ] }
 *          ]
 */
const findDateRanges = (sourceCollection, destinationCollection=[]) => {
  const toDate = dateString => new Date(dateString);
  const datesObjsSortedAsc = [].concat(sourceCollection).map(toDate).sort((a, b) => a === b ? 0 : a > b ? 1 : -1);
  for (let i = 0, l = datesObjsSortedAsc.length, a, d, p, pd; i < l; i++) {
    a = function addToRange() {
      p = [];
      p.push(d);
      destinationCollection.push(p);
    };
    d = toDate(datesObjsSortedAsc[i]);  // Date as Date Object
    if (p) {  // Has p (previous-range)
      pd = p[p.length - 1];  // Previous-date
      // Determine if date is part of range
      if (
        d.getUTCFullYear() === pd.getUTCFullYear() &&               // Same year
        d.getUTCMonth() === pd.getUTCMonth() &&                     // Same month
        ([ 0, -1 ].indexOf(pd.getUTCDate() - d.getUTCDate()) > -1)  // 0-1 day later (duplicate date handling)
      ) {
        // Part of range
        p.push(d);
      } else {
        // New range
        a();
      }
    } else {
      // Initial loop iteration
      a();
    }
  }
  return destinationCollection;
};