kthSmallestElement.js

var kthSmallest = function(root, k) {
  let record = [];
  
  var traverseTree = function(node) {
    record.push(node.val);
    if (node.left) { return traverseTree(node.left) }
    if (node.right) { return traverseTree(node.right) }
  }
  
  traverseTree(root);
  record = record.sort();
  
  return record[k - 1];
};