preorder_search.js

function preorderSearch(node) {
  // Check that a node exists.
  if (node === null) {
    return;
  }
  
  // Print the data of the node.
  console.log(node.data);
  
  // Pass in a reference to the left child node to preorderSearch.
  // Then, pass reference to the right child node to preorderSearch.
  preorderSearch(node.left);
  preorderSearch(node.right);
}