Construct sorted linked list from binary search tree

bst-to-dll.js

let node = function(data) {
  this.data = data;
  this.left = null;
  this.right = null;
}

let head;

let BT2DLL = function(root)
{
  // Base cases
  if (!root) return;

  // Recursively convert right subtree
  BT2DLL(root.right);

  // insert root into DLL
  root.right = head;

  // Change left pointer of previous head
  if (head) head.left = root;

  // Change head of Doubly linked list
  head = root;

  // Recursively convert left subtree
  BT2DLL(root.left);
}

// Utility function for printing double linked list.
let printDLL = function(head)
{
    console.log('Print DLL');
    while (head)
    {
        console.log(head.data);
        head = head.right;
    }
}

preOrder = function(node) {
  if (!node) return null;
  console.log(node.data);
  preOrder(node.left);
  preOrder(node.right);
}

// app

let root = new node(5);
root.left = new node(3);
root.right = new node(6);
root.left.left = new node(1);
root.left.right = new node(4);
root.right.right = new node(8);
root.left.left.left = new node(0);
root.left.left.right = new node(2);
root.right.right.left = new node(7);
root.right.right.right = new node(9);

preOrder(root);
BT2DLL(root);
printDLL(head);