Discussion on: Daily Challenge #93 - Range Extraction

  const solution = x =>
    x.reduce(
      (r, vfirst) =>
      [
        ...r.filter(y => y[0] < vfirst),
        ... (x.indexOf(vfirst - 1) === -1 ? [[vfirst, ...x.map(y => [y]).find(([vlast]) => (
          vlast > vfirst &&
          !x.some(v => v == vlast + 1) &&
          x.every(v => v < vfirst || v >= vlast || x.indexOf(v + 1) > -1)
        )) || [] ]] : []),
        ...r.filter(y => y[0] > vfirst)
      ],
      []
    ).map(y => y.slice(-1)[0] - y[0] >= 2 ? y[0] + '-' + y.slice(-1)[0] : y.join()).join();



  console.log(solution([10]))                                                                           // 10
  console.log(solution([10, 10]))                                                                       // 10
  console.log(solution([10, 10, 9]))                                                                    // 9,10
  console.log(solution([12, 11]))                                                                       // 11,12
  console.log(solution([10, 11]))                                                                       // 10,11
  console.log(solution([12, 11, 10]))                                                                   // 10-12
  console.log(solution([9, 10, 11]))                                                                    // 9-11
  console.log(solution([9, 9, 10, 11]))                                                                 // 9-11
  console.log(solution([9, 10, 9, 11]))                                                                 // 9-11
  console.log(solution([10, 11, 12, 14, 15, 16, 17, 17, 9, 13, 8, 6, 5]))                               // 5,6,8-17
  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
  console.log(solution([6, -6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20]));    // -6,-3-1,3-11,14,15,17-20
  console.log(solution([1, 10, 12, 11, -5]));                                                           // -5, 1, 10, 11, 12