preorder.js

const tree = convertBinaryTree("31,52,,32,64,43,,34".split(","));

console.log(getNodeVal(tree));

function getNodeVal(node, result = []) {
    result.push(node.val);

    node.left && getNodeVal(node.left, result);
    node.right && getNodeVal(node.right, result);
  
  	return result;
}

function convertBinaryTree(arr) {
  let root;
  
  let insertNode = function(parentNode, childNode) {
    if (!childNode || childNode.val == "") return;
    
    if (childNode.val < parentNode.val) {
      if (parentNode.left === null) {
        parentNode.left = childNode;
      } else {
        insertNode(parentNode.left, childNode)
      }
    } else {
      if (parentNode.right === null) {
        parentNode.right = childNode;
      } else {
        insertNode(parentNode.right, childNode)
      }
    }
  }
  
  arr.forEach( val => {
    let node = {
      val: val,
      left: null,
      right: null
    };
    if (root) insertNode(root, node);
    else root = node;
  });
  
  return root;
}