Stack in TypeScript

Stack Node:

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

Stack:

class Stack
{
   private root: StackNode;
   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("Stack is empty.");
       }
       return this.root.value;
   }

   public push(value: any)
   {
       if(this.isEmpty())
       {
           this.root = new StackNode();
           this.root.value = value;
       }
       else
       {
           let tmp = new StackNode();
           tmp.value = value;
           tmp.next = this.root;
           this.root = tmp;
       }
       this.size++;
   }

   public pop(): any
   {
       if(this.isEmpty())
       {
           throw new Error("Stack is empty.");
       }
       let tmp = this.root;
       this.root = this.root.next;
       this.size--;
       return tmp.value;
   }

   public poll(): any
   {
       if(this.isEmpty())
       {
           return null;
       }
       let tmp = this.root;
       this.root = this.root.next;
       this.size--;
       return tmp.value;
   }
}