Convert given binary tree to doubly linked list in typescript

Ts program for Convert given binary tree to doubly linked list. Here problem description and explanation.

/* 
  TypeScript program for
  Convert a given binary tree to doubly linked list
*/
// Binary Tree Node
class LinkNode
{
	public data: number;
	public left: LinkNode;
	public right: LinkNode;
	constructor(data: number)
	{
		// Set node value
		this.data = data;
		this.left = null;
		this.right = null;
	}
}
class BinaryTree
{
	public root: LinkNode;
	constructor()
	{
		// Set initial values
		this.root = null;
	}
	// Display node element of doubly linked list
	public display()
	{
		if (this.root == null)
		{
			console.log("Empty Linked List");
		}
		else
		{
			console.log("Linked List Head to Tail :");
			// Get first node of linked list
			var temp = this.root;
			var tail: LinkNode = null;
			// iterate the linked list
			while (temp != null)
			{
				tail = temp;
				// Display node value
				console.log("  " + temp.data);
				// Visit to next node
				temp = temp.right;
			}
			console.log("\nLinked List Tail to Head :");
			// Get last node of linked list
			temp = tail;
			// iterate the linked list
			while (temp != null)
			{
				// Display node value
				console.log("  " + temp.data);
				// Visit to prev node
				temp = temp.left;
			}
		}
	}
	// Function which is convert binary tree to DLL
	public transform(node: LinkNode, x: LinkNode, y: LinkNode)
	{
		if (node != null)
		{
			// Visit to left subtree
			this.transform(node.left, x, node);
			if (this.root == null)
			{
				// Set the root node when is empty
				this.root = node;
			}
			// Visit to right subtree
			this.transform(node.right, node, y);
			if (node.left == null)
			{
				// Connect left node
				node.left = x;
				if (x != null)
				{
					// Connect right to left node
					x.right = node;
				}
			}
			if (node.right == null)
			{
				// Connect right node
				node.right = y;
				if (y != null)
				{
					// Connect left to right node
					y.left = node;
				}
			}
		}
	}
	public doublyList()
	{
		var node = this.root;
		if (node != null)
		{
			// Reset the root value
			this.root = null;
			// Covert into Doubly linked list
			this.transform(node, null, null);
		}
	}
	public static main(args: string[])
	{
		// Create new tree
		var tree = new BinaryTree();
		/*
		  Binary Tree
		-----------------------
		        1
		       /  \
		      2    3
		     /    /  \
		    4    5    6
		     \    \
		      7    9
		     / 
		    8
		*/
		// Add binary tree nodes
		tree.root = new LinkNode(1);
		tree.root.left = new LinkNode(2);
		tree.root.right = new LinkNode(3);
		tree.root.right.right = new LinkNode(6);
		tree.root.right.left = new LinkNode(5);
		tree.root.right.left.right = new LinkNode(9);
		tree.root.left.left = new LinkNode(4);
		tree.root.left.left.right = new LinkNode(7);
		tree.root.left.left.right.left = new LinkNode(8);
		tree.doublyList();
		tree.display();
	}
}
BinaryTree.main([]);
/*
 file : code.ts
 tsc --target es6 code.ts
 node code.js
 */

Output

Linked List Head to Tail :
  4
  8
  7
  2
  1
  5
  9
  3
  6

Linked List Tail to Head :
  6
  3
  9
  5
  1
  2
  7
  8
  4