Queue in TypeScript

Queue Node:

class QueueNode
{
   public value: any;
   public next: QueueNode;
}

Queue

class Queue
{
   private head: QueueNode;
   private tail: QueueNode;

   private size: number;

   public isEmpty(): boolean
   {
       return this.size <= 0;
   }

   public length():number
   {
       return this.size;
   }

   public peek(): any
   {
       if(this.isEmpty())
       {
           throw new Error("Queue is empty.");
       }
       return this.head.value;
   }

   public enqueue(value: any)
   {
       if(this.isEmpty())
       {
           this.tail = new QueueNode;
           this.tail.value = value;
           this.head = this.tail;
           this.size++;
       }
       else
       {
           this.tail.next = new QueueNode();
           this.tail.next.value = value;
           this.tail = this.tail.next;

           this.size++;
       }
   }

   public dequeue(): any
   {
       if(this.isEmpty())
       {
           throw new Error("Queue is empty.");
       }
       if(this.size == 1)
       {
           let tmp = this.head;
           this.head = null;
           this.tail = null;
           this.size = 0;
           return tmp.value;
       }
       else
       {
           let tmp = this.head;
           this.head = this.head.next;
           this.size--;
           return tmp.value;
       }
   }

   public poll(): any
   {
       if(this.isEmpty())
       {
           return null;
       }
       if(this.size == 1)
       {
           let tmp = this.head;
           this.head = null;
           this.tail = null;
           this.size = 0;
           return tmp.value;
       }
       else
       {
           let tmp = this.head;
           this.head = this.head.next;
           this.size--;
           return tmp.value;
       }
   }
}