Convert sequence of numbers in an array to range of numbers

convert_number_sequence_to_range.js

/**
 * convert sequence of numbers in an array to range of numbers
 * @param {Array} arr
 * @return {Array}
 * @example
 *  var arrayOfNumbers = [ 2, 3, 4, 5, 475, 476, 477, 478, 479, 6, 8, 9, 10, 400, 480 ];
 *  var result = convert_number_sequence_to_range(arrayOfNumbers); // => [ '2-6', '8-10', '400', '475-480' ]
 */
var convert_number_sequence_to_range = function(arr) {
  var sortedArr = arr.sort(function(a, b) {
    return a - b;
  });

  var start = sortedArr[0];
  var prev = start;
  var result = [];
  var rangeSet = false;
  var lastIndex = 0;

  sortedArr.forEach(function(current, i) {
    if (i === 0) {
      result.push('' + current);
      lastIndex = 0;
    } else {
      var next = sortedArr[i + 1] || -1;

      // console.log(prev, current, next);
      if ((prev + 1) === current && current === (next - 1)) {
        if (!rangeSet) {
          result[lastIndex] = '' + result[lastIndex] + '-';
        }
        rangeSet = true;
      } else if ((prev + 1) === current) {
        rangeSet = false;
        result[lastIndex] = '' + result[lastIndex] + current;
        lastIndex++;
      } else if ((prev + 1) !== current && current !== (next - 1)) {
        result[lastIndex] = '' + current;
        lastIndex++;
      } else if (next > current) {
        result[lastIndex] = '' + current;
      }
    }
    prev = current;
  });

  return result;
};

// var arrayOfNumbers = [ 2, 3, 4, 5, 475, 476, 477, 478, 479, 6, 8, 9, 10, 400, 480 ];
// var result = convert_number_sequence_to_range(arrayOfNumbers);
//
// console.log(arrayOfNumbers);
// console.log(result);