取连续数字

需求是取输入数据中的连续数字，将连续数字使用连接符~连接首尾数字

输入：1,2,3, 5, 6, 9, 12, 13, 14
输出： 1 ~ 3, 5 ~ 6, 9, 12 ~ 14

window.onload = function () {
    let input = [1,2,3, 5, 6, 9, 12, 13, 14]
    let output = continuousNum(input);
    console.log('output', output);
  }

  const continuousNum = (arr) => {
    let res = [];
    let str ='';
    let i = 0;
    res[i] = [arr[i]];
    arr.reduce((pre, cur) => {
      if (cur - pre === 1) {
        res[i].push(cur);
      } else {
        i++;
        res[i] = [cur];
      }
      return cur;
    })
    res.forEach(itm => {
      let len = itm.length;
      if ( len > 1) {
        str +=`${itm[0]}~${itm[len-1]},`;
      } else {
        str += `${itm[0]},`;
      }
    })
    return str.substring(0, str.length-1);
  }