addLast(dataOrNode, once = false){
    let node = getNode(dataOrNode);
    if(this.head) {
      this.tail.next = node;
    } else {
      this.head = node;
    }

    while(node) {
      this.tail = node;
      this.length++;
      if(!node.next || once) { break; }
      node = node.next;
    }
    return node;
  }
Add element to the tail
@param dataOrNode
@returns {Node}