Tree - example 4

const postorder = function (node, acc = []) {
  if (!node) return acc;
  const { value, left, right } = node;

  postorder(left, acc);
  postorder(right, acc);
  acc.push(value);

  return acc;
};

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