Range Sum of BST

var rangeSumBST = function(root, low, high) {
    let sum = 0;
    const bfs = (root) => {
        if(root === null) return;
        if(root.val >= low && root.val <= high) {
            sum += root.val;
        }
        bfs(root.left);
        bfs(root.right);
    }
    bfs(root);
    return sum;
};