while문을 합쳐보았습니다!

 function mergeHelper(arr, arr2) {
        let result = [];
        let i = 0;
        let j = 0;

        while (i < arr.length || j < arr2.length) {
          if (arr[i] <= arr2[j] || j >= arr2.length) {
            result.push(arr[i]);
            i++;
          } else if (arr[i] >= arr2[j] || i >= arr.length) {
            result.push(arr2[j]);
            j++;
          }
        }
        return result;
      }