How to sum two linked lists where each node represents a digit

// sum lists where digits are in reverse order
function sum(list1, list2, carry) {
  if (!list1 && !list2 && carry === 0) {
    return null;
  }
  
  const result = new Node();
  let value = !carry ? 0 : carry;
  if (list1) {
    value += list1.data;
  }
  if (list2) {
    value += list2.data;
  }
  
  result.data = value % 10; // second digit of num
  
  // Recurse
  if (list1 || list2) {
    const more = sum(!list1 ? null : list1.next, !list2 ? null : list2.next, value >= 10 ? 1 : 0);
    result.next = more;
  }
  return result;
}
function Node(data) {
  this.next = null;
  this.data = data;
}
Node.prototype.addToTail = function(data) {
  const end = new Node(data);
  let current = this;
  while (current.next !== null) {
    current = current.next;
  }
  current.next = end;
  return end;
};
// sum lists where digits are stored in forward order
function PartialSum() {
  this.sum = null;
  this.carry = 0;
}
function sumForward(list1, list2, carry) {
  const len1 = getLength(list1);
  const len2 = getLength(list2);
  
  // Pad shorter list with zeros
  if (len1 < len2) {
    list1 = padList(list1, len2 - len1);
  } else {
    list2 = padList(list2, len1 - len2);
  }
  
  const sum = addListsHelper(list1, list2);
  
  // if value left over, insert at front of list, otherwise just return linked list
  if (sum.carry === 0) {
    return sum.sum;
  } else {
    const result = insertBefore(sum.sum, sum.carry);
    return result;
  }
}
// helper for adding the lists
function addListsHelper(list1, list2) {
  if (!list1 && !list2) {
    const sum = new PartialSum();
    return sum;
  }
  // add smaller digits recursively
  const sum = addListsHelper(list1.next, list2.next);
  
  // add carry to current data
  const value = sum.carry + list1.data + list2.data;
  
  // insert sum of current digits
  const fullResult = insertBefore(sum.sum, value % 10);
  
  // Return sum so far
  sum.sum = fullResult;
  sum.carry = Math.floor(value / 10);
  return sum;
}
// helper function used to get the length of a linked list
function getLength(list) {
  let total = 0;
  
  let current = list;
  while (current) {
    total++;
    current = current.next;
  }
  
  return total;
}
// helper function used to make list lengths equal
function padList(list, n) {
  let head = list;
  for (let i = 0; i < n; i++) {
    head = insertBefore(head, 0);
  }
  return head;
}
// helper function to insert something in front of a linked list
function insertBefore(target, data) {
  const node = new Node(data);
  if (target) {
    node.next = target;
  }
  return node;
}