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

  return acc;
};

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