Range-Extraction.js

/**
 * Description:
 *
 * A format for expressing an ordered list of integers
 * is to use a comma separated list of either
 *
 * individual integers
 * or a range of integers denoted by the starting
 * integer separated from the end integer in the range
 * by a dash, '-'. The range includes all
 * integers in the interval including both endpoints. It
 * is not considered a range unless it spans at least 3
 * numbers. For example "12,13,15-17"
 *
 * Complete the solution so that it takes a
 * list of integers in increasing order and returns a correctly
 * formatted string in the range format.
 *
 * Example:
 *
 * solution([-10, -9, -8, -6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20]);
 * // returns "-10--8,-6,-3-1,3-5,7-11,14,15,17-20"
 *
 * Courtesy of rosettacode.org
 *
 * Kata URL: https://www.codewars.com/kata/51ba717bb08c1cd60f00002f
 *
 *
 * @param {*} list
 */
function solution(list) {
  let result = list[0] + "";
  let index = 0;
  let range = list.length - 1;
  solve();

  function solve() {
    if (index === range) {
      console.log(result);
      return result;
    }

    if (list[index] + 1 === list[index + 1]) {
      index++;
      solve();
    } else if (list[index] - 1 === list[index - 1]) {
      result += list[index] + "-";
    } else {
      //  result += list[index] + ",";
      index++;
      solve();
    }
  }
}

let a = solution([
  -6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20,
]);

console.log(
  solution([
    -6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20,
  ])
);
// "-6,-3-1,3-5,7-11,14,15,17-20"