数组循环倒叙生成

项目中要用到这种数组
[1, 2, 3, 4, 5, 4, 3, 2, 1, 2, 3, 4, 5, 4, 3, 2, 1, 2, 3, 4, 5, 4, 3, 2, 1, 2, 3, 4, 5]

可以通过一下方法自动生成

const arr = [];
const arr2 = [1, 2, 3, 4, 5];
let len = arr2.length;
const N = 29
let flag = true
let sub = 0

for(let i=0; i < N; i++) {
       if(flag) {
               sub = sub > 0 ? sub : i > len ? 1 : 0
               arr.push(arr2[sub++])
               if(sub === len) {
                       flag = false
                       sub = len - 2
               }
       } else {
               arr.push(arr2[sub--])
               if(sub === -1) {
                       flag = true
               }
       }
}
console.log(arr);

let a = [],k = 0,ad = 1;
    for(let i=1;i<=4;){
      if(k<10){
        if(ad === 1 ){
          a.push(i++)
          if(i === 4){
            ad = 0
          }
          k++
        }else{
          a.push(i--)
          if(i === 1){
            ad = 1
          }
          k++
        }
      }else{ break}
    }
```