[JS] Reverse linked list even subparts

reverseLinkedListSubparts.js

function reverseLinkedList(head, lastElement = null) {
  let prev = lastElement; 
  let cur = head; 
  let next = null; 
  
  while(cur != lastElement) {
    next = cur.next; 
    cur.next = prev;
    prev = cur;
    cur = next;            
  }
  
  return prev;
}
     
function isEven(num) {
    return (num % 2) === 0;
}

function reverse(head) {
  let evenHead = null;
  let prevEvenHead = null;
  let pos = head;
  let prev = null;
  while(pos) {
    if(isEven(pos.data)) {
      if(!evenHead) { 
        evenHead = pos;
        prevEvenHead = prev;
      }
    }
    else { 
      if(evenHead) {
        const newHead = reverseLinkedList(evenHead, pos);
        if(prevEvenHead) prevEvenHead.next = newHead;
        else head = newHead;
        evenHead = null;
        prevEvenHead = null;
      }
    }    
    prev = pos;
    pos = pos.next;
  }
  if(evenHead) {
    const newHead = reverseLinkedList(evenHead);
    if(prevEvenHead) prevEvenHead.next = newHead; 
    else head = newHead;
  }
  
  return head;
}

class Node {
  constructor(x) {
    this.data = x;
    this.next = null;
  }
}