Subtraction of alternate nodes of linked list in typescript

Ts program for Subtraction of alternate nodes of linked list. Here problem description and explanation.

/*
    TypeScript program for
    Subtraction of the alternate nodes of linked list
*/
// Linked list node
class LinkNode
{
	public data: number;
	public next: LinkNode;
	constructor(data: number)
	{
		this.data = data;
		this.next = null;
	}
}
class SingleLL
{
	public head: LinkNode;
	public tail: LinkNode;
	constructor()
	{
		// Set initial value
		this.head = null;
		this.tail = null;
	}
	public insert(data: number)
	{
		var node = new LinkNode(data);
		if (this.head == null)
		{
			// Add first node
			this.head = node;
		}
		else
		{
			// Add node at the end position
			this.tail.next = node;
		}
		// New last node
		this.tail = node;
	}
	// Display linked list element
	public display()
	{
		if (this.head == null)
		{
			return;
		}
		var temp = this.head;
      	var result = "";
		// iterating linked list elements
		while (temp != null)
		{
			result += (" " + temp.data + " →");
			// Visit to next node
			temp = temp.next;
		}
		console.log(result, " NULL");
	}
	// Find the subtraction of all alternate 
	// nodes in linked list
	public alternateSubtraction()
	{
		// Define resultant variables
		var result = 0;
		var counter = 0;
		// Start to first node of linked list
		var temp = this.head;
		// iterating linked list elements
		while (temp != null)
		{
			if (counter % 2 == 0)
			{
				// When get alternate node
				if (result == 0)
				{
					result = temp.data;
				}
				else
				{
					result = result - temp.data;
				}
			}
			// Node counter
			counter++;
			// Visit to next node
			temp = temp.next;
		}
		console.log(" Alternate nodes subtraction : " + result);
	}
	public static main()
	{
		var sll = new SingleLL();
		// Add node in linked list
		// 4 → 7 → 2 → 9 → 1 → 3 → 4 → 3 → 6 → NULL
		sll.insert(4);
		sll.insert(7);
		sll.insert(2);
		sll.insert(9);
		sll.insert(1);
		sll.insert(3);
		sll.insert(4);
		sll.insert(3);
		sll.insert(6);
		// Display of linked list nodes
		sll.display();
		// Test
		sll.alternateSubtraction();
	}
}
SingleLL.main();
/*
 file : code.ts
 tsc --target es6 code.ts
 node code.js
 */

Output

 4 → 7 → 2 → 9 → 1 → 3 → 4 → 3 → 6 →  NULL
 Alternate nodes subtraction : -9