function bfs<T>(tree: BSTNode<T>, fn: (n: T) => void) {
  const queue = [tree];

  while (queue.length) {
    const cur = queue.shift();

    if (cur.left) queue.push(cur.left);
    if (cur.right) queue.push(cur.right);

    fn(cur.value);
  }
}

a = [4, 5, 10, 2, 3, 1];
b = toBST(a);

list = [];
bfs(b, n => list.push(n)); /** [4, 2, 5, 1, 3, 10] */