https://www.geeksforgeeks.org/reverse-a-list-in-groups-of-given-size/

reverse-a-list-in-groups-of-given-size.js

// https://www.geeksforgeeks.org/reverse-a-list-in-groups-of-given-size/
class Node {
  constructor(val, next) {
    this.val = val;
    this.next = next;
  }
}
function printList(head) {
  let tmp = head;
  let arr = [];
  while (tmp) {
    arr.push(tmp.val);
    tmp = tmp.next;  
  }
  return arr;
}
function reverseK(head, K) {
  let tmp = head;
  let next;
  let prev;
  let count = 0;

  while (tmp && count < K) {
    next = tmp.next;
    tmp.next = prev;
    prev = tmp;
    tmp = next;
    count++;
  }
  if (next) {
    head.next = reverseK(next, K);
  }
  return prev;
}

const ex1Head = new Node(1, new Node(2,
  new Node(3, new Node(4, new Node(5, new Node(6, new Node(7, new Node(8))))))));
const res = reverseK(ex1Head, 3);
console.log(printList(res).join(',') === '3,2,1,6,5,4,8,7');