A method to remove duplicates from a linked list

removeLinkedListDuplicates.js

/** 
 * removeLinkedListDuplicates: Removes duplicates from an unsorted linked list.
 * Return value is a linked list with duplicates removed. 
 * @param {Object} linkedList - The linkedList to remove duplicates from.
 * @param {Object} [currentNode] - The currently evaluated node
 * @param {Object} [previouslySeen] - An object storing the values of all previously seen nodes
 */
function removeLinkedListDuplicates(linkedList, currentNode, previouslySeen ) {

  currentNode = currentNode || linkedList.head;
  previouslySeen = previouslySeen || {};

  if (Object.keys(previouslySeen).length === 0) {
    previouslySeen[currentNode.value] = 1;
  }

  if (!currentNode.next) { return linkedList; }

  if (previouslySeen[currentNode.next.value]) {
    currentNode.next = currentNode.next.next;
    if (linkedList.size) { linkedList.size -= 1; }
    if (currentNode.next.previous) { currentNode.next.next.previous = currentNode; }
    if (!currentNode.next) { return linkedList; }
  } else { previouslySeen[currentNode.next.value] = 1; }

  return removeDuplicates(linkedList, currentNode.next, previouslySeen)
}