make number sequence to range sequence 1, 2, 3, 5, 6, 8 => 1-3, 5-6, 8

makeRange.js

  function last(nums) {
      return nums[nums.length - 1];
  }
  function makeRange(nums) {
      return nums.slice().sort(function (a, b) { return a - b; })
          .reduce(function (segs, current) {
          var lastSeg = last(segs);
          var currentSeg = lastSeg || [];
          if (!lastSeg) {
              segs.push(currentSeg);
          }
          var lastNumber = last(currentSeg);
          if (lastNumber === undefined) {
              currentSeg.push(current);
          }
          else if (lastNumber + 1 === current) {
              if (currentSeg.length === 1) {
                  currentSeg.push(current);
              }
              else {
                  currentSeg[1] = current;
              }
          }
          else {
              segs.push([current]);
          }
          return segs;
      }, [])
          .map(function (a) { return a.join('-'); })
          .join(',');
  }
  makeRange([1, 2, 3, 5, 6, 7, 9]);
    makeRange([1, 2, 3, 6, 8]);