Tree - example 2

const preorder = (node, acc = []) => {
  if (!node) return acc;
  const { value, left, right } = node;
  acc.push(value);
  preorder(left, acc);
  preorder(right, acc);

  return acc;
};

console.log(preorder(tree)); //  [0, 1, 3, 6, 7, 4, 2, 5, 8, 9]