// Definition for singly-linked list
function ListNode(val) {
  this.val = val;
  this.next = null;
}

// Function to reverse a singly-linked list
function reverseLinkedList(head) {
  let prev = null;
  let current = head;
  let next = null;

  while (current !== null) {
    next = current.next; // Store the next node in the original list
    current.next = prev; // Reverse the current node's next pointer to point to the previous node
    prev = current; // Move the previous node to the current node
    current = next; // Move the current node to the next node in the original list
  }

  return prev; // The new head of the reversed list
}

// Example usage:
let list = new ListNode(1);
list.next = new ListNode(2);
list.next.next = new ListNode(3);
list.next.next.next = new ListNode(4);

let reversedList = reverseLinkedList(list);